Lecture 15. Enzymes: structure, properties, functions.

Lecture outline:

1. General characteristics of enzymes.

2. The structure of enzymes.

3. Mechanism of enzymatic catalysis.

4. Properties of enzymes.

5. Nomenclature of enzymes.

6. Classification of enzymes.

7. isozymes

8. Kinetics of enzymatic reactions.

9. Units of measurement of enzymatic activity

1. General characteristics of enzymes.

Under normal physiological conditions, biochemical reactions in the body proceed at high speeds, which is ensured by biological catalysts of a protein nature - enzymes.

They are studied by the science of enzymology - the science of enzymes (enzymes), specific proteins - catalysts synthesized by any living cell and activating various biochemical reactions occurring in the body. Some cells can contain up to 1000 different enzymes.

2. The structure of enzymes.

Enzymes are proteins with high molecular weight. Like any proteins, enzymes have primary, secondary, tertiary and quaternary levels of molecular organization. Primary structure is a sequential combination of amino acids and is determined by the hereditary characteristics of the body; it is it that largely characterizes the individual properties of enzymes. Secondary structure enzymes are organized in the form of an alpha helix. Tertiary structure has the form of a globule and participates in the formation of active and other centers. Many enzymes have quaternary structure and represent a union of several subunits, each of which is characterized by three levels of organization of molecules that differ from each other, both in qualitative and quantitative terms.

If enzymes are represented by simple proteins, that is, they consist only of amino acids, they are called simple enzymes. Simple enzymes include pepsin, amylase, lipase (almost all gastrointestinal enzymes).

Complex enzymes consist of protein and non-protein parts. The protein part of the enzyme is called - apoenzyme, non-protein – coenzyme. The coenzyme and apoenzyme form holoenzyme. The coenzyme can connect with the protein part either only for the duration of the reaction, or bind to each other with a permanent strong bond (then the non-protein part is called - prosthetic group). In any case, non-protein components are directly involved in chemical reactions by interacting with the substrate. Coenzymes can be represented by:

Nucleoside triphosphates.

Minerals (zinc, copper, magnesium).

Active forms of vitamins (B 1 is part of the enzyme decarboxylase, B 2 is part of dehydrogenase, B 6 is part of transferase).

Main functions of coenzymes:

Participation in the act of catalysis.

Establishing contact between enzyme and substrate.

Stabilization of the apoenzyme.

The apoenzyme, in turn, enhances the catalytic activity of the non-protein part and determines the specificity of the action of enzymes.

Each enzyme contains several functional centers.

Active center- a zone of an enzyme molecule that specifically interacts with the substrate. The active center is represented by functional groups of several amino acid residues; it is here that the attachment and chemical transformation of the substrate occurs.

Allosteric center or regulatory - this is the zone of the enzyme responsible for the attachment of activators and inhibitors. This center is involved in the regulation of enzyme activity.

These centers are located in different parts of the enzyme molecule.

Enzymes, or enzymes(from lat. Fermentum- starter) - usually protein molecules or RNA molecules (ribozymes) or their complexes that accelerate (catalyze) chemical reactions in living systems. The reactants in a reaction catalyzed by enzymes are called substrates, and the resulting substances are called products. Enzymes are substrate specific (ATPase catalyzes the breakdown of only ATP, and phosphorylase kinase phosphorylates only phosphorylase).

Enzyme activity can be regulated by activators and inhibitors (activators increase, inhibitors decrease).

Protein enzymes are synthesized in ribosomes, and RNA is synthesized in the nucleus.

The terms “enzyme” and “enzyme” have long been used as synonyms (the first mainly in Russian and German scientific literature, the second in English and French).

The science of enzymes is called enzymology, and not enzymology (so as not to mix the roots of words in Latin and Greek).

History of the study

Term enzyme proposed in the 17th century by the chemist van Helmont when discussing the mechanisms of digestion.

In con. XVIII - early XIX centuries It was already known that meat is digested by gastric juice, and astarch is converted into sugar under the influence of saliva. However, the mechanism of these phenomena was unknown.

In the 19th century Louis Pasteur, studying the conversion of carbohydrates into ethyl alcohol under the action of yeast, came to the conclusion that this process (fermentation) is catalyzed by a certain vital force located in yeast cells.

More than a hundred years ago terms enzyme And enzyme reflected different points of view in the theoretical dispute L. Pasteras on the one hand, and M. BertloiY. Liebig - on the other hand, about the nature of alcoholic fermentation. Actually enzymes(from lat. fermentum- sourdough) were called “organized enzymes” (that is, living microorganisms themselves), and the term enzyme(from Greek ἐν- - in- and ζύμη - yeast, leaven) proposed in 1876 by V. Kuehne for “unorganized enzymes” secreted by cells, for example, into the stomach (pepsin) or intestines (trypsin, amylase). Two years after the death of L. Pasteur in 1897, E. Buchner published the work “Alcoholic Fermentation without Yeast Cells,” in which he experimentally showed that cell-free yeast juice carries out alcoholic fermentation in the same way as undestroyed yeast cells. In 1907, he was awarded the Nobel Prize for this work. The first highly purified crystalline enzyme (urease) was isolated in 1926 by J. Sumner. Over the next 10 years, several more enzymes were isolated, and the protein nature of the enzymes was finally proven.

RNA catalytic activity was first discovered in the 1980s in pre-rRNA by Thomas Check, who studied ciliate RNA splicing. Tetrahymena thermophila. The ribozyme turned out to be a section of the Tetrahymena pre-rRNA molecule encoded by the intron of the extrachromosomal rDNA gene; this region performed autosplicing, that is, it cut itself out during rRNA maturation.

Functions of enzymes

Enzymes are present in all living cells and help convert some substances (substrates) into others (products). Enzymes act as catalysts in almost all biochemical reactions occurring in living organisms. By 2013, more than 5,000 different enzymes had been described. They play a vital role in all life processes, directing and regulating the body’s metabolism.

Like all catalysts, enzymes accelerate both forward and reverse reactions, lowering the activation energy of the process. The chemical equilibrium does not shift either in the forward or in the reverse direction. A distinctive feature of enzymes compared to non-protein catalysts is their high specificity—the binding constant of some substrates to protein can reach 10–10 mol/l or less. Each enzyme molecule is capable of performing from several thousand to several million “operations” per second.

For example, one molecule of the enzyme renin, contained in the gastric mucosa of a calf, curdles about 10 6 molecules of milk caseinogen in 10 minutes at a temperature of 37 °C.

Moreover, the efficiency of enzymes is much higher than the efficiency of non-protein catalysts - enzymes speed up reactions by millions and billions of times, non-protein catalysts - by hundreds and thousands of times. See also Catalytically perfect enzyme

Classification of enzymes

Based on the type of reactions they catalyze, enzymes are divided into 6 classes according to the hierarchical classification of enzymes. The classification was proposed by the International Union of Biochemistry and Molecular Biology. Each class contains subclasses, so that the enzyme is described by a set of four numbers separated by dots. For example, pepsi has the name EC 3.4.23.1. The first number roughly describes the mechanism of the reaction catalyzed by the enzyme:

CF 1: Oxidoreductases, catalyzing oxidation or reduction. Example: catalase, alcohol dehydrogenase.

