Squirrels- natural polypeptides with a huge molecular weight. They are part of all living organisms and perform various biological functions.

The structure of the protein.

Proteins have 4 levels of structure:

• primary structure of a protein- linear sequence of amino acids in the polypeptide chain, folded in space:

• protein secondary structure- conformation of the polypeptide chain, because twisting in space due to hydrogen bonds between NH And SO groups. There are 2 installation methods: α -spiral and β - structure.

• protein tertiary structure is a three-dimensional representation of a swirling α - spiral or β -structures in space:

This structure is formed by disulfide bridges -S-S- between cysteine ​​residues. Oppositely charged ions participate in the formation of such a structure.

• quaternary protein structure formed by the interaction between different polypeptide chains:

Protein synthesis.

The synthesis is based on the solid-phase method, in which the first amino acid is fixed on a polymer carrier, and new amino acids are sequentially sutured to it. The polymer is then separated from the polypeptide chain.

The physical properties of the protein.

The physical properties of the protein are determined by the structure, so the proteins are divided into globular(soluble in water) and fibrillar(insoluble in water).

Chemical properties of proteins.

1. Protein denaturation(destruction of the secondary and tertiary structure with the preservation of the primary). An example of denaturation is the curdling of egg whites when eggs are boiled.

2. Protein hydrolysis- irreversible destruction of the primary structure in an acidic or alkaline solution with the formation of amino acids. This way you can determine the quantitative composition of proteins.

3. Qualitative reactions:

Biuret reaction- interaction of the peptide bond and salts of copper (II) in an alkaline solution. At the end of the reaction, the solution turns purple.

xantoprotein reaction- when reacted with nitric acid, a yellow color is observed.

The biological significance of protein.

1. Proteins are a building material; muscles, bones, and tissues are built from it.

2. Proteins - receptors. They transmit and receive signals from neighboring cells from the environment.

3. Proteins play an important role in the body's immune system.

4. Proteins perform transport functions and carry molecules or ions to the place of synthesis or accumulation. (Hemoglobin carries oxygen to tissues.)

5. Proteins - catalysts - enzymes. These are very powerful selective catalysts that speed up reactions millions of times.

There are a number of amino acids that cannot be synthesized in the body - irreplaceable, they are obtained only with food: tizine, phenylalanine, methinine, valine, leucine, tryptophan, isoleucine, threonine.

As you know, proteins are the basis for the origin of life on our planet. But it was the coacervate drop, consisting of peptide molecules, that became the basis for the birth of living things. This is beyond doubt, because the analysis of the internal composition of any representative of the biomass shows that these substances are found in everything: plants, animals, microorganisms, fungi, viruses. Moreover, they are very diverse and macromolecular in nature.

These structures have four names, all of them are synonyms:

• proteins;
• proteins;
• polypeptides;
• peptides.

protein molecules

Their number is truly incalculable. In this case, all protein molecules can be divided into two large groups:

• simple - consist only of amino acid sequences connected by peptide bonds;
• complex - the structure and structure of the protein are characterized by additional protolytic (prosthetic) groups, also called cofactors.

Moreover, complex molecules also have their own classification.

Gradation of complex peptides

1. Glycoproteins are closely related compounds of protein and carbohydrate. Prosthetic groups of mucopolysaccharides are woven into the structure of the molecule.
2. Lipoproteins are a complex compound of protein and lipid.
3. Metalloproteins - metal ions (iron, manganese, copper and others) act as a prosthetic group.
4. Nucleoproteins - the connection of protein and nucleic acids (DNA, RNA).
5. Phosphoproteins - the conformation of a protein and an orthophosphoric acid residue.
6. Chromoproteins are very similar to metalloproteins, however, the element that is part of the prosthetic group is a whole colored complex (red - hemoglobin, green - chlorophyll, and so on).

Each group considered has a different structure and properties of proteins. The functions they perform also vary depending on the type of molecule.

Chemical structure of proteins

From this point of view, proteins are a long, massive chain of amino acid residues interconnected by specific bonds called peptide bonds. From the side structures of the acids depart branches - radicals. This structure of the molecule was discovered by E. Fischer at the beginning of the 21st century.

Later, proteins, the structure and functions of proteins were studied in more detail. It became clear that there are only 20 amino acids that form the structure of the peptide, but they can be combined in a variety of ways. Hence the diversity of polypeptide structures. In addition, in the process of life and performance of their functions, proteins are able to undergo a number of chemical transformations. As a result, they change the structure, and a completely new type of connection appears.

To break the peptide bond, that is, to break the protein, the structure of the chains, you need to choose very harsh conditions (the action of high temperatures, acids or alkalis, a catalyst). This is due to the high strength in the molecule, namely in the peptide group.

The detection of the protein structure in the laboratory is carried out using the biuret reaction - exposure to the freshly precipitated polypeptide (II). The complex of the peptide group and the copper ion gives a bright violet color.

There are four main structural organizations, each of which has its own structural features of proteins.

Organization levels: primary structure

As mentioned above, a peptide is a sequence of amino acid residues with or without inclusions, coenzymes. So the primary name is such a structure of the molecule, which is natural, natural, is truly amino acids connected by peptide bonds, and nothing more. That is, a polypeptide of a linear structure. At the same time, the structural features of proteins of such a plan are that such a combination of acids is decisive for the performance of the functions of a protein molecule. Due to the presence of these features, it is possible not only to identify the peptide, but also to predict the properties and role of a completely new, not yet discovered. Examples of peptides with a natural primary structure are insulin, pepsin, chymotrypsin, and others.

Secondary conformation

The structure and properties of proteins in this category change somewhat. Such a structure can be formed initially from nature or when the primary structure is exposed to severe hydrolysis, temperature, or other conditions.

This conformation has three varieties:

1. Smooth, regular, stereoregular coils built from amino acid residues that twist around the main axis of the connection. They are held together only by those arising between the oxygen of one peptide group and the hydrogen of another. Moreover, the structure is considered correct due to the fact that the turns are evenly repeated every 4 links. Such a structure can be either left-handed or right-handed. But in most known proteins, the dextrorotatory isomer predominates. Such conformations are called alpha structures.
2. The composition and structure of proteins of the following type differs from the previous one in that hydrogen bonds are formed not between residues adjacent to one side of the molecule, but between significantly distant, and at a sufficiently large distance. For this reason, the entire structure takes the form of several wavy, serpentine polypeptide chains. There is one feature that a protein must exhibit. The structure of amino acids on the branches should be as short as possible, like glycine or alanine, for example. This type of secondary conformation is called beta sheets for the ability to seem to stick together when forming a common structure.
3. Biology refers to the third type of protein structure as complex, scattered, disordered fragments that do not have stereoregularity and are capable of changing the structure under the influence of external conditions.

No examples of proteins having a secondary structure by nature have been identified.

Tertiary education

This is a fairly complex conformation called a "globule". What is such a protein? Its structure is based on the secondary structure, however, new types of interactions between the atoms of the groups are added, and the whole molecule seems to curl up, thus focusing on the fact that the hydrophilic groups are directed inside the globule, and the hydrophobic ones are outward.

This explains the charge of the protein molecule in colloidal solutions of water. What types of interactions are present here?

1. Hydrogen bonds - remain unchanged between the same parts as in the secondary structure.
2. interactions - occur when the polypeptide is dissolved in water.
3. Ionic attraction - formed between differently charged groups of amino acid residues (radicals).
4. Covalent interactions - are able to form between specific acid sites - cysteine ​​molecules, or rather, their tails.

Thus, the composition and structure of proteins with a tertiary structure can be described as polypeptide chains folded into globules that retain and stabilize their conformation due to various types of chemical interactions. Examples of such peptides: phosphoglycerate kenase, tRNA, alpha-keratin, silk fibroin, and others.

Quaternary structure

This is one of the most complex globules that proteins form. The structure and functions of proteins of this kind are very versatile and specific.

What is such a conformation? These are several (in some cases dozens) large and small polypeptide chains that are formed independently of each other. But then, due to the same interactions that we considered for the tertiary structure, all these peptides twist and intertwine with each other. In this way, complex conformational globules are obtained, which can contain metal atoms, lipid groups, and carbohydrate groups. Examples of such proteins are DNA polymerase, tobacco virus envelope, hemoglobin, and others.

All the peptide structures we have considered have their own identification methods in the laboratory, based on modern possibilities of using chromatography, centrifugation, electron and optical microscopy, and high computer technologies.

Functions performed

The structure and function of proteins are closely correlated with each other. That is, each peptide plays a certain role, unique and specific. There are also those who are able to perform several significant operations in one living cell at once. However, it is possible to express in a generalized form the main functions of protein molecules in the organisms of living beings:

1. Ensuring movement. Unicellular organisms, or organelles, or some types of cells are capable of locomotion, contraction, movement. This is provided by proteins that are part of the structure of their motor apparatus: cilia, flagella, cytoplasmic membrane. If we talk about cells incapable of moving, then proteins can contribute to their contraction (muscle myosin).
2. Nutritional or reserve function. It is the accumulation of protein molecules in the eggs, embryos and seeds of plants to further replenish the missing nutrients. When cleaved, peptides give amino acids and biologically active substances that are necessary for the normal development of living organisms.
3. Energy function. In addition to carbohydrates, proteins can also give strength to the body. With the breakdown of 1 g of the peptide, 17.6 kJ of useful energy is released in the form of adenosine triphosphoric acid (ATP), which is spent on vital processes.
4. Signal and It consists in the implementation of careful monitoring of ongoing processes and the transmission of signals from cells to tissues, from them to organs, from the latter to systems, and so on. A typical example is insulin, which strictly fixes the amount of glucose in the blood.
5. receptor function. It is carried out by changing the conformation of the peptide on one side of the membrane and involving the other end in the restructuring. At the same time, the signal and the necessary information are transmitted. Most often, such proteins are built into the cytoplasmic membranes of cells and exercise strict control over all substances passing through it. They also alert you to chemical and physical changes in the environment.
6. Transport function of peptides. It is carried out by channel proteins and carrier proteins. Their role is obvious - transporting the necessary molecules to places with a low concentration from parts with a high one. A typical example is the transport of oxygen and carbon dioxide through organs and tissues by the protein hemoglobin. They also carry out the delivery of compounds with a low molecular weight through the cell membrane inside.
7. structural function. One of the most important of those that protein performs. The structure of all cells, their organelles is provided precisely by peptides. They, like a frame, set the shape and structure. In addition, they support it and modify it if necessary. Therefore, for growth and development, all living organisms need proteins in the diet. These peptides include elastin, tubulin, collagen, actin, keratin and others.
8. catalytic function. Enzymes do it. Numerous and varied, they accelerate all chemical and biochemical reactions in the body. Without their participation, an ordinary apple in the stomach could be digested in only two days, with a high probability of rotting. Under the action of catalase, peroxidase and other enzymes, this process takes two hours. In general, it is thanks to this role of proteins that anabolism and catabolism are carried out, that is, plastic and

Protective role

There are several types of threats that proteins are designed to protect the body from.

First, traumatic reagents, gases, molecules, substances of various spectrums of action. Peptides are able to enter into chemical interaction with them, converting them into a harmless form or simply neutralizing them.

Secondly, there is a physical threat from wounds - if the fibrinogen protein is not transformed into fibrin in time at the site of injury, then the blood will not clot, which means that blockage will not occur. Then, on the contrary, you will need the plasmin peptide, which is capable of resolving the clot and restoring the patency of the vessel.

Thirdly, the threat to immunity. The structure and significance of proteins that form immune defenses are extremely important. Antibodies, immunoglobulins, interferons are all important and significant elements of the human lymphatic and immune system. Any foreign particle, harmful molecule, dead part of the cell or the whole structure is subjected to immediate investigation by the peptide compound. That is why a person can independently, without the help of medicines, daily protect himself from infections and simple viruses.

Physical properties

The structure of a cell protein is very specific and depends on the function performed. But the physical properties of all peptides are similar and boil down to the following characteristics.

1. The weight of the molecule is up to 1,000,000 Daltons.
2. Colloidal systems are formed in an aqueous solution. There, the structure acquires a charge that can vary depending on the acidity of the medium.
3. When exposed to harsh conditions (irradiation, acid or alkali, temperature, and so on), they are able to move to other levels of conformations, that is, to denature. This process is irreversible in 90% of cases. However, there is also a reverse shift - renaturation.

These are the main properties of the physical characteristics of peptides.

No. 1. Proteins: peptide bond, their detection.

Proteins are macromolecules of linear polyamides formed by a-amino acids as a result of a polycondensation reaction in biological objects.

Squirrels are macromolecular compounds built from amino acids. 20 amino acids are involved in making proteins. They link together into long chains that form the backbone of a large molecular weight protein molecule.

Functions of proteins in the body

The combination of peculiar chemical and physical properties of proteins provides this particular class of organic compounds with a central role in the phenomena of life.

Proteins have the following biological properties, or perform the following main functions in living organisms:

1. Catalytic function of proteins. All biological catalysts - enzymes are proteins. To date, thousands of enzymes have been characterized, many of them isolated in crystalline form. Almost all enzymes are powerful catalysts, increasing the rates of reactions by at least a million times. This function of proteins is unique, not characteristic of other polymeric molecules.

2. Nutritional (reserve function of proteins). These are, first of all, proteins intended for nutrition of the developing embryo: milk casein, egg ovalbumin, storage proteins of plant seeds. A number of other proteins are undoubtedly used in the body as a source of amino acids, which, in turn, are precursors of biologically active substances that regulate the metabolic process.

3. Transport function of proteins. Many small molecules and ions are transported by specific proteins. For example, the respiratory function of blood, namely the transport of oxygen, is performed by hemoglobin molecules, a protein in red blood cells. Serum albumins are involved in lipid transport. A number of other whey proteins form complexes with fats, copper, iron, thyroxine, vitamin A and other compounds, ensuring their delivery to the appropriate organs.

4. Protective function of proteins. The main function of protection is performed by the immunological system, which provides the synthesis of specific protective proteins - antibodies - in response to the entry of bacteria, toxins or viruses (antigens) into the body. Antibodies bind antigens, interacting with them, and thereby neutralize their biological effect and maintain the normal state of the body. The coagulation of a blood plasma protein - fibrinogen - and the formation of a blood clot that protects against blood loss during injuries is another example of the protective function of proteins.

5. Contractile function of proteins. Many proteins are involved in the act of muscle contraction and relaxation. The main role in these processes is played by actin and myosin - specific proteins of muscle tissue. The contractile function is also inherent in the proteins of subcellular structures, which provides the finest processes of cell vital activity,

6. Structural function of proteins. Proteins with this function rank first among other proteins in the human body. Structural proteins such as collagen are widely distributed in connective tissue; keratin in hair, nails, skin; elastin - in the vascular walls, etc.

7. Hormonal (regulatory) function of proteins. Metabolism in the body is regulated by various mechanisms. In this regulation, an important place is occupied by hormones produced by endocrine glands. A number of hormones are represented by proteins or polypeptides, for example, hormones of the pituitary gland, pancreas, etc.

Peptide bond

Formally, the formation of a protein macromolecule can be represented as a polycondensation reaction of α-amino acids.

