An important property of proteins is their ability to denature. This concept refers to phenomena associated with an irreversible change in the secondary, tertiary and quaternary structures of a protein under the influence of heat, acids, alkalis, UV rays, ionizing radiation, ultrasound, etc. In other words, denaturation is an irreversible violation of the native spatial configuration of a protein molecule, accompanied by significant changes in the biological and physico-chemical properties of proteins.

Since relatively weak bonds are partially involved in the formation of secondary and tertiary structures, the physical state of the protein depends to a large extent on temperature, pH, the presence of salts, and other factors. Heating, for example, causes the polypeptide chain of a protein molecule to straighten; some chemicals break hydrogen bonds. A change in pH also causes the breaking of bonds, and electrostatic instability is manifested in this case.

Proteins under the influence of various physical and chemical factors lose their original (native) properties. Outwardly, this is expressed in their coagulation and precipitation. An example of such a phenomenon is the coagulation of milk albumin during boiling. Non-hydrolytic irreversible violation of the native structure of the protein is called denaturation. In this case, mainly hydrogen bonds are torn, the spatial structure of the protein changes, but the rupture of covalent bonds in the protein molecule does not occur.

Denaturation leads to the unfolding of the protein molecule, and it passes into a more or less disordered state (it no longer has any helices, no layers, or any other types of regular chain packing). In the denatured state, the amide groups of the peptide chain form hydrogen bonds with the surrounding water molecules; there are much more such hydrogen bonds than intramolecular ones.

Whipping egg white, cream turns them into a foam consisting of air bubbles surrounded by thin protein films, the formation of which is accompanied by the deployment of polypeptide chains as a result of breaking bonds during mechanical action. Thus, during the formation of films, partial or complete denaturation of the protein occurs. This type of denaturation is called surface protein denaturation.

For culinary processes, thermal denaturation of proteins is of particular importance. The mechanism of thermal denaturation of proteins can be considered using the example of globular proteins.

The main globular protein molecule consists of one or more polypeptide chains, folded and forming coils. Such a structure is stabilized by weak bonds, among which hydrogen bonds play an important role, forming transverse bridges between parallel peptide chains or their folds.

When proteins are heated, an increased movement of polypeptide chains or folds begins, which leads to the breaking of fragile bonds between them. The protein unfolds and acquires an unusual, unnatural shape, hydrogen and other bonds are established in places unusual for this molecule, and the configuration of the molecule changes. As a result, the folds unfold and rearrange, accompanied by a redistribution of polar and nonpolar groups, and nonpolar radicals are concentrated on the surface of the globules, reducing their hydrophilicity. During denaturation, proteins become insoluble and, to a greater or lesser extent, lose their ability to swell.

During thermal denaturation of proteins, an active role belongs to water, which is involved in the formation of a new conformational structure of the denatured protein. Completely dehydrated proteins do not denature even when heated for a long time. The denaturing effect of external influences is the stronger, the higher the hydration of proteins and the lower their concentration in solution.

At pH values ​​close to the IEP of the protein, the maximum dehydration of the protein occurs. The most complete denaturation is carried out in the IEP of the protein. The shift of pH in one direction or another from the IEP of the protein contributes to an increase in its thermal stability and weakening of denaturation processes.

The temperature of protein denaturation rises in the presence of other thermostable proteins and certain substances of a non-protein nature, such as sucrose. This property of proteins is used when, during heat treatment, it is necessary to increase the temperature of the mixture (for example, when pasteurizing ice cream, making egg-butter creams), preventing separation or structure formation in the protein colloidal system.

The appearance on the surface of a protein molecule after denaturation of previously hidden radicals or functional groups changes the physicochemical and biological properties of proteins. As a result of denaturation, the properties of proteins change irreversibly.

It is impossible to make dough from heated flour, and cutlets from boiled meat, since denatured proteins do not have the ability to hydrate and form viscous elastic-plastic masses suitable for molding semi-finished products.

The loss of the ability to hydrate is explained by the loss of native properties of proteins, the most important of which is pronounced hydrophilicity (high affinity for water), and is associated with a change in the conformation of polypeptide chains in the protein molecule as a result of denaturation.

The swelling and solubility of proteins in water are due to the presence on the surface of protein molecules of a large number of hydrophilic groups (COOH, OH, NH 2) capable of binding a significant amount of water.

As already noted, the ability of various native food proteins to dissolve in any solvent (water, neutral salt solutions, weak alkali solutions, alcohol, etc.) is used to separate or isolate a certain protein fraction (for research or food purposes). Denatured proteins do not have such differences, they are all equally insoluble and cannot swell in water. An exception to this general rule is the fibrillar collagen of meat and fish, which, after thermal denaturation and destruction to glutin, is able to dissolve in hot water.

