ChapterIV.3.

Enzymes

Metabolism in the body can be defined as the totality of all chemical transformations to which compounds coming from outside undergo. These transformations include all known types of chemical reactions: intermolecular transfer of functional groups, hydrolytic and non-hydrolytic cleavage of chemical bonds, intramolecular rearrangement, new formation of chemical bonds and redox reactions. Such reactions occur in the body at extremely high speed only in the presence of catalysts. All biological catalysts are substances of protein nature and are called enzymes (hereinafter F) or enzymes (E).

Enzymes are not components of reactions, but only accelerate the achievement of equilibrium by increasing the rate of both direct and reverse conversion. Acceleration of the reaction occurs due to a decrease in the activation energy - the energy barrier that separates one state of the system (initial chemical compound) from another (reaction product).

Enzymes speed up a variety of reactions in the body. So, from the point of view of traditional chemistry, the reaction of eliminating water from carbonic acid with the formation of CO 2 requires the participation of an enzyme, because without it, it proceeds too slowly to regulate blood pH. Thanks to the catalytic action of enzymes in the body, it becomes possible for reactions to occur that without a catalyst would proceed hundreds and thousands of times slower.

Properties of enzymes

1. Influence on the rate of a chemical reaction: enzymes increase the rate of a chemical reaction, but are not consumed themselves.

The rate of a reaction is the change in the concentration of reaction components per unit time. If it goes in the forward direction, then it is proportional to the concentration of the reactants, if in the opposite direction, then it is proportional to the concentration of the reaction products. The ratio of the rates of forward and reverse reactions is called the equilibrium constant. Enzymes cannot change the values ​​of the equilibrium constant, but the state of equilibrium occurs faster in the presence of enzymes.

2. Specificity of enzyme action. 2-3 thousand reactions take place in the cells of the body, each of which is catalyzed by a specific enzyme. The specificity of an enzyme's action is the ability to accelerate the course of one specific reaction without affecting the speed of others, even very similar ones.

There are:

Absolute– when F catalyzes only one specific reaction ( arginase– breakdown of arginine)

Relative(group special) – F catalyzes a certain class of reactions (for example, hydrolytic cleavage) or reactions involving a certain class of substances.

The specificity of enzymes is due to their unique amino acid sequence, which determines the conformation of the active center that interacts with the reaction components.

Substance, chemical transformation which is catalyzed by an enzyme called substrate ( S ) .

3. Enzyme activity – the ability to accelerate the reaction rate to varying degrees. Activity is expressed in:

1) International units of activity - (IU) the amount of enzyme that catalyzes the conversion of 1 µM of substrate in 1 minute.

2) Catalach (kat) - the amount of catalyst (enzyme) capable of converting 1 mole of substrate in 1 s.

3) Specific activity - the number of activity units (any of the above) in the test sample to the total mass of protein in this sample.

4) Less commonly used is molar activity - the number of substrate molecules converted by one enzyme molecule per minute.

Activity depends primarily on temperature . This or that enzyme exhibits its greatest activity at the optimal temperature. For F of a living organism, this value is in the range +37.0 - +39.0° C, depending on the type of animal. As the temperature decreases, the Brownian motion slows down, the diffusion rate decreases and, consequently, the process of complex formation between the enzyme and the reaction components (substrates) slows down. If the temperature rises above +40 - +50° The enzyme molecule, which is a protein, undergoes a process of denaturation. In this case, the rate of the chemical reaction noticeably drops (Fig. 4.3.1.).

Enzyme activity also depends on pH of the environment . For most of them, there is a certain optimal pH value at which their activity is maximum. Since a cell contains hundreds of enzymes and each of them has its own pH limits, pH changes are one of the important factors in the regulation of enzymatic activity. So, as a result of one chemical reaction with the participation of a certain enzyme, the pH value of which lies in the range of 7.0 - 7.2, a product is formed that is an acid. In this case, the pH value shifts to the region of 5.5 – 6.0. The activity of the enzyme decreases sharply, the rate of product formation slows down, but at the same time another enzyme is activated, for which these pH values ​​are optimal and the product of the first reaction undergoes further chemical transformation. (Another example about pepsin and trypsin).