CF 2: Transferases, catalyzing the transfer of chemical groups from one substrate molecule to another. Among transferases, kinases that transfer a phosphate group, usually from an ATP molecule, are especially distinguished.

CF 3: Hydrolases, catalyzing hydrolyzchemical bonds. Example: esterases, pepsin, trypsin, amylase, lipoprotein lipase.

CF 4: Lyases, catalyzing the breaking of chemical bonds without hydrolysis with the formation of a double bond in one of the products.

CF 5: Isomerases, catalyzing structural or geometric changes in the substrate molecule.

CF 6: Ligases, catalyzing the formation of chemical bonds between substrates due to ATP hydrolysis. Example: DNA polymerase.

Oxyreductases- these are enzymes that catalyze oxidation and reduction reactions, i.e. transfer of electrons from donor to acceptor. Oxidation is the removal of hydrogen atoms from the substrate, and reduction is the addition of hydrogen atoms to the acceptor.

Oxidoreductases include: dehydrases, oxidases, oxygenases, hydroxylases, peroxidases, catalases. For example, the enzyme alcohol dehydrogenase catalyzes the reaction converting alcohol into aldehyde.

Oxidoreductases that transfer a hydrogen atom or electrons directly to oxygen atoms are called aerobic dehydrogenases (oxidases), while oxidoreductases that transfer a hydrogen atom or electrons from one component of the respiratory chain of enzymes to another are called anaerobic dehydrogenases. A common variant of the redox process in cells is the oxidation of hydrogen atoms of the substrate with the participation of oxyreductases. Oxidoreductases are two-component enzymes in which the same coenzyme can bind to different apoenzymes. For example, many oxidoreductases contain NAD and NADP as coenzymes. At the end of the numerous class of oxireductases (at position 11) there are enzymes such as catalases and peroxidases. Of the total number of proteins in cell peroxisomes, up to 40 percent are catalase. Catalase and peroxidase break down hydrogen peroxide in the following reactions: H2O2 + H2O2 = O2 + 2H2O H2O2 + HO – R – OH = O=R=O + 2H2O From these equations, both the analogy and the significant difference between these reactions and enzymes immediately become clear . In this sense, catalase cleavage of hydrogen peroxide is a special case of a peroxidase reaction, where hydrogen peroxide serves as both a substrate and an acceptor in the first reaction.

Transferases- a separate class of enzymes that catalyze the transfer of functional groups and molecular residues from one molecule to another. Widely distributed in plant and animal organisms, they participate in the transformation of carbohydrates, lipids, nucleic acids and amino acids.

Reactions catalyzed by transferases generally look like this:

A-X + B ↔ A + B-X.

Molecule A here acts as a donor of a group of atoms ( X), and the molecule B is an acceptor of the group. Often one of the coenzymes acts as a donor in such transfer reactions. Many of the reactions catalyzed by transferases are reversible. Systematic names of class enzymes are formed according to the following scheme:

"donor:acceptor + group + transferase».

Or slightly more general names are used, when the name of the enzyme includes the name of either the donor or acceptor of the group:

"donor + group + transferase" or "acceptor + group + transferase».

For example, aspartate aminotransferase catalyzes the transfer of the amine group from the glutamic acid molecule, catechol-O-methyltransferase transfers the methyl group of S-adenosylmethionine to the benzene ring of various catecholamines, and ahistone acetyltransferase transfers the acetyl group from acetyl-coenzyme A to histone in the process of transcription activation.

In addition, enzymes of subgroup 7 of transferases that transfer a phosphoric acid residue using ATP as a donor of the phosphate group are often also called kinases; aminotransferases (subgroup 6) are often called transaminases

Hydrolases(KF3) are a class of enzymes that catalyze hydrolytic covalent bonds. The general form of a reaction catalyzed by a hydrolase is as follows:

A–B + H 2 O → A–OH + B–H

The systematic name of hydrolases includes name of fissilesubstrate followed by the addition -hydrolase. However, as a rule, in a trivial name the word hydrolase is omitted and only the suffix “-aza” remains.

The most important representatives

Esterases: nuclease, phosphodiesterase, lipase, phosphatase;

Glycosidases: amylase, lysozyme, etc.;

Proteases: trypsin, chymotrypsin, elastase, thrombin, renin, etc.;

Acid anhydride hydrolase (helicase, GTPase)

Being catalysts, enzymes accelerate both forward and reverse reactions, therefore, for example, lyases are able to catalyze the reverse reaction - addition at double bonds.

Liases- a separate class of enzymes that catalyze reactions of non-hydrolytic and non-oxidative cleavage of various chemical bonds ( C-C, C-O, C-N, C-S and others) of the substrate, reversible reactions of formation and cleavage of double bonds, accompanied by the elimination or addition of groups of atoms at its place, as well as the formation of cyclic structures.

In general, the names of enzymes are formed according to the scheme “ substrate+ lyase.” However, more often the name takes into account the subclass of the enzyme. Lyases differ from other enzymes in that catalyzed reactions involve two substrates in one direction, but only one in the reverse reaction. The name of the enzyme contains the words “decarboxylase” and “aldolase” or “lyase” (pyruvate decarboxylase, oxalate decarboxylase, oxaloacetate decarboxylase, threonine aldolase, phenylserine aldolase, isocitrate lyase, alanine lyase, ATP citrate lyase etc.), and for enzymes that catalyze reactions of water abstraction from the substrate - “dehydratase” (carbonate dehydratase, citrate dehydratase, serine dehydratase, etc.). In cases where only the reverse reaction is detected, or this direction in the reactions is more significant, the word “synthase” is present in the name of the enzymes (malate synthase, 2-isopropylmalate synthase, citrate synthase, hydroxymethylglutaryl-CoA synthase, etc.) .

Examples: histidine decarboxylase, fumarate hydratase.

Isomerases- enzymes that catalyze structural transformations of isomers (racemization or epimerization). Isomerases catalyze reactions similar to the following: A → B, where B is an isomer of A.

The name of the enzyme contains the word " racemase" (alanine racemase, methionine racemase, hydroxyproline racemase, lactate racemase, etc.), " epimerase" (aldose-1-epimerase, ribulose phosphate-4-epimerase, UDP-glucuronate-4-epimerase, etc.), " isomerase" (ribose phosphate isomerase, xylose isomerase, glucosamine phosphate isomerase, enoyl-CoA isomerase, etc.), " mutase"(phosphoglycerate mutase, methylaspartate mutase, phosphoglucomutase, etc.).

Ligaza(lat. ligāre- cross-link, connect) - an enzyme that catalyzes the joining of two molecules to form a new chemical bond ( ligation). In this case, the elimination (hydrolysis) of a small chemical group from one of the molecules usually occurs.

Ligases belong to the EC 6 enzyme class.

In molecular biology, subclass 6.5 ligases are classified into RNA ligases and DNA ligases.

DNA ligases

DNA ligase performing DNA repair

DNA ligases- enzymes (EC 6.5.1.1) that catalyze the covalent cross-linking of DNA strands in a duplex during replication, repair and recombination. They form phosphodiester bridges between the 5"-phosphoryl and 3"-hydroxyl groups of neighboring deoxynucleotides at DNA breaks or between two DNA molecules. To form these bridges, ligases use the energy of the hydrolysis of the pyrophosphoryl bond of ATP. One of the most common commercially available enzymes is bacteriophage T4 DNA ligase.

