Millions of chemical reactions take place in the cell of any living organism. 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 - sourdough. They can also be called enzymes from the Greek en zyme - "in yeast."
Enzymes are biologically active substances, so any reaction that occurs in the cell is not without their participation. These substances act as catalysts. Accordingly, any enzyme has two main properties:
1) The enzyme accelerates the biochemical reaction, but it is not consumed.
2) The value of the equilibrium constant does not change, but only the achievement of this value is accelerated.
Enzymes accelerate 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.
A variety of enzymes allows you to diversify the regulation of cell metabolism. In any cascade of reactions involved many enzymes of various classes. Biological catalysts are highly selective due to a certain conformation of the molecule. Since enzymes in most cases have 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 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 proper functioning of the enzymes is achieved by high specificity for a particular substrate. This means that the catalyst can accelerate only a certain reaction and no more, even very similar. According to the degree of specificity, the following groups of enzymes are distinguished:
1) Enzymes with absolute specificity when only one 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 the biocatalyst begins from the moment of attachment of its active center to the substrate. At the same time, they talk about a complementary interaction like a lock and a key. This refers to the complete coincidence of the shape of the active center with the substrate, which makes it possible to accelerate the reaction.
The next step is the course of the reaction itself. Its speed increases due to the action of the enzymatic complex. Ultimately, we get an enzyme that is associated with reaction products.
The final stage is the disconnection of the 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 the substrate, E is the enzyme, and P is the product.
Enzyme classification
In the human body you can find a huge number of enzymes. All knowledge about their functions and work was systematized, and as a result, a single classification appeared, thanks to which it is easy to determine what this or that catalyst is intended for. Here are 6 main classes of enzymes, as well as examples of some subgroups.
- Oxidoreductases.
Enzymes of this class catalyze redox reactions. A total of 17 subgroups are distinguished. Oxidoreductases usually have a non-protein part, represented by vitamin or heme.
The following subgroups are often found among oxidoreductases:
a) Dehydrogenases. The biochemistry of dehydrogenase enzymes consists in the removal of hydrogen atoms and their transfer to another substrate. This subgroup is most often found in reactions of respiration, photosynthesis. In the composition of dehydrogenases, coenzyme is necessarily present in the form of NAD / NADP or FAD / FMN flavoproteins. Often there are metal ions. Enzymes such as cytochrome reductases, pyruvate dehydrogenase, isocitrate dehydrogenase, as well as many liver enzymes (lactate dehydrogenase, glutamate dehydrogenase, etc.) can serve as examples.
b) oxidase. A number of enzymes catalyze the addition of oxygen to hydrogen, as a result of which the reaction products may 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.
g) oxygenase. These biocatalysts accelerate the attachment of oxygen to the substrate. Dopamine hydroxylase is one example of such enzymes.
2. Transferase.
The task of the enzymes of this group is to transfer radicals from the donor substance to the recipient substance.
a) Methyl transferases. DNA methyl transferases are the main enzymes that control the process of DNA replication . Nucleotide methylation plays a large role in the regulation of nucleic acid.
b) Acyltransferases. Enzymes of this subgroup transport an acyl group from one molecule to another. Examples of acyltransferases: lecithin cholesterol acyltransferase (transfers the functional group from fatty acid to cholesterol), lysophosphatidylcholinacyltransferase (the acyl group is transferred to lysophosphatidylcholine).
c) Aminotransferases - 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 transferring an amino group.
g) Phosphotransferases. 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, which attach phosphorus residues to hexoses (most often glucose) and to aspartic acid, respectively.
3. Hydrolases are a class of enzymes that catalyze the cleavage of bonds in a molecule followed by the addition of water. Substances that belong to this group are the main digestive enzymes.
a) Esterases - break ether connections. An example is lipases that break down fats.
b) Glycosidase. The biochemistry of enzymes of this series is 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, carboixipeptidase.
d) Amidases - cleave amide bonds. Examples: arginase, urease, glutaminase, etc. Many amidase enzymes are found in the ornithine cycle.
4. Liases are enzymes that are similar in function to hydrolases, however, when the bonds are broken down in the molecules, water is not expended. Enzymes of this class always have a non-protein part, for example, in the form of vitamins B1 or B6.
a) Decarboxylases. These enzymes act on the CC bond. Examples include glutamate decarboxylase or pyruvate decarboxylase.
b) Hydratases and dehydratases are enzymes that catalyze the cleavage reaction of CO bonds.
c) Amidin-lyases - destroy -N bonds. Example: arginine succinate lyase.
d) P-O lyase. Such enzymes typically 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 their individual, only characteristic 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 area of the protein along which the enzyme joins the substrate. Depending on the conformation of the protein molecule, the catalytic center can take a variety of forms, which should correspond to the substrate in the same way as a key lock. Such a complex structure explains that the enzymatic protein is in a tertiary or quaternary state.
