These are biopolymers whose monomers are amino acids.

Amino acids are low molecular weight organic compounds containing carboxyl (-COOH) and amine (-NH 2) groups that are bonded to the same carbon atom. A side chain is attached to the carbon atom - a radical that gives each amino acid certain properties.

Most amino acids have one carboxyl group and one amino group; these amino acids are called neutral. There are, however, also basic amino acids- with more than one amino group, as well as acidic amino acids- with more than one carboxyl group.

There are about 200 amino acids known to be found in living organisms, but only 20 of them are found in proteins. These are the so-called basic or proteinogenic amino acids.

Depending on the radical, basic amino acids are divided into 3 groups:

1. Non-polar (alanine, methionine, valine, proline, leucine, isoleucine, tryptophan, phenylalanine);
2. Polar uncharged (asparagine, glutamine, serine, glycine, tyrosine, threonine, cysteine);
3. Charged (arginine, histidine, lysine - positively; aspartic and glutamic acid - negative).

Amino acid side chains (radical) can be hydrophobic or hydrophilic and impart corresponding properties to proteins.

In plants, all essential amino acids are synthesized from the primary products of photosynthesis. Humans and animals are not able to synthesize a number of proteinogenic amino acids and must receive them in finished form along with food. Such amino acids are called essential. These include lysine, valine, leucine, isoleucine, threonine, phenylalanine, tryptophan, methionine; arginine and histidine are essential for children.

In solution, amino acids can act as both acids and bases, i.e. they are amphoteric compounds. The carboxyl group (-COOH) can donate a proton, functioning as an acid, and the amine group (-NH2) can accept a proton, thus exhibiting the properties of a base.

The amino group of one amino acid is capable of reacting with the carboxyl group of another amino acid. The resulting molecule is dipeptide, and the -CO-NH- bond is called a peptide bond.

At one end of the dipeptide molecule there is a free amino group, and at the other there is a free carboxyl group. Thanks to this, the dipeptide can attach other amino acids to itself, forming oligopeptides. If many amino acids (more than 10) are combined in this way, then polypeptide.

Peptides play an important role in the body. Many aligopeptides are hormones. These are oxytocin, vasopressin, thyrotropin-releasing hormone, thyrotropin, etc. Oligopeptides also include bradykidin (pain peptide) and some opiates (“natural drugs” of humans), which perform the function of pain relief. Taking drugs destroys the body's opiate system, so a drug addict without a dose of drugs experiences severe pain - “withdrawal”, which is normally relieved by opiates.

Oligopeptides include some antibiotics (for example, gramicidin S).

Many hormones (insulin, adrenocorticotropic hormone, etc.), antibiotics (for example, gramicidin A), toxins (for example, diphtheria toxin) are polypeptides.

Proteins are polypeptides, the molecule of which contains from 50 to several thousand amino acids with a molecular weight of over 10,000.

Each protein has its own special spatial structure in a certain environment. When characterizing the spatial (three-dimensional) structure, four levels of organization of protein molecules are distinguished.

Primary structure- sequence of amino acids in a polypeptide chain. The primary structure is specific for each protein and is determined by genetic information, i.e. depends on the sequence of nucleotides in the section of the DNA molecule encoding the protein. All properties and functions of proteins depend on the primary structure. Replacing one single amino acid in protein molecules or changing their arrangement usually entails a change in protein function. Since proteins contain 20 types of amino acids, the number of options for their combinations in the sex and peptide chain is truly limitless, which provides a huge number of types of proteins in living cells.

In living cells, protein molecules or individual sections of them are not an elongated chain, but are twisted into a spiral, reminiscent of an extended spring (the so-called α-helix) or folded into a folded layer (β-layer). Secondary structure arises as a result of the formation of hydrogen bonds between the -CO- and -NH 2 -groups of two peptide bonds within one polypeptide chain (helical configuration) or between two polypeptide chains (folded layers).

The keratin protein has a completely α-helical configuration. It is the structural protein of hair, fur, nails, claws, beaks, feathers and horns. The spiral secondary structure is characteristic, in addition to keratin, of such fibrillar (thread-like) proteins as myosin, fibrinogen, and collagen.

In most proteins, the helical and non-helical sections of the polypeptide chain fold into a three-dimensional spherical formation - a globule (characteristic of globular proteins). A globule of a certain configuration is tertiary structure squirrel. The tertiary structure is stabilized by ionic, hydrogen bonds, covalent disulfide bonds (which are formed between the sulfur atoms that make up cysteine), as well as hydrophobic interactions. The most important in the emergence of tertiary structure are hydrophobic interactions; In this case, the protein folds in such a way that its hydrophobic side chains are hidden inside the molecule, i.e., they are protected from contact with water, and the hydrophilic side chains, on the contrary, are exposed outside.

Many proteins with a particularly complex structure consist of several polypeptide chains held together in the molecule due to hydrophobic interactions, as well as with the help of hydrogen and ionic bonds - arises quaternary structure. This structure is found, for example, in the globular protein hemoglobin. Its molecule consists of four separate polypeptide subunits (protomers) located in the tertiary structure, and a non-protein part - heme. Only in such a structure is hemoglobin able to perform its transport function.

Under the influence of various chemical and physical factors (treatment with alcohol, acetone, acids, alkalis, high temperature, irradiation, high pressure, etc.), the tertiary and quaternary structure of the protein changes due to the rupture of hydrogen and ionic bonds. The process of disrupting the native (natural) structure of a protein is called denaturation. In this case, there is a decrease in protein solubility, a change in the shape and size of molecules, loss of enzymatic activity, etc. The denaturation process is sometimes reversible, that is, the return of normal environmental conditions can be accompanied by the spontaneous restoration of the natural structure of the protein. This process is called renaturation. It follows that all features of the structure and functioning of a protein macromolecule are determined by its primary structure.

Based on their chemical composition, proteins are divided into simple and complex. TO simple include proteins consisting only of amino acids, and complex- containing a protein part and a non-protein part (prostatic) - metal ions, carbohydrates, lipids, etc. Simple proteins are serum albumin, immunoglobulin (antibodies), fibrin, some enzymes (trypsin), etc. Complex proteins are all proteolipids and glycoproteins, hemoglobin , most enzymes, etc.

Functions of proteins

Structural.

Proteins are part of cell membranes and cell organelles. The walls of blood vessels, cartilage, tendons, hair, nails, and claws in higher animals consist mainly of proteins.

Catalytic (enzymatic).