Mammalian DNA ligases

In mammals, three main types of DNA ligases are classified.

DNA ligase I ligates Okazaki fragments during replication of the lagging DNA strand and is involved in excision repair.

DNA ligase III, in complex with the XRCC1 protein, is involved in excision repair and recombination.

DNA ligase IV, in complex with XRCC4, catalyzes the final step of non-homologous end joining (NHEJ) of DNA double-strand breaks. Also required for V(D)J recombination of immunoglobulin genes.

Previously, another type of ligase was isolated - DNA ligase II, which was later recognized as an artifact of protein isolation, namely the proteolysis product of DNA ligase III.

Enzyme naming conventions

Enzymes are usually named by the type of reaction they catalyze, adding the suffix -aza to the name of the substrate( For example, lactase is an enzyme involved in the conversion of lactose). Thus, different enzymes performing the same function will have the same name. Such enzymes are distinguished by other properties, for example, by optimal pH (alkaline phosphatase) or localization in the cell (membrane ATPase).

Structure and mechanism of action of enzymes

The activity of enzymes is determined by their three-dimensional structure.

Like all proteins, enzymes are synthesized as a linear chain of amino acids that folds in a specific way. Each sequence of amino acids folds in a special way, and the resulting molecule (protein globule) has unique properties. Several protein chains can be combined to form a protein complex. The tertiary structure of proteins is destroyed when heated or exposed to certain chemicals.

Active site of enzymes

The study of the mechanism of a chemical reaction catalyzed by an enzyme, along with the determination of intermediate and final products at different stages of the reaction, implies precise knowledge of the geometry of the tertiary structure of the enzyme, the nature of the functional groups of its molecule, providing specificity of action and high catalytic activity on a given substrate, as well as the chemical nature of the region (regions) of the molecule an enzyme that provides a high rate of catalytic reaction. Typically, the substrate molecules involved in enzymatic reactions are relatively small in size compared to enzyme molecules. Thus, during the formation of enzyme-substrate complexes, only limited fragments of the amino acid sequence of the polypeptide chain enter into direct chemical interaction - the “active center” - a unique combination of amino acid residues in the enzyme molecule, ensuring direct interaction with the substrate molecule and direct participation in the act of catalysis.

The active center is conventionally divided into:

catalytic center - directly chemically interacting with the substrate;

binding center (contact or “anchor” site) - providing specific affinity for the substrate and the formation of the enzyme-substrate complex.

To catalyze a reaction, an enzyme must bind to one or more substrates. The protein chain of the enzyme folds in such a way that a gap, or depression, is formed on the surface of the globule where substrates bind. This region is called the substrate binding site. It usually coincides with or is close to the active site of the enzyme. Some enzymes also contain binding sites for cofactors or metal ions.

The enzyme combines with the substrate:

cleans the substrate from water “coat”

arranges reacting substrate molecules in space in the manner necessary for the reaction to occur

prepares substrate molecules for reaction (for example, polarizes).

Usually, the enzyme attaches to the substrate through ionic or hydrogen bonds, rarely through covalent bonds. At the end of the reaction, its product (or products) are separated from the enzyme.

As a result, the enzyme reduces the activation energy of the reaction. This is because in the presence of the enzyme the reaction follows a different path (actually a different reaction occurs), for example:

In the absence of an enzyme:

In the presence of an enzyme:

• AF+B = AVF

  AVF = AB+F

where A, B are substrates, AB is the reaction product, F is the enzyme.

Enzymes cannot independently provide energy for endergonic reactions (which require energy to occur). Therefore, enzymes that carry out such reactions couple them with exergonic reactions that release more energy. For example, biopolymer synthesis reactions are often coupled with ATP hydrolysis reactions.

The active centers of some enzymes are characterized by the phenomenon of cooperativity.

Specificity

Enzymes generally exhibit high specificity for their substrates (substrate specificity). This is achieved by partial complementarity between the shape, charge distribution and hydrophobic regions on the substrate molecule and the substrate binding site on the enzyme. Enzymes also typically exhibit high levels of stereospecificity (forming only one of the possible stereoisomers as a product or using only one stereoisomer as a substrate), regioselectivity (forming or breaking a chemical bond at only one of the possible positions of the substrate), and chemoselectivity (catalyzing only one chemical reaction from several possible for given conditions). Despite the overall high level of specificity, the degree of substrate and reaction specificity of enzymes may vary. For example, endopeptidase trypsin only breaks the peptide bond after arginine or lysine if they are not followed by a proline, is much less specific and can break the peptide bond following many amino acids.

In 1890, Emil Fischer proposed that the specificity of enzymes is determined by the exact match between the form of the enzyme and the substrate. This assumption is called the key-lock model. The enzyme combines with the substrate to form a short-lived enzyme-substrate complex. However, although this model explains the high specificity of enzymes, it does not explain the phenomenon of transition state stabilization that is observed in practice.

Induced correspondence model

In 1958, Daniel Koshland proposed a modification of the key-lock model. Enzymes are generally not rigid, but flexible molecules. The active site of an enzyme can change conformation after binding a substrate. The amino acid side groups of the active site assume a position that allows the enzyme to perform its catalytic function. In some cases, the substrate molecule also changes conformation after binding at the active site. Unlike the key-lock model, the induced-fit model explains not only the specificity of enzymes, but also the stabilization of the transition state. This model is called the “glove hand”.

Modifications

Many enzymes undergo modifications after the synthesis of the protein chain, without which the enzyme does not fully exhibit its activity. Such modifications are called post-translational modifications (processing). One of the most common types of modification is the addition of chemical groups to side residues of the polypeptide chain. For example, the addition of a phosphoric acid residue is called phosphorylation and is catalyzed by the enzyme kinase. Many eukaryotic enzymes are glycosylated, that is, modified by oligomers of carbohydrate nature.

Another common type of post-translational modification is cleavage of the polypeptide chain. For example, chymotrypsin (a protease involved in digestion) is obtained by cleaving a polypeptide region from chymotrypsinogen. Chymotrypsinogen is an inactive precursor of chymotrypsin and is synthesized in the pancreas. The inactive form is transported to the stomach, where it is converted into chymotrypsin. This mechanism is necessary in order to avoid the splitting of the pancreas and other tissues before the enzyme enters the stomach. The inactive enzyme precursor is also called a "zymogen".

Enzyme cofactors

Some enzymes perform the catalytic function on their own, without any additional components. However, there are enzymes that require non-protein components to carry out catalysis. Cofactors can be either inorganic molecules (metal ions, iron-sulfur clusters, etc.) or organic (for example, flavinyl hem). Organic cofactors that are tightly bound to an enzyme are also called prosthetic groups. Organic cofactors that can be separated from the enzyme are called coenzymes.

An enzyme that requires the presence of a cofactor for catalytic activity, but is not bound to it, is called an apo enzyme. An apo enzyme in combination with a cofactor is called a holo enzyme. Most cofactors are bound to the enzyme by non-covalent but rather strong interactions. There are also prosthetic groups that are covalently bound to the enzyme, for example, thiamine pyrophosphate in pyruvate dehydrogenase.

Regulation of enzymes

Some enzymes have small molecule binding sites and may be substrates or products of the metabolic pathway in which the enzyme enters. They decrease or increase the activity of the enzyme, which creates the opportunity for feedback.