Chemical nature of enzymes. The structure of the enzyme. Active and allosteric centers

All enzymes are proteins with molecular weight from 15,000 to several million Yes. According to their chemical structure they are distinguished simple enzymes (consisting only of AA) and complex enzymes (have a non-protein part or a prosthetic group). The protein part is called - apoenzyme, and non-protein, if it is covalently linked to the apoenzyme, it is called coenzyme, and if the bond is non-covalent (ionic, hydrogen) – cofactor . The functions of the prosthetic group are as follows: participation in the act of catalysis, contact between the enzyme and the substrate, stabilization of the enzyme molecule in space.

The role of cofactor is usually played by inorganic substances - ions of zinc, copper, potassium, magnesium, calcium, iron, molybdenum.

Coenzymes can be considered as an integral part of the enzyme molecule. This organic matter, among which there are: nucleotides ( ATP, UMF, etc.), vitamins or their derivatives ( TDF– from thiamine ( IN 1), FMN– from riboflavin ( AT 2), coenzyme A– from pantothenic acid ( AT 3), NAD, etc.) and tetrapyrrole coenzymes - hemes.

In the process of catalyzing a reaction, not the entire enzyme molecule comes into contact with the substrate, but a certain part of it, which is called active center. This zone of the molecule does not consist of a sequence of amino acids, but is formed by twisting the protein molecule into a tertiary structure. Individual sections of amino acids come closer to each other, forming a specific configuration of the active center. An important feature of the structure of the active center is that its surface is complementary to the surface of the substrate, i.e. AK residues of this zone of the enzyme are capable of entering into chemical reaction with certain substrate groups. One can imagine that The active site of the enzyme coincides with the structure of the substrate like a key and a lock.

IN active center two zones are distinguished: binding center, responsible for substrate attachment, and catalytic center, responsible for the chemical transformation of the substrate. The catalytic center of most enzymes includes AAs such as Ser, Cys, His, Tyr, Lys. Complex enzymes have a cofactor or coenzyme at the catalytic center.

In addition to the active center, a number of enzymes are equipped with a regulatory (allosteric) center. Substances that affect its catalytic activity interact with this zone of the enzyme.

Mechanism of action of enzymes

The act of catalysis consists of three successive stages.

1. Formation of an enzyme-substrate complex upon interaction through the active center.

2. Binding of the substrate occurs at several points in the active center, which leads to a change in the structure of the substrate and its deformation due to changes in the bond energy in the molecule. This is the second stage and is called substrate activation. In this case, a certain chemical modification of the substrate occurs and it is converted into a new product or products.

3. As a result of this transformation, the new substance (product) loses its ability to be retained in the active center of the enzyme and the enzyme-substrate, or rather, enzyme-product complex dissociates (breaks up).

Types of catalytic reactions:

A+E = AE = BE = E + B

A+B +E = AE+B = ABE = AB + E

AB+E = ABE = A+B+E, where E is the enzyme, A and B are substrates or reaction products.

Enzymatic effectors - substances that change the rate of enzymatic catalysis and thereby regulate metabolism. Among them there are inhibitors - slow down the reaction rate and activators - accelerating the enzymatic reaction.

Depending on the mechanism of reaction inhibition, competitive and non-competitive inhibitors are distinguished. The structure of the competitive inhibitor molecule is similar to the structure of the substrate and coincides with the surface of the active center like a key and a lock (or almost coincides). The degree of this similarity may even be higher than with the substrate.

If A+E = AE = BE = E + B, then I+E = IE¹

The concentration of the enzyme capable of catalysis decreases and the rate of formation of reaction products drops sharply (Fig. 4.3.2.).

A large number of competitive inhibitors act as chemical substances endogenous and exogenous origin (i.e. formed in the body and coming from the outside - xenobiotics, respectively). Endogenous substances are regulators of metabolism and are called antimetabolites. Many of them are used in the treatment of oncological and microbial diseases, as. they inhibit key metabolic reactions of microorganisms (sulfonamides) and tumor cells. But with an excess of substrate and a low concentration of the competitive inhibitor, its effect is canceled.