Enzyme proteins catalyze all chemical reactions in the body. They ensure the breakdown of nutrients in the digestive tract, carbon fixation during photosynthesis, matrix synthesis reactions, etc.

Transport.

Proteins are capable of attaching and transporting various substances. Blood albumins transport fatty acids, globulins transport metal ions and hormones. Hemoglobin carries oxygen and carbon dioxide.

Protein molecules that make up the plasma membrane take part in the transport of substances into and out of the cell.

Protective.

It is performed by immunoglobulins (antibodies) in the blood, which provide the body’s immune defense. Fibrinogen and thrombin are involved in blood clotting and prevent bleeding.

Contractile.

It is ensured by the movement of filaments of actin and myosin proteins relative to each other in muscles and inside cells. The sliding of microtubules, built from the protein tubulin, explains the movement of cilia and flagella.

Regulatory.

Many hormones are oligopeptides or proteins, for example: insulin, glucagon, adenocorticotropic hormone, etc.

Receptor.

Some proteins embedded in the cell membrane are able to change their structure in response to the external environment. This is how signals are received from the external environment and information is transmitted into the cell. An example would be phytochrome- a light-sensitive protein that regulates the photoperiodic response of plants, and opsin- component rhodopsin, a pigment found in the cells of the retina.

Primary structure - sequence of amino acids in a polypeptide chain. In a protein molecule, when alternating rigid (peptide bond) and flexible (α-carbon atom) sections, a compact arrangement of the chain in space is formed.

Akobori method is to use phenylhydrazine. Phenylhydrazine breaks peptide bonds in the protein and attaches to all amino acids except the C-terminal one. Subsequent chromatographic analysis makes it possible to recognize the C-terminal amino acid in the protein.

The study of the primary structure is important general biological and medical significance:

1. the primary structure determines subsequent protein structures.
2. knowledge of the primary structure of a protein is necessary for the artificial synthesis of proteins.
3. the primary structure determines species specificity, for example, in the insulin protein, usually in the middle of the molecule in various animal species and humans, a replacement occurs, as a rule, of 3 equivalent amino acids.
4. changes in the primary structure can lead to many diseases, for example, sickle cell anemia, in which in hemoglobin in the β chain at position 6, glutamic acid is replaced by valine. This replacement with an unequal amino acid leads to disruption of hemoglobin function and the appearance of sickle-shaped red blood cells.

Secondary structure - a regularly repeating pattern of arrangement of a polypeptide chain in space. Most often, 2 types of secondary structure are found in proteins: α - helix and β - structure.

α - helix in 1951 it was studied by L. Pauling using the X-ray diffraction method. It is a right-handed helical structure, in one turn of which 3.6 amino acids fit. The spiral pitch (the distance between adjacent turns) is 0.54 n.m. The α-helix is ​​fixed by hydrogen bonds, which are closed between the peptide bonds formed by every 4th amino acid. The secondary α - structure folds spontaneously and is determined by the primary structure of the protein. The proportion of regions arranged in a helical structure varies in different proteins. For example, in hemoglobin and myoglobin, the α - structural fold predominates, which reduces the size of the protein molecule by 4 times.

β-structure has the shape of an “accordion” and is stabilized by hydrogen bonds between distant sections of one polypeptide chain or between several protein molecules. There are parallel β structures, in which the N and C ends correspond to each other, and antiparallel structures. An example of proteins that predominantly contain β structures are immunoglobulins.

The secondary structure is studied by X-ray diffraction analysis and by studying the absorption of ultraviolet rays by the protein (the greater the proportion of α - structures, the greater the absorption).

The secondary structure is destroyed during denaturation.

Tertiary structure - with a form of spatial arrangement of the polypeptide chain specific for each protein. This structure is formed spontaneously and is determined by the primary structure. The tertiary structure significantly increases the compactness of the protein by tens. Non-covalent bonds (hydrophobic, ionic) and covalent (disulfide) bonds are involved in the formation of the tertiary structure.

Tertiary structure determines the biological activity and physicochemical properties of proteins. If the tertiary structure is disrupted, the protein loses its biological activity.

Methods for studying the tertiary structure are X-ray diffraction analysis and determination of the chemical activity of individual amino acid radicals in a protein. The tertiary structure of the myoglobin protein was first studied by J. Kendrew (1957). M. Perutz (1959) studied the structure of hemoglobin.

The tertiary structure of proteins includes α - helical, β - folded structures, β - loops (in which the polypeptide chain is bent by 180 0) and the so-called disordered coil. For example, the insulin protein contains 57% α - helical regions, 6% β - folded structures, 10% of the molecule is arranged in the form of β - loops and 27% of the molecule is a disordered coil.

The totality of primary, secondary, tertiary is conformation protein molecule. The lifetime (native) conformation is formed spontaneously and its formation is called folding. The conformation of proteins is very unstable and is formed with the participation of special proteins - chaperones(companions). Chaperones are able to bind to partially denatured proteins that are in an unstable state and restore their native conformation. Chaperones are classified by molecular weight (60 - 100 cd.). The most studied are Sh-60, Sh-70 and Sh-90. For example, Sh-70 interacts with proteins rich in hydrophobic radicals and protects them from high-temperature denaturation. In general, chaperones shield the main proteins of the body, prevent denaturation and promote the formation of conformation, facilitate the transport of denatured proteins into lysosomes, and participate in the process of protein synthesis.

According to conformation, all proteins are divided into three groups:

• fibrillar proteins: collagen, elastin, fibroin.
• Globular proteins: hemoglobin, albumin, globulin.
• Mixed proteins: myosin.

Tertiary structure is inherent in all proteins. Only oligomeric proteins, which contain several subunits, protomers, have a quaternary structure. A protomer is considered a separate polypeptide chain, a subunit is the functionally active part of an oligomeric protein. A subunit may contain either one protomer or several.

Quaternary structure - the number and relative arrangement of subunits in oligomeric proteins. Only oligomeric proteins, which contain several subunits, protomers, have a quaternary structure. A protomer is considered a separate polypeptide chain, a subunit is the functionally active part of an oligomeric protein. A subunit may contain one protomer or multiple protomers.

The formation of the quaternary structure involves weak non-covalent bonds (hydrophobic, ionic, hydrogen). The quaternary structure of proteins forms spontaneously and is easily broken when denatured. Individual subunits in an oligomeric protein interact with each other, which leads to changes in the tertiary structure of individual protomers. This phenomenon is called cooperative changes in protomer conformation and is usually accompanied by an increase in protein activity.