From a chemical point of view, proteins are high-molecular nitrogen-containing organic compounds (polyamides), whose molecules are built from amino acid residues. Protein monomers are α-amino acids, a common feature of which is the presence of a carboxyl group -COOH and an amino group -NH 2 at the second carbon atom (α-carbon atom):

Based on the results of studying the products of protein hydrolysis and put forward by A.Ya. Danilevsky's ideas about the role of peptide bonds -CO-NH- in the construction of a protein molecule, the German scientist E. Fischer proposed at the beginning of the 20th century the peptide theory of the structure of proteins. According to this theory, proteins are linear polymers of α-amino acids linked by a peptide bond - polypeptides:

In each peptide, one terminal amino acid residue has a free α-amino group (N-terminus) and the other has a free α-carboxyl group (C-terminus). The structure of peptides is usually depicted starting from the N-terminal amino acid. In this case, amino acid residues are indicated by symbols. For example: Ala-Tyr-Leu-Ser-Tyr- - Cys. This entry denotes a peptide in which the N-terminal α-amino acid is ­ lyatsya alanine, and the C-terminal - cysteine. When reading such a record, the endings of the names of all acids, except for the last ones, change to - "yl": alanyl-tyrosyl-leucyl-seryl-tyrosyl--cysteine. The length of the peptide chain in peptides and proteins found in the body ranges from two to hundreds and thousands of amino acid residues.

No. 2. Classification of simple proteins.

TO simple (proteins) include proteins that, when hydrolyzed, give only amino acids.

Proteinoids ____simple proteins of animal origin, insoluble in water, salt solutions, dilute acids and alkalis. They perform mainly supporting functions (for example, collagen, keratin

protamines - positively charged nuclear proteins, with a molecular weight of 10-12 kDa. Approximately 80% are composed of alkaline amino acids, which makes it possible for them to interact with nucleic acids through ionic bonds. They take part in the regulation of gene activity. Well soluble in water;

histones - nuclear proteins that play an important role in the regulation of gene activity. They are found in all eukaryotic cells, and are divided into 5 classes, differing in molecular weight and amino acid. The molecular weight of histones is in the range from 11 to 22 kDa, and the differences in amino acid composition relate to lysine and arginine, the content of which varies from 11 to 29% and from 2 to 14%, respectively;

prolamins - insoluble in water, but soluble in 70% alcohol, chemical structure features - a lot of proline, glutamic acid, no lysine ,

glutelins - soluble in alkaline solutions ,

globulins - proteins that are insoluble in water and in a semi-saturated solution of ammonium sulphate, but soluble in aqueous solutions of salts, alkalis and acids. Molecular weight - 90-100 kDa;

albumins - proteins of animal and plant tissues, soluble in water and saline solutions. The molecular weight is 69 kDa;

scleroproteins - proteins of the supporting tissues of animals

Examples of simple proteins are silk fibroin, egg serum albumin, pepsin, etc.

No. 3. Methods for isolation and precipitation (purification) of proteins.

No. 4. Proteins as polyelectrolytes. Isoelectric point of a protein.

Proteins are amphoteric polyelectrolytes, i.e. exhibit both acidic and basic properties. This is due to the presence in protein molecules of amino acid radicals capable of ionization, as well as free α-amino and α-carboxyl groups at the ends of peptide chains. Acidic properties of the protein are given by acidic amino acids (aspartic, glutamic), and alkaline properties - by basic amino acids (lysine, arginine, histidine).

The charge of a protein molecule depends on the ionization of acidic and basic groups of amino acid radicals. Depending on the ratio of negative and positive groups, the protein molecule as a whole acquires a total positive or negative charge. When a protein solution is acidified, the degree of ionization of anionic groups decreases, while that of cationic groups increases; when alkalized - vice versa. At a certain pH value, the number of positively and negatively charged groups becomes the same, and the isoelectric state of the protein appears (the total charge is 0). The pH value at which the protein is in the isoelectric state is called the isoelectric point and is denoted pI, similar to amino acids. For most proteins, pI lies in the range of 5.5-7.0, which indicates a certain predominance of acidic amino acids in proteins. However, there are also alkaline proteins, for example, salmin - the main protein from salmon milt (pl=12). In addition, there are proteins that have a very low pI value, for example, pepsin, an enzyme of gastric juice (pl=l). At the isoelectric point, proteins are very unstable and precipitate easily, having the least solubility.

If the protein is not in an isoelectric state, then in an electric field its molecules will move towards the cathode or anode, depending on the sign of the total charge and at a speed proportional to its value; this is the essence of the electrophoresis method. This method can separate proteins with different pI values.

Although proteins have buffer properties, their capacity at physiological pH values ​​is limited. The exception is proteins containing a lot of histidine, since only the histidine radical has buffer properties in the pH range of 6-8. There are very few of these proteins. For example, hemoglobin, containing almost 8% histidine, is a powerful intracellular buffer in red blood cells, maintaining the pH of the blood at a constant level.

No. 5. Physico-chemical properties of proteins.

Proteins have different chemical, physical and biological properties, which are determined by the amino acid composition and spatial organization of each protein. The chemical reactions of proteins are very diverse, they are due to the presence of NH 2 -, COOH groups and radicals of various nature. These are reactions of nitration, acylation, alkylation, esterification, redox and others. Proteins have acid-base, buffer, colloidal and osmotic properties.

Acid-base properties of proteins

Chemical properties. With weak heating of aqueous solutions of proteins, denaturation occurs. This creates a precipitate.

When proteins are heated with acids, hydrolysis occurs, and a mixture of amino acids is formed.

Physico-chemical properties of proteins

Proteins have a high molecular weight.

The charge of a protein molecule. All proteins have at least one free -NH and -COOH group.

Protein solutions- colloidal solutions with different properties. Proteins are acidic and basic. Acidic proteins contain a lot of glu and asp, which have additional carboxyl and fewer amino groups. There are many lys and args in alkaline proteins. Each protein molecule in an aqueous solution is surrounded by a hydration shell, since proteins have many hydrophilic groups (-COOH, -OH, -NH 2, -SH) due to amino acids. In aqueous solutions, the protein molecule has a charge. The charge of protein in water can change depending on the pH.

Protein precipitation. Proteins have a hydration shell, a charge that prevents sticking. For deposition, it is necessary to remove the hydrate shell and charge.

1. Hydration. The process of hydration means the binding of water by proteins, while they exhibit hydrophilic properties: they swell, their mass and volume increase. Swelling of the protein is accompanied by its partial dissolution. The hydrophilicity of individual proteins depends on their structure. The hydrophilic amide (–CO–NH–, peptide bond), amine (NH2) and carboxyl (COOH) groups present in the composition and located on the surface of the protein macromolecule attract water molecules, strictly orienting them to the surface of the molecule. Surrounding the protein globules, the hydrate (water) shell prevents the stability of protein solutions. At the isoelectric point, proteins have the least ability to bind water; the hydration shell around the protein molecules is destroyed, so they combine to form large aggregates. Aggregation of protein molecules also occurs when they are dehydrated with some organic solvents, such as ethyl alcohol. This leads to the precipitation of proteins. When the pH of the medium changes, the protein macromolecule becomes charged, and its hydration capacity changes.

Precipitation reactions are divided into two types.

Salting out of proteins: (NH 4)SO 4 - only the hydration shell is removed, the protein retains all types of its structure, all bonds, retains its native properties. Such proteins can then be re-dissolved and used.

Precipitation with loss of native protein properties is an irreversible process. The hydration shell and charge are removed from the protein, various properties in the protein are violated. For example, salts of copper, mercury, arsenic, iron, concentrated inorganic acids - HNO 3 , H 2 SO 4 , HCl, organic acids, alkaloids - tannins, mercury iodide. The addition of organic solvents lowers the degree of hydration and leads to precipitation of the protein. Acetone is used as such solvent. Proteins are also precipitated with the help of salts, for example, ammonium sulfate. The principle of this method is based on the fact that with an increase in the salt concentration in the solution, the ionic atmospheres formed by the protein counterions are compressed, which contributes to their convergence to a critical distance, at which the intermolecular forces of van der Waals attraction outweigh the Coulomb forces of repulsion of the counterions. This leads to the adhesion of protein particles and their precipitation.

When boiling, protein molecules begin to move randomly, collide, the charge is removed, and the hydration shell decreases.

To detect proteins in solution, the following are used:

color reactions;

precipitation reactions.

Methods for isolation and purification of proteins.

homogenization- the cells are ground to a homogeneous mass;

extraction of proteins with water or water-salt solutions;

1. salting out;

   electrophoresis;

   chromatography: adsorption, splitting;

   ultracentrifugation.

Structural organization of proteins.

Primary Structure- determined by the sequence of amino acids in the peptide chain, stabilized by covalent peptide bonds (insulin, pepsin, chymotrypsin).

secondary structure- spatial structure of the protein. This is either a spiral or a folding. Hydrogen bonds are created.

Tertiary structure globular and fibrillar proteins. They stabilize hydrogen bonds, electrostatic forces (COO-, NH3+), hydrophobic forces, sulfide bridges, are determined by the primary structure. Globular proteins - all enzymes, hemoglobin, myoglobin. Fibrillar proteins - collagen, myosin, actin.

Quaternary structure- found only in some proteins. Such proteins are built from several peptides. Each peptide has its own primary, secondary, tertiary structure, called protomers. Several protomers join together to form one molecule. One protomer does not function as a protein, but only in conjunction with other protomers.

Example: hemoglobin \u003d -globule + -globule - carries O 2 in the aggregate, and not separately.

Protein can renature. This requires a very short exposure to agents.

6) Methods for detecting proteins.

Proteins are high-molecular biological polymers, the structural (monomeric) units of which are -amino acids. Amino acids in proteins are linked to each other by peptide bonds. the formation of which occurs due to the carboxyl group standing at -carbon atom of one amino acid and -amine group of another amino acid with the release of a water molecule. The monomeric units of proteins are called amino acid residues.

Peptides, polypeptides and proteins differ not only in quantity, composition, but also in the sequence of amino acid residues, physicochemical properties and functions performed in the body. The molecular weight of proteins varies from 6 thousand to 1 million or more. The chemical and physical properties of proteins are determined by the chemical nature and physico-chemical properties of the radicals that make up their amino acid residues. Methods for the detection and quantification of proteins in biological objects and food products, as well as their isolation from tissues and biological fluids, are based on the physical and chemical properties of these compounds.

Proteins when interacting with certain chemicals give colored compounds. The formation of these compounds occurs with the participation of amino acid radicals, their specific groups or peptide bonds. Color reactions allow you to set the presence of a protein in a biological object or solution and prove the presence certain amino acids in a protein molecule. On the basis of color reactions, some methods for the quantitative determination of proteins and amino acids have been developed.

Consider universal biuret and ninhydrin reactions, since all proteins give them. Xantoprotein reaction, Fohl reaction and others are specific, since they are due to the radical groups of certain amino acids in the protein molecule.

Color reactions allow you to establish the presence of a protein in the material under study and the presence of certain amino acids in its molecules.

Biuret reaction. The reaction is due to the presence in proteins, peptides, polypeptides peptide bonds, which in an alkaline medium form with copper(II) ions complex compounds colored in purple (with a red or blue tinge) color. The color is due to the presence of at least two groups in the molecule -CO-NH- connected directly to each other or with the participation of a carbon or nitrogen atom.

Copper (II) ions are connected by two ionic bonds with =C─O ˉ groups and four coordination bonds with nitrogen atoms (=N−).

The color intensity depends on the amount of protein in the solution. This makes it possible to use this reaction for the quantitative determination of protein. The color of the colored solutions depends on the length of the polypeptide chain. Proteins give a blue-violet color; the products of their hydrolysis (poly- and oligopeptides) are red or pink in color. The biuret reaction is given not only by proteins, peptides and polypeptides, but also by biuret (NH 2 -CO-NH-CO-NH 2), oxamide (NH 2 -CO-CO-NH 2), histidine.

The complex compound of copper (II) with peptide groups formed in an alkaline medium has the following structure:

Ninhydrin reaction. In this reaction, solutions of protein, polypeptides, peptides and free α-amino acids, when heated with ninhydrin, give a blue, blue-violet or pink-violet color. The color in this reaction develops due to the α-amino group.

-amino acids react very easily with ninhydrin. Along with them, Rueman's blue-violet is also formed by proteins, peptides, primary amines, ammonia, and some other compounds. Secondary amines, such as proline and hydroxyproline, give a yellow color.

The ninhydrin reaction is widely used to detect and quantify amino acids.

xantoprotein reaction. This reaction indicates the presence of aromatic amino acid residues in proteins - tyrosine, phenylalanine, tryptophan. It is based on the nitration of the benzene ring of the radicals of these amino acids with the formation of yellow-colored nitro compounds (Greek "Xanthos" - yellow). Using tyrosine as an example, this reaction can be described in the form of the following equations.

In an alkaline environment, nitro derivatives of amino acids form salts of the quinoid structure, colored orange. The xantoprotein reaction is given by benzene and its homologues, phenol and other aromatic compounds.

Reactions to amino acids containing a thiol group in a reduced or oxidized state (cysteine, cystine).

Fohl's reaction. When boiled with alkali, sulfur is easily split off from cysteine ​​in the form of hydrogen sulfide, which in an alkaline medium forms sodium sulfide:

In this regard, the reactions for determining thiol-containing amino acids in solution are divided into two stages:

The transition of sulfur from organic to inorganic state

Detection of sulfur in solution

To detect sodium sulfide, lead acetate is used, which, when interacting with sodium hydroxide, turns into its plumbite:

Pb(CH 3 COO) 2 + 2NaOH Pb(ONa) 2 + 2CH 3 COOH

As a result of the interaction of sulfur ions and lead, black or brown lead sulfide is formed:

Na 2 S + Pb(ONa) 2 + 2 H 2 O PbS (black precipitate) + 4NaOH

To determine sulfur-containing amino acids, an equal volume of sodium hydroxide and a few drops of lead acetate solution are added to the test solution. With intensive boiling for 3-5 minutes, the liquid turns black.

The presence of cystine can be determined using this reaction, since cystine is easily reduced to cysteine.

Millon reaction:

This is a reaction to the amino acid tyrosine.

Free phenolic hydroxyls of tyrosine molecules, when interacting with salts, give compounds of the mercury salt of the nitro derivative of tyrosine, colored pinkish red:

Pauli reaction for histidine and tyrosine . The Pauli reaction makes it possible to detect the amino acids histidine and tyrosine in the protein, which form cherry-red complex compounds with diazobenzenesulfonic acid. Diazobenzenesulfonic acid is formed in the reaction of diazotization when sulfanilic acid reacts with sodium nitrite in an acidic medium:

An equal volume of an acidic solution of sulfanilic acid (prepared using hydrochloric acid) and a double volume of sodium nitrite solution are added to the test solution, mixed thoroughly and soda (sodium carbonate) is immediately added. After stirring, the mixture turns cherry red, provided that histidine or tyrosine is present in the test solution.

Adamkevich-Hopkins-Kohl (Schulz-Raspail) reaction to tryptophan (reaction to the indole group). Tryptophan reacts in an acidic environment with aldehydes, forming colored condensation products. The reaction proceeds due to the interaction of the indole ring of tryptophan with aldehyde. It is known that formaldehyde is formed from glyoxylic acid in the presence of sulfuric acid:

R
Solutions containing tryptophan in the presence of glyoxylic and sulfuric acids give a red-violet color.