As a result of denaturation, proteins lose their biological activity. In plant and animal raw materials used in public catering establishments, the activity of most protein substances is preserved. So, as a result of the activity of enzymes, fruits ripen during storage (and sometimes overripe), potatoes and root crops germinate. The activity of enzymes is especially evident in potato tubers when stored in the light: the surface of the tubers acquires a green color and a bitter taste, respectively, as a result of the synthesis of chlorophyll and the formation of the poisonous glycoside solanine.

In raw meat, tissue enzymes are also in an active state, participating in meat autolysis (post-slaughter maturation). This property is used for practical purposes. Complete inactivation of acid phosphatase occurs when the temperature in the geometric center of the meat product reaches 80 °C, which corresponds to the temperature of pasteurization (death of vegetative forms of bacteria).

In a native protein, peptide groups are shielded by an outer hydration shell or are located inside the protein globule and are thus protected from external influences. During denaturation, the protein loses its hydration shell, which facilitates the access of digestive enzymes of the gastrointestinal tract to functional groups. Protein is digested faster.

In addition, sometimes the inhibitory function of a protein disappears after denaturation. So, some egg proteins negatively affect the digestion process: avidin in the intestine binds biotin (vitamin H), which is involved in the regulation of the nervous system and neuro-reflex activity; Ovomucoid inhibits the action of trypsin (pancreatic enzyme). That is why raw egg proteins are not only poorly digested, but also partially absorbed in an undigested form, which can cause allergies, reduce the digestibility of other food components and impair the absorption of calcium compounds. Upon denaturation, these proteins lose their antienzymatic properties.

During denaturation, the protein loses its hydration shell, as a result of which many functional groups and peptide bonds of the protein molecule appear on the surface and the protein becomes more reactive.

As a result of thermal denaturation of the protein, the aggregation of protein molecules occurs. Since the hydration shell around the protein molecule is broken, individual protein molecules combine to form larger particles and can no longer stay in solution. The process of protein folding begins, as a result of which new molecular bonds are formed.

The interaction of denatured protein molecules in solutions and gels proceeds differently. In weakly concentrated protein solutions during thermal denaturation, the aggregation of protein molecules occurs through the formation of intermolecular bonds, both strong, for example, disulfide, and weak (but numerous) hydrogen bonds. As a result, large particles are formed. Further aggregation of particles leads to stratification of the colloidal system, the formation of protein flakes that precipitate or float to the surface of the liquid, often with the formation of foam (for example, the precipitation of denatured lactalbumin flakes during boiling milk; the formation of flakes and foam from denaturing proteins on the surface of meat and fish broths). The concentration of proteins in such solutions does not exceed 1%.

In more concentrated protein solutions, denaturation of proteins forms a continuous gel that retains all the water contained in the colloidal system. As a result of aggregation of denatured protein molecules, a structured protein system is formed. Denaturation of proteins in concentrated solutions with the formation of a continuous gel occurs during the heat treatment of meat, fish (sarcoplasm proteins), chicken eggs and various mixtures based on them. The exact concentrations of proteins at which their solutions form a continuous gel as a result of heating are unknown. Given that the ability of proteins to gel depends on the configuration (asymmetry) of the molecules and the nature of the intermolecular bonds formed in this case, it must be assumed that these concentrations are different for different proteins.

For example, to prepare omelettes, 38 ... 75% of milk is added to egg melange. The lower limits refer to fried omelettes, the upper limits to steamed omelettes. For the preparation of egg white omelettes used in dietary nutrition, milk is added in an amount of 40%, regardless of the method of heat treatment, since the concentration of proteins in the egg white is much lower than in the yolk.

Some proteins, which are more or less watered gels, denaturate during denaturation, as a result of which they are dehydrated with the separation of liquid into the environment. The protein gel subjected to heating, as a rule, is characterized by a smaller volume, mass, plasticity, increased mechanical strength and greater elasticity compared to the original gel of native proteins. Similar changes in proteins are observed during the heat treatment of meat, fish (myofibril proteins), cooking cereals, legumes, pasta, and baking dough products.

Gels and jellies are solid non-fluid structured systems formed as a result of the action of molecular cohesive forces between colloidal particles or macromolecules of polymers. The cells of spatial grids of gels and jellies are usually filled with a solvent.

Thus, gels are colloidal systems or solutions of macromolecular compounds (HMCs) that have lost their fluidity due to the appearance in them of certain internal structures in the form of a spatial mesh frame, the cells of which are filled with a dispersion medium. Since the dispersion medium contained in the cells loses its mobility, it is called immobilized.