Oligomeric proteins have a number of features compared to monomeric proteins.

• They have a very compact packing and a relatively small interface surface, therefore, being located intracellularly, they bind less water
• Their activity is regulated in the body. Protomers are usually inactive, but oligomeric proteins are much more active.
• If the same type of protomers are involved in the synthesis of an oligomeric protein, this saves genetic material (several identical protomers are “stamped” on a short section of DNA)
• They are functionally more adapted to the conditions of the body.

The functionality of oligomeric proteins is illustrated by comparing the proteins hemoglobin and myoglobin, which are involved in the transport of oxygen to tissues. Hemoglobin of erythrocytes is an oligomeric protein that includes 4 polypeptide chains. Muscle myoglobin is a monomeric protein that includes 1 polypeptide chain. The oxygen saturation curve of myoglobin indicates its direct dependence on oxygen concentration. For hemoglobin, the oxygen saturation curve is S-shaped. This is due to a gradual sequential change in the structure (conformation) of each of the 4 protomers in the composition of hemoglobin, as a result of which the affinity of hemoglobin for oxygen sharply increases. This nature of hemoglobin saturation with oxygen sharply increases its oxygen capacity compared to myoglobin.

A special position among proteins is occupied by domain proteins .

Domains are structurally and functionally separate sections of one polypeptide chain. Domains can be responsible for the interaction of a protein with various substances - ligands (low-molecular substances, DNA, RNA, polysaccharides, etc.). Examples of domain proteins are serum albumin, immunoglobulins, and some enzymes (pancreatic trypsin).

Due to the high selectivity of proteins, they can be combined into complexes, which are most often called multienzyme complexes - these are structural associations of several enzymes that catalyze individual stages of a complex chemical process. Example: pyruvate dehydrogenase complex (PDC), a complex of three types of enzymes that catalyzes the oxidation of pyruvic acid (PVA).

It is possible to specifically combine not only individual proteins, but also proteins with lipids (fats) during the formation of cell membranes, and proteins with nucleic acids during the formation of chromatin.

Physicochemical properties of proteins.

They are largely determined by the conformation of the protein molecule (primary - tertiary structure of the protein). The physicochemical properties of proteins appear in solutions.

Solubility proteins varies from protein to protein.

In general, the solubility of proteins is high, but varies among different types of proteins. It is influenced by the following factors:

• shape of the protein molecule (globular proteins are more soluble than fibrillar proteins)
• the nature of the protein amino acid radical, the ratio of polar non-polar radicals (the more polar hydrophilic radicals in the protein, the better its solubility)
• solvent properties, presence of salts. A low concentration of salts (KCL, NaCl) sometimes increases the solubility of proteins. For example, albumins are better soluble in pure distilled water, globulins are dissolved only in the presence of 10% salts (KCL, NaCl). The connective tissue proteins collagen and elastin are insoluble in either water or saline solutions.

Molecular weight proteins is quite large, ranging from 6,000 to 1,000,000. For example, the molecular weight of hemoglobin is 68,000, albumin is 100,000, ribonuclease is about 14,000, myosin is 500,000.

Methods for determining the molar mass of proteins must be gentle and not destroy protein molecules. For example, the ebullioscopic method, based on measuring the boiling point of solutions, is not applicable to proteins. The most accurate methods for determining the molecular weight of proteins are the ultracentrifugation method and the X-ray diffraction method.

Ultracentrifugation method(sedimentation) is based on a change in the rate of sedimentation of proteins of different molecular weights when rotating protein solutions at high speed. The molecular weight of proteins found by this method is designated by the Svedberg unit (S = 10 -13 c.)

X-ray diffraction method allows you to calculate molecular weight by analyzing multiple X-ray images of a protein molecule.

Electrophoretic method is based on the dependence of the speed of protein movement in a constant electric field on the molecular weight of the protein (electrophoretic mobility is higher for proteins with lower molecular weight)

Chromatographic method is based on the different rates of passage of different proteins through molecular gel “sieves”.

Large molecules larger than the pore size of the gel pass through the gel faster than smaller protein molecules that are retained within the gel grains.

Electron microscopic method carried out by comparing the size of a protein molecule with reference samples of known mass.

Chemical methods associated with the characteristics of the chemical composition of proteins

Shape of protein molecules different. Protein molecules can be fibrillar or globular in shape. Fibrillar proteins have a thread-like molecular shape. They are generally insoluble in water and dilute saline solutions. Fibrillar proteins include the main structural proteins of connective tissue: collagen, keratin, elastin. In globular proteins, the polypeptide chains are tightly coiled into compact spherical structures. Most globular proteins are highly soluble in water and weak saline solutions. Globular proteins include enzymes, antibodies, albumins, and hemoglobin. Some proteins have an intermediate type of molecule, containing both thread-like and spherical regions. An example of such proteins is the muscle protein myosin, which is soluble in saline solutions.

Sizes of protein molecules are in the range from 1 to 100 nm, close to the sizes of colloidal particles. Because of this, protein solutions have the properties of both true solutions and colloidal solutions.

Many molecular kinetic properties of protein solutions are similar to properties of colloidal solutions .

• The slow rate of diffusion of proteins necessary for their exchange.
• Inability of proteins to pass through semipermeable membranes. In compartments with a high protein concentration, excess hydrostatic pressure is created due to the one-way movement of water molecules through the semi-permeable membrane towards the high protein concentration. The excess pressure created by proteins is called oncotic pressure. It is an important factor determining the movement of water between tissues, blood, and intestines.
• The high viscosity of proteins is due to various intermolecular interactions of large protein molecules. Increased blood viscosity, in particular, increases the load on the heart muscle.
• Some proteins are able to form gels, which increases the strength of the proteins (eg collagen).

Optical properties of proteins determined by the size of protein molecules, the structure of amino acid radicals in proteins, the presence of peptide bonds and alpha-helical regions in proteins.

• Protein solutions have the effect of light refraction (refraction) and light scattering. These properties are due to the large size of protein molecules, commensurate with the wavelength of the visible part of the spectrum. In this case, short blue rays are scattered to a greater extent than longer wavelength red rays. The degree of refraction is proportional to the concentration of the protein solution.
• Protein solutions absorb ultraviolet rays in the range of 190-230 nm due to the presence of peptide bonds and in the range of 260-280 nm due to the presence of cyclic amino acids in proteins. The degree of UV absorption is proportional to the protein concentration in solution.
• Protein solutions can rotate the plane of polarized light, which is due to the optical activity of the amino acids contained in the protein and the presence of alpha-helical sections in it. There is a direct relationship between the polarization of light and the concentration of proteins in solution.