Glyoxylic acid is always present in small amounts in glacial acetic acid. Therefore, the reaction can be carried out using acetic acid. At the same time, an equal volume of glacial (concentrated) acetic acid is added to the test solution and gently heated until the precipitate dissolves. After cooling, a volume of concentrated sulfuric acid equal to the added volume of glyoxylic acid is added to the mixture carefully along the wall (to avoid mixing liquids). After 5-10 minutes, the formation of a red-violet ring is observed at the interface between the two layers. If you mix the layers, the contents of the dish will evenly turn purple.

TO

condensation of tryptophan with formaldehyde:

The condensation product is oxidized to bis-2-tryptophanylcarbinol, which in the presence of mineral acids forms blue-violet salts:

7) Classification of proteins. Methods for studying the amino acid composition.

Strict nomenclature and classification of proteins still does not exist. The names of proteins are given randomly, most often taking into account the source of protein isolation or taking into account its solubility in certain solvents, the shape of the molecule, etc.

Proteins are classified according to composition, particle shape, solubility, amino acid composition, origin, etc.

1. Composition Proteins are divided into two large groups: simple and complex proteins.

Simple (proteins) include proteins that give only amino acids upon hydrolysis (proteinoids, protamines, histones, prolamins, glutelins, globulins, albumins). Examples of simple proteins are silk fibroin, egg serum albumin, pepsin, etc.

Complex (proteids) include proteins composed of a simple protein and an additional (prosthetic) group of non-protein nature. The group of complex proteins is divided into several subgroups depending on the nature of the non-protein component:

Metalloproteins containing in their composition metals (Fe, Cu, Mg, etc.) associated directly with the polypeptide chain;

Phosphoproteins - contain residues of phosphoric acid, which are attached to the protein molecule by ester bonds at the site of the hydroxyl groups of serine, threonine;

Glycoproteins - their prosthetic groups are carbohydrates;

Chromoproteins - consist of a simple protein and a colored non-protein compound associated with it, all chromoproteins are biologically very active; as prosthetic groups, they may contain derivatives of porphyrin, isoalloxazine, and carotene;

Lipoproteins - prosthetic group lipids - triglycerides (fats) and phosphatides;

Nucleoproteins are proteins that consist of a single protein and a nucleic acid linked to it. These proteins play a colossal role in the life of the body and will be discussed below. They are part of any cell, some nucleoproteins exist in nature in the form of special particles with pathogenic activity (viruses).

2. Particle shape- proteins are divided into fibrillar (thread-like) and globular (spherical) (see page 30).

3. By solubility and characteristics of the amino acid composition the following groups of simple proteins are distinguished:

Proteinoids - proteins of supporting tissues (bones, cartilage, ligaments, tendons, hair, nails, skin, etc.). These are mainly fibrillar proteins with a large molecular weight (> 150,000 Da), insoluble in common solvents: water, salt and water-alcohol mixtures. They dissolve only in specific solvents;

Protamines (the simplest proteins) are proteins that are soluble in water and contain 80-90% arginine and a limited set (6-8) of other amino acids, are present in the milk of various fish. Due to the high content of arginine, they have basic properties, their molecular weight is relatively small and is approximately equal to 4000-12000 Da. They are a protein component in the composition of nucleoproteins;

Histones are highly soluble in water and dilute solutions of acids (0.1 N), are distinguished by a high content of amino acids: arginine, lysine and histidine (at least 30%) and therefore have basic properties. These proteins are found in significant amounts in the nuclei of cells as part of nucleoproteins and play an important role in the regulation of nucleic acid metabolism. The molecular weight of histones is small and equal to 11000-24000 Da;

Globulins are proteins that are insoluble in water and saline solutions with a salt concentration of more than 7%. Globulins are completely precipitated at 50% saturation of the solution with ammonium sulfate. These proteins are characterized by a high content of glycine (3.5%), their molecular weight > 100,000 Da. Globulins are weakly acidic or neutral proteins (p1=6-7.3);

Albumins are proteins that are highly soluble in water and strong saline solutions, and the salt concentration (NH 4) 2 S0 4 should not exceed 50% of saturation. At higher concentrations, albumins are salted out. Compared to globulins, these proteins contain three times less glycine and have a molecular weight of 40,000-70,000 Da. Albumins have an excess negative charge and acidic properties (pl=4.7) due to the high content of glutamic acid;

Prolamins are a group of plant proteins found in the gluten of cereals. They are soluble only in 60-80% aqueous solution of ethyl alcohol. Prolamins have a characteristic amino acid composition: they contain a lot (20-50%) of glutamic acid and proline (10-15%), which is why they got their name. Their molecular weight is over 100,000 Da;

Glutelins - vegetable proteins are insoluble in water, salt solutions and ethanol, but soluble in dilute (0.1 N) solutions of alkalis and acids. In terms of amino acid composition and molecular weight, they are similar to prolamins, but contain more arginine and less proline.

Methods for studying the amino acid composition

Proteins are broken down into amino acids by enzymes in the digestive juices. Two important conclusions were made: 1) proteins contain amino acids; 2) methods of hydrolysis can be used to study the chemical, in particular amino acid, composition of proteins.

To study the amino acid composition of proteins, a combination of acidic (HCl), alkaline [Ba(OH) 2 ], and, more rarely, enzymatic hydrolysis, or one of them, is used. It has been established that during the hydrolysis of a pure protein that does not contain impurities, 20 different α-amino acids are released. All other amino acids discovered in the tissues of animals, plants and microorganisms (more than 300) exist in nature in a free state or in the form of short peptides or complexes with other organic substances.

The first step in determining the primary structure of proteins is the qualitative and quantitative assessment of the amino acid composition of a given individual protein. It must be remembered that for the study you need to have a certain amount of pure protein, without impurities of other proteins or peptides.

Acid hydrolysis of protein

To determine the amino acid composition, it is necessary to destroy all peptide bonds in the protein. The analyzed protein is hydrolyzed in 6 mol/l HC1 at a temperature of about 110 °C for 24 hours. As a result of this treatment, peptide bonds in the protein are destroyed, and only free amino acids are present in the hydrolyzate. In addition, glutamine and asparagine are hydrolyzed to glutamic and aspartic acids (i.e., the amide bond in the radical is broken and the amino group is cleaved off from them).

Separation of amino acids using ion exchange chromatography

The mixture of amino acids obtained by acid hydrolysis of proteins is separated in a column with a cation exchange resin. Such a synthetic resin contains negatively charged groups (for example, sulfonic acid residues -SO 3 -) strongly associated with it, to which Na + ions are attached (Fig. 1-4).

A mixture of amino acids is introduced into the cation exchanger in an acidic environment (pH 3.0), where the amino acids are mainly cations, i. carry a positive charge. Positively charged amino acids attach to negatively charged resin particles. The greater the total charge of the amino acid, the stronger its bond with the resin. Thus, the amino acids lysine, arginine, and histidine bind most strongly to the cation exchanger, while aspartic and glutamic acids bind the most weakly.

The release of amino acids from the column is carried out by eluting (eluting) them with a buffer solution with increasing ionic strength (ie, with increasing NaCl concentration) and pH. With an increase in pH, amino acids lose a proton, as a result, their positive charge decreases, and hence the bond strength with negatively charged resin particles.

Each amino acid exits the column at a specific pH and ionic strength. By collecting the solution (eluate) from the lower end of the column in the form of small portions, fractions containing individual amino acids can be obtained.

(for more details on "hydrolysis" see question #10)

8) Chemical bonds in the protein structure.

9) The concept of the hierarchy and structural organization of proteins. (see question #12)

10) Protein hydrolysis. Reaction chemistry (stepping, catalysts, reagents, reaction conditions) - a complete description of hydrolysis.

11) Chemical transformations of proteins.

Denaturation and renaturation

When protein solutions are heated to 60-80% or under the action of reagents that destroy non-covalent bonds in proteins, the tertiary (quaternary) and secondary structure of the protein molecule is destroyed, it takes the form of a random random coil to a greater or lesser extent. This process is called denaturation. Acids, alkalis, alcohols, phenols, urea, guanidine chloride, etc. can be used as denaturing reagents. The essence of their action is that they form hydrogen bonds with = NH and = CO - groups of the peptide backbone and with acid groups of amino acid radicals, replacing their own intramolecular hydrogen bonds in the protein, as a result of which the secondary and tertiary structures change. During denaturation, the solubility of the protein decreases, it “coagulates” (for example, when boiling a chicken egg), and the biological activity of the protein is lost. Based on this, for example, the use of an aqueous solution of carbolic acid (phenol) as an antiseptic. Under certain conditions, with slow cooling of a solution of a denatured protein, renaturation occurs - the restoration of the original (native) conformation. This confirms the fact that the nature of the folding of the peptide chain is determined by the primary structure.

The process of denaturation of an individual protein molecule, leading to the disintegration of its "rigid" three-dimensional structure, is sometimes called the melting of the molecule. Almost any noticeable change in external conditions, such as heating or a significant change in pH, leads to a consistent violation of the quaternary, tertiary and secondary structures of the protein. Usually, denaturation is caused by an increase in temperature, the action of strong acids and alkalis, salts of heavy metals, certain solvents (alcohol), radiation, etc.

Denaturation often leads to the process of aggregation of protein particles into larger ones in a colloidal solution of protein molecules. Visually, this looks, for example, as the formation of a "protein" when frying eggs.

Renaturation is the reverse process of denaturation, in which proteins return to their natural structure. It should be noted that not all proteins are able to renature; in most proteins, denaturation is irreversible. If, during protein denaturation, physicochemical changes are associated with the transition of the polypeptide chain from a densely packed (ordered) state to a disordered state, then during renaturation, the ability of proteins to self-organize is manifested, the path of which is predetermined by the sequence of amino acids in the polypeptide chain, that is, its primary structure determined by hereditary information . In living cells, this information is probably decisive for the transformation of a disordered polypeptide chain during or after its biosynthesis on the ribosome into the structure of a native protein molecule. When double-stranded DNA molecules are heated to a temperature of about 100 ° C, the hydrogen bonds between the bases are broken, and the complementary strands diverge - the DNA denatures. However, upon slow cooling, the complementary strands can reconnect into a regular double helix. This ability of DNA to renature is used to produce artificial DNA hybrid molecules.

Natural protein bodies are endowed with a certain, strictly defined spatial configuration and have a number of characteristic physicochemical and biological properties at physiological temperatures and pH values. Under the influence of various physical and chemical factors, proteins undergo coagulation and precipitate, losing their native properties. Thus, denaturation should be understood as a violation of the general plan of the unique structure of the native protein molecule, mainly its tertiary structure, leading to the loss of its characteristic properties (solubility, electrophoretic mobility, biological activity, etc.). Most proteins denature when their solutions are heated above 50–60°C.

External manifestations of denaturation are reduced to a loss of solubility, especially at the isoelectric point, an increase in the viscosity of protein solutions, an increase in the number of free functional SH-groups, and a change in the nature of X-ray scattering. The most characteristic sign of denaturation is a sharp decrease or complete loss by the protein of its biological activity (catalytic, antigenic or hormonal). During protein denaturation caused by 8M urea or another agent, mostly non-covalent bonds (in particular, hydrophobic interactions and hydrogen bonds) are destroyed. Disulfide bonds are broken in the presence of the reducing agent mercaptoethanol, while the peptide bonds of the backbone of the polypeptide chain itself are not affected. Under these conditions, globules of native protein molecules unfold and random and disordered structures are formed (Fig.)

Denaturation of a protein molecule (scheme).

a - initial state; b - beginning reversible violation of the molecular structure; c - irreversible deployment of the polypeptide chain.

Denaturation and renaturation of ribonuclease (according to Anfinsen).

a - deployment (urea + mercaptoethanol); b - refolding.

1. Protein hydrolysis: H+

[− NH2─CH─ CO─NH─CH─CO − ]n +2nH2O → n NH2 − CH − COOH + n NH2 ─ CH ─ COOH

│ │ ‌‌│ │

Amino acid 1 amino acid 2

2. Precipitation of proteins:

a) reversible

Protein in solution ↔ protein precipitate. Occurs under the action of solutions of salts Na+, K+

b) irreversible (denaturation)

During denaturation under the influence of external factors (temperature; mechanical action - pressure, rubbing, shaking, ultrasound; the action of chemical agents - acids, alkalis, etc.), a change occurs in the secondary, tertiary and quaternary structures of the protein macromolecule, i.e. its native spatial structure. The primary structure, and, consequently, the chemical composition of the protein does not change.

During denaturation, the physical properties of proteins change: solubility decreases, biological activity is lost. At the same time, the activity of some chemical groups increases, the effect of proteolytic enzymes on proteins is facilitated, and, consequently, it is more easily hydrolyzed.

For example, albumin - egg white - at a temperature of 60-70 ° is precipitated from a solution (coagulates), losing the ability to dissolve in water.

Scheme of the process of protein denaturation (destruction of the tertiary and secondary structures of protein molecules)

3. Burning proteins

Proteins burn with the formation of nitrogen, carbon dioxide, water, and some other substances. Burning is accompanied by the characteristic smell of burnt feathers.

4. Color (qualitative) reactions to proteins:

a) xantoprotein reaction (for amino acid residues containing benzene rings):

Protein + HNO3 (conc.) → yellow color

b) biuret reaction (for peptide bonds):

Protein + CuSO4 (sat) + NaOH (conc) → bright purple color

c) cysteine ​​reaction (for amino acid residues containing sulfur):

Protein + NaOH + Pb(CH3COO)2 → Black staining

Proteins are the basis of all life on Earth and perform various functions in organisms.

Salting out proteins

Salting out is the process of isolating proteins from aqueous solutions with neutral solutions of concentrated salts of alkali and alkaline earth metals. When high concentrations of salts are added to the protein solution, the dehydration of the protein particles and the removal of the charge occur, while the proteins precipitate. The degree of protein precipitation depends on the ionic strength of the precipitant solution, the size of the particles of the protein molecule, the magnitude of its charge, and hydrophilicity. Different proteins precipitate at different salt concentrations. Therefore, in sediments obtained by gradually increasing the concentration of salts, individual proteins are in different fractions. Salting out of proteins is a reversible process, and after the salt is removed, the protein regains its natural properties. Therefore, salting out is used in clinical practice in the separation of blood serum proteins, as well as in the isolation and purification of various proteins.

Added anions and cations destroy the hydrated protein shell of proteins, which is one of the stability factors of protein solutions. Most often, solutions of Na and ammonium sulfates are used. Many proteins differ in the size of the hydration shell and the magnitude of the charge. Each protein has its own salting out zone. After removal of the salting out agent, the protein retains its biological activity and physicochemical properties. In clinical practice, the salting out method is used to separate globulins (with the addition of 50% ammonium sulfate (NH4)2SO4 a precipitate forms) and albumins (with the addition of 100% ammonium sulfate (NH4)2SO4 a precipitate forms).

Salting out is influenced by:

1) nature and concentration of salt;

2) pH environments;

3) temperature.

The main role is played by the valencies of the ions.