Inhibition by end product

Metabolic pathway is a chain of sequential enzymatic reactions. Often the end product of a metabolic pathway is an inhibitor of an enzyme that accelerates the first reaction in that metabolic pathway. If there is too much of the final product, then it acts as an inhibitor for the very first enzyme, and if after this there is too little of the final product, then the first enzyme is activated again. Thus, inhibition by the final product according to the principle of negative feedback is an important way of maintaining homeostasis (relative constancy of the internal environmental conditions of the body).

Influence of environmental conditions on enzyme activity

The activity of enzymes depends on the conditions in the cell or body - pressure, acidity of the environment, temperature, concentration of dissolved salts (ionic strength of the solution), etc.

Multiple Forms of Enzymes

The multiple forms of enzymes can be divided into two categories:

Isoenzymes

Proper plural forms (true)

Isoenzymes- these are enzymes, the synthesis of which is encoded by different genes, they have different primary structures and different properties, but they catalyze the same reaction. Types of isoenzymes:

Organ - glycolysis enzymes in the liver and muscles.

Cellular - malate dehydrogenase cytoplasmic and mitochondrial (the enzymes are different, but they catalyze the same reaction).

Hybrid - enzymes with a quaternary structure, formed as a result of non-covalent binding of individual subunits (lactate dehydrogenase - 4 subunits of 2 types).

Mutant - formed as a result of a single gene mutation.

Alloenzymes are encoded by different alleles of the same gene.

Actually plural forms(true) are enzymes, the synthesis of which is encoded by the same allele of the same gene, they have the same primary structure and properties, but after synthesis on ribosomachons they undergo modifications and become different, although they catalyze the same reaction.

Isoenzymes are distinct at the genetic level and differ from the primary sequence, and true multiple forms become distinct at the post-translational level.

Medical significance

The connection between enzymes and hereditary metabolic diseases was first established by A. Garrod in the 1910s. Garrod called diseases associated with enzyme defects “inborn errors of metabolism.”

If a mutation occurs in the gene encoding a particular enzyme, the amino acid sequence of the enzyme may change. Moreover, as a result of most mutations, its catalytic activity decreases or disappears completely. If an organism receives two such mutant genes (one from each parent), the chemical reaction catalyzed by this enzyme stops occurring in the body. For example, the appearance of albinos is associated with the cessation of the production of the enzyme tyrosinase, which is responsible for one of the stages of the synthesis of the dark pigment melanin. Phenylketonuria is associated with reduced or absent activity of the enzyme phenylalanine-4-hydroxylase in the liver.

Currently, hundreds of hereditary diseases associated with enzyme defects are known. Methods have been developed for the treatment and prevention of many of these diseases.

Practical use

Enzymes are widely used in the national economy - food, textile industries, pharmacology and medicine. Most drugs affect the course of enzymatic processes in the body, starting or stopping certain reactions.

The scope of use of enzymes in scientific research and medicine is even wider.

Millions of chemical reactions take place in the cell of any living organism. Each of them is of great importance, so it is important to maintain the speed of biological processes at a high level. Almost every reaction is catalyzed by its own enzyme. What are enzymes? What is their role in the cell?

Enzymes. Definition

The term "enzyme" comes from the Latin fermentum - leaven. They can also be called enzymes from the Greek en zyme - “in yeast”.

Enzymes are biologically active substances, so any reaction occurring in a cell cannot occur without their participation. These substances act as catalysts. Accordingly, any enzyme has two main properties:

1) The enzyme accelerates the biochemical reaction, but is not consumed.

2) The value of the equilibrium constant does not change, but only accelerates the achievement of this value.

Enzymes speed up biochemical reactions a thousand, and in some cases a million, times. This means that in the absence of the enzymatic apparatus, all intracellular processes will practically stop, and the cell itself will die. Therefore, the role of enzymes as biologically active substances is great.

The variety of enzymes allows for versatile regulation of cell metabolism. Many enzymes of different classes take part in any reaction cascade. Biological catalysts are highly selective due to the specific conformation of the molecule. Since enzymes in most cases are of a protein nature, they are located in a tertiary or quaternary structure. This is again explained by the specificity of the molecule.

Functions of enzymes in the cell

The main task of the enzyme is to accelerate the corresponding reaction. Any cascade of processes, from the decomposition of hydrogen peroxide to glycolysis, requires the presence of a biological catalyst.

The correct functioning of enzymes is achieved by high specificity to a specific substrate. This means that a catalyst can only accelerate a certain reaction and no other, even very similar ones. According to the degree of specificity, the following groups of enzymes are distinguished:

1) Enzymes with absolute specificity, when only one single reaction is catalyzed. For example, collagenase breaks down collagen, and maltase breaks down maltose.

2) Enzymes with relative specificity. This includes substances that can catalyze a certain class of reactions, for example, hydrolytic cleavage.

The work of a biocatalyst begins from the moment its active center attaches to the substrate. In this case, they talk about complementary interaction like a lock and key. Here we mean the complete coincidence of the shape of the active center with the substrate, which makes it possible to accelerate the reaction.

The next stage is the reaction itself. Its speed increases due to the action of an enzymatic complex. Ultimately, we get an enzyme that is associated with the reaction products.

The final stage is the detachment of the reaction products from the enzyme, after which the active center again becomes free for the next job.

Schematically, the work of the enzyme at each stage can be written as follows:

1) S + E ——> SE

2) SE ——> SP

3) SP ——> S + P, where S is the substrate, E is the enzyme, and P is the product.

Classification of enzymes

A huge number of enzymes can be found in the human body. All knowledge about their functions and operation was systematized, and as a result, a single classification emerged, thanks to which you can easily determine what a particular catalyst is intended for. The 6 main classes of enzymes are presented here, as well as examples of some of the subgroups.

1. Oxidoreductases.

Enzymes of this class catalyze redox reactions. A total of 17 subgroups are distinguished. Oxidoreductases usually have a non-protein part, represented by a vitamin or heme.

Among the oxidoreductases, the following subgroups are often found:

a) Dehydrogenases. The biochemistry of dehydrogenase enzymes involves the removal of hydrogen atoms and their transfer to another substrate. This subgroup is most often found in the reactions of respiration and photosynthesis. Dehydrogenases necessarily contain a coenzyme in the form of NAD/NADP or flavoproteins FAD/FMN. Metal ions are often found. Examples include enzymes such as cytochrome reductase, pyruvate dehydrogenase, isocitrate dehydrogenase, as well as many liver enzymes (lactate dehydrogenase, glutamate dehydrogenase, etc.).

b) Oxidases. A number of enzymes catalyze the addition of oxygen to hydrogen, as a result of which the reaction products can be water or hydrogen peroxide (H 2 0, H 2 0 2). Examples of enzymes: cytochrome oxidase, tyrosinase.

c) Peroxidases and catalases are enzymes that catalyze the decomposition of H 2 O 2 into oxygen and water.

d) Oxygenases. These biocatalysts accelerate the addition of oxygen to the substrate. Dopamine hydroxylase is one example of such enzymes.