Proteins, being molecular solutions, have properties of true solutions . Being true solutions, protein solutions are highly stable.

Squirrels are high-molecular biopolymer organic compounds whose monomers are amino acids. Proteins were identified as a separate class of biological molecules in the 18th century. as a result of the work of the French chemist A. de Fourcroix. First described proteins and proposed a name proteins, which in the modern sense means protein, the Dutch chemist E. J. Berzelius. The first isolation of protein (in the form of gluten) from wheat flour was carried out by J. Beccari. A feature of protein research at the beginning of the 21st century. simultaneous acquisition of data on the protein composition of whole cells, tissues or organisms, which is a separate science - proteomics .

Molecular weight of proteins from 5000 to 150000 Yes and more.

One of the largest single proteins is titin(component of muscle sarcomeres), containing more than 29 thousand amino acids and has a molecular weight of 3,000,000 Da. But the largest proteins by mass (more than 40,000,000 Da) are characteristic of viruses.

Chemical composition . Proteins consist of C, Η, O, N; in some proteins is S, some proteins form complexes with other molecules that contain P, Fe, Zn, Cu. Proteins are biopolymers of 20 different monomers - natural basic amino acids. Proteins can form interpolymer complexes with carbohydrates, lipids, nucleic acids, phosphoric acid, etc.

Physico-chemical properties. Due to the presence of free amino groups and carboxyl groups, proteins are characterized by all the properties of acids and bases ( amphoteric properties). The dissociation of amino acids and carboxyl protein groups determines the electrophoretic mobility of proteins. At low pH values ​​of the protein solution, positively charged amino groups predominate in it, so the proteins are in cationic form. At high pH values, negatively charged COOH groups predominate and proteins will be in anionic form. At some intermediate pH value, amino groups and carboxyl groups can interact with each other, then the sum of the charges is zero, and the proteins remain motionless in the electric field ( electrical properties). High molecular weight provides protein solutions with properties characteristic of colloidal systems, namely: the ability to form gels, high viscosity, low diffusion rate, high degree of swelling, due to which they bind about 80-90% of all water in the body ( colloidal properties). Protein breakdown occurs under the action of acids, alkalis or specific hydrolase enzymes, which break them down into peptides and amino acids. Synthesis is carried out from amino acids with a template principle using messenger RNA. Under the influence of various chemicals, proteins can coagulate and precipitate, losing their natural properties. The absence of a charge and a hydration shell contributes to the convergence of protein molecules, their sticking together and precipitation. This phenomenon is called coagulation, it can be reverse and irreversible. Irreversible coagulation can be considered as protein denaturation. Denaturation is the process of disrupting the natural structure of proteins. At the same time, the solubility of the protein decreases, the shape and size of the molecules change, etc. The denaturation process is reversible, that is, the return to normal conditions is accompanied by restoration

tion of the natural structure of the protein. This process is called renaturation . It follows that the characteristics of a protein are determined by its primary structure. But the process of destruction of the primary structure of proteins is always irreversible, it is called destruction . The properties of proteins depend on the structure, composition and sequence of amino acids.

Structure of proteins. Protein molecules are linear polymers consisting of amino acids. In addition to the amino acid sequence of the polypeptide chain (primary structure), the three-dimensional structure (secondary tertiary and quaternary) is extremely important for the functioning of proteins, which is contained as a result of the interaction of structures below the levels and is formed during the process of protein folding. The three-dimensional structure of proteins under normal natural conditions, under which proteins perform their biological functions, is called sewn on condition protein, and the structure itself is native conformation There are four levels of protein structure.

Levels of organization of protein molecules

Primary structure encoded by the corresponding gene, is specific for each individual protein and to the greatest extent determines the properties of the formed protein. Secondary structure is a helix shape (α-structure) or a folded sheet structure (β-conformation) and is a thermodynamically stable state of a polypeptide chain and a simple conformation structure of biomolecules. An example of proteins with a secondary structure in the form of a spiral are keratin proteins (form hair, nails, feathers, etc.) and in the form of a folded sheet - fibroin (silk protein). In the secondary structure, α-helical regions often alternate with linear ones. Tertiary structure occurs automatically as a result of the interaction of amino acid residues with water molecules. In this case, hydrophobic radicals are “drawn” into the protein molecule, and hydrophilic groups are oriented towards the solvent. In this way, a compact protein molecule is formed, inside which there are practically no water molecules. Proteins with a tertiary structure include myoglobin. Quaternary structure arises as a result of the combination of several subunits ( protomers), which together fulfill a common

function. This combination is called a protein complex ( multimer, or epimer). Typical proteins of quaternary structure are hemoglobin, STM, and some enzymes.

The final structure can be very complex, and the process of its adoption into the newly synthesized polypeptide chain requires some time. The process of a protein adopting a structure is called coagulation, or Folding. Many proteins are unable to complete folding on their own and reach their native state, often through interaction with other cellular proteins. Such proteins require external assistance from proteins of a special class - molecular chaperones. Most proteins acquire the correct conformation only under certain environmental conditions. When these conditions change, the protein denatures, changing its conformation. Factors that cause changes in the conformation of proteins are heat, radiation, strong acids, strong bases, concentrated salts, heavy metals, organic solvents and the like.

Types of chemical bonds in proteins. Amino acids are capable of forming a number of chemical bonds (peptide, disulfide, hydrogen, ionic, hydrophobic) with various functional groups, and this property is very important for the structure and functions of proteins.

Peptide bond - it is a covalent nitrogen-carbon polar bond that is formed by the interaction of NH 2 one amino acid with COOH of another with the release of water. This acid amide bond (-CO-NH-) is the main chemical bond of protein molecules and determines their primary structure and conformation. The compound formed by the condensation of two amino acids is a dipeptide. At one end of this molecule there is an amino group, at the other - a free carboxyl group. Thanks to this, the dipeptide can attach other amino acids to itself.

A disulfide bond is a covalent polar bond that is formed by the interaction of sulfhydryl groups (-SH) radicals of sulfur-containing amino acids cysteine. This connection (-S-S-) can occur both between different sections of one polypeptide chain and between different chains, determining the characteristics of protein molecules. The stability of many proteins is largely determined by the number of these bonds, as if they “stitch” the molecules, giving them strength and insolubility (for example, in skin collagen, hair keratin, wool).