12) Features of the organization of the primary, secondary, tertiary structure of the protein.

At present, the existence of four levels of structural organization of a protein molecule has been experimentally proven: primary, secondary, tertiary and quaternary structure.

The content of the article

PROTEINS (Article 1)- a class of biological polymers present in every living organism. With the participation of proteins, the main processes that ensure the vital activity of the body take place: respiration, digestion, muscle contraction, transmission of nerve impulses. Bone tissue, skin, hair, horn formations of living beings are composed of proteins. For most mammals, the growth and development of the organism occurs due to products containing proteins as a food component. The role of proteins in the body and, accordingly, their structure is very diverse.

The composition of proteins.

All proteins are polymers, the chains of which are assembled from fragments of amino acids. Amino acids are organic compounds containing in their composition (in accordance with the name) an NH 2 amino group and an organic acid, i.e. carboxyl, COOH group. Of the entire variety of existing amino acids (theoretically, the number of possible amino acids is unlimited), only those that have only one carbon atom between the amino group and the carboxyl group participate in the formation of proteins. In general, the amino acids involved in the formation of proteins can be represented by the formula: H 2 N–CH(R)–COOH. The R group attached to the carbon atom (the one between the amino and carboxyl groups) determines the difference between the amino acids that make up proteins. This group can consist only of carbon and hydrogen atoms, but more often contains, in addition to C and H, various functional (capable of further transformations) groups, for example, HO-, H 2 N-, etc. There is also an option when R = H.

The organisms of living beings contain more than 100 different amino acids, however, not all are used in the construction of proteins, but only 20, the so-called "fundamental". In table. 1 shows their names (most of the names have developed historically), the structural formula, as well as the widely used abbreviation. All structural formulas are arranged in the table so that the main fragment of the amino acid is on the right.

Table 1. AMINO ACIDS INVOLVED IN THE CREATION OF PROTEINS

Name	Structure	Designation
GLYCINE		GLI
ALANIN		ALA
VALIN		SHAFT
LEUCINE		LEI
ISOLEUCINE		ILE
SERIN		SER
THREONINE		TRE
CYSTEINE		CIS
METIONINE		MET
LYSINE		LIZ
ARGININE		ARG
ASPARAGIC ACID		ACH
ASPARAGIN		ACH
GLUTAMIC ACID		GLU
GLUTAMINE		GLN
phenylalanine		hair dryer
TYROSINE		TIR
tryptophan		THREE
HISTIDINE		GIS
PROLINE		PRO
In international practice, the abbreviated designation of the listed amino acids using Latin three-letter or one-letter abbreviations is accepted, for example, glycine - Gly or G, alanine - Ala or A.

Among these twenty amino acids (Table 1), only proline contains an NH group (instead of NH 2) next to the COOH carboxyl group, since it is part of the cyclic fragment.

Eight amino acids (valine, leucine, isoleucine, threonine, methionine, lysine, phenylalanine and tryptophan), placed in the table on a gray background, are called essential, since the body must constantly receive them with protein food for normal growth and development.

A protein molecule is formed as a result of the sequential connection of amino acids, while the carboxyl group of one acid interacts with the amino group of the neighboring molecule, as a result, a –CO–NH– peptide bond is formed and a water molecule is released. On fig. 1 shows the serial connection of alanine, valine and glycine.

Rice. one SERIAL CONNECTION OF AMINO ACIDS during the formation of a protein molecule. The path from the terminal amino group H 2 N to the terminal carboxyl group COOH was chosen as the main direction of the polymer chain.

To compactly describe the structure of a protein molecule, the abbreviations for amino acids (Table 1, third column) involved in the formation of the polymer chain are used. The fragment of the molecule shown in Fig. 1 is written as follows: H 2 N-ALA-VAL-GLY-COOH.

Protein molecules contain from 50 to 1500 amino acid residues (shorter chains are called polypeptides). The individuality of a protein is determined by the set of amino acids that make up the polymer chain and, no less important, by the order of their alternation along the chain. For example, the insulin molecule consists of 51 amino acid residues (it is one of the shortest chain proteins) and consists of two interconnected parallel chains of unequal length. The sequence of amino acid fragments is shown in fig. 2.

Rice. 2 INSULIN MOLECULE, built from 51 amino acid residues, fragments of the same amino acids are marked with the corresponding background color. The cysteine ​​amino acid residues (abbreviated designation CIS) contained in the chain form disulfide bridges -S-S-, which link two polymer molecules, or form jumpers within one chain.

Molecules of the amino acid cysteine ​​(Table 1) contain reactive sulfhydride groups -SH, which interact with each other, forming disulfide bridges -S-S-. The role of cysteine ​​in the world of proteins is special, with its participation, cross-links are formed between polymeric protein molecules.

The combination of amino acids into a polymer chain occurs in a living organism under the control of nucleic acids, it is they that provide a strict assembly order and regulate the fixed length of the polymer molecule ().

The structure of proteins.

The composition of the protein molecule, presented in the form of alternating amino acid residues (Fig. 2), is called the primary structure of the protein. Between the imino groups HN and carbonyl groups CO present in the polymer chain, hydrogen bonds arise (), as a result, the protein molecule acquires a certain spatial shape, called the secondary structure. The most common are two types of secondary structure in proteins.

The first option, called the α-helix, is implemented using hydrogen bonds within one polymer molecule. The geometric parameters of the molecule, determined by the bond lengths and bond angles, are such that the formation of hydrogen bonds is possible for the H-N and C=O groups, between which there are two peptide fragments H-N-C=O (Fig. 3).

The composition of the polypeptide chain shown in fig. 3 is written in abbreviated form as follows:

H 2 N-ALA VAL-ALA-LEY-ALA-ALA-ALA-ALA-VAL-ALA-ALA-ALA-COOH.

As a result of the contracting action of hydrogen bonds, the molecule takes the form of a helix - the so-called α-helix, it is depicted as a curved helical ribbon passing through the atoms that form the polymer chain (Fig. 4)

Rice. 4 3D MODEL OF A PROTEIN MOLECULE in the form of an α-helix. Hydrogen bonds are shown as green dotted lines. The cylindrical shape of the spiral is visible at a certain angle of rotation (hydrogen atoms are not shown in the figure). The color of individual atoms is given in accordance with international rules, which recommend black for carbon atoms, blue for nitrogen, red for oxygen, and yellow for sulfur (white color is recommended for hydrogen atoms not shown in the figure, in this case the entire structure depicted on a dark background).

Another variant of the secondary structure, called the β-structure, is also formed with the participation of hydrogen bonds, the difference is that the H-N and C=O groups of two or more polymer chains located in parallel interact. Since the polypeptide chain has a direction (Fig. 1), variants are possible when the direction of the chains is the same (parallel β-structure, Fig. 5), or they are opposite (antiparallel β-structure, Fig. 6).

Polymer chains of various compositions can participate in the formation of the β-structure, while the organic groups framing the polymer chain (Ph, CH 2 OH, etc.) in most cases play a secondary role, the mutual arrangement of the H-N and C=O groups is decisive. Since the H-N and C=O groups are directed in different directions relative to the polymer chain (up and down in the figure), simultaneous interaction of three or more chains becomes possible.

The composition of the first polypeptide chain in Fig. five:

H 2 N-LEI-ALA-FEN-GLI-ALA-ALA-COOH

The composition of the second and third chain:

H 2 N-GLY-ALA-SER-GLY-TRE-ALA-COOH

The composition of the polypeptide chains shown in fig. 6, the same as in Fig. 5, the difference is that the second chain has the opposite (in comparison with Fig. 5) direction.

It is possible to form a β-structure inside one molecule, when the chain fragment in a certain section turns out to be rotated by 180°, in this case, two branches of one molecule have the opposite direction, as a result, an antiparallel β-structure is formed (Fig. 7).

The structure shown in fig. 7 in a flat image, shown in fig. 8 in the form of a three-dimensional model. Sections of the β-structure are usually denoted in a simplified way by a flat wavy ribbon that passes through the atoms that form the polymer chain.

In the structure of many proteins, sections of the α-helix and ribbon-like β-structures alternate, as well as single polypeptide chains. Their mutual arrangement and alternation in the polymer chain is called the tertiary structure of the protein.

Methods for depicting the structure of proteins are shown below using the plant protein crambin as an example. Structural formulas of proteins, often containing up to hundreds of amino acid fragments, are complex, cumbersome and difficult to understand, therefore, sometimes simplified structural formulas are used - without symbols of chemical elements (Fig. 9, option A), but at the same time they retain the color of valence strokes in accordance with international rules (Fig. 4). In this case, the formula is presented not in a flat, but in a spatial image, which corresponds to the real structure of the molecule. This method makes it possible, for example, to distinguish between disulfide bridges (similar to those in insulin, Fig. 2), phenyl groups in the side frame of the chain, etc. The image of molecules in the form of three-dimensional models (balls connected by rods) is somewhat clearer (Fig. 9, option B). However, both methods do not allow showing the tertiary structure, so the American biophysicist Jane Richardson proposed to represent α-structures as spirally twisted ribbons (see Fig. 4), β-structures as flat wavy ribbons (Fig. 8), and connecting them single chains - in the form of thin bundles, each type of structure has its own color. This method of depicting the tertiary structure of a protein is now widely used (Fig. 9, variant B). Sometimes, for greater information content, a tertiary structure and a simplified structural formula are shown together (Fig. 9, variant D). There are also modifications of the method proposed by Richardson: α-helices are depicted as cylinders, and β-structures are in the form of flat arrows indicating the direction of the chain (Fig. 9, option E). Less common is the method in which the entire molecule is depicted as a bundle, where unequal structures are distinguished by different colors, and disulfide bridges are shown as yellow bridges (Fig. 9, variant E).

Option B is the most convenient for perception, when, when depicting the tertiary structure, the structural features of the protein (amino acid fragments, their alternation order, hydrogen bonds) are not indicated, while it is assumed that all proteins contain “details” taken from a standard set of twenty amino acids ( Table 1). The main task in depicting a tertiary structure is to show the spatial arrangement and alternation of secondary structures.

Rice. nine VARIOUS VERSIONS OF IMAGE OF THE STRUCTURE OF THE CRUMBIN PROTEIN.
A is a structural formula in a spatial image.
B - structure in the form of a three-dimensional model.
B is the tertiary structure of the molecule.
G - a combination of options A and B.
E - simplified image of the tertiary structure.
E - tertiary structure with disulfide bridges.

The most convenient for perception is a three-dimensional tertiary structure (option B), freed from the details of the structural formula.

A protein molecule that has a tertiary structure, as a rule, takes on a certain configuration, which is formed by polar (electrostatic) interactions and hydrogen bonds. As a result, the molecule takes the form of a compact coil - globular proteins (globules, lat. ball), or filamentous - fibrillar proteins (fibra, lat. fiber).

An example of a globular structure is the protein albumin, the protein of a chicken egg belongs to the class of albumins. The polymeric chain of albumin is assembled mainly from alanine, aspartic acid, glycine, and cysteine, alternating in a certain order. The tertiary structure contains α-helices connected by single chains (Fig. 10).

Rice. 10 GLOBULAR STRUCTURE OF ALBUMIN

An example of a fibrillar structure is the fibroin protein. They contain a large amount of glycine, alanine and serine residues (every second amino acid residue is glycine); cysteine ​​residues containing sulfhydride groups are absent. Fibroin, the main component of natural silk and cobwebs, contains β-structures connected by single chains (Fig. 11).

Rice. eleven FIBRILLARY PROTEIN FIBROIN

The possibility of forming a tertiary structure of a certain type is inherent in the primary structure of the protein, i.e. determined in advance by the order of alternation of amino acid residues. From certain sets of such residues, α-helices predominantly arise (there are quite a lot of such sets), another set leads to the appearance of β-structures, single chains are characterized by their composition.

Some protein molecules, while retaining a tertiary structure, are able to combine into large supramolecular aggregates, while they are held together by polar interactions, as well as hydrogen bonds. Such formations are called the quaternary structure of the protein. For example, the protein ferritin, which consists mainly of leucine, glutamic acid, aspartic acid and histidine (ferricin contains all 20 amino acid residues in varying amounts) forms a tertiary structure of four parallel-laid α-helices. When molecules are combined into a single ensemble (Fig. 12), a quaternary structure is formed, which can include up to 24 ferritin molecules.

Fig.12 FORMATION OF THE QUATERNARY STRUCTURE OF THE GLOBULAR PROTEIN FERRITIN

Another example of supramolecular formations is the structure of collagen. It is a fibrillar protein whose chains are built mainly of glycine alternating with proline and lysine. The structure contains single chains, triple α-helices, alternating with ribbon-like β-structures stacked in parallel bundles (Fig. 13).

Fig.13 SUPRAMOLECULAR STRUCTURE OF COLLAGEN FIBRILLARY PROTEIN

Chemical properties of proteins.

Under the action of organic solvents, waste products of some bacteria (lactic acid fermentation) or with an increase in temperature, secondary and tertiary structures are destroyed without damaging its primary structure, as a result, the protein loses solubility and loses biological activity, this process is called denaturation, that is, the loss of natural properties, for example, the curdling of sour milk, the coagulated protein of a boiled chicken egg. At elevated temperatures, the proteins of living organisms (in particular, microorganisms) quickly denature. Such proteins are not able to participate in biological processes, as a result, microorganisms die, so boiled (or pasteurized) milk can be stored longer.

Peptide bonds H-N-C=O, forming the polymer chain of the protein molecule, are hydrolyzed in the presence of acids or alkalis, and the polymer chain breaks, which, ultimately, can lead to the original amino acids. Peptide bonds included in α-helices or β-structures are more resistant to hydrolysis and various chemical attack (compared to the same bonds in single chains). A more delicate disassembly of the protein molecule into its constituent amino acids is carried out in an anhydrous medium using hydrazine H 2 N–NH 2, while all amino acid fragments, except for the last one, form the so-called carboxylic acid hydrazides containing the fragment C (O)–HN–NH 2 ( Fig. 14).

Rice. fourteen. POLYPEPTIDE CLEAVAGE

Such an analysis can provide information about the amino acid composition of a protein, but it is more important to know their sequence in a protein molecule. One of the methods widely used for this purpose is the action of phenylisothiocyanate (FITC) on the polypeptide chain, which in an alkaline medium attaches to the polypeptide (from the end that contains the amino group), and when the reaction of the medium changes to acidic, it detaches from the chain, taking with it fragment of one amino acid (Fig. 15).

Rice. 15 SEQUENTIAL POLYPEPTIDE Cleavage

Many special methods have been developed for such an analysis, including those that begin to “disassemble” a protein molecule into its constituent components, starting from the carboxyl end.

Cross S-S disulfide bridges (formed by the interaction of cysteine ​​residues, Figs. 2 and 9) are cleaved, converting them into HS groups by the action of various reducing agents. The action of oxidizing agents (oxygen or hydrogen peroxide) leads again to the formation of disulfide bridges (Fig. 16).

Rice. 16. Cleavage of disulfide bridges

To create additional cross-links in proteins, the reactivity of amino and carboxyl groups is used. More accessible for various interactions are the amino groups that are in the side frame of the chain - fragments of lysine, asparagine, lysine, proline (Table 1). When such amino groups interact with formaldehyde, the process of condensation occurs and cross-bridges –NH–CH2–NH– appear (Fig. 17).