Gels are very widespread in nature: they include many building materials (concrete, cement, clay suspensions), soils, some minerals (agate, opal), various food products (flour, dough, bread, jelly, marmalade, jelly), gelatin , rubber, tissues of living organisms and many other materials of animate and inanimate nature.

Depending on the concentration of the dispersion medium, gels are usually subdivided into lyogels, coagels, and xerogels (aerogels).

Liquid-rich gels containing little dry matter (up to 1 ... 2%) are called diogels. Typical diogels include jelly, jelly (jelly), curdled milk, soap solutions, etc.

Gelatinous precipitates obtained during the coagulation of some hydrophobic sols, as well as flocculent precipitates formed by salting out HMS solutions, are called coagels. The content of dry matter in coagels reaches 80%. However, very liquid-poor flakes and microcrystalline powders formed during the coagulation of typical hydrophobic colloids (hydrosols of gold, silver, platinum, sulfides) do not belong to coagels.

Liquid-poor or completely dry gels are called xero-gels. Examples of xerogels are dry gelatin sheets, wood glue in tiles, starch, rubber. Complex xerogels include many food products (flour, crackers, biscuits). Highly porous xerogels are also called aerogels, since air serves as a dispersion medium in them. Aerogels include many sorbents (silica gel), solid catalysts for chemical reactions.

Depending on the nature of the dispersed phase and the ability to swell, it is customary to distinguish between brittle and elastic gels. Elastic gels we will call jellies.

In the hereditary disease phenylketonuria, the body is deficient in phenylalanine hydroxylase (EC 1.14.3.1). As a result, the catabolism of phenylalanine does not go to the final products through tyrosine, but enters a side pathway of deamination with the formation of phenylpyruvic acid. The accumulation of the latter, together with phenylalanine, leads to a serious illness in children, accompanied by dementia. With albinism, there is a defect in diphenol oxidase (EC 1.10.3.1.), with alkaptonuria - homogentisinate oxidase (EC 17.1.5.), with xanthonuria - xanthine oxidase (EC

1.2.3.2.), etc.

1.5. Protein denaturation

The inherent properties of proteins associated with the features of the conformation of their molecules change significantly if this conformation is disturbed during protein denaturation.

By denaturation is meant the transformation of a biologically active, so-called native3 protein into a form in which its natural properties such as solubility, electrophoretic activity, enzymatic activity, etc. are preserved. are lost.

Denaturation is a characteristic feature of proteins and is not observed in amino acids and low molecular weight peptides. Denaturation, as a rule, is associated with a violation of the tertiary and partially secondary structure of the protein molecule and is not accompanied by any changes in the primary structure. Therefore, it is natural that during protein denaturation, mainly hydrogen bonds and disulfide bridges in the protein molecule are destroyed.

Denaturing agents are divided into physical and chemical. Physical factors include heating (over 50-60 ° C), high pressure, ultrasound, etc., chemical factors - H + and OH - ions (usually at pH below 4 and above 10 - denaturation), organic solvents (acetone , alcohol), urea, salts of heavy metals, etc. Proteins are also denatured under the influence of detergents (from Latin Detergeo - crush, break, clean), which have a soap-like effect, although in most cases the denatured protein remains in a soluble form. Dehydration, drying proteins at room temperature usually entails complete denaturation. All this indicates a wide variety of denaturing agents and their mechanism of action.

3 The native conformation of a protein is the characteristic three-dimensional structure of a protein, in which it is stable and exhibits biological activity under certain physical conditions (temperature, pH, etc.).

The tertiary structure of a protein is the way in which a polypeptide chain is folded in three dimensions. This conformation arises due to the formation of chemical bonds between amino acid radicals remote from each other. This process is carried out with the participation of the molecular mechanisms of the cell and plays a huge role in giving proteins functional activity.

Features of the tertiary structure

The following types of chemical interactions are characteristic of the tertiary structure of proteins:

• ionic;
• hydrogen;
• hydrophobic;
• van der Waals;
• disulfide.

All these bonds (except for the covalent disulfide bond) are very weak, but due to the quantity they stabilize the spatial shape of the molecule.

In fact, the third level of polypeptide chain folding is a combination of various elements of the secondary structure (α-helices; β-folded layers and loops), which are oriented in space due to chemical interactions between side amino acid radicals. For a schematic representation of the tertiary structure of a protein, α-helices are indicated by cylinders or helical lines, folded layers by arrows, and loops by simple lines.

The nature of the tertiary conformation is determined by the sequence of amino acids in the chain, therefore, under equal conditions, two molecules with the same primary structure will correspond to the same spatial arrangement. This conformation ensures the functional activity of the protein and is called native.