Hydrogen bond - This is a polar bond that occurs when electropositive hydrogen interacts with electronegative oxygen in the hydroxyl, carboxyl and amine groups of different amino acids. These connections (-HE) much weaker than peptide, disulfide and ionic ones, but due to their quantity (they occur between groups that are most abundant in protein molecules), they become very important in stabilizing the structure of protein molecules.

An ionic bond is an electrostatic polar bond that occurs between the ionized, positively charged amino group of one amino acid and the ionized, negatively charged carboxyl group of another amino acid. This salt connection (-COO - HN 3+ -) can combine both turns of one or more polypeptide chains in proteins of tertiary structure, and turns of different chains in proteins of quaternary structure. In an aqueous environment, ionic bonds are much weaker than peptide bonds and can be broken by changes in pH.

Hydrophobic interactions - This is a non-polar bond between amino acid radicals that do not carry an electrical charge and do not dissolve in water. The approach of these radicals is due to the nature of the interaction of hydrophobic groups (-CH3, -C2H5, etc.) with water. These connections (-R-R-) even weaker than hydrogen ones, they support the tertiary and quaternary structure of proteins.

BIOLOGY + Hemoglobin (from Greek Naita - blood and "lat. Globus - ball") - complex iron-containing protein of animal and human erythrocytes; is able to bind to oxygen, ensuring its transfer to tissues. In addition, hemoglobin is capable of binding a small amount of CO in tissues and releasing it in the lungs. Hemoglobin is a complex protein of the chromoprotein class and contains 1) protein part - globin, which consists of four protomers - two identical a-chains and two identical β-chains, 2) non-protein part - heme, which is represented by four prosthetic groups with a coordination center in the form of Fe 2+ . The subunits are united by hydrogen, ionic bonds, but the main contribution to this interaction is made by hydrophobic interactions. The normal hemoglobin content in human blood is: in men - 130-170 g/l, in women - 120-150 g/l, in children - 120-140 g/l. Hemoglobin is highly toxic when a significant amount of it enters the blood plasma from red blood cells (for example, when transfusion of incompatible blood) . Given the high toxicity of free hemoglobin, the body has special systems for binding and neutralizing it. In particular, one of the components of the hemoglobin neutralization system is a special plasma protein, haptoglobin, which specifically binds free globin and globin in the composition of hemoglobin.

It is customary to distinguish four levels of structural organization of a protein molecule: primary, secondary, tertiary and quaternary structure. Let's look at the features of each of these levels.

2.1.1. The primary structure of a protein is the sequence of alternating amino acids in a polypeptide chain. This structure is formed by peptide bonds between the α-amino and α-carboxyl groups of amino acids (see 1.4.2). Keep in mind that even small changes in the primary structure of a protein can significantly change its properties. An example of diseases that develop as a result of changes in the primary structure of a protein are hemoglobinopathies (hemoglobinoses).

Hemoglobin A (Hb A) is present in the red blood cells of healthy adults. Some people's blood contains abnormal (altered) hemoglobin - hemoglobin (Hb S). The only difference between the primary structure of Hb S and Hb A is the replacement of the hydrophilic glutamic acid residue with a hydrophobic valine residue in the terminal region of their β-chains:

As you know, the main function of hemoglobin is to transport oxygen to tissues. Under conditions of reduced partial pressure of O2, the solubility of hemoglobin S in water and its ability to bind and transport oxygen decreases. The red blood cells take on a sickle shape and are quickly destroyed, resulting in anemia (sickle cell anemia).

It has been established that the sequence of amino acid residues of the polypeptide chain of a protein carries the information necessary for the formation of the spatial structure of the protein. It has been established that each polypeptide sequence corresponds to only one stable variant of the spatial structure. The process of folding a polypeptide chain into a regular three-dimensional structure is called folding

Until recently, it was believed that the formation of the spatial structure of a protein occurs spontaneously, without the participation of any components. However, relatively recently it was discovered that this is true only for relatively small proteins (about 100 amino acid residues). In the process of folding larger proteins, special proteins take part - chaperones, which create the possibility of rapid formation of the correct spatial structure of the protein.

2.1.2. Protein secondary structure represents a method of folding a polypeptide chain into a helical or other conformation. In this case, hydrogen bonds are formed between the CO and NH groups of the peptide backbone of one chain or adjacent polypeptide chains. Several types of secondary structure of peptide chains are known, among which the main ones are the α-helix and β-sheet layer.

α-Helix- rigid structure, looks like a rod. The inner part of this rod is created by a tightly twisted peptide backbone, the amino acid radicals are directed outward. In this case, the CO group of each amino acid residue interacts with the NH group of the fourth residue from it. There are 3.6 amino acid residues per turn of the helix, and the helix pitch is 0.54 nm (Figure 2.1).

Figure 2.1.α-Helix.

Some amino acids prevent the chain from folding into an α-helix, and at their location the continuity of the helix is ​​disrupted. These amino acids include proline (in which the nitrogen atom is part of a rigid ring structure and rotation around the N - C α bond becomes impossible), as well as amino acids with charged radicals that electrostatically or mechanically prevent the formation of an α-helix. If there are two such radicals (or more) within one turn (about 4 amino acid residues), they interact and deform the helix.

β-fold layer differs from an α-helix in that it is flat rather than rod-shaped. Formed by hydrogen bonds within one or more polypeptide chains. The peptide chains can be arranged in the same direction (parallel) or in opposite directions (antiparallel), resembling the bellows of an accordion. Lateral radicals are located above and below the plane of the layer.

Figure 2.2.β-folded layer.

Note that the type of secondary structure of a protein is determined by its primary structure. For example, at the location of the proline residue (the atoms of the pyrrolidine ring in proline lie in the same plane), the peptide chain bends, and hydrogen bonds between amino acids are not formed. Therefore, proteins with a high proline content (for example, collagen) are not able to form an α-helix. Amino acid radicals, which carry an electrical charge, also prevent helicalization.

2.1.3. The tertiary structure of a protein is the distribution in space of all the atoms of the protein molecule, or in other words, spatial packing of a helical polypeptide chain. The main role in the formation of the tertiary structure of a protein is played by hydrogen, ionic, hydrophobic and disulfide bonds, which are formed as a result of the interaction between amino acid radicals.

Based on the shape of the molecule and the characteristics of the formation of the tertiary structure, proteins are divided into globular and fibrillar.