Rice. 17 CREATION OF ADDITIONAL TRANSVERSAL BRIDGES BETWEEN PROTEIN MOLECULES.

The terminal carboxyl groups of the protein are able to react with complex compounds of some polyvalent metals (chromium compounds are more often used), and cross-links also occur. Both processes are used in leather tanning.

The role of proteins in the body.

The role of proteins in the body is diverse.

Enzymes(fermentatio lat. - fermentation), their other name is enzymes (en zumh greek. - in yeast) - these are proteins with catalytic activity, they are able to increase the speed of biochemical processes by thousands of times. Under the action of enzymes, the constituent components of food: proteins, fats and carbohydrates are broken down into simpler compounds, from which new macromolecules are then synthesized, which are necessary for a certain type of body. Enzymes also take part in many biochemical processes of synthesis, for example, in the synthesis of proteins (some proteins help to synthesize others).

Enzymes are not only highly efficient catalysts, but also selective (direct the reaction strictly in the given direction). In their presence, the reaction proceeds with almost 100% yield without the formation of by-products and, at the same time, the flow conditions are mild: normal atmospheric pressure and temperature of a living organism. For comparison, the synthesis of ammonia from hydrogen and nitrogen in the presence of a catalyst, activated iron, is carried out at 400–500°C and a pressure of 30 MPa, the yield of ammonia is 15–25% per cycle. Enzymes are considered unsurpassed catalysts.

Intensive study of enzymes began in the middle of the 19th century; more than 2,000 different enzymes have now been studied; this is the most diverse class of proteins.

The names of enzymes are as follows: the name of the reagent with which the enzyme interacts, or the name of the catalyzed reaction, is added with the ending -aza, for example, arginase decomposes arginine (Table 1), decarboxylase catalyzes decarboxylation, i.e. elimination of CO 2 from the carboxyl group:

– COOH → – CH + CO 2

Often, to more accurately indicate the role of an enzyme, both the object and the type of reaction are indicated in its name, for example, alcohol dehydrogenase is an enzyme that dehydrogenates alcohols.

For some enzymes discovered quite a long time ago, the historical name (without the ending -aza) has been preserved, for example, pepsin (pepsis, Greek. digestion) and trypsin (thrypsis Greek. liquefaction), these enzymes break down proteins.

For systematization, enzymes are combined into large classes, the classification is based on the type of reaction, the classes are named according to the general principle - the name of the reaction and the ending - aza. Some of these classes are listed below.

Oxidoreductase are enzymes that catalyze redox reactions. The dehydrogenases included in this class carry out proton transfer, for example, alcohol dehydrogenase (ADH) oxidizes alcohols to aldehydes, the subsequent oxidation of aldehydes to carboxylic acids is catalyzed by aldehyde dehydrogenases (ALDH). Both processes occur in the body during the processing of ethanol into acetic acid (Fig. 18).

Rice. eighteen TWO-STAGE OXIDATION OF ETHANOL to acetic acid

It is not ethanol that has a narcotic effect, but the intermediate product acetaldehyde, the lower the activity of the ALDH enzyme, the slower the second stage passes - the oxidation of acetaldehyde to acetic acid, and the longer and stronger the intoxicating effect from ingestion of ethanol. The analysis showed that more than 80% of the representatives of the yellow race have a relatively low activity of ALDH and therefore a markedly more severe alcohol tolerance. The reason for this innate reduced activity of ALDH is that part of the glutamic acid residues in the “attenuated” ALDH molecule is replaced by lysine fragments (Table 1).

Transferases- enzymes that catalyze the transfer of functional groups, for example, transiminase catalyzes the transfer of an amino group.

Hydrolases are enzymes that catalyze hydrolysis. The previously mentioned trypsin and pepsin hydrolyze peptide bonds, and lipases cleave the ester bond in fats:

–RC(O)OR 1 + H 2 O → –RC(O)OH + HOR 1

Liase- enzymes that catalyze reactions that take place in a non-hydrolytic way, as a result of such reactions, C-C, C-O, C-N bonds are broken and new bonds are formed. The enzyme decarboxylase belongs to this class

Isomerases- enzymes that catalyze isomerization, for example, the conversion of maleic acid to fumaric acid (Fig. 19), this is an example of cis-trans isomerization ().

Rice. 19. ISOMERIZATION OF MALEIC ACID into fumaric acid in the presence of the enzyme.

In the work of enzymes, the general principle is observed, according to which there is always a structural correspondence between the enzyme and the reagent of the accelerated reaction. According to the figurative expression of one of the founders of the doctrine of enzymes, the reagent approaches the enzyme like a key to a lock. In this regard, each enzyme catalyzes a certain chemical reaction or a group of reactions of the same type. Sometimes an enzyme can act on a single compound, such as urease (uron Greek. - urine) catalyzes only the hydrolysis of urea:

(H 2 N) 2 C \u003d O + H 2 O \u003d CO 2 + 2NH 3

The most subtle selectivity is shown by enzymes that distinguish between optically active antipodes - left- and right-handed isomers. L-arginase acts only on levorotatory arginine and does not affect the dextrorotatory isomer. L-lactate dehydrogenase acts only on the levorotatory esters of lactic acid, the so-called lactates (lactis lat. milk), while D-lactate dehydrogenase only breaks down D-lactates.

Most of the enzymes act not on one, but on a group of related compounds, for example, trypsin "prefers" to cleave the peptide bonds formed by lysine and arginine (Table 1.)

The catalytic properties of some enzymes, such as hydrolases, are determined solely by the structure of the protein molecule itself, another class of enzymes - oxidoreductases (for example, alcohol dehydrogenase) can only be active in the presence of non-protein molecules associated with them - vitamins that activate Mg, Ca, Zn, Mn and fragments of nucleic acids (Fig. 20).

Rice. twenty ALCOHOLD DEHYDROGENASE MOLECULE

Transport proteins bind and transport various molecules or ions through cell membranes (both inside and outside the cell), as well as from one organ to another.

For example, hemoglobin binds oxygen as blood passes through the lungs and delivers it to various body tissues, where oxygen is released and then used to oxidize food components, this process serves as an energy source (sometimes the term "burning" of food in the body is used).

In addition to the protein part, hemoglobin contains a complex compound of iron with a cyclic porphyrin molecule (porphyros Greek. - purple), which determines the red color of the blood. It is this complex (Fig. 21, left) that plays the role of an oxygen carrier. In hemoglobin, the iron porphyrin complex is located inside the protein molecule and is retained by polar interactions, as well as by a coordination bond with nitrogen in histidine (Table 1), which is part of the protein. The O2 molecule, which is carried by hemoglobin, is attached via a coordination bond to the iron atom from the side opposite to that to which histidine is attached (Fig. 21, right).

Rice. 21 STRUCTURE OF THE IRON COMPLEX

The structure of the complex is shown on the right in the form of a three-dimensional model. The complex is held in the protein molecule by a coordination bond (dashed blue line) between the Fe atom and the N atom in histidine, which is part of the protein. The O 2 molecule, which is carried by hemoglobin, is coordinated (red dotted line) to the Fe atom from the opposite country of the planar complex.

Hemoglobin is one of the most studied proteins, it consists of a-helices connected by single chains and contains four iron complexes. Thus, hemoglobin is like a voluminous package for the transfer of four oxygen molecules at once. The form of hemoglobin corresponds to globular proteins (Fig. 22).

Rice. 22 GLOBULAR FORM OF HEMOGLOBIN

The main "advantage" of hemoglobin is that the addition of oxygen and its subsequent splitting off during transmission to various tissues and organs takes place quickly. Carbon monoxide, CO (carbon monoxide), binds to Fe in hemoglobin even faster, but, unlike O 2 , forms a complex that is difficult to break down. As a result, such hemoglobin is not able to bind O 2, which leads (when large amounts of carbon monoxide are inhaled) to the death of the body from suffocation.

The second function of hemoglobin is the transfer of exhaled CO 2, but not the iron atom, but the H 2 of the N-group of the protein is involved in the process of temporary binding of carbon dioxide.

The "performance" of proteins depends on their structure, for example, replacing the only amino acid residue of glutamic acid in the hemoglobin polypeptide chain with a valine residue (a rarely observed congenital anomaly) leads to a disease called sickle cell anemia.

There are also transport proteins that can bind fats, glucose, amino acids and carry them both inside and outside the cells.

Transport proteins of a special type do not carry the substances themselves, but act as a “transport regulator”, passing certain substances through the membrane (the outer wall of the cell). Such proteins are often called membrane proteins. They have the shape of a hollow cylinder and, being embedded in the membrane wall, ensure the movement of some polar molecules or ions into the cell. An example of a membrane protein is porin (Fig. 23).

Rice. 23 PORIN PROTEIN

Food and storage proteins, as the name implies, serve as sources of internal nutrition, more often for the embryos of plants and animals, as well as in the early stages of development of young organisms. Dietary proteins include albumin (Fig. 10) - the main component of egg white, as well as casein - the main protein of milk. Under the action of the enzyme pepsin, casein curdles in the stomach, which ensures its retention in the digestive tract and efficient absorption. Casein contains fragments of all the amino acids needed by the body.

In ferritin (Fig. 12), which is contained in the tissues of animals, iron ions are stored.

Myoglobin is also a storage protein, which resembles hemoglobin in composition and structure. Myoglobin is concentrated mainly in the muscles, its main role is the storage of oxygen, which hemoglobin gives it. It is rapidly saturated with oxygen (much faster than hemoglobin), and then gradually transfers it to various tissues.

Structural proteins perform a protective function (skin) or support - they hold the body together and give it strength (cartilage and tendons). Their main component is the fibrillar protein collagen (Fig. 11), the most common protein of the animal world, in the body of mammals, it accounts for almost 30% of the total mass of proteins. Collagen has a high tensile strength (the strength of the skin is known), but due to the low content of cross-links in skin collagen, animal skins are not very suitable in their raw form for the manufacture of various products. To reduce the swelling of the skin in water, shrinkage during drying, as well as to increase the strength in the watered state and increase the elasticity in collagen, additional cross-links are created (Fig. 15a), this is the so-called tanning process of the skin.

In living organisms, collagen molecules that have arisen in the process of growth and development of the organism are not updated and are not replaced by newly synthesized ones. As the body ages, the number of cross-links in collagen increases, which leads to a decrease in its elasticity, and since renewal does not occur, age-related changes appear - an increase in the fragility of cartilage and tendons, the appearance of wrinkles on the skin.

Articular ligaments contain elastin, a structural protein that easily stretches in two dimensions. The resilin protein, which is located at the points of hinge attachment of the wings in some insects, has the greatest elasticity.

Horn formations - hair, nails, feathers, consisting mainly of keratin protein (Fig. 24). Its main difference is the noticeable content of cysteine ​​​​residues, which form disulfide bridges, which gives high elasticity (the ability to restore its original shape after deformation) to hair, as well as woolen fabrics.

Rice. 24. FRAGMENT OF FIBRILLAR PROTEIN KERATIN

For an irreversible change in the shape of a keratin object, you must first destroy the disulfide bridges with the help of a reducing agent, give it a new shape, and then re-create the disulfide bridges with the help of an oxidizing agent (Fig. 16), this is how, for example, perming hair is done.

With an increase in the content of cysteine ​​residues in keratin and, accordingly, an increase in the number of disulfide bridges, the ability to deform disappears, but high strength appears at the same time (up to 18% of cysteine ​​fragments are contained in the horns of ungulates and turtle shells). Mammals have up to 30 different types of keratin.

The keratin-related fibrillar protein fibroin, which is secreted by silkworm caterpillars when curling a cocoon, as well as by spiders when weaving a web, contains only β-structures connected by single chains (Fig. 11). Unlike keratin, fibroin does not have transverse disulfide bridges, it has a very strong tensile strength (strength per unit cross-section of some web samples is higher than that of steel cables). Due to the absence of cross-links, fibroin is inelastic (it is known that woolen fabrics are almost indelible, and silk fabrics are easily wrinkled).

regulatory proteins.

Regulatory proteins, more commonly called, are involved in various physiological processes. For example, the hormone insulin (Fig. 25) consists of two α-chains connected by disulfide bridges. Insulin regulates metabolic processes involving glucose, its absence leads to diabetes.

Rice. 25 PROTEIN INSULIN

The pituitary gland of the brain synthesizes a hormone that regulates the growth of the body. There are regulatory proteins that control the biosynthesis of various enzymes in the body.

Contractile and motor proteins give the body the ability to contract, change shape and move, primarily, we are talking about muscles. 40% of the mass of all proteins contained in the muscles is myosin (mys, myos, Greek. - muscle). Its molecule contains both a fibrillar and a globular part (Fig. 26)

Rice. 26 MYOSIN MOLECULE

Such molecules combine into large aggregates containing 300–400 molecules.

When the concentration of calcium ions changes in the space surrounding muscle fibers, a reversible change in the conformation of molecules occurs - a change in the shape of the chain due to the rotation of individual fragments around valence bonds. This leads to muscle contraction and relaxation, the signal to change the concentration of calcium ions comes from the nerve endings in the muscle fibers. Artificial muscle contraction can be caused by the action of electrical impulses, leading to a sharp change in the concentration of calcium ions, this is the basis for stimulating the heart muscle to restore the work of the heart.

Protective proteins allow you to protect the body from the invasion of attacking bacteria, viruses and from the penetration of foreign proteins (the generalized name of foreign bodies is antigens). The role of protective proteins is performed by immunoglobulins (their other name is antibodies), they recognize antigens that have penetrated the body and firmly bind to them. In the body of mammals, including humans, there are five classes of immunoglobulins: M, G, A, D and E, their structure, as the name implies, is globular, in addition, they are all built in a similar way. The molecular organization of antibodies is shown below using class G immunoglobulin as an example (Fig. 27). The molecule contains four polypeptide chains connected by three S-S disulfide bridges (in Fig. 27 they are shown with thickened valence bonds and large S symbols), in addition, each polymer chain contains intrachain disulfide bridges. Two large polymer chains (highlighted in blue) contain 400–600 amino acid residues. The other two chains (highlighted in green) are almost half as long, containing approximately 220 amino acid residues. All four chains are located in such a way that the terminal H 2 N-groups are directed in one direction.

Rice. 27 SCHEMATIC DRAWING OF THE STRUCTURE OF IMMUNOGLOBULIN

After the body comes into contact with a foreign protein (antigen), the cells of the immune system begin to produce immunoglobulins (antibodies), which accumulate in the blood serum. At the first stage, the main work is done by chain sections containing terminal H 2 N (in Fig. 27, the corresponding sections are marked in light blue and light green). These are antigen capture sites. In the process of immunoglobulin synthesis, these sites are formed in such a way that their structure and configuration correspond as much as possible to the structure of the approaching antigen (like a key to a lock, like enzymes, but the tasks in this case are different). Thus, for each antigen, a strictly individual antibody is created as an immune response. Not a single known protein can change its structure so “plastically” depending on external factors, in addition to immunoglobulins. Enzymes solve the problem of structural conformity to the reagent in a different way - with the help of a gigantic set of various enzymes for all possible cases, and immunoglobulins each time rebuild the "working tool". Moreover, the hinge region of the immunoglobulin (Fig. 27) provides the two capture regions with some independent mobility, as a result, the immunoglobulin molecule can immediately “find” the two most convenient regions for capture in the antigen in order to securely fix it, this resembles the actions of a crustacean creature.