The second type of inhibitors is non-competitive. They interact with the enzyme outside the active site and excess substrate does not affect their inhibitory ability, as is the case with competitive inhibitors. These inhibitors interact either with certain groups of the enzyme (heavy metals bind to the thiol groups of Cys) or most often with the regulatory center, which reduces the binding ability of the active center. The actual process of inhibition is the complete or partial suppression of enzyme activity while maintaining its primary and spatial structure.

A distinction is also made between reversible and irreversible inhibition. Irreversible inhibitors inactivate the enzyme by forming a chemical bond with its AK or other structural components. This is usually a covalent bond to one of the active site sites. Such a complex practically does not dissociate under physiological conditions. In another case, the inhibitor disrupts the conformational structure of the enzyme molecule and causes its denaturation.

The effect of reversible inhibitors can be removed when there is an excess of substrate or under the influence of substances that change the chemical structure of the inhibitor. Competitive and non-competitive inhibitors are in most cases reversible.

In addition to inhibitors, activators of enzymatic catalysis are also known. They:

1) protect the enzyme molecule from inactivating influences,

2) form a complex with the substrate that binds more actively to the active center of F,

3) interacting with an enzyme that has a quaternary structure, they separate its subunits and thereby open up access for the substrate to the active center.

Distribution of enzymes in the body

Enzymes involved in the synthesis of proteins, nucleic acids and energy metabolism enzymes are present in all cells of the body. But cells that perform special functions also contain special enzymes. Thus, the cells of the islets of Langerhans in the pancreas contain enzymes that catalyze the synthesis of the hormones insulin and glucagon. Enzymes that are characteristic only of the cells of certain organs are called organ-specific: arginase and urokinase- liver, acid phosphatase- prostate. By changing the concentration of such enzymes in the blood, the presence of pathologies in these organs is judged.

In a cell, individual enzymes are distributed throughout the cytoplasm, others are embedded in the membranes of mitochondria and the endoplasmic reticulum, such enzymes form compartments, in which certain, closely interconnected stages of metabolism occur.

Many enzymes are formed in cells and secreted into anatomical cavities in an inactive state - these are proenzymes. Proteolytic enzymes (that break down proteins) are often formed as proenzymes. Then, under the influence of pH or other enzymes and substrates, their chemical modification occurs and the active center becomes accessible to the substrates.

There are also isoenzymes - enzymes that differ in molecular structure, but perform the same function.

Nomenclature and classification of enzymes

The name of the enzyme is formed from the following parts:

1. name of the substrate with which it interacts

2. nature of the catalyzed reaction

3. name of the enzyme class (but this is optional)

4. suffix -aza-

pyruvate - decarboxyl - aza, succinate - dehydrogen - aza

Since about 3 thousand enzymes are already known, they need to be classified. Currently, an international classification of enzymes has been adopted, which is based on the type of reaction catalyzed. There are 6 classes, which in turn are divided into a number of subclasses (presented only selectively in this book):

1. Oxidoreductases. Catalyze redox reactions. They are divided into 17 subclasses. All enzymes contain a non-protein part in the form of heme or derivatives of vitamins B2, B5. The substrate undergoing oxidation acts as a hydrogen donor.

1.1. Dehydrogenases remove hydrogen from one substrate and transfer it to other substrates. Coenzymes NAD, NADP, FAD, FMN. They accept the hydrogen removed by the enzyme, transforming it into a reduced form (NADH, NADPH, FADH) and transfer it to another enzyme-substrate complex, where they release it.

1.2. Oxidases - catalyze the transfer of hydrogen to oxygen to form water or H 2 O 2. F. Cytochrome oxidase respiratory chain.

RH + NAD H + O 2 = ROH + NAD + H 2 O

1.3. Monoxidases - cytochrome P450. According to its structure, it is both a hemoprotein and a flavoprotein. It hydroxylates lipophilic xenobiotics (according to the mechanism described above).

1.4. PeroxidasesAnd catalase- catalyze the decomposition of hydrogen peroxide, which is formed during metabolic reactions.