2. Transferases.

The task of enzymes of this group is to transfer radicals from a donor substance to a recipient substance.

a) Methyltransferases. DNA methyltransferases are the main enzymes that control the process of nucleotide replication and play a large role in regulating the functioning of nucleic acids.

b) Acyltransferases. Enzymes of this subgroup transport an acyl group from one molecule to another. Examples of acyltransferases: lecithin cholesterol acyltransferase (transfers a functional group from a fatty acid to cholesterol), lysophosphatidylcholine acyltransferase (transfers an acyl group to lysophosphatidylcholine).

c) Aminotransferases are enzymes that are involved in the conversion of amino acids. Examples of enzymes: alanine aminotransferase, which catalyzes the synthesis of alanine from pyruvate and glutamate by amino group transfer.

d) Phosphotransferases. Enzymes of this subgroup catalyze the addition of a phosphate group. Another name for phosphotransferases, kinases, is much more common. Examples include enzymes such as hexokinases and aspartate kinases, which add phosphorus residues to hexoses (most often glucose) and aspartic acid, respectively.

3. Hydrolases - a class of enzymes that catalyze the cleavage of bonds in a molecule with the subsequent addition of water. Substances that belong to this group are the main digestive enzymes.

a) Esterases - break ether bonds. An example is lipases, which break down fats.

b) Glycosidases. The biochemistry of enzymes of this series consists in the destruction of glycosidic bonds of polymers (polysaccharides and oligosaccharides). Examples: amylase, sucrase, maltase.

c) Peptidases are enzymes that catalyze the breakdown of proteins into amino acids. Peptidases include enzymes such as pepsins, trypsin, chymotrypsin, and carboxypeptidase.

d) Amidases - cleave amide bonds. Examples: arginase, urease, glutaminase, etc. Many amidase enzymes are found in

4. Lyases are enzymes that are similar in function to hydrolases, but the cleavage of bonds in molecules does not require water. Enzymes of this class always contain a non-protein part, for example, in the form of vitamins B1 or B6.

a) Decarboxylase. These enzymes act on the C-C bond. Examples include glutamate decarboxylase or pyruvate decarboxylase.

b) Hydratases and dehydratases are enzymes that catalyze the reaction of cleavage of C-O bonds.

c) Amidine lyases - destroy C-N bonds. Example: arginine succinate lyase.

d) P-O lyase. Such enzymes, as a rule, cleave a phosphate group from a substrate substance. Example: adenylate cyclase.

The biochemistry of enzymes is based on their structure

The abilities of each enzyme are determined by its individual, unique structure. Any enzyme is first and foremost a protein, and its structure and degree of folding play a decisive role in determining its function.

Each biocatalyst is characterized by the presence of an active center, which, in turn, is divided into several independent functional areas:

1) The catalytic center is a special region of the protein through which the enzyme attaches to the substrate. Depending on the conformation of the protein molecule, the catalytic center can take on a variety of shapes, which must fit the substrate just like a lock fits a key. This complex structure explains what is in the tertiary or quaternary state.

2) Adsorption center - acts as a “holder”. Here, first of all, the connection between the enzyme molecule and the substrate molecule occurs. However, the bonds formed by the adsorption center are very weak, which means that the catalytic reaction at this stage is reversible.

3) Allosteric centers can be located both in the active center and over the entire surface of the enzyme as a whole. Their function is to regulate the functioning of the enzyme. Regulation occurs with the help of inhibitor molecules and activator molecules.

Activator proteins, by binding to the enzyme molecule, speed up its work. Inhibitors, on the other hand, inhibit catalytic activity, and this can happen in two ways: either the molecule binds to an allosteric site in the region of the active site of the enzyme (competitive inhibition), or it attaches to another region of the protein (non-competitive inhibition). considered more effective. After all, this closes the place for the substrate to bind to the enzyme, and this process is possible only in the case of an almost complete coincidence of the shape of the inhibitor molecule and the active center.

An enzyme often consists not only of amino acids, but also of other organic and inorganic substances. Accordingly, apoenzyme is the protein part, coenzyme is the organic part, and cofactor is the inorganic part. The coenzyme can be represented by carbohydrates, fats, nucleic acids, and vitamins. In turn, a cofactor is most often auxiliary metal ions. The activity of enzymes is determined by its structure: additional substances included in the composition change the catalytic properties. Various types of enzymes are the result of a combination of all the listed factors in the formation of the complex.

Regulation of enzymes

Enzymes as biologically active substances are not always necessary for the body. The biochemistry of enzymes is such that they can, if catalyzed excessively, harm a living cell. To prevent the harmful effects of enzymes on the body, it is necessary to somehow regulate their work.

Since enzymes are protein in nature, they are easily destroyed at high temperatures. The denaturation process is reversible, but it can significantly affect the performance of substances.

pH also plays a big role in regulation. The highest enzyme activity is usually observed at neutral pH values ​​(7.0-7.2). There are also enzymes that work only in acidic environments or only in alkaline environments. Thus, a low pH is maintained in cellular lysosomes, at which the activity of hydrolytic enzymes is maximum. If they accidentally enter the cytoplasm, where the environment is already closer to neutral, their activity will decrease. This protection against “self-eating” is based on the peculiarities of the work of hydrolases.

It is worth mentioning the importance of coenzyme and cofactor in the composition of enzymes. The presence of vitamins or metal ions significantly affects the functioning of some specific enzymes.

Enzyme nomenclature

All enzymes in the body are usually named depending on their belonging to any of the classes, as well as on the substrate with which they react. Sometimes not one, but two substrates are used in the name.

Examples of the names of some enzymes:

1. Liver enzymes: lactate dehydrogenase, glutamate dehydrogenase.
2. Full systematic name of the enzyme: lactate-NAD+-oxidoreductase.

Trivial names that do not adhere to the rules of nomenclature have also been preserved. Examples are digestive enzymes: trypsin, chymotrypsin, pepsin.

Enzyme synthesis process

The functions of enzymes are determined at the genetic level. Since the molecule is, by and large, a protein, its synthesis exactly repeats the processes of transcription and translation.

Enzyme synthesis occurs according to the following scheme. First, information about the desired enzyme is read from DNA, resulting in the formation of mRNA. Messenger RNA encodes all the amino acids that make up the enzyme. Regulation of enzymes can also occur at the DNA level: if the product of the catalyzed reaction is sufficient, gene transcription stops and vice versa, if there is a need for the product, the transcription process is activated.

After the mRNA has entered the cytoplasm of the cell, the next stage begins - translation. On the ribosomes of the endoplasmic reticulum, the primary chain is synthesized, consisting of amino acids connected by peptide bonds. However, the protein molecule in the primary structure cannot yet perform its enzymatic functions.

The activity of enzymes depends on the structure of the protein. On the same EPS, protein twisting occurs, as a result of which first secondary and then tertiary structures are formed. The synthesis of some enzymes stops already at this stage, but to activate catalytic activity it is often necessary to add a coenzyme and a cofactor.

In certain areas of the endoplasmic reticulum, the organic components of the enzyme are added: monosaccharides, nucleic acids, fats, vitamins. Some enzymes cannot work without the presence of a coenzyme.

The cofactor plays a crucial role in the formation of Some enzyme functions are available only when the protein reaches a domain organization. Therefore, the presence of a quaternary structure, in which the connecting link between several protein globules is a metal ion, is very important for them.