Next, a chain of successive reactions of the body's immune system is turned on, immunoglobulins of other classes are connected, as a result, the foreign protein is deactivated, and then the antigen (foreign microorganism or toxin) is destroyed and removed.

After contact with the antigen, the maximum concentration of immunoglobulin is reached (depending on the nature of the antigen and the individual characteristics of the organism itself) within a few hours (sometimes several days). The body retains the memory of such contact, and when attacked again with the same antigen, immunoglobulins accumulate in the blood serum much faster and in greater quantities - acquired immunity occurs.

The above classification of proteins is somewhat arbitrary, for example, the thrombin protein, mentioned among protective proteins, is essentially an enzyme that catalyzes the hydrolysis of peptide bonds, that is, it belongs to the class of proteases.

Protective proteins are often referred to as snake venom proteins and the toxic proteins of some plants, since their task is to protect the body from damage.

There are proteins whose functions are so unique that it makes it difficult to classify them. For example, the protein monellin, found in an African plant, is very sweet tasting and has been the subject of research as a non-toxic substance that can be used in place of sugar to prevent obesity. The blood plasma of some Antarctic fish contains proteins with antifreeze properties that keep the blood of these fish from freezing.

Artificial synthesis of proteins.

The condensation of amino acids leading to a polypeptide chain is a well-studied process. It is possible to carry out, for example, the condensation of any one amino acid or a mixture of acids and obtain, respectively, a polymer containing the same units, or different units, alternating in random order. Such polymers bear little resemblance to natural polypeptides and do not possess biological activity. The main task is to connect amino acids in a strictly defined, pre-planned order in order to reproduce the sequence of amino acid residues in natural proteins. The American scientist Robert Merrifield proposed an original method that made it possible to solve such a problem. The essence of the method is that the first amino acid is attached to an insoluble polymer gel that contains reactive groups that can combine with –COOH – groups of the amino acid. Cross-linked polystyrene with chloromethyl groups introduced into it was taken as such a polymeric substrate. So that the amino acid taken for the reaction does not react with itself and so that it does not join the H 2 N-group to the substrate, the amino group of this acid is pre-blocked with a bulky substituent [(C 4 H 9) 3] 3 OS (O) -group. After the amino acid has attached to the polymeric support, the blocking group is removed and another amino acid is introduced into the reaction mixture, in which the H 2 N group is also previously blocked. In such a system, only the interaction of the H 2 N-group of the first amino acid and the –COOH group of the second acid is possible, which is carried out in the presence of catalysts (phosphonium salts). Then the whole scheme is repeated, introducing the third amino acid (Fig. 28).

Rice. 28. SYNTHESIS SCHEME OF POLYPEPTIDE CHAINS

In the last step, the resulting polypeptide chains are separated from the polystyrene support. Now the whole process is automated, there are automatic peptide synthesizers that operate according to the described scheme. Many peptides used in medicine and agriculture have been synthesized by this method. It was also possible to obtain improved analogs of natural peptides with selective and enhanced action. Some small proteins have been synthesized, such as the hormone insulin and some enzymes.

There are also methods of protein synthesis that replicate natural processes: fragments of nucleic acids are synthesized that are configured to produce certain proteins, then these fragments are inserted into a living organism (for example, into a bacterium), after which the body begins to produce the desired protein. In this way, significant amounts of hard-to-reach proteins and peptides, as well as their analogues, are now obtained.

Proteins as food sources.

Proteins in a living organism are constantly broken down into their original amino acids (with the indispensable participation of enzymes), some amino acids pass into others, then proteins are synthesized again (also with the participation of enzymes), i.e. the body is constantly renewing itself. Some proteins (collagen of the skin, hair) are not renewed, the body continuously loses them and instead synthesizes new ones. Proteins as food sources perform two main functions: they supply the body with building material for the synthesis of new protein molecules and, in addition, supply the body with energy (sources of calories).

Carnivorous mammals (including humans) get the necessary proteins from plant and animal foods. None of the proteins obtained from food is integrated into the body in an unchanged form. In the digestive tract, all absorbed proteins are broken down to amino acids, and proteins necessary for a particular organism are already built from them, while the remaining 12 can be synthesized from 8 essential acids (Table 1) in the body if they are not supplied in sufficient quantities with food, but essential acids must be supplied with food without fail. Sulfur atoms in cysteine ​​are obtained by the body with the essential amino acid methionine. Part of the proteins breaks down, releasing the energy necessary to maintain life, and the nitrogen contained in them is excreted from the body with urine. Usually the human body loses 25–30 g of protein per day, so protein foods must always be present in the right amount. The minimum daily requirement for protein is 37 g for men and 29 g for women, but the recommended intake is almost twice as high. When evaluating foods, it is important to consider protein quality. In the absence or low content of essential amino acids, the protein is considered of low value, so such proteins should be consumed in greater quantities. So, the proteins of legumes contain little methionine, and the proteins of wheat and corn are low in lysine (both amino acids are essential). Animal proteins (excluding collagens) are classified as complete foods. A complete set of all essential acids contains milk casein, as well as cottage cheese and cheese prepared from it, so a vegetarian diet, if it is very strict, i.e. “dairy-free”, requires increased consumption of legumes, nuts and mushrooms to supply the body with essential amino acids in the right amount.

Synthetic amino acids and proteins are also used as food products, adding them to feed, which contain essential amino acids in small quantities. There are bacteria that can process and assimilate oil hydrocarbons, in this case, for the full synthesis of proteins, they need to be fed with nitrogen-containing compounds (ammonia or nitrates). The protein obtained in this way is used as feed for livestock and poultry. A set of enzymes, carbohydrases, are often added to animal feed, which catalyze the hydrolysis of carbohydrate food components that are difficult to decompose (cell walls of cereal crops), as a result of which plant foods are more fully absorbed.

Mikhail Levitsky

PROTEINS (Article 2)

(proteins), a class of complex nitrogen-containing compounds, the most characteristic and important (along with nucleic acids) components of living matter. Proteins perform many and varied functions. Most proteins are enzymes that catalyze chemical reactions. Many hormones that regulate physiological processes are also proteins. Structural proteins such as collagen and keratin are the main components of bone tissue, hair and nails. The contractile proteins of muscles have the ability to change their length, using chemical energy to perform mechanical work. Proteins are antibodies that bind and neutralize toxic substances. Some proteins that can respond to external influences (light, smell) serve as receptors in the sense organs that perceive irritation. Many proteins located inside the cell and on the cell membrane perform regulatory functions.

In the first half of the 19th century many chemists, and among them primarily J. von Liebig, gradually came to the conclusion that proteins are a special class of nitrogenous compounds. The name "proteins" (from the Greek protos - the first) was proposed in 1840 by the Dutch chemist G. Mulder.

PHYSICAL PROPERTIES

Proteins are white in the solid state, but colorless in solution, unless they carry some chromophore (colored) group, such as hemoglobin. The solubility in water of different proteins varies greatly. It also varies with pH and with the concentration of salts in the solution, so that one can choose the conditions under which one protein will selectively precipitate in the presence of other proteins. This "salting out" method is widely used to isolate and purify proteins. The purified protein often precipitates out of solution as crystals.

In comparison with other compounds, the molecular weight of proteins is very large - from several thousand to many millions of daltons. Therefore, during ultracentrifugation, proteins are precipitated, and, moreover, at different rates. Due to the presence of positively and negatively charged groups in protein molecules, they move at different speeds in an electric field. This is the basis of electrophoresis, a method used to isolate individual proteins from complex mixtures. Purification of proteins is also carried out by chromatography.

CHEMICAL PROPERTIES

Structure.

Proteins are polymers, i.e. molecules built like chains from repeating monomer units, or subunits, whose role is played by alpha-amino acids. General formula of amino acids

where R is a hydrogen atom or some organic group.

A protein molecule (polypeptide chain) may consist of only a relatively small number of amino acids or several thousand monomer units. The connection of amino acids in a chain is possible because each of them has two different chemical groups: an amino group with basic properties, NH2, and an acidic carboxyl group, COOH. Both of these groups are attached to the a carbon atom. The carboxyl group of one amino acid can form an amide (peptide) bond with the amino group of another amino acid:

After two amino acids have been connected in this way, the chain can be extended by adding a third to the second amino acid, and so on. As can be seen from the above equation, when a peptide bond is formed, a water molecule is released. In the presence of acids, alkalis or proteolytic enzymes, the reaction proceeds in the opposite direction: the polypeptide chain is cleaved into amino acids with the addition of water. This reaction is called hydrolysis. Hydrolysis proceeds spontaneously, and energy is required to combine amino acids into a polypeptide chain.

A carboxyl group and an amide group (or an imide group similar to it - in the case of the proline amino acid) are present in all amino acids, while the differences between amino acids are determined by the nature of that group, or "side chain", which is indicated above by the letter R. The role of the side chain can be played by one a hydrogen atom, like the amino acid glycine, and some bulky grouping, like histidine and tryptophan. Some side chains are chemically inert, while others are highly reactive.

Many thousands of different amino acids can be synthesized, and many different amino acids occur in nature, but only 20 types of amino acids are used for protein synthesis: alanine, arginine, asparagine, aspartic acid, valine, histidine, glycine, glutamine, glutamic acid, isoleucine, leucine, lysine , methionine, proline, serine, tyrosine, threonine, tryptophan, phenylalanine and cysteine ​​(in proteins, cysteine ​​may be present as a dimer - cystine). True, there are other amino acids in some proteins, in addition to the regularly occurring twenty, but they are formed as a result of modification of any of the twenty listed after it has been included in the protein.

optical activity.

All amino acids, with the exception of glycine, have four different groups attached to the α-carbon atom. In terms of geometry, four different groups can be attached in two ways, and accordingly there are two possible configurations, or two isomers, related to each other as an object to its mirror image, i.e. like left hand to right. One configuration is called left, or left-handed (L), and the other right-handed, or right-handed (D), because the two such isomers differ in the direction of rotation of the plane of polarized light. Only L-amino acids occur in proteins (the exception is glycine; it can only be represented in one form, since two of its four groups are the same), and they all have optical activity (since there is only one isomer). D-amino acids are rare in nature; they are found in some antibiotics and the cell wall of bacteria.

The sequence of amino acids.

Amino acids in the polypeptide chain are not randomly arranged, but in a certain fixed order, and it is this order that determines the functions and properties of the protein. By varying the order of the 20 types of amino acids, you can get a huge number of different proteins, just like you can make up many different texts from the letters of the alphabet.

In the past, determining the amino acid sequence of a protein often took several years. Direct determination is still a rather laborious task, although devices have been created that allow it to be carried out automatically. It is usually easier to determine the nucleotide sequence of the corresponding gene and derive the amino acid sequence of the protein from it. To date, the amino acid sequences of many hundreds of proteins have already been determined. The functions of decoded proteins are usually known, and this helps to imagine the possible functions of similar proteins formed, for example, in malignant neoplasms.

Complex proteins.

Proteins consisting of only amino acids are called simple. Often, however, a metal atom or some chemical compound that is not an amino acid is attached to the polypeptide chain. Such proteins are called complex. An example is hemoglobin: it contains iron porphyrin, which gives it its red color and allows it to act as an oxygen carrier.

The names of most complex proteins contain an indication of the nature of the attached groups: sugars are present in glycoproteins, fats in lipoproteins. If the catalytic activity of the enzyme depends on the attached group, then it is called a prosthetic group. Often, some vitamin plays the role of a prosthetic group or is part of it. Vitamin A, for example, attached to one of the proteins of the retina, determines its sensitivity to light.

Tertiary structure.

What is important is not so much the amino acid sequence of the protein (primary structure), but the way it is laid in space. Along the entire length of the polypeptide chain, hydrogen ions form regular hydrogen bonds, which give it the shape of a spiral or layer (secondary structure). From the combination of such helices and layers, a compact form of the next order arises - the tertiary structure of the protein. Around the bonds that hold the monomeric links of the chain, rotations through small angles are possible. Therefore, from a purely geometric point of view, the number of possible configurations for any polypeptide chain is infinitely large. In reality, each protein normally exists in only one configuration, determined by its amino acid sequence. This structure is not rigid, it seems to "breathe" - it oscillates around a certain average configuration. The chain is folded into a configuration in which the free energy (the ability to do work) is minimal, just as a released spring is compressed only to a state corresponding to a minimum of free energy. Often, one part of the chain is rigidly linked to the other by disulfide (–S–S–) bonds between two cysteine ​​residues. This is partly why cysteine ​​among amino acids plays a particularly important role.

The complexity of the structure of proteins is so great that it is not yet possible to calculate the tertiary structure of a protein, even if its amino acid sequence is known. But if it is possible to obtain protein crystals, then its tertiary structure can be determined by X-ray diffraction.

In structural, contractile, and some other proteins, the chains are elongated and several slightly folded chains lying side by side form fibrils; fibrils, in turn, fold into larger formations - fibers. However, most proteins in solution are globular: the chains are coiled in a globule, like yarn in a ball. Free energy with this configuration is minimal, since hydrophobic ("water-repelling") amino acids are hidden inside the globule, and hydrophilic ("water-attracting") amino acids are on its surface.

Many proteins are complexes of several polypeptide chains. This structure is called the quaternary structure of the protein. The hemoglobin molecule, for example, is made up of four subunits, each of which is a globular protein.

Structural proteins, due to their linear configuration, form fibers in which the tensile strength is very high, while the globular configuration allows proteins to enter into specific interactions with other compounds. On the surface of the globule, with the correct laying of chains, cavities of a certain shape appear, in which reactive chemical groups are located. If this protein is an enzyme, then another, usually smaller, molecule of some substance enters such a cavity, just as a key enters a lock; in this case, the configuration of the electron cloud of the molecule changes under the influence of chemical groups located in the cavity, and this forces it to react in a certain way. In this way, the enzyme catalyzes the reaction. Antibody molecules also have cavities in which various foreign substances bind and are thereby rendered harmless. The "key and lock" model, which explains the interaction of proteins with other compounds, makes it possible to understand the specificity of enzymes and antibodies, i.e. their ability to react only with certain compounds.

Proteins in different types of organisms.

Proteins that perform the same function in different plant and animal species and therefore bear the same name also have a similar configuration. They, however, differ somewhat in their amino acid sequence. As species diverge from a common ancestor, some amino acids in certain positions are replaced by mutations with others. Harmful mutations that cause hereditary diseases are discarded by natural selection, but beneficial or at least neutral ones can be preserved. The closer two biological species are to each other, the less differences are found in their proteins.

Some proteins change relatively quickly, others are quite conservative. The latter include, for example, cytochrome c, a respiratory enzyme found in most living organisms. In humans and chimpanzees, its amino acid sequences are identical, while in cytochrome c of wheat, only 38% of the amino acids turned out to be different. Even when comparing humans and bacteria, the similarities of cytochromes with (the differences here affect 65% of amino acids) can still be seen, although the common ancestor of bacteria and humans lived on Earth about two billion years ago. Nowadays, comparison of amino acid sequences is often used to build a phylogenetic (genealogical) tree that reflects the evolutionary relationships between different organisms.