Globular proteins- have a spherical or ellipsoidal molecule shape (globule). During the formation of a globule, hydrophobic amino acid radicals are immersed in the internal regions, while hydrophilic radicals are located on the surface of the molecule. When interacting with the aqueous phase, polar radicals form numerous hydrogen bonds. Proteins are held in a dissolved state due to their charge and hydration shell. In the body, globular proteins perform dynamic functions (transport, enzymatic, regulatory, protective). Globular proteins include:

• Albumen - blood plasma protein; contains many glutamate and aspartate residues; precipitates at 100% saturation of the solution with ammonium sulfate.
• Globulins - blood plasma proteins; Compared to albumin, they have a higher molecular weight and contain fewer glutamate and aspartate residues; they precipitate at 50% saturation of the solution with ammonium sulfate.
• Histones - are part of cell nuclei, where they form a complex with DNA. Contains many arginine and lysine residues.

Fibrillar proteins- have a thread-like shape (fibrils), form fibers and bundles of fibers. There are many covalent cross-links between adjacent polypeptide chains. Insoluble in water. The transition into solution is prevented by non-polar amino acid radicals and cross-links between peptide chains. In the body they perform mainly a structural function, providing mechanical strength to tissues. Fibrillar proteins include:

• Collagen - connective tissue protein. Its composition is dominated by the amino acids glycine, proline, and hydroxyproline.
• Elastin - more elastic than collagen, it is part of the walls of arteries and lung tissue; its composition is dominated by the amino acids glycine, alanine, and valine.
• Keratin - protein of the epidermis and skin derivatives; the amino acid cysteine ​​predominates in its structure.

2.1.4. The quaternary structure of a protein is the arrangement in space of interacting subunits formed by individual polypeptide chains of the protein. Quaternary structure is the highest level of organization of a protein molecule, and it is also optional - more than half of the known proteins do not have it. Proteins with a quaternary structure are also called oligomeric proteins, and the polypeptide chains included in their composition are subunits or protomers. In some proteins, such subunits are the same or have a similar structure, while other proteins consist of subunits with chains of different types.

Each of the protomers is synthesized as a separate polypeptide chain, which folds into a globule and then combines with others through self-assembly. Each subunit contains regions that can interact with corresponding regions of other subunits. These interactions are carried out through hydrogen, ionic and hydrophobic bonds between amino acid radicals that are part of different chains.

Oligomeric proteins can exist in several stable conformations and have allosteric properties, that is, they are capable of transitioning from one conformation to another with a change in their functional activity. Examples of oligomeric proteins include erythrocyte protein hemoglobin, enzyme phosphofructokinase and many others.

The structural organization and functioning of oligomeric proteins will be discussed in more detail in the next topic using hemoglobin as an example (Figure 2.3).

Figure 2.3. Spatial structure of hemoglobin. Its molecule consists of four pairwise identical subunits, designated by the letters α and β. The non-protein part of hemoglobin, heme, is shown in blue.

Proteins are also known whose molecules consist of two or more polypeptide chains connected by disulfide bonds (insulin, thrombin). Such proteins cannot be oligomeric. Such proteins are formed from a single polypeptide chain as a result of partial proteolysis - local cleavage of peptide bonds. Such proteins do not possess allosteric properties characteristic of oligomeric proteins.

There are four levels of structural organization of proteins: primary, secondary, tertiary and quaternary. Each level has its own characteristics.

The primary structure of proteins is a linear polypeptide chain of amino acids connected by peptide bonds. Primary structure is the simplest level of structural organization of a protein molecule. High stability is given to it by covalent peptide bonds between the α-amino group of one amino acid and the α-carboxyl group of another amino acid. [show] .

If the imino group of proline or hydroxyproline is involved in the formation of a peptide bond, then it has a different form [show] .

When peptide bonds form in cells, the carboxyl group of one amino acid is first activated, and then it combines with the amino group of another. Laboratory synthesis of polypeptides is carried out in approximately the same way.

A peptide bond is a repeating fragment of a polypeptide chain. It has a number of features that affect not only the shape of the primary structure, but also the higher levels of organization of the polypeptide chain:

• coplanarity - all atoms included in the peptide group are in the same plane;
• the ability to exist in two resonance forms (keto or enol form);
• trans position of the substituents relative to the C-N bond;
• the ability to form hydrogen bonds, and each of the peptide groups can form two hydrogen bonds with other groups, including peptide ones.

The exception is peptide groups involving the amino group of proline or hydroxyproline. They are only able to form one hydrogen bond (see above). This affects the formation of the secondary structure of the protein. The polypeptide chain in the area where proline or hydroxyproline is located easily bends, since it is not held, as usual, by a second hydrogen bond.

Nomenclature of peptides and polypeptides . The name of peptides is made up of the names of their constituent amino acids. Two amino acids make a dipeptide, three make a tripeptide, four make a tetrapeptide, etc. Each peptide or polypeptide chain of any length has an N-terminal amino acid containing a free amino group and a C-terminal amino acid containing a free carboxyl group. When naming polypeptides, all amino acids are listed sequentially, starting with the N-terminal one, replacing in their names, except for the C-terminal one, the suffix -in with -yl (since the amino acids in peptides no longer have a carboxyl group, but a carbonyl one). For example, the name shown in Fig. 1 tripeptide - leuc silt phenylalane silt threon in.

Features of the primary structure of the protein . In the backbone of the polypeptide chain, rigid structures (flat peptide groups) alternate with relatively mobile regions (-CHR), which are capable of rotating around bonds. Such structural features of the polypeptide chain affect its spatial arrangement.

Secondary structure is a way of folding a polypeptide chain into an ordered structure due to the formation of hydrogen bonds between peptide groups of the same chain or adjacent polypeptide chains. According to their configuration, secondary structures are divided into helical (α-helix) and layered-folded (β-structure and cross-β-form).

α-Helix. This is a type of secondary protein structure that looks like a regular helix, formed due to interpeptide hydrogen bonds within one polypeptide chain. The model of the structure of the α-helix (Fig. 2), which takes into account all the properties of the peptide bond, was proposed by Pauling and Corey. Main features of the α-helix:

• helical configuration of the polypeptide chain having helical symmetry;
• the formation of hydrogen bonds between the peptide groups of each first and fourth amino acid residue;
• regularity of spiral turns;
• the equivalence of all amino acid residues in the α-helix, regardless of the structure of their side radicals;
• side radicals of amino acids do not participate in the formation of the α-helix.

