Biochemistry of enzymes. Structure, properties and functions

In the cell of any living organism, millions of chemical reactions take place. Each of them is of great importance, therefore it is important to maintain the speed of biological processes at a high level. Almost every reaction is catalyzed by its 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 that takes place in the cell does not do without their participation. These substances act as catalysts. Accordingly, any enzyme has two main properties:

1) Enzyme accelerates the biochemical reaction, but it is not consumed.

2) The magnitude of the equilibrium constant does not change, but only the acceleration of this value is accelerated.

Enzymes accelerate biochemical reactions in a thousand, and in some cases a million times. This means that in the absence of an 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.

A variety of enzymes allows a variety of ways to regulate the metabolism of the cell. In any cascade of reactions, many enzymes of different classes participate. Biological catalysts are highly selective due to a certain conformation of the molecule. Because enzymes in most cases are of a protein nature, they are 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 the acceleration of the corresponding reaction. Any cascade of processes, from the decomposition of hydrogen peroxide to glycolysis, requires the presence of a biological catalyst.

Proper work of enzymes is achieved by high specificity to a certain substrate. This means that the catalyst can only accelerate a specific reaction and no more, even very similar. By 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 cleaves collagen, and maltase splits maltose.

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

The work of the biocatalyst begins from the moment of attachment of its active center to the substrate. Thus they speak of a complementary interaction like a lock and a 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 consists in the course of the reaction itself. Its rate increases due to the action of the enzymatic complex. In the end, we get an enzyme, which is associated with the products of the reaction.

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

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 a substrate, E is an enzyme, and P is a product.

Classification of enzymes

In the human body, you can find a huge number of enzymes. All knowledge of their functions and work has been systematized, and as a result, a single classification has emerged, thanks to which it is easy to determine what the catalyst is for. Here are 6 basic classes of enzymes, as well as examples of some subgroups.

1. Oxidoreductase.

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

Among the oxidoreductases, the following subgroups are often found:

A) Dehydrogenases. Biochemistry of enzymes-dehydrogenases consists in splitting off hydrogen atoms and transferring them to another substrate. This subgroup is most often found in reactions of respiration, photosynthesis. In the dehydrogenases necessarily present 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) Oxidase. 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 - 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. Dopaminhydroxylase is one example of such enzymes.

2. Transferase.

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

A) Methyltransferases. DNA methyltransferases are the main enzymes that control the process of DNA replication. Methylation of nucleotides plays an important role in the regulation of the operation of the nucleic acid.

B) Acyltransferases. The enzymes of this subgroup transport the acyl group from one molecule to another. Examples of acyltransferases: lecithin cholesterol acyltransferase (transferring a functional group from fatty acid to cholesterol), lysophosphatidyl cholinacyltransferase (the acyl group is transferred to lysophosphatidylcholine).

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

D) Phosphotransferase. The enzymes of this subgroup catalyze the addition of a phosphate group. Another name for phosphotransferases, kinases, is much more common. Examples are enzymes such as hexokinases and aspartate kinases that attach phosphorus residues to hexoses (most often to glucose) and to 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 basic enzymes of digestion.

A) Esterases - rupture of ethereal bonds. An example is lipases, which break down fats.

B) glycosidases. Biochemistry of enzymes of this series consists in the destruction of glycosidic bonds of polymers (polysaccharides and oligosaccharides). Examples: amylase, sucrose, maltase.

C) Peptidases - enzymes that catalyze the destruction of proteins to amino acids. Peptidases include enzymes such as pepsins, trypsin, chymotrypsin, and carboxypeptidase.

D) Amidases - cleavage amide bonds. Examples: arginase, urease, glutaminase, etc. Many amidase enzymes occur in the ornithine cycle.

4. Lyases - enzymes, similar to hydrolases, but water does not expose when molecules are broken down. Enzymes of this class always have 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 are glutamate decarboxylase or pyruvate decarboxylase.

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

C) Amide-lyase - destroy the C-N bond. Example: arginine succinate lyase.

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

Biochemistry of enzymes is based on their structure

The capabilities of each enzyme are determined by an individual, only its inherent structure. Any enzyme is, first of all, 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 is attached to the substrate. Depending on the conformation of the protein molecule, the catalytic center can take a diverse form, which must correspond to the substrate in the same way as the lock to the key. Such a complex structure explains that the enzymatic protein is in the tertiary or quaternary state.

2) Adsorption Center - serves as a "holder". Here, first of all, there is a connection between the molecule of the enzyme and the molecule-substrate. However, the bonds that form the adsorption center are very weak, and therefore the catalytic reaction at this stage is reversible.