Multiple Forms of Enzymes

There are situations when it is necessary to have several enzymes that catalyze the same reaction, but differ from each other in some parameters. For example, an enzyme can work at 20 degrees, but at 0 degrees it will no longer be able to perform its functions. What should a living organism do in such a situation at low ambient temperatures?

This problem is easily solved by the presence of several enzymes that catalyze the same reaction, but operate under different conditions. There are two types of multiple forms of enzymes:

1. Isoenzymes. Such proteins are encoded by different genes, consist of different amino acids, but catalyze the same reaction.
2. True plural forms. These proteins are transcribed from the same gene, but modification of the peptides occurs on the ribosomes. The output is several forms of the same enzyme.

As a result, the first type of multiple forms is formed at the genetic level, while the second type is formed at the post-translational level.

The importance of enzymes

In medicine, it comes down to the release of new medicines, which already contain substances in the required quantities. Scientists have not yet found a way to stimulate the synthesis of missing enzymes in the body, but today there are widespread drugs that can temporarily compensate for their deficiency.

Various enzymes in the cell catalyze a large number of reactions associated with maintaining life. One of these enisms are representatives of the group of nucleases: endonucleases and exonucleases. Their job is to maintain a constant level of nucleic acids in the cell and remove damaged DNA and RNA.

Don't forget about the phenomenon of blood clotting. As an effective protective measure, this process is controlled by a number of enzymes. The main one is thrombin, which converts the inactive fibrinogen protein into active fibrin. Its threads create a kind of network that clogs the site of damage to the vessel, thereby preventing excessive blood loss.

Enzymes are used in winemaking, brewing, and the production of many fermented milk products. Yeast can be used to produce alcohol from glucose, but an extract from it is sufficient for this process to proceed successfully.

Interesting facts you didn't know about

All enzymes in the body have a huge mass - from 5000 to 1,000,000 Da. This is due to the presence of protein in the molecule. For comparison: the molecular weight of glucose is 180 Da, and carbon dioxide is only 44 Da.

To date, more than 2000 enzymes have been discovered that have been found in the cells of various organisms. However, most of these substances have not yet been fully studied.

Enzyme activity is used to produce effective washing powders. Here, enzymes perform the same role as in the body: they break down organic matter, and this property helps in the fight against stains. It is recommended to use such washing powder at a temperature no higher than 50 degrees, otherwise denaturation may occur.

According to statistics, 20% of people around the world suffer from a deficiency of any of the enzymes.

The properties of enzymes were known for a very long time, but only in 1897 did people realize that not the yeast itself, but an extract from its cells, could be used to ferment sugar into alcohol.

Enzymes (from lat. Fermentum - fermentation) , or enzymes (from Greek Ep - inside, sume - leaven) - protein compounds that are biological catalysts. The science of enzymes is called enzymology. Enzyme molecules are proteins or ribonucleic acid (RNA). RNA enzymes are called ribozymes and are considered the original form of enzymes that were replaced by protein enzymes during evolution.

Structural and functional organization. Enzyme molecules are larger in size than substrate molecules and have a complex spatial configuration, mainly a globular structure.

Due to the large size of enzyme molecules, a strong electric field arises, in which: a) enzymes acquire an asymmetric shape, weakens bonds and causes a change in their structure; b) orientation of substrate molecules becomes possible. The functional organization of enzymes is associated with the center - this is a special small section of the protein molecule that can bind the substrate and thus ensure the catalytic activity of the enzyme. The active center of simple enzymes is a combination of certain amino acids in the chain to form a kind of “pocket” in which catalytic transformations of the substrate occur. In complex enzymes, the number of active centers is equal to the number of subunits, and these are cofactors with adjacent protein functional groups. In addition to the active center, some enzymes have an allosteric center that regulates the functioning of the active center.

Properties . There are certain common and distinctive features between enzymes and inorganic catalysts. What they have in common is that they: a) can catalyze only thermodynamically possible reactions and accelerate only those reactions that can occur without them, but at a lower rate; b) are not used during the reaction and are not part of the final products; b) do not shift the chemical equilibrium, but only accelerate its onset. Enzymes also have some specific properties that inorganic catalysts do not have.

Enzymes are not destroyed in reactions, so a very small amount of them causes the transformation of a large amount of substrate (for example, 1 molecule of catalase can break down more than 5 million molecules of H2O2 in 1 minute). The zones accelerate the rate of chemical reactions under normal conditions, but are not consumed themselves. All this together determines the properties of enzymes such as high biological activity. The optimal action of most enzymes occurs at a temperature of 37-40 ° C. With increasing temperature, the activity of enzymes decreases and subsequently stops completely, and beyond + 80 ° C they are destroyed. At low temperatures (below 0 ° C), enzymes stop their action, but are not destroyed. So, enzymes are characterized thermal sensitivity.

Enzymes exhibit their activity at a certain concentration of H ions, therefore they speak of pH dependence. The optimal action of most enzymes is observed in an environment close to neutral.

A property like specificity or selectivity manifests itself in the fact that each enzyme acts on a specific substrate, catalyzing only one “its” reaction. The selectivity of enzyme action is determined by the protein component.

Enzymes are catalysts with controlled activity that can be significantly altered by certain chemical compounds that increase or decrease the rate of the reaction being catalyzed. Metal cations and anions act as activators

acids, organic substances, and inhibitors - cations of heavy metals, etc. This property was called controllability of action (allostericity). Enzymes are formed only when a substrate appears that induces its synthesis ( inducibility), and “switching off” the action of enzymes is usually carried out by an excess of assimilation products ( repressiveness). Enzymatic reactions are reversible, which is due to the ability of enzymes to catalyze forward and reverse reactions. For example, lipase can, under certain conditions, break down fat into glycerol and fatty acids, as well as catalyze its synthesis from breakdown products ( recurrence of action).

Mechanism of action. To understand the mechanism of action of enzymes on the occurrence of chemical reactions, it is important active center theory, lock and key hypothesis And induced fit hypothesis. According to active center theory, in the molecule of each enzyme there are one or more regions in which biocatalysis occurs due to close contact between the enzyme and the substrate. Key-lock hypothesis(1890, E. Fischer) explains the specificity of enzymes by matching the shape of the enzyme (lock) and the substrate (key). The enzyme combines with the substrate to form a temporary enzyme-substrate complex. Induced correspondence hypothesis(1958, D. Koshland). is based on the assertion that enzymes are flexible molecules, due to which the configuration of the active center in them undergoes changes in the presence of a substrate, that is, the enzyme orients its functional groups so as to ensure the greatest catalytic activity. The substrate molecule, when attached to the enzyme, also changes its configuration to increase reactivity.

Diversity . In modern enzymology, over 3000 enzymes are known. Enzymes are generally classified according to their chemical composition and the type of reactions they influence. The classification of enzymes by chemical composition includes simple and complex enzymes. Simple enzymes (one-component) - contain only the protein part. Most enzymes in this group can crystallize. An example of simple enzymes is ribonuclease, hydrolases (amylase, lipase, protease), urease, etc. Complex enzymes (two-component) - consist of apoenzyme And cofactor. The protein component that determines the specificity of complex enzymes and is synthesized, as a rule, by the body and is sensitive to temperature is an apoenzyme. A non-protein component that determines the activity of complex enzymes and, as a rule, enters the body in the form of precursors or in finished form, and remains stable under unfavorable conditions, is a cofactor. Cofactors can be either inorganic molecules (for example, metal ions) or organic molecules (for example, flavin). Organic cofactors that are permanently associated with the enzyme are called prosthetic groups. Organic cofactors that can be separated from the enzyme are called coenzymes. complex enzymes are oxidoreductases (for example, catalase), ligases (for example, DNA polymerase, tRNA synthetases), lyases, etc.