Externally, the α-helix looks like a slightly stretched spiral of an electric stove. The regularity of hydrogen bonds between the first and fourth peptide groups determines the regularity of the turns of the polypeptide chain. The height of one turn, or the pitch of the α-helix, is 0.54 nm; it includes 3.6 amino acid residues, i.e., each amino acid residue moves along the axis (the height of one amino acid residue) by 0.15 nm (0.54:3.6 = 0.15 nm), which allows us to talk about equivalence of all amino acid residues in the α-helix. The regularity period of an α-helix is ​​5 turns or 18 amino acid residues; the length of one period is 2.7 nm. Rice. 3. Pauling-Corey a-helix model

β-Structure. This is a type of secondary structure that has a slightly curved configuration of the polypeptide chain and is formed by interpeptide hydrogen bonds within individual sections of one polypeptide chain or adjacent polypeptide chains. It is also called a layered-fold structure. There are varieties of β-structures. The limited layered regions formed by one polypeptide chain of a protein are called cross-β form (short β structure). Hydrogen bonds in the cross-β form are formed between the peptide groups of the loops of the polypeptide chain. Another type - the complete β-structure - is characteristic of the entire polypeptide chain, which has an elongated shape and is held by interpeptide hydrogen bonds between adjacent parallel polypeptide chains (Fig. 3). This structure resembles the bellows of an accordion. Moreover, variants of β-structures are possible: they can be formed by parallel chains (the N-terminal ends of the polypeptide chains are directed in the same direction) and antiparallel (the N-terminal ends are directed in different directions). The side radicals of one layer are placed between the side radicals of another layer.

In proteins, transitions from α-structures to β-structures and back are possible due to the rearrangement of hydrogen bonds. Instead of regular interpeptide hydrogen bonds along the chain (thanks to which the polypeptide chain is twisted into a spiral), the helical sections unwind and hydrogen bonds close between the elongated fragments of the polypeptide chains. This transition is found in keratin, the protein of hair. When washing hair with alkaline detergents, the helical structure of β-keratin is easily destroyed and it turns into α-keratin (curly hair straightens).

The destruction of the regular secondary structures of proteins (α-helices and β-structures), by analogy with the melting of a crystal, is called the “melting” of polypeptides. In this case, hydrogen bonds are broken, and the polypeptide chains take the form of a random tangle. Consequently, the stability of secondary structures is determined by interpeptide hydrogen bonds. Other types of bonds take almost no part in this, with the exception of disulfide bonds along the polypeptide chain at the locations of cysteine ​​residues. Short peptides are closed into cycles due to disulfide bonds. Many proteins contain both α-helical regions and β-structures. There are almost no natural proteins consisting of 100% α-helix (the exception is paramyosin, a muscle protein that is 96-100% α-helix), while synthetic polypeptides have 100% helix.

Other proteins have varying degrees of coiling. A high frequency of α-helical structures is observed in paramyosin, myoglobin, and hemoglobin. In contrast, in trypsin, a ribonuclease, a significant part of the polypeptide chain is folded into layered β-structures. Proteins of supporting tissues: keratin (protein of hair, wool), collagen (protein of tendons, skin), fibroin (protein of natural silk) have a β-configuration of polypeptide chains. The different degrees of helicity of the polypeptide chains of proteins indicate that, obviously, there are forces that partially disrupt the helicity or “break” the regular folding of the polypeptide chain. The reason for this is a more compact folding of the protein polypeptide chain in a certain volume, i.e., into a tertiary structure.

Protein tertiary structure

The tertiary structure of a protein is the way the polypeptide chain is arranged in space. Based on the shape of their tertiary structure, proteins are mainly divided into globular and fibrillar. Globular proteins most often have an ellipsoid shape, and fibrillar (thread-like) proteins have an elongated shape (rod or spindle shape).

However, the configuration of the tertiary structure of proteins does not yet give reason to think that fibrillar proteins have only a β-structure, and globular proteins have an α-helical structure. There are fibrillar proteins that have a helical, rather than layered, folded secondary structure. For example, α-keratin and paramyosin (protein of the obturator muscle of mollusks), tropomyosins (proteins of skeletal muscles) belong to fibrillar proteins (have a rod shape), and their secondary structure is α-helix; in contrast, globular proteins may contain a large number of β-structures.

Spiralization of a linear polypeptide chain reduces its size by approximately 4 times; and packing into the tertiary structure makes it tens of times more compact than the original chain.

Bonds that stabilize the tertiary structure of a protein . Bonds between side radicals of amino acids play a role in stabilizing the tertiary structure. These connections can be divided into:

• strong (covalent) [show] .

  Covalent bonds include disulfide bonds (-S-S-) between the side radicals of cysteines located in different parts of the polypeptide chain; isopeptide, or pseudopeptide, - between the amino groups of side radicals of lysine, arginine, and not α-amino groups, and COOH groups of side radicals of aspartic, glutamic and aminocitric acids, and not α-carboxyl groups of amino acids. Hence the name of this type of bond - peptide-like. A rare ester bond is formed by the COOH group of dicarboxylic amino acids (aspartic, glutamic) and the OH group of hydroxyamino acids (serine, threonine).

• weak (polar and van der Waals) [show] .

  TO polar bonds include hydrogen and ionic. Hydrogen bonds, as usual, occur between the -NH 2 , -OH or -SH group of the side radical of one amino acid and the carboxyl group of another. Ionic, or electrostatic, bonds are formed when the charged groups of side radicals -NH + 3 (lysine, arginine, histidine) and -COO - (aspartic and glutamic acids) come into contact.

  Non-polar, or van der Waals, bonds formed between hydrocarbon radicals of amino acids. Hydrophobic radicals of the amino acids alanine, valine, isoleucine, methionine, phenylalanine interact with each other in an aqueous environment. Weak van der Waals bonds promote the formation of a hydrophobic core of nonpolar radicals inside the protein globule. The more nonpolar amino acids there are, the greater the role van der Waals bonds play in the folding of the polypeptide chain.

Numerous bonds between the side radicals of amino acids determine the spatial configuration of the protein molecule.