In the process of folding the protein molecule, the components of the active center approach each other, which in the primary structure can be significantly removed from each other.

For single-stranded proteins, the tertiary structure is the final functional form. Complex multi-subunit proteins form a quaternary structure that characterizes the arrangement of several chains in relation to each other.

Characterization of chemical bonds in the tertiary structure of a protein

To a large extent, the folding of the polypeptide chain is due to the ratio of hydrophilic and hydrophobic radicals. The former tend to interact with hydrogen (a constituent element of water) and therefore are on the surface, while hydrophobic regions, on the contrary, rush to the center of the molecule. This conformation is energetically the most favorable. As a result, a globule with a hydrophobic core is formed.

Hydrophilic radicals, which nevertheless fall into the center of the molecule, interact with each other to form ionic or hydrogen bonds. Ionic bonds can occur between oppositely charged amino acid radicals, which are:

• cationic groups of arginine, lysine or histidine (have a positive charge);
• carboxyl groups of glutamic and aspartic acid radicals (have a negative charge).

Hydrogen bonds are formed by the interaction of uncharged (OH, SH, CONH 2) and charged hydrophilic groups. Covalent bonds (the strongest in the tertiary conformation) arise between the SH groups of cysteine ​​residues, forming the so-called disulfide bridges. Typically, these groups are spaced apart in a linear chain and approach each other only during the stacking process. Disulfide bonds are not characteristic of most intracellular proteins.

conformational lability

Since the bonds that form the tertiary structure of a protein are very weak, the Brownian movement of atoms in an amino acid chain can cause them to break and form in new places. This leads to a slight change in the spatial shape of individual sections of the molecule, but does not violate the native conformation of the protein. This phenomenon is called conformational lability. The latter plays a huge role in the physiology of cellular processes.

The conformation of a protein is affected by its interactions with other molecules or changes in the physicochemical parameters of the environment.

How is the tertiary structure of a protein formed?

The process of folding a protein into its native form is called folding. This phenomenon is based on the desire of a molecule to adopt a conformation with a minimum value of free energy.

No protein needs intermediary instructors who will determine the tertiary structure. The stacking scheme is initially "recorded" in the sequence of amino acids.

However, under normal conditions, in order for a large protein molecule to adopt a native conformation corresponding to the primary structure, it would take more than a trillion years. Nevertheless, in a living cell, this process lasts only a few tens of minutes. Such a significant reduction in time is provided by the participation in folding of specialized auxiliary proteins - foldases and chaperones.

The folding of small protein molecules (up to 100 amino acids in a chain) occurs quite quickly and without the participation of intermediaries, which was shown by in vitro experiments.

Folding factors

The accessory proteins involved in folding are divided into two groups:

• foldases - have catalytic activity, are required in an amount significantly inferior to the concentration of the substrate (like other enzymes);
• chaperones are proteins with various mechanisms of action; they are needed at a concentration comparable to the amount of the folded substrate.

Both types of factors are involved in folding, but are not part of the final product.

The group of foldases is represented by 2 enzymes:

• Protein disulfide isomerase (PDI) - controls the correct formation of disulfide bonds in proteins with a large number of cysteine ​​residues. This function is very important, since covalent interactions are very strong, and in the event of erroneous connections, the protein would not be able to rearrange itself and adopt the native conformation.
• Peptidyl-prolyl-cis-trans-isomerase - provides a change in the configuration of radicals located on the sides of proline, which changes the nature of the bend of the polypeptide chain in this area.

Thus, foldases play a corrective role in the formation of the tertiary conformation of the protein molecule.

Chaperones

Chaperones are otherwise called or stress. This is due to a significant increase in their secretion with negative effects on the cell (temperature, radiation, heavy metals, etc.).

Chaperones belong to three protein families: hsp60, hsp70 and hsp90. These proteins perform many functions, including:

• protection of proteins from denaturation;
• exclusion of the interaction of newly synthesized proteins with each other;
• prevention of the formation of incorrect weak bonds between radicals and their labialization (correction).

Thus, chaperones contribute to the rapid acquisition of an energetically correct conformation, eliminating the random enumeration of many variants and protecting still immature protein molecules from unnecessary interaction with each other. In addition, chaperones provide:

• some types of protein transport;
• refolding control (restoration of the tertiary structure after its loss);
• maintaining the state of unfinished folding (for some proteins).

In the latter case, the chaperone molecule remains bound to the protein after the folding process is complete.

Denaturation

Violation of the tertiary structure of the protein under the influence of any factors is called denaturation. The loss of the native conformation occurs when a large number of weak bonds that stabilize the molecule are broken. In this case, the protein loses its specific function, but retains its primary structure (peptide bonds are not destroyed during denaturation).