Enzymatic reactions are divided into anabolic (synthesis reactions) and catabolic (decomposition reactions), and the totality of all these processes in a living system is called metabolism. Within these groups of processes, types of enzymatic reactions are distinguished, according to which enzymes are divided into 6 classes: oxidoreductases, transferases, hydrolases, lyases, isomerases And ligases

1. Oxidoreductases catalyze redox reactions (transfer of electrons and H atoms from one substrate to another).

2. Transferases accelerate transfer reactions (transfer of chemical groups from one substrate to another).

3. Hydrolases are enzymes of hydrolysis reactions (splitting of substrates with the participation of water).

4. Lyases catalyze reactions of non-hydrolytic decomposition (cleavage of substrates without the participation of water with the formation of a double bond and without the use of ATP energy).

5. Isomerases affect the rate of isomerization reactions (intramolecular movement of various groups).

6. Ligases catalyze synthesis reactions (the combination of molecules using the energy of ATP and the formation of new bonds).

An enzyme is usually named by the type of reaction it catalyzes by adding the suffix -aza to the name of the substrate (for example, lactase is an enzyme involved in the conversion of lactose).

Meanings. Enzymes provide chemical transformations of substances due to the reduction activation energy, that is, in reducing the level of energy required to provide reactivity to a molecule (for example, to break the bond between nitrogen and Carbon in laboratory conditions, about 210 kJ is required, while in a biosystem only 42-50 kJ are spent on this). Enzymes present in all living cells contribute to the conversion of some substances (substrates) to others (products). Enzymes act as catalysts in almost all biochemical reactions occurring in living organisms - they catalyze about 4000 chemically separate bioreactions. Enzymes play a vital role in all life processes, directing or regulating the body's metabolism. Enzymes are widely used in agriculture.

Some examples of the use of enzymes in human activities

industry	enzymes	Usage
food industry	Pectinase	For lighting fruit juices
	glucose oxidase	For preserving meat, juices, beer as an antiszhysnyuvach
		To break down starch into glucose, which is fermented by yeast during bread baking
	Pepsin, trypsin	For the production of “ready-made” cereals and baby food products
		For cheese production
Light industry	Peptihydrolysis	For softening leathers and removing hair from them
pharmaceutical industry		To remove plaque in toothpastes
	collagenase	For cleaning wounds from burns, frostbite, varicose ulcers as part of ointments and new types of dressings
Chemical industry	bacterial proteases	For washing clothes using biopowders with enzyme additives
Agriculture	cellulase	Feed enzymes to increase the nutritional value of feed
	bacterial proteases	To obtain feed proteins
Genetic Engineering	Ligases and restriction enzymes	For cutting and stitching DNA molecules in order to modify their hereditary information
cosmetic industry	Calagenases	For skin rejuvenation in creams and masks

Nucleic acids are compounds that connect the past with the future.

ENZYMES
organic substances of a protein nature that are synthesized in cells and many times accelerate the reactions occurring in them without undergoing chemical transformations. Substances that have a similar effect also exist in inanimate nature and are called catalysts. Enzymes (from the Latin fermentum - fermentation, leaven) are sometimes called enzymes (from the Greek en - inside, zyme - leaven). All living cells contain a very large set of enzymes, the catalytic activity of which determines the functioning of the cells. Almost each of the many different reactions occurring in a cell requires the participation of a specific enzyme. The study of the chemical properties of enzymes and the reactions they catalyze is a special, very important area of ​​biochemistry - enzymology. Many enzymes are in a free state in the cell, simply dissolved in the cytoplasm; others are associated with complex, highly organized structures. There are also enzymes that are normally located outside the cell; Thus, enzymes that catalyze the breakdown of starch and proteins are secreted by the pancreas into the intestine. Secreted by enzymes and many microorganisms. The first data on enzymes were obtained from the study of fermentation and digestion processes. L. Pasteur made a great contribution to the study of fermentation, but he believed that only living cells could carry out the corresponding reactions. At the beginning of the 20th century. E. Buchner showed that the fermentation of sucrose to form carbon dioxide and ethyl alcohol can be catalyzed by cell-free yeast extract. This important discovery stimulated the isolation and study of cellular enzymes. In 1926, J. Sumner from Cornell University (USA) isolated urease; it was the first enzyme obtained in almost pure form. Since then, more than 700 enzymes have been discovered and isolated, but many more exist in living organisms. The identification, isolation and study of the properties of individual enzymes occupy a central place in modern enzymology. Enzymes involved in fundamental energy conversion processes, such as the breakdown of sugars and the formation and hydrolysis of the high-energy compound adenosine triphosphate (ATP), are present in all types of cells - animal, plant, bacterial. However, there are enzymes that are produced only in the tissues of certain organisms. Thus, enzymes involved in cellulose synthesis are found in plant cells, but not in animal cells. Thus, it is important to distinguish between “universal” enzymes and enzymes specific to certain cell types. Generally speaking, the more specialized a cell is, the more likely it is that it will synthesize the set of enzymes needed to perform a particular cellular function.
Enzymes are like proteins. All enzymes are proteins, simple or complex (i.e., containing, along with the protein component, a non-protein part).
See also PROTEINS. Enzymes are large molecules, with molecular weights ranging from 10,000 to over 1,000,000 daltons (Da). For comparison, we indicate that masses of known substances: glucose - 180, carbon dioxide - 44, amino acids - from 75 to 204 Da. Enzymes that catalyze the same chemical reactions, but isolated from different types of cells, differ in properties and composition, but usually have a certain similarity in structure. The structural features of enzymes necessary for their functioning are easily lost. Thus, when heated, a restructuring of the protein chain occurs, accompanied by a loss of catalytic activity. The alkaline or acidic properties of the solution are also important. Most enzymes work best in solutions whose pH is close to 7, when the concentration of H+ and OH- ions is approximately the same. This is due to the fact that the structure of protein molecules, and therefore the activity of enzymes, strongly depends on the concentration of hydrogen ions in the medium. Not all proteins present in living organisms are enzymes. Thus, a different function is performed by structural proteins, many specific blood proteins, protein hormones, etc.
Coenzymes and substrates. Many large molecular weight enzymes exhibit catalytic activity only in the presence of specific low molecular weight substances called coenzymes (or cofactors). Most vitamins and many minerals play the role of coenzymes; that is why they must enter the body with food. Vitamins PP (nicotinic acid, or niacin) and riboflavin, for example, are part of the coenzymes necessary for the functioning of dehydrogenases. Zinc is a coenzyme of carbonic anhydrase, an enzyme that catalyzes the release of carbon dioxide from the blood, which is removed from the body along with exhaled air. Iron and copper serve as components of the respiratory enzyme cytochrome oxidase. The substance that undergoes transformation in the presence of an enzyme is called a substrate. The substrate attaches to an enzyme, which accelerates the breaking of some chemical bonds in its molecule and the creation of others; the resulting product is detached from the enzyme. This process is represented as follows:

The product can also be considered a substrate, since all enzymatic reactions are reversible to one degree or another. True, the equilibrium is usually shifted towards the formation of the product, and the reverse reaction can be difficult to detect.
Mechanism of action of enzymes. The rate of an enzymatic reaction depends on the substrate concentration [[S]] and the amount of enzyme present. These quantities determine how many enzyme molecules will combine with the substrate, and the rate of the reaction catalyzed by this enzyme depends on the content of the enzyme-substrate complex. In most situations of interest to biochemists, the enzyme concentration is very low and the substrate is present in excess. In addition, biochemists study processes that have reached a steady state, in which the formation of an enzyme-substrate complex is balanced by its transformation into a product. Under these conditions, the dependence of the rate (v) of the enzymatic transformation of the substrate on its concentration [[S]] is described by the Michaelis-Menten equation:

where KM is the Michaelis constant, characterizing the activity of the enzyme, V is the maximum reaction rate at a given total enzyme concentration. From this equation it follows that at small [[S]], the reaction rate increases in proportion to the concentration of the substrate. However, with a sufficiently large increase in the latter, this proportionality disappears: the reaction rate ceases to depend on [[S]] - saturation occurs when all enzyme molecules are occupied by the substrate. Elucidation of the mechanisms of action of enzymes in all details is a matter for the future, but some of their important features have already been established. Each enzyme has one or more active sites to which the substrate binds. These centers are highly specific, i.e. “recognize” only “their” substrate or closely related compounds. The active center is formed by special chemical groups in the enzyme molecule, oriented relative to each other in a certain way. The loss of enzymatic activity that occurs so easily is associated precisely with a change in the mutual orientation of these groups. The substrate molecule associated with the enzyme undergoes changes, as a result of which some chemical bonds are broken and other chemical bonds are formed. For this process to occur, energy is needed; the role of the enzyme is to lower the energy barrier that the substrate must overcome to be converted into a product. How exactly such a reduction is ensured has not been fully established.
Enzymatic reactions and energy. The release of energy from nutrient metabolism, such as the oxidation of the six-carbon sugar glucose to form carbon dioxide and water, occurs through a series of concerted enzymatic reactions. In animal cells, 10 different enzymes are involved in the conversion of glucose into pyruvic acid (pyruvate) or lactic acid (lactate). This process is called glycolysis. The first reaction, phosphorylation of glucose, requires the participation of ATP. The conversion of each molecule of glucose into two molecules of pyruvic acid requires two molecules of ATP, but at the intermediate stages 4 molecules of ATP are formed from adenosine diphosphate (ADP), so the whole process produces 2 molecules of ATP. Next, pyruvic acid is oxidized to carbon dioxide and water with the participation of enzymes associated with mitochondria. These transformations form a cycle called the tricarboxylic acid cycle or the citric acid cycle.
See also METABOLISM. The oxidation of one substance is always associated with the reduction of another: the first gives up a hydrogen atom, and the second adds it. These processes are catalyzed by dehydrogenases, which ensure the transfer of hydrogen atoms from substrates to coenzymes. In the tricarboxylic acid cycle, some specific dehydrogenases oxidize substrates to form a reduced form of the coenzyme (nicotinamide dinucleotide, designated NAD), while others oxidize the reduced coenzyme (NADCH), reducing other respiratory enzymes, including cytochromes (iron-containing hemoproteins), in which the iron atom alternates between oxidized, then reduced. Ultimately, the reduced form of cytochrome oxidase, one of the key iron-containing enzymes, is oxidized by oxygen entering our body with inhaled air. When sugar burns (oxidation by atmospheric oxygen), its carbon atoms directly interact with oxygen, forming carbon dioxide. Unlike combustion, when sugar is oxidized in the body, oxygen oxidizes the cytochrome oxidase iron itself, but its oxidative potential is ultimately used to completely oxidize the sugars in a multi-step process mediated by enzymes. At certain stages of oxidation, the energy contained in nutrients is released mainly in small portions and can be stored in the phosphate bonds of ATP. Remarkable enzymes take part in this, which couple oxidative reactions (providing energy) with reactions of ATP formation (storing energy). This conjugation process is known as oxidative phosphorylation. Without coupled enzymatic reactions, life in the forms we know would not be possible. Enzymes also perform many other functions. They catalyze a variety of synthesis reactions, including the formation of tissue proteins, fats and carbohydrates. Entire enzyme systems are used to synthesize the vast array of chemical compounds found in complex organisms. This requires energy, and in all cases its source is phosphorylated compounds such as ATP.

Enzymes and digestion. Enzymes are necessary participants in the digestion process. Only low molecular weight compounds can pass through the intestinal wall and enter the bloodstream, so food components must first be broken down into small molecules. This occurs during the enzymatic hydrolysis (breakdown) of proteins into amino acids, starch into sugars, fats into fatty acids and glycerol. Protein hydrolysis is catalyzed by the enzyme pepsin, found in the stomach. A number of highly effective digestive enzymes are secreted into the intestine by the pancreas. These are trypsin and chymotrypsin, which hydrolyze proteins; lipase, which breaks down fats; amylase, which catalyzes the breakdown of starch. Pepsin, trypsin and chymotrypsin are secreted in an inactive form, in the form of the so-called. zymogens (proenzymes), and become active only in the stomach and intestines. This explains why these enzymes do not destroy pancreatic and stomach cells. The walls of the stomach and intestines are protected from digestive enzymes and a layer of mucus. Several important digestive enzymes are secreted by cells of the small intestine. Most of the energy stored in plant foods, such as grass or hay, is concentrated in cellulose, which is broken down by the enzyme cellulase. This enzyme is not synthesized in the body of herbivores, and ruminants, such as cattle and sheep, can eat food containing cellulose only because cellulase is produced by microorganisms that populate the first section of the stomach - the rumen. Termites also use microorganisms to digest food. Enzymes are used in the food, pharmaceutical, chemical and textile industries. An example is a plant enzyme obtained from papaya and used to tenderize meat. Enzymes are also added to washing powders.
Enzymes in medicine and agriculture. Awareness of the key role of enzymes in all cellular processes has led to their widespread use in medicine and agriculture. The normal functioning of any plant and animal organism depends on the efficient functioning of enzymes. The action of many toxic substances (poisons) is based on their ability to inhibit enzymes; A number of medications have the same effect. Often the effect of a drug or toxic substance can be traced by its selective effect on the functioning of a certain enzyme in the body as a whole or in a particular tissue. For example, powerful organophosphorus insecticides and nerve gases developed for military purposes have their destructive effect by blocking the work of enzymes - primarily cholinesterase, which plays an important role in the transmission of nerve impulses. To better understand the mechanism of action of drugs on enzyme systems, it is useful to consider how some enzyme inhibitors work. Many inhibitors bind to the active site of the enzyme - the same site with which the substrate interacts. In such inhibitors, the most important structural features are close to the structural features of the substrate, and if both the substrate and the inhibitor are present in the reaction medium, there is competition between them for binding to the enzyme; Moreover, the higher the concentration of the substrate, the more successfully it competes with the inhibitor. Inhibitors of another type induce conformational changes in the enzyme molecule, which involve functionally important chemical groups. Studying the mechanism of action of inhibitors helps chemists create new drugs.