Features of the organization of protein tertiary structure . The conformation of the tertiary structure of the polypeptide chain is determined by the properties of the side radicals of the amino acids included in it (which do not have a noticeable effect on the formation of primary and secondary structures) and the microenvironment, i.e., the environment. When folded, the polypeptide chain of a protein tends to take on an energetically favorable form, characterized by a minimum of free energy. Therefore, nonpolar R-groups, “avoiding” water, form, as it were, the internal part of the tertiary structure of the protein, where the main part of the hydrophobic residues of the polypeptide chain is located. There are almost no water molecules in the center of the protein globule. The polar (hydrophilic) R groups of the amino acid are located outside this hydrophobic core and are surrounded by water molecules. The polypeptide chain is intricately bent in three-dimensional space. When it bends, the secondary helical conformation is disrupted. The chain “breaks” at weak points where proline or hydroxyproline are located, since these amino acids are more mobile in the chain, forming only one hydrogen bond with other peptide groups. Another bend site is glycine, which has a small R group (hydrogen). Therefore, the R-groups of other amino acids, when stacked, tend to occupy the free space at the location of glycine. A number of amino acids - alanine, leucine, glutamate, histidine - contribute to the preservation of stable helical structures in protein, and such as methionine, valine, isoleucine, aspartic acid favor the formation of β-structures. In a protein molecule with a tertiary configuration, there are regions in the form of α-helices (helical), β-structures (layered) and a random coil. Only the correct spatial arrangement of the protein makes it active; its violation leads to changes in protein properties and loss of biological activity.

Quaternary protein structure

Proteins consisting of one polypeptide chain have only tertiary structure. These include myoglobin - a muscle tissue protein involved in the binding of oxygen, a number of enzymes (lysozyme, pepsin, trypsin, etc.). However, some proteins are built from several polypeptide chains, each of which has a tertiary structure. For such proteins, the concept of quaternary structure has been introduced, which is the organization of several polypeptide chains with a tertiary structure into a single functional protein molecule. Such a protein with a quaternary structure is called an oligomer, and its polypeptide chains with a tertiary structure are called protomers or subunits (Fig. 4).

At the quaternary level of organization, proteins retain the basic configuration of the tertiary structure (globular or fibrillar). For example, hemoglobin is a protein with a quaternary structure and consists of four subunits. Each of the subunits is a globular protein and, in general, hemoglobin also has a globular configuration. Hair and wool proteins - keratins, related in tertiary structure to fibrillar proteins, have a fibrillar conformation and a quaternary structure.

Stabilization of protein quaternary structure . All proteins that have a quaternary structure are isolated in the form of individual macromolecules that do not break down into subunits. Contacts between the surfaces of subunits are possible only due to the polar groups of amino acid residues, since during the formation of the tertiary structure of each of the polypeptide chains, the side radicals of non-polar amino acids (which make up the majority of all proteinogenic amino acids) are hidden inside the subunit. Numerous ionic (salt), hydrogen, and in some cases disulfide bonds are formed between their polar groups, which firmly hold the subunits in the form of an organized complex. The use of substances that break hydrogen bonds or substances that reduce disulfide bridges causes disaggregation of protomers and destruction of the quaternary structure of the protein. In table 1 summarizes the data on the bonds that stabilize different levels of organization of the protein molecule [show] .

Features of the structural organization of some fibrillar proteins

The structural organization of fibrillar proteins has a number of features compared to globular proteins. These features can be seen in the example of keratin, fibroin and collagen. Keratins exist in α- and β-conformations. α-Keratins and fibroin have a layered-folded secondary structure, however, in keratin the chains are parallel, and in fibroin they are antiparallel (see Fig. 3); In addition, keratin contains interchain disulfide bonds, while fibroin does not have them. Breakage of disulfide bonds leads to separation of polypeptide chains in keratins. On the contrary, the formation of the maximum number of disulfide bonds in keratins through exposure to oxidizing agents creates a strong spatial structure. In general, in fibrillar proteins, unlike globular proteins, it is sometimes difficult to strictly distinguish between different levels of organization. If we accept (as for a globular protein) that the tertiary structure should be formed by laying one polypeptide chain in space, and the quaternary structure by several chains, then in fibrillar proteins several polypeptide chains are involved already during the formation of the secondary structure. A typical example of a fibrillar protein is collagen, which is one of the most abundant proteins in the human body (about 1/3 of the mass of all proteins). It is found in tissues that have high strength and low extensibility (bones, tendons, skin, teeth, etc.). In collagen, a third of the amino acid residues are glycine, and about a quarter or slightly more are proline or hydroxyproline.

The isolated polypeptide chain of collagen (primary structure) looks like a broken line. It contains about 1000 amino acids and has a molecular weight of about 10 5 (Fig. 5, a, b). The polypeptide chain is built from a repeating trio of amino acids (triplet) of the following composition: gly-A-B, where A and B are any amino acids other than glycine (most often proline and hydroxyproline). Collagen polypeptide chains (or α-chains) during the formation of secondary and tertiary structures (Fig. 5, c and d) cannot produce typical α-helices with helical symmetry. Proline, hydroxyproline and glycine (antihelical amino acids) interfere with this. Therefore, three α-chains form, as it were, twisted spirals, like three threads wrapping around a cylinder. Three helical α chains form a repeating collagen structure called tropocollagen (Fig. 5d). Tropocollagen in its organization is the tertiary structure of collagen. The flat rings of proline and hydroxyproline regularly alternating along the chain give it rigidity, as do the interchain bonds between the α-chains of tropocollagen (which is why collagen is resistant to stretching). Tropocollagen is essentially a subunit of collagen fibrils. The laying of tropocollagen subunits into the quaternary structure of collagen occurs in a stepwise manner (Fig. 5e).

Stabilization of collagen structures occurs due to interchain hydrogen, ionic and van der Waals bonds and a small number of covalent bonds.

The α-chains of collagen have different chemical structures. There are different types of α 1 chains (I, II, III, IV) and α 2 chains. Depending on which α 1 - and α 2 -chains are involved in the formation of the three-stranded helix of tropocollagen, four types of collagen are distinguished:

• the first type - two α 1 (I) and one α 2 chain;
• the second type - three α 1 (II) chains;
• third type - three α 1 (III) chains;
• fourth type - three α 1 (IV) chains.

The most common collagen is the first type: it is found in bone tissue, skin, tendons; type 2 collagen is found in cartilage tissue, etc. One type of tissue can contain different types of collagen.

The ordered aggregation of collagen structures, their rigidity and inertness ensure the high strength of collagen fibers. Collagen proteins also contain carbohydrate components, i.e. they are protein-carbohydrate complexes.

Collagen is an extracellular protein that is formed by connective tissue cells found in all organs. Therefore, with damage to collagen (or disruption of its formation), multiple violations of the supporting functions of the connective tissue of organs occur.