During denaturation, a spatial increase in the protein molecule occurs, and hydrophobic regions again come to the surface. The polypeptide chain acquires the conformation of a random coil, the shape of which depends on which bonds of the protein's tertiary structure have been broken. In this form, the molecule is more susceptible to the effects of proteolytic enzymes.

Factors that violate the tertiary structure

There are a number of physical and chemical influences that can cause denaturation. These include:

• temperature above 50 degrees;
• radiation;
• change in the pH of the medium;
• heavy metal salts;
• some organic compounds;
• detergents.

After the termination of the denaturing effect, the protein can restore the tertiary structure. This process is called renaturation or refolding. Under in vitro conditions, this is possible only for small proteins. In a living cell, refolding is provided by chaperones.

Squirrels- high-molecular organic compounds, consisting of residues of α-amino acids.

AT protein composition includes carbon, hydrogen, nitrogen, oxygen, sulfur. Some proteins form complexes with other molecules containing phosphorus, iron, zinc and copper.

Proteins have a large molecular weight: egg albumin - 36,000, hemoglobin - 152,000, myosin - 500,000. For comparison: the molecular weight of alcohol is 46, acetic acid - 60, benzene - 78.

Amino acid composition of proteins

Squirrels- non-periodic polymers, the monomers of which are α-amino acids. Usually, 20 types of α-amino acids are called protein monomers, although more than 170 of them have been found in cells and tissues.

Depending on whether amino acids can be synthesized in the body of humans and other animals, there are: non-essential amino acids- can be synthesized essential amino acids- cannot be synthesized. Essential amino acids must be ingested with food. Plants synthesize all kinds of amino acids.

Depending on the amino acid composition, proteins are: complete- contain the entire set of amino acids; defective- some amino acids are absent in their composition. If proteins are made up of only amino acids, they are called simple. If proteins contain, in addition to amino acids, also a non-amino acid component (a prosthetic group), they are called complex. The prosthetic group can be represented by metals (metalloproteins), carbohydrates (glycoproteins), lipids (lipoproteins), nucleic acids (nucleoproteins).

All amino acids contain: 1) a carboxyl group (-COOH), 2) an amino group (-NH 2), 3) a radical or R-group (the rest of the molecule). The structure of the radical in different types of amino acids is different. Depending on the number of amino groups and carboxyl groups that make up amino acids, there are: neutral amino acids having one carboxyl group and one amino group; basic amino acids having more than one amino group; acidic amino acids having more than one carboxyl group.

Amino acids are amphoteric compounds, since in solution they can act as both acids and bases. In aqueous solutions, amino acids exist in different ionic forms.

Peptide bond

Peptides- organic substances consisting of amino acid residues connected by a peptide bond.

The formation of peptides occurs as a result of the condensation reaction of amino acids. When the amino group of one amino acid interacts with the carboxyl group of another, a covalent nitrogen-carbon bond arises between them, which is called peptide. Depending on the number of amino acid residues that make up the peptide, there are dipeptides, tripeptides, tetrapeptides etc. The formation of a peptide bond can be repeated many times. This leads to the formation polypeptides. At one end of the peptide there is a free amino group (it is called the N-terminus), and at the other end there is a free carboxyl group (it is called the C-terminus).

Spatial organization of protein molecules

The performance of certain specific functions by proteins depends on the spatial configuration of their molecules, in addition, it is energetically unfavorable for the cell to keep proteins in an expanded form, in the form of a chain, therefore, polypeptide chains undergo folding, acquiring a certain three-dimensional structure, or conformation. Allocate 4 levels spatial organization of proteins.

Primary structure of a protein- the sequence of amino acid residues in the polypeptide chain that makes up the protein molecule. The bond between amino acids is peptide.

If a protein molecule consists of only 10 amino acid residues, then the number of theoretically possible variants of protein molecules that differ in the order of alternation of amino acids is 10 20 . With 20 amino acids, you can make even more diverse combinations of them. About ten thousand different proteins have been found in the human body, which differ both from each other and from the proteins of other organisms.

It is the primary structure of the protein molecule that determines the properties of the protein molecules and its spatial configuration. The replacement of just one amino acid for another in the polypeptide chain leads to a change in the properties and functions of the protein. For example, the replacement of the sixth glutamine amino acid in the β-subunit of hemoglobin with valine leads to the fact that the hemoglobin molecule as a whole cannot perform its main function - oxygen transport; in such cases, a person develops a disease - sickle cell anemia.

secondary structure- ordered folding of the polypeptide chain into a spiral (looks like a stretched spring). The coils of the helix are strengthened by hydrogen bonds between carboxyl groups and amino groups. Almost all CO and NH groups take part in the formation of hydrogen bonds. They are weaker than peptide ones, but, repeating many times, they impart stability and rigidity to this configuration. At the level of the secondary structure, there are proteins: fibroin (silk, web), keratin (hair, nails), collagen (tendons).