1.5. Oxygenases - catalyze reactions of oxygen addition to the substrate.

2. Transferases - catalyze transfer various radicals from a donor molecule to an acceptor molecule.

A A+ E + B = E A+ A + B = E + B A+ A

2.1. Methyltransferase (CH 3 -).

2.2.Carboxyl- and carbamoyltransferases.

2.2. Acyltransferases – Coenzyme A (transfer of acyl group - R -C=O).

Example: synthesis of the neurotransmitter acetylcholine (see chapter “Protein Metabolism”).

2.3. Hexosyltransferases catalyze the transfer of glycosyl residues.

Example: the cleavage of a glucose molecule from glycogen under the influence of phosphorylases.

2.4. Aminotransferases - transfer of amino groups

R 1- CO - R 2 + R 1 - CH - N.H. 3 - R 2 = R 1 - CH - N.H. 3 - R 2 + R 1- CO - R 2

playing important role in the transformation of AK. The common coenzyme is pyridoxal phosphate.

Example: alanine aminotransferase(ALT): pyruvate + glutamate = alanine + alpha-ketoglutarate (see chapter “Protein Metabolism”).

2.5. Phosphotransferase (kinase) - catalyze the transfer of a phosphoric acid residue. In most cases, the phosphate donor is ATP. Enzymes of this class mainly take part in the breakdown of glucose.

Example: Hexo(gluco)kinase.

3. Hydrolases - catalyze hydrolysis reactions, i.e. splitting of substances with addition at the site where the water bond is broken. This class includes mainly digestive enzymes; they are single-component (do not contain a non-protein part)

R1-R2 +H 2 O = R1H + R2OH

3.1. Esterases - break down ester bonds. This is a large subclass of enzymes that catalyze the hydrolysis of thiol esters and phosphoesters.
Example: NH 2 ).

Example: arginase(urea cycle).

4.Lyases - catalyze reactions of molecular splitting without adding water. These enzymes have a non-protein part in the form of thiamine pyrophosphate (B 1) and pyridoxal phosphate (B 6).

4.1. C-C bond lyases. They are usually called decarboxylases.

Example: pyruvate decarboxylase.

5.Isomerases - catalyze isomerization reactions.

Example: phosphopentose isomerase, pentose phosphate isomerase(enzymes of the non-oxidative branch of the pentose phosphate pathway).

6.Ligases catalyze reactions for the synthesis of more complex substances from simpler ones. Such reactions require the energy of ATP. “Synthetase” is added to the name of such enzymes.

REFERENCES FOR THE CHAPTER IV.3.

1. Byshevsky A. Sh., Tersenov O. A. Biochemistry for the doctor // Ekaterinburg: Uralsky Rabochiy, 1994, 384 pp.;

2. Knorre D. G., Myzina S. D. Biological chemistry. – M.: Higher. school 1998, 479 pp.;

3. Filippovich Yu. B., Egorova T. A., Sevastyanova G. A. Workshop on general biochemistry // M.: Enlightenment, 1982, 311 pp.;

4. Leninger A. Biochemistry. Molecular basis of cell structure and functions // M.: Mir, 1974, 956 pp.;

5. Pustovalova L.M. Workshop on biochemistry // Rostov-on-Don: Phoenix, 1999, 540 p.

1. change the free energy of the reaction

2. inhibit the reverse reaction

3. change the equilibrium constant of the reaction

4. direct the reaction along a bypass path with lower values ​​of activation energy for intermediate reactions

102. A change in the conformation of an enzyme molecule can occur:

2. only when pH changes

103. A change in the degree of ionization of the functional groups of an enzyme occurs when:

1. only when temperature changes

2. only when pH changes

3. only when both conditions change

4. does not occur with any changes

104. Hydrolysis of peptide bonds occurs when:

1. only when temperature changes

2. only when pH changes

3. when both conditions change

4. does not occur under any changes in temperature and pH

105. Violation of weak bonds in an enzyme molecule occurs when:

2. only when pH changes

3. when both conditions change

4. does not occur with any changes

106 Pepsin exhibits optimal activity at pH values:

1. 1,5-2,5

107. The optimum pH for the operation of most enzymes is:

1. pH< 4,0

3. 6,0 < pH < 8,0

108. Choose the correct ones from the following statements:

1. all enzymes exhibit maximum activity at pH = 7

2. most enzymes exhibit maximum activity at a pH close to neutral

3. pepsin exhibits maximum activity at pH = 1.5-2.5

109. Using the Michaelis-Menten equation, you can calculate:

4. change in free energy during a chemical reaction

V = V max x [S] / K m + [S]