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

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

Enzyme often consists not only of amino acids, but also of other organic and inorganic substances. Accordingly, the apoenzyme is isolated - the protein part, the coenzyme is the organic part, and the cofactor is the inorganic part. Coenzyme can be represented by ulcers, fats, nucleic acids, vitamins. In turn, the cofactor is most often auxiliary metal ions. The activity of enzymes is determined by its structure: the additional substances that make up the composition change the catalytic properties. A variety of types of enzymes is the result of combining all of the above factors of complex formation.

Regulation of enzymes

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

Because enzymes are protein-like, they easily break down at high temperatures. The process of denaturation is reversible, but it can significantly affect the work of substances.

PH also plays a big role in regulation. The highest activity of enzymes, as a rule, is observed at neutral pH values (7.0-7.2). Also there are enzymes that work only in acidic medium or only in alkaline. Thus, low pH is maintained in cell lysosomes, at which the activity of hydrolytic enzymes is maximal. In the event of their accidental entry into the cytoplasm, where the medium is closer to neutral, their activity will decrease. This protection against "self-abuse" is based on the features 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 certain specific enzymes.

Nomenclature of enzymes

All the enzymes of the body are called, depending on their belonging to any of the classes, and also on the substrate with which they react. Sometimes the systematic nomenclature uses not one but two substrates in the name.

Examples of the names of certain enzymes:

1. Enzymes of the liver: lactate-dehydrogenase, glutamate-dehydrogenase.
2. The full systematic name of the enzyme is lactate-NAD + -hydroxydoreductase.

Preserved and trivial names that do not adhere to the rules of the nomenclature. Examples are digestive enzymes: trypsin, chymotrypsin, pepsin.

The process of enzyme synthesis

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

Synthesis of enzymes occurs according to the following scheme. First, information on the desired enzyme is read from DNA, resulting in the formation of mRNA. The matrix RNA encodes all the amino acids that make up the enzyme. Regulation of enzymes can occur at the level of DNA: if the product of the catalyzed reaction is sufficient, the transcription of the gene stops and vice versa, if the need arises for a product, the transcription process becomes active.

After the mRNA is released into the cytoplasm of the cell, the next stage begins - translation. On the ribosomes of the endoplasmic reticulum , a primary chain is synthesized, consisting of amino acids linked by peptide bonds. However, the protein molecule in the primary structure can not yet fulfill its enzymatic functions.

The activity of enzymes depends on the structure of the protein. On the same EPS, the protein is twisted, resulting in the formation of a secondary structure and then a tertiary structure. Synthesis of some enzymes stops already at this stage, however, to activate catalytic activity, it is often necessary to attach coenzyme and cofactor.

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

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

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, the enzyme can work at 20 degrees, but at 0 degrees it can no longer perform its functions. What should a living organism do in a similar situation at low ambient temperatures?

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

1. Isozymes. Such proteins are encoded by different genes, consist of different amino acids, but catalyze the same reaction.
2. True multiple forms. These proteins are transcribed from the same gene, however, on the ribosomes, the peptides are modified. At the output, several forms of the same enzyme are obtained.

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

The importance of enzymes

The use of enzymes in medicine is reduced to the production of new drugs, in which the substances are already in the right quantities. Scientists have not yet found a way to stimulate the synthesis of missing enzymes in the body, but today drugs are widely used that can temporarily fill their deficiency.

Different enzymes in the cell catalyze a large number of reactions associated with maintaining vital activity. One of such ennoms are representatives of the nuclease group: endonuclease and exonuclease. Their work is to maintain a constant level of nucleic acids in the cell, remove damaged DNA and RNA.

Do not forget about the phenomenon of blood clotting. Being an effective measure of protection, this process is controlled by a number of enzymes. The main one is thrombin, which transfers inactive protein fibrinogen to 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, obtaining many fermented milk products. Yeast can be used to produce alcohol from glucose, however, an extract from them is sufficient for the successful course of this process.

Interesting facts about which you did not know

- All the enzymes of 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 the carbon dioxide content is only 44 Da.

- To date, more than 2,000 enzymes have been discovered that have been found in the cells of various organisms. However, most of these substances are not yet fully understood.

- The activity of enzymes is used to obtain effective detergent powders. Here, enzymes perform the same role as in the body: they destroy organic substances, and this property helps in the fight against stains. It is recommended to use a similar detergent at a temperature of no higher than 50 degrees, otherwise the denaturation process may occur.

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

- They knew about the properties of enzymes a very long time, but only in 1897 people realized that for fermenting sugar into alcohol, you can use not the yeast itself, but an extract from their cells.