Tertiary structure- packing of polypeptide chains into globules, resulting from the occurrence of chemical bonds (hydrogen, ionic, disulfide) and the establishment of hydrophobic interactions between radicals of amino acid residues. The main role in the formation of the tertiary structure is played by hydrophilic-hydrophobic interactions. In aqueous solutions, hydrophobic radicals tend to hide from water, grouping inside the globule, while hydrophilic radicals tend to appear on the surface of the molecule as a result of hydration (interaction with water dipoles). In some proteins, the tertiary structure is stabilized by disulfide covalent bonds that form between the sulfur atoms of the two cysteine ​​residues. At the level of the tertiary structure, there are enzymes, antibodies, some hormones.

Quaternary structure characteristic of complex proteins, the molecules of which are formed by two or more globules. Subunits are held in the molecule by ionic, hydrophobic, and electrostatic interactions. Sometimes, during the formation of a quaternary structure, disulfide bonds occur between subunits. The most studied protein with a quaternary structure is hemoglobin. It is formed by two α-subunits (141 amino acid residues) and two β-subunits (146 amino acid residues). Each subunit is associated with a heme molecule containing iron.

If for some reason the spatial conformation of proteins deviates from normal, the protein cannot perform its functions. For example, the cause of "mad cow disease" (spongiform encephalopathy) is an abnormal conformation of prions, the surface proteins of nerve cells.

Protein Properties

The amino acid composition, the structure of the protein molecule determine its properties. Proteins combine basic and acidic properties determined by amino acid radicals: the more acidic amino acids in a protein, the more pronounced its acidic properties. The ability to give and attach H + determine buffer properties of proteins; one of the most powerful buffers is hemoglobin in erythrocytes, which maintains the pH of the blood at a constant level. There are soluble proteins (fibrinogen), there are insoluble proteins that perform mechanical functions (fibroin, keratin, collagen). There are chemically active proteins (enzymes), there are chemically inactive, resistant to various environmental conditions and extremely unstable.

External factors (heat, ultraviolet radiation, heavy metals and their salts, pH changes, radiation, dehydration)

can cause a violation of the structural organization of the protein molecule. The process of losing the three-dimensional conformation inherent in a given protein molecule is called denaturation. The cause of denaturation is the breaking of bonds that stabilize a particular protein structure. Initially, the weakest ties are torn, and when conditions become tougher, even stronger ones. Therefore, first the quaternary, then the tertiary and secondary structures are lost. A change in the spatial configuration leads to a change in the properties of the protein and, as a result, makes it impossible for the protein to perform its biological functions. If denaturation is not accompanied by the destruction of the primary structure, then it can be reversible, in this case, self-healing of the conformation characteristic of the protein occurs. Such denaturation is subjected, for example, to membrane receptor proteins. The process of restoring the structure of a protein after denaturation is called renaturation. If the restoration of the spatial configuration of the protein is impossible, then denaturation is called irreversible.

Functions of proteins

Function	Examples and explanations
Construction	Proteins are involved in the formation of cellular and extracellular structures: they are part of cell membranes (lipoproteins, glycoproteins), hair (keratin), tendons (collagen), etc.
Transport	The blood protein hemoglobin attaches oxygen and transports it from the lungs to all tissues and organs, and from them carbon dioxide transfers to the lungs; The composition of cell membranes includes special proteins that provide an active and strictly selective transfer of certain substances and ions from the cell to the external environment and back.
Regulatory	Protein hormones are involved in the regulation of metabolic processes. For example, the hormone insulin regulates blood glucose levels, promotes glycogen synthesis, and increases the formation of fats from carbohydrates.
Protective	In response to the penetration of foreign proteins or microorganisms (antigens) into the body, special proteins are formed - antibodies that can bind and neutralize them. Fibrin, formed from fibrinogen, helps to stop bleeding.
Motor	The contractile proteins actin and myosin provide muscle contraction in multicellular animals.
Signal	Molecules of proteins are embedded in the surface membrane of the cell, capable of changing their tertiary structure in response to the action of environmental factors, thus receiving signals from the external environment and transmitting commands to the cell.
Reserve	In the body of animals, proteins, as a rule, are not stored, with the exception of egg albumin, milk casein. But thanks to proteins in the body, some substances can be stored in reserve, for example, during the breakdown of hemoglobin, iron is not excreted from the body, but is stored, forming a complex with the ferritin protein.
Energy	With the breakdown of 1 g of protein to the final products, 17.6 kJ is released. First, proteins break down into amino acids, and then to the end products - water, carbon dioxide and ammonia. However, proteins are used as an energy source only when other sources (carbohydrates and fats) are used up.
catalytic	One of the most important functions of proteins. Provided with proteins - enzymes that accelerate the biochemical reactions that occur in cells. For example, ribulose biphosphate carboxylase catalyzes CO2 fixation during photosynthesis.