1. activation energy of a chemical reaction

2. rate of enzyme-catalyzed reaction

3. energy barrier of a chemical reaction

111. Choose the correct answers: The Michaelis constant (K m) is:

2. May have different meaning for isoenzymes

3. The value at which all enzyme molecules are in the ES form

4. The greater its value, the greater the affinity of the enzyme for the substrate

112. Choose the correct answers: The Michaelis constant (K m) is:

1. Kinetics parameter enzymatic reaction

2. The value at which all enzyme molecules are in the ES form

3. The greater its value, the greater the affinity of the enzyme for the substrate

4. Substrate concentration at which half of the maximum reaction rate (V max) is achieved

113. Name the features of the structure and functioning of allosteric enzymes:

3. upon interaction with ligands, a cooperative change in the conformation of subunits is observed

4. upon interaction with ligands, a cooperative change in the conformation of subunits is observed

114. Name the features of the structure and functioning of allosteric enzymes:

1. as a rule, they are oligomeric proteins

2. as a rule, they are not oligomeric proteins

3. exhibit regulatory properties during the dissociation of the molecule into protomers

4. upon interaction with ligands, a cooperative change in the conformation of subunits is observed

Enzymes - biological catalysts, without whose participation not a single life process is complete. Boni are characterized by the ability to: react with a certain substance - substrate; accelerate biochemical reactions that usually proceed very slowly; act at very low concentrations of the substrate, while not requiring energy input from the outside; functioning of cotton wool depending on the temperature and pH of the environment.

Biological catalysis celebrated extremely< высокой эффективностью и способностью ферментов четкие < выделять вещество, с которой они взаимодействуют.

The enzyme molecule contains a group of particularly active amino acids that form the active center of the enzyme (129), which can quickly interact only with the corresponding substance - the substrate (130). In this case, the substrate is specific for a particular enzyme and is suitable, both in its structure and physical and chemical properties, to the active center “like a key to a lock”, and therefore the reaction of the substrate with the active center is instantaneous. As a result of the reaction, an enzyme appears - a substrate complex, which then easily breaks down, forming new products. The substances formed are immediately separated from the enzyme, which restores its structure and becomes capable of carrying out the same reaction again. After a second, the enzyme reacts with millions of substrate molecules without being destroyed.

Thanks to the enzyme bio chemical reactions are possible with a very small concentration of the substance in the cell, which is extremely important, especially in cases where the body gets rid of harmful substances. The enzyme catalase, already known to you, destroys in one second the same number of hydrogen peroxide molecules as it would under normal conditions take 300 years.

Each enzyme catalyzes only a specific reaction. It should be noted that it does not determine the possibility of the reaction itself, but only accelerates it millions of times, making its speed “cosmic”. Further transformation of the substance formed as a result of one enzymatic reaction is carried out by a second enzyme, then a third, etc. The cells of animals and plants contain thousands of different enzymes, so they not only accelerate thousands of chemical reactions, but also control their progress.

The speed of the enzyme action depends on the temperature (effective - about +40 ° C) and certain pH values ​​​​of the solution specific to the particular enzyme. Most enzymes have a pH value between 6.6 and 8.0, although there are exceptions. (Remember at what pH values ​​certain enzymes work best.)

An increase in temperature to +50 ° C leads to the destruction of the active center of the enzyme and it forever loses the ability to perform its functions. This is due to the fact that an irreversible disruption of the tertiary structure of the protein occurs, and after cooling the enzyme molecule does not restore its structure. This explains why even short exposure to high temperatures kills living beings. However, there are organisms whose enzymes have adapted to high temperatures. For example, in Africa, in hot springs with a water temperature of about +60 ° C, a representative of the class of crustaceans Thermosbaena amazing lives and reproduces, and some bacteria even live in reservoirs where the water temperature is more than 70 ° C.