Denaturation.

The synthesized protein molecule, folding, acquires its own configuration. This configuration, however, can be destroyed by heating, by changing the pH, by the action of organic solvents, and even by simply agitating the solution until bubbles appear on its surface. A protein altered in this way is called denatured; it loses its biological activity and usually becomes insoluble. Well-known examples of denatured protein are boiled eggs or whipped cream. Small proteins, containing only about a hundred amino acids, are able to renature, i.e. reacquire the original configuration. But most of the proteins are simply transformed into a mass of tangled polypeptide chains and do not restore their previous configuration.

One of the main difficulties in isolating active proteins is their extreme sensitivity to denaturation. This property of proteins finds useful application in the preservation of food products: high temperature irreversibly denatures the enzymes of microorganisms, and the microorganisms die.

PROTEIN SYNTHESIS

For protein synthesis, a living organism must have a system of enzymes capable of attaching one amino acid to another. A source of information is also needed that would determine which amino acids should be connected. Since there are thousands of types of proteins in the body, and each of them consists of an average of several hundred amino acids, the information required must be truly enormous. It is stored (similar to how a record is stored on a magnetic tape) in the nucleic acid molecules that make up genes.

Enzyme activation.

A polypeptide chain synthesized from amino acids is not always a protein in its final form. Many enzymes are first synthesized as inactive precursors and become active only after another enzyme removes a few amino acids from one end of the chain. Some of the digestive enzymes, such as trypsin, are synthesized in this inactive form; these enzymes are activated in the digestive tract as a result of the removal of the terminal fragment of the chain. The hormone insulin, whose molecule in its active form consists of two short chains, is synthesized in the form of a single chain, the so-called. proinsulin. Then the middle part of this chain is removed, and the remaining fragments bind to each other, forming an active hormone molecule. Complex proteins are formed only after a certain chemical group is attached to the protein, and this attachment often also requires an enzyme.

Metabolic circulation.

After feeding an animal with amino acids labeled with radioactive isotopes of carbon, nitrogen or hydrogen, the label is quickly incorporated into its proteins. If labeled amino acids cease to enter the body, then the amount of label in proteins begins to decrease. These experiments show that the resulting proteins are not stored in the body until the end of life. All of them, with a few exceptions, are in a dynamic state, constantly decomposing to amino acids, and then re-synthesized.

Some proteins break down when cells die and are destroyed. This happens all the time, for example, with red blood cells and epithelial cells lining the inner surface of the intestine. In addition, the breakdown and resynthesis of proteins also occur in living cells. Oddly enough, less is known about the breakdown of proteins than about their synthesis. What is clear, however, is that proteolytic enzymes are involved in the breakdown, similar to those that break down proteins into amino acids in the digestive tract.

The half-life of different proteins is different - from several hours to many months. The only exception is collagen molecules. Once formed, they remain stable and are not renewed or replaced. Over time, however, some of their properties, in particular elasticity, change, and since they are not renewed, certain age-related changes, such as the appearance of wrinkles on the skin, are the result of this.

synthetic proteins.

Chemists have long since learned how to polymerize amino acids, but the amino acids combine randomly, so that the products of such polymerization bear little resemblance to natural ones. True, it is possible to combine amino acids in a given order, which makes it possible to obtain some biologically active proteins, in particular insulin. The process is quite complicated, and in this way it is possible to obtain only those proteins whose molecules contain about a hundred amino acids. It is preferable instead to synthesize or isolate the nucleotide sequence of a gene corresponding to the desired amino acid sequence, and then introduce this gene into a bacterium, which will produce by replication a large amount of the desired product. This method, however, also has its drawbacks.

PROTEINS AND NUTRITION

When proteins in the body are broken down into amino acids, these amino acids can be reused for protein synthesis. At the same time, the amino acids themselves are subject to decay, so that they are not fully utilized. It is also clear that during growth, pregnancy, and wound healing, protein synthesis must exceed degradation. The body continuously loses some proteins; these are the proteins of hair, nails and the surface layer of the skin. Therefore, for the synthesis of proteins, each organism must receive amino acids from food.

Sources of amino acids.

Green plants synthesize all 20 amino acids found in proteins from CO2, water and ammonia or nitrates. Many bacteria are also able to synthesize amino acids in the presence of sugar (or some equivalent) and fixed nitrogen, but sugar is ultimately supplied by green plants. In animals, the ability to synthesize amino acids is limited; they obtain amino acids by eating green plants or other animals. In the digestive tract, the absorbed proteins are broken down into amino acids, the latter are absorbed, and the proteins characteristic of the given organism are built from them. None of the absorbed protein is incorporated into body structures as such. The only exception is that in many mammals, part of maternal antibodies can pass intact through the placenta into the fetal circulation, and through mother's milk (especially in ruminants) be transferred to the newborn immediately after birth.

Need for proteins.

It is clear that in order to maintain life, the body must receive a certain amount of protein from food. However, the size of this need depends on a number of factors. The body needs food both as a source of energy (calories) and as a material for building its structures. In the first place is the need for energy. This means that when there are few carbohydrates and fats in the diet, dietary proteins are used not for the synthesis of their own proteins, but as a source of calories. With prolonged fasting, even your own proteins are spent to meet energy needs. If there are enough carbohydrates in the diet, then protein intake can be reduced.

nitrogen balance.

On average approx. 16% of the total protein mass is nitrogen. When the amino acids that make up proteins are broken down, the nitrogen contained in them is excreted from the body in the urine and (to a lesser extent) in the feces in the form of various nitrogenous compounds. Therefore, it is convenient to use such an indicator as nitrogen balance to assess the quality of protein nutrition, i.e. the difference (in grams) between the amount of nitrogen taken into the body and the amount of nitrogen excreted per day. With normal nutrition in an adult, these amounts are equal. In a growing organism, the amount of excreted nitrogen is less than the amount of incoming, i.e. the balance is positive. With a lack of protein in the diet, the balance is negative. If there are enough calories in the diet, but the proteins are completely absent in it, the body saves proteins. At the same time, protein metabolism slows down, and the re-utilization of amino acids in protein synthesis proceeds as efficiently as possible. However, losses are inevitable, and nitrogenous compounds are still excreted in the urine and partly in the feces. The amount of nitrogen excreted from the body per day during protein starvation can serve as a measure of the daily lack of protein. It is natural to assume that by introducing into the diet an amount of protein equivalent to this deficiency, it is possible to restore the nitrogen balance. However, it is not. Having received this amount of protein, the body begins to use amino acids less efficiently, so some additional protein is required to restore the nitrogen balance.

If the amount of protein in the diet exceeds what is necessary to maintain nitrogen balance, then there seems to be no harm from this. Excess amino acids are simply used as a source of energy. A particularly striking example is the Eskimo, who consume little carbohydrate and about ten times more protein than is required to maintain nitrogen balance. In most cases, however, using protein as an energy source is not beneficial, since you can get many more calories from a given amount of carbohydrates than from the same amount of protein. In poor countries, the population receives the necessary calories from carbohydrates and consumes a minimum amount of protein.

If the body receives the required number of calories in the form of non-protein foods, then the minimum amount of protein that maintains the nitrogen balance is approx. 30 g per day. Approximately as much protein is contained in four slices of bread or 0.5 liters of milk. A slightly larger amount is usually considered optimal; recommended from 50 to 70 g.

Essential amino acids.

Until now, protein has been considered as a whole. Meanwhile, in order for protein synthesis to take place, all the necessary amino acids must be present in the body. Some of the amino acids the body of the animal itself is able to synthesize. They are called interchangeable, since they do not have to be present in the diet, it is only important that, in general, the intake of protein as a source of nitrogen is sufficient; then, with a shortage of non-essential amino acids, the body can synthesize them at the expense of those that are present in excess. The remaining "essential" amino acids cannot be synthesized and must be ingested with food. Essential for humans are valine, leucine, isoleucine, threonine, methionine, phenylalanine, tryptophan, histidine, lysine, and arginine. (Although arginine can be synthesized in the body, it is considered an essential amino acid because newborns and growing children produce insufficient amounts of it. On the other hand, for a person of mature age, the intake of some of these amino acids from food may become optional.)

This list of essential amino acids is approximately the same in other vertebrates and even in insects. The nutritional value of proteins is usually determined by feeding them to growing rats and monitoring the weight gain of the animals.

The nutritional value of proteins.

The nutritional value of a protein is determined by the essential amino acid that is most deficient. Let's illustrate this with an example. The proteins of our body contain an average of approx. 2% tryptophan (by weight). Let's say that the diet includes 10 g of protein containing 1% tryptophan, and that there are enough other essential amino acids in it. In our case, 10 g of this defective protein is essentially equivalent to 5 g of a complete one; the remaining 5 g can only serve as a source of energy. Note that, since amino acids are practically not stored in the body, and in order for protein synthesis to take place, all amino acids must be present simultaneously, the effect of the intake of essential amino acids can be detected only if all of them enter the body at the same time.

The average composition of most animal proteins is close to the average composition of proteins in the human body, so we are unlikely to face amino acid deficiency if our diet is rich in foods such as meat, eggs, milk and cheese. However, there are proteins, such as gelatin (a product of collagen denaturation), which contain very few essential amino acids. Vegetable proteins, although they are better than gelatin in this sense, are also poor in essential amino acids; especially little in them lysine and tryptophan. However, a purely vegetarian diet is by no means unhealthy, unless it consumes a slightly larger amount of vegetable proteins, sufficient to provide the body with essential amino acids. Most protein is found in plants in seeds, especially in the seeds of wheat and various legumes. Young shoots, such as asparagus, are also rich in protein.

Synthetic proteins in the diet.

By adding small amounts of synthetic essential amino acids or proteins rich in them to incomplete proteins, such as corn proteins, one can significantly increase the nutritional value of the latter, i.e. thereby increasing the amount of protein consumed. Another possibility is to grow bacteria or yeasts on petroleum hydrocarbons with the addition of nitrates or ammonia as a source of nitrogen. The microbial protein obtained in this way can serve as feed for poultry or livestock, or can be directly consumed by humans. The third, widely used, method uses the physiology of ruminants. In ruminants, in the initial section of the stomach, the so-called. In the rumen, there are special forms of bacteria and protozoa that convert defective plant proteins into more complete microbial proteins, and these, in turn, after digestion and absorption, turn into animal proteins. Urea, a cheap synthetic nitrogen-containing compound, can be added to livestock feed. Microorganisms living in the rumen use urea nitrogen to convert carbohydrates (of which there is much more in the feed) into protein. About a third of all nitrogen in livestock feed can come in the form of urea, which in essence means, to a certain extent, chemical protein synthesis.

Proteins, or proteins, are complex, high-molecular organic compounds consisting of amino acids. They represent the main, most important part of all cells and tissues of animal and plant organisms, without which vital physiological processes cannot be carried out. Proteins are not the same in composition and properties in different animal and plant organisms and in different cells and tissues of the same organism. Proteins of different molecular composition dissolve differently in and in aqueous salt solutions; they do not dissolve in organic solvents. Due to the presence of acidic and basic groups in the protein molecule, it has a neutral reaction.

Proteins form numerous compounds with any chemical substances, which determines their special importance in chemical reactions occurring in the body and representing the basis of all manifestations of life and its protection from harmful influences. Proteins form the basis of enzymes, antibodies, hemoglobin, myoglobin, many hormones, and form complex complexes with vitamins.

Entering into compounds with fats and carbohydrates, proteins can be converted in the body during their breakdown into fats and carbohydrates. In the animal body, they are synthesized only from amino acids and their complexes - polypeptides, and they cannot be formed from inorganic compounds, fats and carbohydrates. Outside the body, many low-molecular biologically active protein substances are synthesized, similar to those found in the body, for example, some hormones.

General information about proteins and their classification

Proteins are the most important bioorganic compounds, which, along with nucleic acids, occupy a special role in living matter - life is impossible without these compounds, since, according to F. Engels, life is a special existence of protein bodies, etc.

"Proteins are natural biopolymers that are products of the polycondensation reaction of natural alpha-amino acids."

Natural alpha-amino acids 18-23, their combination forms an infinite number of varieties of protein molecules, providing a variety of different organisms. Even for individual individuals of organisms of this species, their own proteins are characteristic, and a number of proteins are found in many organisms.

Proteins are characterized by the following elemental composition: they are formed by carbon, hydrogen, oxygen, nitrogen, sulfur and some other chemical elements. The main feature of protein molecules is the mandatory presence of nitrogen in them (in addition to C, H, O atoms).

In protein molecules, a “peptide” bond is realized, that is, a bond between the C atom of the carbonyl group and the nitrogen atom of the amino group, which determines some features of protein molecules. The side chains of the protein molecule contain a large number of radicals and functional groups, which "makes" the protein molecule polyfunctional, capable of a significant variety of physicochemical and biochemical properties.

Due to the wide variety of protein molecules and the complexity of their composition and properties, proteins have several different classifications based on different features. Let's consider some of them.

I. Two groups of proteins are distinguished by composition:

1. Proteins (simple proteins; their molecule is formed only by a protein, for example, egg albumin).

2. Proteins are complex proteins, the molecules of which consist of protein and non-protein components.

Proteins are divided into several groups, the most important of which are:

1) glycoproteins (a complex combination of protein and carbohydrate);

2) lipoproteins (a complex of protein molecules and fats (lipids);

3) nucleoproteins (a complex of protein molecules and nucleic acid molecules).

II. There are two groups of proteins according to the shape of the molecule:

1. Globular proteins - a protein molecule has a spherical shape (globule shape), for example, egg albumin molecules; such proteins are either soluble in water or capable of forming colloidal solutions.

2. Fibrillar proteins - the molecules of these substances are in the form of filaments (fibrils), for example, muscle myosin, silk fibroin. Fibrillar proteins are insoluble in water, they form structures that implement contractile, mechanical, shaping and protective functions, as well as the body's ability to move in space.

III. By solubility in various solvents, proteins are divided into several groups, of which the most important are the following:

1. Water soluble.

2. Fat soluble.

There are other classifications of proteins.

Brief description of natural alpha-amino acids

Natural alpha-amino acids are a type of amino acids. An amino acid is a polyfunctional organic substance containing in its composition at least two functional groups - an amino group (-NH 2) and a carboxyl (carboxylic, the latter is more correct) group (-COOH).

Alpha amino acids are amino acids in which the amino and carboxyl groups are located on the same carbon atom. Their general formula is NH 2 CH(R)COOH. Below are the formulas for some natural alpha-amino acids; they are written in a form convenient for writing the equations of the polycondensation reaction and are used when it is necessary to write the equations (schemes) of reactions for obtaining certain polypeptides:

1) glycine (aminoacetic acid) - MH 2 CH 2 COOH;

2) alanine - NH 2 CH (CH 3) COOH;

3) phenylalanine - NH 2 CH (CH 2 C 6 H 5) COOH;

4) serine - NH 2 CH (CH 2 OH) COOH;

5) aspartic acid - NH 2 CH (CH 2 COOH) COOH;

6) cysteine ​​- NH 2 CH (CH 2 SH) COOH, etc.