Enzymes

Enzymes, or enzymes, is a special class of proteins that are biological catalysts. Thanks to enzymes, biochemical reactions proceed at a tremendous speed. The rate of enzymatic reactions is tens of thousands of times (and sometimes millions) higher than the rate of reactions involving inorganic catalysts. The substance on which an enzyme acts is called substrate.

Enzymes are globular proteins structural features Enzymes can be divided into two groups: simple and complex. simple enzymes are simple proteins, i.e. consist only of amino acids. Complex enzymes are complex proteins, i.e. in addition to the protein part, they include a group of non-protein nature - cofactor. For some enzymes, vitamins act as cofactors. In the enzyme molecule, a special part is isolated, called the active center. active center- a small section of the enzyme (from three to twelve amino acid residues), where the binding of the substrate or substrates occurs with the formation of an enzyme-substrate complex. Upon completion of the reaction, the enzyme-substrate complex decomposes into an enzyme and a reaction product(s). Some enzymes have (other than active) allosteric centers- sites to which regulators of the rate of enzyme work are attached ( allosteric enzymes).

Enzymatic catalysis reactions are characterized by: 1) high efficiency, 2) strict selectivity and direction of action, 3) substrate specificity, 4) fine and precise regulation. The substrate and reaction specificity of enzymatic catalysis reactions is explained by the hypotheses of E. Fischer (1890) and D. Koshland (1959).

E. Fisher (key-lock hypothesis) suggested that the spatial configurations of the active site of the enzyme and the substrate should correspond exactly to each other. The substrate is compared to the "key", the enzyme - to the "lock".

D. Koshland (hypothesis "hand-glove") suggested that the spatial correspondence between the structure of the substrate and the active center of the enzyme is created only at the moment of their interaction with each other. This hypothesis is also called induced fit hypothesis.

The rate of enzymatic reactions depends on: 1) temperature, 2) enzyme concentration, 3) substrate concentration, 4) pH. It should be emphasized that since enzymes are proteins, their activity is highest under physiologically normal conditions.

Most enzymes can only work at temperatures between 0 and 40°C. Within these limits, the reaction rate increases by about 2 times for every 10 °C rise in temperature. At temperatures above 40 °C, the protein undergoes denaturation and the activity of the enzyme decreases. At temperatures close to freezing, the enzymes are inactivated.

With an increase in the amount of substrate, the rate of the enzymatic reaction increases until the number of substrate molecules becomes equal to the number of enzyme molecules. With a further increase in the amount of substrate, the rate will not increase, since the active sites of the enzyme are saturated. An increase in the enzyme concentration leads to an increase in catalytic activity, since a larger number of substrate molecules undergo transformations per unit time.

For each enzyme, there is an optimal pH value at which it exhibits maximum activity (pepsin - 2.0, salivary amylase - 6.8, pancreatic lipase - 9.0). At higher or lower pH values, the activity of the enzyme decreases. With sharp shifts in pH, the enzyme denatures.

The speed of allosteric enzymes is regulated by substances that attach to allosteric centers. If these substances speed up the reaction, they are called activators if they slow down - inhibitors.

Enzyme classification

According to the type of catalyzed chemical transformations, enzymes are divided into 6 classes:

1. oxidoreductase(transfer of hydrogen, oxygen or electron atoms from one substance to another - dehydrogenase),
2. transferase(transfer of a methyl, acyl, phosphate or amino group from one substance to another - transaminase),
3. hydrolases(hydrolysis reactions in which two products are formed from the substrate - amylase, lipase),
4. lyases(non-hydrolytic addition to the substrate or the elimination of a group of atoms from it, while C-C, C-N, C-O, C-S bonds can be broken - decarboxylase),
5. isomerase(intramolecular rearrangement - isomerase),
6. ligases(the connection of two molecules as a result of the formation of C-C, C-N, C-O, C-S bonds - synthetase).

Classes are in turn subdivided into subclasses and subsubclasses. In the current international classification, each enzyme has a specific code, consisting of four numbers separated by dots. The first number is the class, the second is the subclass, the third is the subclass, the fourth is the serial number of the enzyme in this subclass, for example, the arginase code is 3.5.3.1.