The destruction of the enzyme structure can be caused by poisons that enter the body even in very small quantities. These substances, called inhibitors (from the Latin Inhibio - I restrain), irreversibly combine with the active center of the enzyme and thus block its activity.

One of the most powerful poisons, as is known, is cyanide (salts hydrocyanic acid HCN), blocking the work of the respiratory enzyme cytochrome oxidase. Therefore, even a small amount of this substance, once in the body, causes death from suffocation. Inhibitors are heavy metal ions (Hg2 +, Pb2 +), as well as arsenic compounds, which form compounds with amino acids included in the active center of the enzyme.

Enzymes- proteins that accelerate chemical reactions. All enzymes are globular proteins. When reacting are not spent. They have all the properties of proteins.

In addition to enzymes, some RNAs (ribozymes) have catalytic activity.

Differ:

1. Specificity of action.

2.High efficiency.

3.Ability to regulate.

There are 6 classes of enzymes.

Enzyme classes:

1. Oxyreductases catalyze ORR with the participation of 2 substrates (transfer of electrons or hydrogen atoms from one substrate to another).

Dehydrogenases - catalyze reactions of hydrogen abstraction (dehydrogenation). NAD+, NADP+, FAD, FMN act as electron acceptors.

The electron acceptor oxidase is molecular oxygen.

Oxygenases (hydroxylases) - an oxygen atom from an oxygen molecule attaches to a substrate.

2. Transferases - catalyze the transfer of functional groups from one compound to another. They are divided depending on the groups being transferred.

3.Hydrolases - catalyze hydrolysis reactions (cleavage covalent bond with the addition of a water molecule at the break point).

4.Lyases—cleavage of a certain group (CO2, H2O, NH2, SH2) from the substrate in a non-hydrolytic way.

5.Isomerases - catalyze various intramolecular transformations. If a group is transferred within one molecule, the enzyme is called a mutase.

6. Ligases (synthetases) - reactions of two molecules joining each other to form a covalent bond. The process is associated with the rupture of the ATP bond or other high-energy compound. If ATP synthetase, if not ATP synthase.

Enzyme active site- a combination of the substrate binding site and the catalytic site. Consists of amino acid residues.

Substrate binding site- a site in which the substrate binds to the enzyme using non-covalent bonds, forming an enzyme-substrate complex.

Catalytic site- the area where the substrate undergoes a chemical transformation into a product.

Cofactor- a non-protein compound that converts the enzyme into active form(most often these are metal ions).

Coenzyme- a protein compound that converts the enzyme into its active form (vitamin derivative).

Cofactors and coenzymes either form the tertiary structure of the protein-enzyme, which ensures its specificity to the substrate. Or they are involved in the reaction as an additional substrate (mainly coenzymes).

Mechanism of enzyme reaction with substrate:

1. The enzyme binds to the substrate in the active center (in complex proteins, a cofactor is located in the active center).

2. In the area of ​​the active center, a chemical transformation of the substrate occurs and a reaction product is formed.

3. The resulting reaction product loses its complementarity and is disconnected from the enzyme.

The molecule of each enzyme has the conformation necessary for its action, only under certain conditions external conditions(PH, temperature, etc.).

Types of enzyme specificity:

Substrate specificity:

1. Absolute - catalyzes the transformation of only one substrate.

2. Group - catalyzes similar transformations in several structurally similar substrates.

3. Stereospecificity - if the substrate has several stereoisomers, the enzyme exhibits absolute specificity to only one of them (D-sugars, L-amino acids, cis-trans isomers).

Catalytic specificity:

Catalysis of the attached substrate along one of the possible conversion pathways. The same substance can be converted into different products by the action of different enzymes.

Catalytic efficiency (enzyme turnover number) is the number of substrate molecules converted into a product by one enzyme molecule in 1 second.

The phenomenon of specificity of transformation pathways - the same substrate can be converted into different substances under the action of different enzymes.