Some natural alpha-amino acids contain two amino groups (for example, lysine), two carboxylic groups (for example, aspartic and glutamic acids), hydroxide (OH) groups (for example, tyrosine), and can be cyclic (for example, proline).

According to the nature of the influence of natural alpha-amino acids on metabolism, they are divided into interchangeable and irreplaceable. Essential amino acids must be ingested with food.

Brief description of the structure of protein molecules

Proteins, in addition to their complex composition, are also characterized by a complex structure of protein molecules. There are four types of structures of protein molecules.

1. The primary structure is characterized by the order of arrangement of alpha-amino acid residues in the polypeptide chain. For example, a tetrapeptide (a polypeptide formed by the polycondensation of four amino acid molecules) ala-phen-thyro-serine is a sequence of alanine, phenylalanine, tyrosine and serine residues linked to each other by a peptide bond.

2. The secondary structure of a protein molecule is the spatial arrangement of the polypeptide chain. It can be different, but the most common is the alpha helix, characterized by a certain "pitch" of the helix, size and distance between the individual turns of the helix.

The stability of the secondary structure of the protein molecule is ensured by the emergence of various chemical bonds between the individual turns of the helix. The most important role among them belongs to the hydrogen bond (implemented by drawing the nucleus of the atom of groups - NH 2 or \u003d NH into the electron shell of oxygen or nitrogen atoms), ionic bonding (implemented due to the electrostatic interaction of ions -COO - and - NH + 3 or \u003d NH + 2) and other types of communication.

3. The tertiary structure of protein molecules is characterized by the spatial arrangement of the alpha helix, or another structure. The stability of such structures is determined by the same types of connection as the secondary structure. As a result of the implementation of the tertiary structure, a “subunit” of the protein molecule appears, which is typical for very complex molecules, and for relatively simple molecules, the tertiary structure is final.

4. The quaternary structure of a protein molecule is the spatial arrangement of subunits of protein molecules. It is characteristic of complex proteins, such as hemoglobin.

Considering the question of the structure of protein molecules, it is necessary to distinguish between the structure of a living protein - the native structure and the structure of a dead protein. A protein in living matter (native protein) is different from a protein that has been exposed to a condition in which it may lose the properties of a living protein. A shallow impact is called denaturation, in which the properties of a living protein can be restored in the future. One type of denaturation is reversible coagulation. With irreversible coagulation, the native protein is converted into a "dead protein".

Brief description of the physical, physico-chemical and chemical properties of the protein

The properties of protein molecules are of great importance for the realization of their biological and ecological properties. So, according to the state of aggregation, proteins are classified as solids, which can be soluble or insoluble in water or other solvents. Much in the bioecological role of proteins is determined by physical properties. Thus, the ability of protein molecules to form colloidal systems determines their building, catalytic and other functions. The insolubility of proteins in water and other solvents, their fibrillarity determines the protective and shaping functions, etc.

The physicochemical properties of proteins include their ability to denature and coagulate. Coagulation manifests itself in colloidal systems, which are the basis of any living substance. During coagulation, the particles become larger due to their sticking together. Coagulation can be hidden (it can only be observed under a microscope) and explicit - its sign is the precipitation of protein. Coagulation is irreversible, when the structure of the colloidal system is not restored after the termination of the action of the coagulating factor, and reversible, when the colloidal system is restored after the removal of the coagulating factor.

An example of reversible coagulation is the precipitation of egg albumin protein under the action of salt solutions, while the protein precipitate dissolves when the solution is diluted or when the precipitate is transferred to distilled water.

An example of irreversible coagulation is the destruction of the colloidal structure of albumin protein when heated to the boiling point of water. At (complete) death, living matter turns into dead matter due to irreversible coagulation of the entire system.

The chemical properties of proteins are very diverse due to the presence of a large number of functional groups in protein molecules, as well as due to the presence of peptide and other bonds in protein molecules. From an ecological and biological point of view, the ability of protein molecules to hydrolysis is of greatest importance (in this case, a mixture of natural alpha-amino acids that participated in the formation of this molecule is ultimately obtained, this mixture may contain other substances if the protein was a protein), to oxidation (its products may be carbon dioxide, water, nitrogen compounds, such as urea, phosphorus compounds, etc.).

Proteins burn with the release of the smell of "burnt horn" or "burnt feathers", which is necessary to know when conducting environmental experiments. Various color reactions to protein are known (biuret, xantoprotein, etc.), more about them in the course of chemistry.

Brief description of the ecological and biological functions of proteins

It is necessary to distinguish between the ecological and biological role of proteins in cells and in the body as a whole.

Ecological and biological role of proteins in cells

Due to the fact that proteins (along with nucleic acids) are the substances of life, their functions in cells are very diverse.

1. The most important function of protein molecules is the structural function, which consists in the fact that protein is the most important component of all structures that form a cell, in which it is part of a complex of various chemical compounds.

2. Protein is the most important reagent in the course of a huge variety of biochemical reactions that ensure the normal functioning of living matter, therefore it is characterized by a reagent function.

3. In living matter, reactions are possible only in the presence of biological catalysts - enzymes, and as established by biochemical studies, they are of a protein nature, therefore proteins also perform a catalytic function.

4. If necessary, proteins are oxidized in organisms and, at the same time, are released, due to which ATP is synthesized, i.e. proteins also perform an energy function, but due to the fact that these substances are of particular value to organisms (due to their complex composition), the energy function of proteins is realized by organisms only under critical conditions.

5. Proteins can also perform a storage function, since they are a kind of “canned food” of substances and energy for organisms (especially plants) that ensure their initial development (for animals - intrauterine, for plants - the development of embryos before the appearance of a young organism - a seedling).

A number of protein functions are characteristic of both cells and the organism as a whole, therefore, they are discussed below.

Ecological and biological role of proteins in organisms (in general)

1. Proteins form special structures in cells and organisms (together with other substances) that are able to perceive signals from the environment in the form of irritations, due to which a state of “excitation” arises, to which the body responds with a certain reaction, i.e. for proteins both in the cell and in the body as a whole, a perceiving function is characteristic.

2. Proteins are also characterized by a conductive function (both in cells and in the body as a whole), consisting in the fact that the excitation that has arisen in certain structures of the cell (organism) is transmitted to the corresponding center (cell or organism), in which a certain reaction is formed ( response) of an organism or cell to an incoming signal.

3. Many organisms are capable of moving in space, which is possible due to the ability of cell or organism structures to contract, and this is possible because the proteins of the fibrillar structure have a contractile function.

4. For heterotrophic organisms, proteins, both separately and in mixture with other substances, are food products, that is, they are characterized by a trophic function.

Brief description of protein transformations in heterotrophic organisms on the example of a human

Proteins in the composition of food enter the oral cavity, where they are moistened with saliva, crushed with teeth and turned into a homogeneous mass (with thorough chewing), and through the pharynx and esophagus enter the stomach (before entering the latter, nothing happens with proteins as compounds).

In the stomach, the food bolus is saturated with gastric juice, which is the secret of the gastric glands. Gastric juice is an aqueous system containing hydrogen chloride and enzymes, the most important of which (for proteins) is pepsin. Pepsin in an acidic environment causes the process of hydrolysis of proteins to peptones. The food gruel then enters the first section of the small intestine - the duodenum, into which the pancreatic duct opens, which secretes pancreatic juice, which has an alkaline environment and a complex of enzymes, of which trypsin accelerates the process of protein hydrolysis and leads it to the end, i.e. until the appearance mixtures of natural alpha-amino acids (they are soluble and can be absorbed into the blood by intestinal villi).

This mixture of amino acids enters the interstitial fluid, and from there - into the cells of the body, in which they (amino acids) enter into various transformations. One part of these compounds is directly used for the synthesis of proteins characteristic of a given organism, the second part undergoes transamination or deamination, giving new compounds necessary for the body, the third part is oxidized and is a source of energy necessary for the body to realize its vital functions.

It is necessary to note some features of intracellular transformations of proteins. If the organism is heterotrophic and unicellular, then the proteins in the food enter the cells into the cytoplasm or special digestive vacuoles, where they undergo hydrolysis under the action of enzymes, and then everything proceeds as described for amino acids in cells. Cellular structures are constantly updated, so the "old" protein is replaced by a "new" one, while the first one is hydrolyzed to obtain a mixture of amino acids.

Autotrophic organisms have their own characteristics in the transformation of proteins. Primary proteins (in meristem cells) are synthesized from amino acids, which are synthesized from the products of transformations of primary carbohydrates (they arose during photosynthesis) and inorganic nitrogen-containing substances (nitrates or ammonium salts). The replacement of protein structures in long-living cells of autotrophic organisms does not differ from that of heterotrophic organisms.

Nitrogen balance

Proteins, consisting of amino acids, are the basic compounds that are inherent in the processes of life. Therefore, it is extremely important to take into account the metabolism of proteins and their cleavage products.

There is very little nitrogen in the composition of sweat, so usually sweat analysis for nitrogen content is not done. The amount of nitrogen supplied with food and the amount of nitrogen contained in urine and feces are multiplied by 6.25 (16%) and the second is subtracted from the first value. As a result, the amount of nitrogen that enters the body and is absorbed by it is determined.

When the amount of nitrogen that enters the body with food is equal to the amount of nitrogen in the urine and feces, i.e., formed during deamination, then there is a nitrogen balance. Nitrogen balance is characteristic, as a rule, of an adult healthy organism.

When the amount of nitrogen entering the body is greater than the amount of nitrogen released, then there is a positive nitrogen balance, i.e., the amount of protein that has entered the body is greater than the amount of protein that has undergone decay. A positive nitrogen balance is characteristic of a growing healthy organism.

When the intake of protein from food increases, the amount of nitrogen excreted in the urine also increases.

And, finally, when the amount of nitrogen entering the body is less than the amount of nitrogen released, then there is a negative nitrogen balance, in which the breakdown of the protein exceeds its synthesis and the protein that is part of the body is destroyed. This happens with protein starvation and when the amino acids necessary for the body do not come. A negative nitrogen balance was also found after the action of high doses of ionizing radiation, which cause an increased breakdown of proteins in organs and tissues.

The problem of protein optimum

The minimum amount of food proteins needed to replenish the degraded proteins of the body, or the amount of breakdown of body proteins with exclusively carbohydrate nutrition, is referred to as the wear factor. In an adult, the smallest value of this coefficient is about 30 g of protein per day. However, this amount is not enough.

Fats and carbohydrates affect the consumption of proteins beyond the minimum required for plastic purposes, since they release the amount of energy that was required to break down proteins above the minimum. Carbohydrates with normal nutrition reduce the breakdown of proteins by 3-3.5 times more than with complete starvation.

For an adult with a mixed diet containing a sufficient amount of carbohydrates and fats, and a body weight of 70 kg, the protein rate per day is 105 g.

The amount of protein that fully ensures the growth and vital activity of the body is designated as the protein optimum and is equal to 100-125 g of protein per day for a person with light work, up to 165 g for hard work, and 220-230 g for very hard work.

The amount of protein per day should be at least 17% of the total amount of food by weight, and 14% by energy.

Complete and incomplete proteins

Proteins that enter the body with food are divided into biologically complete and biologically inferior.

Biologically complete proteins are those proteins that contain in sufficient quantities all the amino acids necessary for protein synthesis of the animal organism. The composition of complete proteins necessary for the growth of the body includes the following essential amino acids: lysine, tryptophan, threonine, leucine, isoleucine, histidine, arginine, valine, methionine, phenylalanine. Other amino acids, hormones, etc. can be formed from these amino acids. Tyrosine is formed from phenylalanine, the hormones thyroxine and adrenaline are transformed from tyrosine, and histamine is formed from histidine. Methionine is involved in the formation of thyroid hormones and is necessary for the formation of choline, cysteine ​​and glutathione. It is necessary for redox processes, nitrogen metabolism, absorption of fats, normal brain activity. Lysine is involved in hematopoiesis, promotes the growth of the body. Tryptophan is also necessary for growth; it is involved in the formation of serotonin, vitamin PP, and in tissue synthesis. Lysine, cystine and valine excite cardiac activity. The low content of cystine in food retards hair growth, increases blood sugar.

Biologically inferior proteins are those proteins that lack even one amino acid that cannot be synthesized by animal organisms.

The biological value of protein is measured by the amount of protein in the body, which is formed from 100 g of food protein.

Proteins of animal origin, contained in meat, eggs and milk, are the most complete (70-95%). Proteins of plant origin have a lower biological value, such as proteins from rye bread, corn (60%), potatoes, yeast (67%).

Protein of animal origin - gelatin, which does not contain tryptophan and tyrosine, is defective. Wheat and barley are low in lysine, corn is low in lysine and tryptophan.

Some amino acids replace each other, for example, phenylalanine replaces tyrosine.

Two incomplete proteins, which lack various amino acids, together can make up a complete protein diet.

The role of the liver in protein synthesis

The liver synthesizes proteins contained in blood plasma: albumins, globulins (with the exception of gamma globulins), fibrinogen, nucleic acids and numerous enzymes, some of which are synthesized only in the liver, such as enzymes involved in the formation of urea.

Proteins synthesized in the body are part of organs, tissues and cells, enzymes and hormones (the plastic value of proteins), but are not stored by the body in the form of various protein compounds. Therefore, that part of the proteins that has no plastic significance is deaminated with the participation of enzymes - it breaks down with the release of energy into various nitrogenous products. The half-life of liver proteins is 10 days.

Protein nutrition under various conditions

Unsplit protein cannot be absorbed by the body except through the alimentary canal. Protein introduced outside the alimentary canal (parenterally) causes a protective reaction on the part of the body.

Amino acids of the split protein and their compounds - polypeptides - are brought to the cells of the body, in which, under the influence of enzymes, protein synthesis occurs continuously throughout life. Food proteins are mainly plastic value.

During the growth period of the body - in childhood and adolescence - protein synthesis is especially high. As we age, protein synthesis decreases. Consequently, in the process of growth, retention occurs, or a delay in the body of the chemicals that make up proteins.

The study of metabolism using isotopes showed that in some organs within 2-3 days approximately half of all proteins undergo decay and the same amount of proteins is re-synthesized by the body (resynthesis). In each, in each organism, specific proteins are synthesized that differ from the proteins of other tissues and other organisms.

Like fats and carbohydrates, amino acids that are not used to build the body are broken down to release energy.

Amino acids, which are formed from the proteins of dying, decaying cells of the body, also undergo transformations with the release of energy.

Under normal conditions, the amount of protein required per day for an adult is 1.5-2.0 g per 1 kg of body weight, in conditions of prolonged cold 3.0-3.5 g, with very hard physical work 3.0-3.5 G.

An increase in the amount of proteins to more than 3.0-3.5 g per 1 kg of body weight disrupts the activity of the nervous system, liver and kidneys.

Lipids, their classification and physiological role

Lipids are substances that are insoluble in water and dissolve in organic compounds (alcohol, chloroform, etc.). Lipids include neutral fats, fat-like substances (lipoids) and some vitamins (A, D, E, K). Lipids have plastic significance and are part of all cells and sex hormones.

Especially a lot of lipids in the cells of the nervous system and adrenal glands. A significant part of them is used by the body as an energy material.