Go to lectures number 2"The structure and functions of carbohydrates and lipids"

Go to lectures №4"The structure and functions of ATP nucleic acids"

1. What is the name of the process of violation of the natural structure of a protein, in which its primary structure is preserved? The action of what factors can lead to a violation of the structure of protein molecules?

The process of violation of the natural structure of proteins under the influence of any factors without destroying the primary structure is called denaturation. Denaturation occurs due to the breaking of hydrogen, ionic, disulfide and other bonds. In this case, their quaternary, tertiary, and even secondary structure may be lost.

2. How do fibrillar proteins differ from globular ones? Give examples of fibrillar and globular proteins

These proteins differ in the shape of the molecules. Molecules of globular proteins have a rounded shape, fibrillar proteins are characterized by an elongated, filamentous shape of the molecules. So, globular proteins are globulins and blood albumins, fibrinogen, hemoglobin. Fibrillar proteins - keratin, collagen, myosin, elastin, etc.

3. Name the main biological functions of proteins, give relevant examples.

structural function. Proteins are part of all cells and tissues of living organisms. The elements of the cytoskeleton, the contractile elements of muscle fibers are built from proteins. Cartilage and tendons are predominantly made up of proteins. They contain the protein collagen. The most important structural component of feathers, hair, nails, claws, horns, hooves in animals is the protein keratin.

Enzymatic (catalytic) function. Enzymes are biological catalysts, i.e., substances that speed up the flow of chemical reactions in living organisms. Enzymes are involved in the synthesis and breakdown of various substances. They provide carbon fixation during photosynthesis, breakdown of nutrients in the digestive tract, etc.

transport function. Many proteins are able to attach and carry various substances. Hemoglobin binds and transports oxygen and carbon dioxide.

Contractile (motor) function. Contractile proteins provide the ability of cells, tissues, organs and whole organisms to change shape and move. So, actin and myosin provide muscle work and non-muscle intracellular contractions.

regulatory function. Some peptides and proteins are hormones. They influence various physiological processes. For example, insulin and glucagon regulate blood glucose, and somatotropin (growth hormone) regulates growth and physical development.

Signal function. Some cell membrane proteins are able to change their structure in response to external factors. With the help of these proteins, signals from the external environment are received and information is transmitted to the cell.

protective function. Proteins protect the body from invasion by foreign organisms and from damage.

toxic function. Many living organisms secrete proteins-toxins, which are poisons for other organisms. Toxins are synthesized in the body of a number of animals, fungi, plants, microorganisms.

Energy function. After being broken down into amino acids, proteins can serve as a source of energy in the cell. With complete oxidation of 1 g of protein, 17.6 kJ of energy is released.

storage function. Reserve proteins are stored in plant seeds, which are used during germination by the embryo, and then by the seedling as a source of nitrogen.

4. What are enzymes? Why would most of the biochemical processes in the cell be impossible without their participation?

Enzymes are biological catalysts, i.e., substances that speed up the flow of chemical reactions in living organisms. Unlike conventional chemical catalysts, enzymes are specific, that is, each enzyme accelerates only one specific reaction or acts only on a specific type of bond.

5. What is the specificity of enzymes? What is its reason? Why do enzymes function actively only in a certain range of temperature, pH, and other factors?

Enzymes accelerate chemical reactions due to close interaction with substrate molecules - the initial reactants. Not the entire enzyme molecule interacts with the substrate, but only a small part of it - the active center. Most often it is formed by several amino acid residues. The shape and chemical structure of the active center are such that only certain substrates can bind to it due to the correspondence of their spatial structures. Enzymes are proteins, therefore, they actively work only in a certain range of pH, temperature, and other factors.

6. Why are proteins, as a rule, used as energy sources only in extreme cases, when carbohydrates and fats are exhausted in cells?

Proteins are used as a last resort, since they are part of all cells and tissues of living organisms. The elements of the cytoskeleton, the contractile elements of muscle fibers are built from proteins.

7. In many bacteria, para-aminobenzoic acid (PABA) is involved in the synthesis of substances necessary for normal growth and reproduction. At the same time, sulfonamides, substances similar in structure to PABA, are used in medicine to treat a number of bacterial infections. What do you think the therapeutic effect of sulfonamides is based on?

PABA is necessary for the formation of growth factors in the microbial cell - folic acid and purine bases involved in the construction of nucleic acids, without which the growth and reproduction of microbes is impossible. According to the structure of PABA, it resembles sulfonamides, therefore, with an excess of the latter, its activity is suppressed. Microbes deprived of PABA stop dividing and growing, and then they are achieved by the protective forces of the macroorganism.