The rate of enzymatic reactions (V) is measured by the loss of substrate (S) or increase of product (P) per unit time. The change in the rate of an enzymatic reaction is directly proportional to the change in enzyme concentration at the saturating concentration of the substrate.

Enzymes are protein substances (see Proteins) that accelerate vital chemical reactions in the cells of organisms. Being catalysts, they form unstable intermediate compounds with the starting substances: these compounds, breaking down, give the final product of this reaction and release enzymes.

The action of some enzymes is easy to observe experimentally. Thus, the enzyme catalase significantly accelerates the decomposition of hydrogen peroxide into water and oxygen. This is a vital reaction, since hydrogen peroxide is formed as a result of metabolism in the cell and in itself has a harmful effect on the cell. Catalase is found in almost all cells of animal and plant organisms.

There are a lot of enzymes known, and each of them accelerates only one reaction or groups of similar reactions. This feature of enzymes is called specificity or selectivity (selectivity) of action. The direction of their action allows the body to quickly and accurately perform complex chemical work to rearrange food molecules into the compounds it needs.

Already in the mouth, during chewing food, under the influence of the amylase enzyme, complex sugars, in particular starch, begin to decompose into less complex ones. This work will be continued in the intestine by the enzymes carbohydrases. In the stomach and intestines, food proteins undergo decomposition with the participation of pepsin, trypsin, and chymotrypsin. Fats are broken down into glycerol and carboxylic acids (or their salts) under the influence of enzymes called lipases. All these decomposition reactions proceed according to the same principle: a certain chemical bond in a molecule of protein, carbohydrate or fat, and the released valences are used to attach groups and ions from water molecules. The process of hydrolysis occurs. For a protein molecule, this reaction can be represented as follows:

Enzymes are known that have a different effect on molecules. Some of them accelerate redox reactions: they promote the transfer of an electron from one molecule (oxidized) to another (reduced). There are enzymes that link molecules together; enzymes that transfer large and complex groups of atoms from one molecule to another, etc.

Having a rich set of enzyme catalysts, the cell decomposes the molecules of food proteins, fats and carbohydrates into small fragments and from them rebuilds protein and other molecules that will exactly meet the needs of a given organism. That is why the great Russian physiologist I.P. Pavlov called enzymes the carriers of life.

Activity more enzymes are determined by the structure of the protein molecule. A certain spatial arrangement of amino acid residues forming a chain-like protein molecule (polypeptide chain, see Peptides) creates the conditions for the enzyme-catalyzed reaction to occur. A long chain of amino acid residues is folded into a complex ball so that amino acids located far from each other in the chain may be neighbors. Some of the groups of amino acid residues that arise in this way exhibit catalytic properties and form the active center of the enzyme.

Pepsin and chymotrypsin, which take part in digestion, can serve as an example of enzymes in which the active group is part of a protein molecule.

Other enzymes require non-protein substances - so-called cofactors - to be active. A cofactor can be a metal ion (zinc, manganese, calcium, etc.) or a molecule of an organic compound; in the latter case it is often called a coenzyme. Sometimes the presence of both metal ions and coenzymes is necessary for the enzyme to act. In some cases, the coenzyme is very tightly connected to the protein; this is observed, for example, in the enzyme catalase, where the coenzyme is complex compound iron In some enzymes, coenzymes are substances similar in molecular structure to vitamins. Vitamins are thus precursors of coenzymes. Thus, from vitamin (thiamine) in cells, thiamine pyrophosphate is formed - a coenzyme of an important enzyme (it is called decarboxylase), which converts pyruvic acid into carbon monoxide (IV) and acetaldehyde; The vitamin produces coenzymes of flavin enzymes, which carry out electron transfer in cells - one of the stages of oxidation of nutrients; The vitamin produces coenzymes necessary in the process of hematopoiesis, etc.

IN last years so-called immobilized (stationary) enzymes are widely used. To speed up the desired reaction, they are fixed on the surface of an inert “carrier”. Silica gel is usually used as a porous white mass, the composition is silicon (IV) oxide or polymer materials. The starting substances are filtered through this mass. The enzyme quickly and accurately performs highly specific “chemical work”, resulting in products that contain almost no foreign compounds.