2) Adsorption center - acts as a “holder". Here, first of all, there is a connection between the enzyme molecule and the substrate molecule. 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 on the entire surface of the enzyme as a whole. Their function is the regulation of the enzyme. Regulation occurs with inhibitor molecules and activator molecules.
Activating 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 region of the active center of the enzyme (competitive inhibition), or it joins another region of the protein (non-competitive inhibition). Competitive inhibition is considered more effective. Indeed, this closes the place for binding of the substrate to the enzyme, and this process is possible only in the case of almost complete coincidence of the form of the inhibitor molecule and the active center.
The 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 - the organic part, and the cofactor - the inorganic part. Coenzyme can be represented by ulgovody, fats, nucleic acids, vitamins. In turn, a 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 their catalytic properties. Various types of enzymes are the result of combining all of the above factors of complex formation.
Enzyme regulation
Enzymes as biologically active substances are not always necessary for the body. The biochemistry of the enzymes is such that, in the event of excessive catalysis, they can 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 work of substances.
pH also plays a large role in regulation. The highest enzyme activity, as a rule, is observed at neutral pH values (7.0-7.2). There are also enzymes that work only in an acidic environment or only in an alkaline one. So, in cell lysosomes, a low pH is maintained at which the activity of hydrolytic enzymes is maximum. If they accidentally enter the cytoplasm, where the medium is already closer to neutral, their activity will decrease. Such protection against "self-eating" is based on the features 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 certain specific enzymes.
Enzyme Nomenclature
All enzymes of the body are usually called depending on their belonging to any of the classes, as well as on the substrate with which they react. Sometimes, according to the systematic nomenclature , not one, but two substrates are used in the name.
Examples of the name of some enzymes:
- Enzymes of the liver: lactate dehydrogenase, glutamate dehydrogenase.
- The full systematic name of the enzyme is: lactate-NAD + -oxidoreductase.
Preserved and trivial names that do not adhere to the rules of nomenclature. Digestive enzymes are examples: trypsin, chymotrypsin, pepsin.
Enzyme synthesis process
The functions of enzymes are determined at the genetic level. Since a molecule is by and large a protein, its synthesis exactly repeats the processes of transcription and translation.
The synthesis of enzymes occurs according to the following scheme. First, information about the desired enzyme is read from the DNA, as a result of which mRNA is formed. Matrix RNA encodes all amino acids that make up the enzyme. Enzyme regulation can also occur at the DNA level: if the product of the catalyzed reaction is sufficient, gene transcription ceases and vice versa, if a need arose for the product, the transcription process is activated.
After mRNA has entered 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 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. At the same EPS, protein twisting occurs, as a result of which secondary and then tertiary structures are formed. The synthesis of some enzymes stops already at this stage, however, to activate catalytic activity, it is often necessary to attach a coenzyme and cofactor.
In certain areas of the endoplasmic reticulum, the organic components of the enzyme are attached: monosaccharides, nucleic acids, fats, vitamins. Some enzymes cannot work without coenzyme.
The cofactor plays a crucial role in the formation of the quaternary structure of the protein. Some enzyme functions are only available when a protein domain organization is reached. Therefore, it is very important for them to have a quaternary structure 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 any 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 to do in a similar situation to a living organism at low ambient temperatures?
This problem is easily solved by the presence of several enzymes that catalyze the same reaction, but work under different conditions. There are two types of multiple forms of enzymes:
- Isozymes. Such proteins are encoded by different genes, composed of different amino acids, but they catalyze the same reaction.
- True plural forms. These proteins are transcribed from the same gene; however, peptides are modified on ribosomes. 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, while the second - at the post-translational.
The value of enzymes
The use of enzymes in medicine is reduced to the release 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 widespread that can temporarily make up for their lack.
Various enzymes in the cell catalyze a large number of reactions associated with the maintenance of life. One of these enisms is the group of nucleases: endonuclease and exonuclease. Their job is to maintain a constant level of nucleic acids in the cell, removing damaged DNA and RNA.
Do not forget about such a thing as blood coagulation. Being an effective measure of protection, this process is under the control of a number of enzymes. The main one is thrombin, which converts an inactive fibrinogen protein into active fibrin. Its threads create a kind of network that clogs the place of damage to the vessel, thereby preventing excessive blood loss.
Enzymes are used in winemaking, brewing, and the production of many dairy products. To obtain alcohol from glucose, yeast can be used, however, for the successful course of this process, an extract from them is sufficient.
Interesting facts that you did not know about
- All enzymes in the body have a huge mass - from 5,000 to 1,000,000 Yes. This is due to the presence of protein in the composition of the molecule. For comparison: the molecular weight of glucose is 180 Da, and carbon dioxide 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 washing powders. Here, enzymes play the same role as in the body: they destroy organic matter, and this property helps in the fight against stains. It is recommended to use a similar washing powder at a temperature not exceeding 50 degrees, otherwise the denaturation process may go on.
- According to statistics, 20% of people around the world suffer from a lack of any of the enzymes.
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