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 cage?
Enzymes. Definition
The term "enzyme" comes from the Latin fermentum - leaven. They may also be called enzymes, from the Greek en zyme, "in yeast".
Enzymes are biologically active substances, so any reaction that takes place in a cell cannot do without their participation. These substances act as catalysts. Accordingly, any enzyme has two main properties:
1) The enzyme speeds up 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 by 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.
Diversity of enzymes allows you to diversify the regulation of cell metabolism. In any cascade of reactions, many enzymes of various classes take part. 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 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 an enzyme is to speed up the corresponding reaction. Any cascade of processes, from the decomposition of hydrogen peroxide to glycolysis, requires the presence of a biological catalyst.
Proper functioning of enzymes is achieved by high specificity for a particular substrate. This means that a catalyst can only speed up a certain reaction and no other, even a very similar one. 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 m altase breaks down m altose.
2) Enzymes with relative specificity. This includes substances that can catalyze a certain class of reactions, such as hydrolytic cleavage.
The work of a biocatalyst begins from the moment its active center is attached to the substrate. In this case, one speaks 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 step is the reaction itself. Its speed increases due to the action of the enzymatic complex. In the end, we get an enzyme that is associated with the products of the reaction.
The final stage is the detachment 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.
Classification of enzymes
In the human body, you can find a huge amount 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 the 6 main classes of enzymes, as well as examples of some of the subgroups.
Oxidoreductases
Enzymes of this class catalyze redox reactions. There are 17 subgroups in total. Oxidoreductases usually have a non-protein part, represented by a vitamin or heme.
The following subgroups are often found among oxidoreductases:
a) Dehydrogenases. The biochemistry of dehydrogenase enzymes consists in the elimination of hydrogen atoms and their transfer to another substrate. This subgroup is most often found in respiratory reactions,photosynthesis. The composition of dehydrogenases necessarily contains a coenzyme in the form of NAD / NADP or flavoproteins FAD / FMN. Often there are metal ions. Examples include enzymes such as cytochrome reductases, pyruvate dehydrogenase, isocitrate dehydrogenase, and 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 (H20, H20 2). Examples of enzymes: cytochrome oxidase, tyrosinase.
c) Peroxidases and catalase are enzymes that catalyze the breakdown of H2O2 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 the enzymes of this group is to transfer radicals from the donor substance to the recipient substance.
a) Methyltransferase. DNA methyltransferases are the main enzymes that control the process of DNA replication. Nucleotide methylation plays an important role in the regulation of nucleic acid function.
b) Acyltransferases. Enzymes of this subgroup transport the acyl group from one molecule to another. Examples of acyltransferases: lecithincholesterol acyltransferase (transfers a functional group from a fatty acid to cholesterol), lysophosphatidylcholine acyltransferase (acyl group is transferred 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 are enzymes such as hexokinases and aspartate kinases, which add 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 enzymes of digestion.
a) Esterases - break ethereal 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, m altase.
c) Peptidases are enzymes that catalyze the breakdown of proteins into amino acids. Peptidases include enzymes such as pepsins, trypsin, chymotrypsin, carboxypeptidase.
d) Amidases - split amide bonds. Examples: arginase, urease, glutaminase, etc. Many amidase enzymes occur in the ornithine cycle.
4. Lyases are enzymes similar in function to hydrolases, however, water is not consumed during the cleavage of bonds in molecules. Enzymes of this class always contain a non-protein part, for example, in the form of vitamins B1 or B6.
a) Decarboxylases. These enzymes act on the C-C bond. Examples areserve as glutamate decarboxylase or pyruvate decarboxylase.
b) Hydratases and dehydratases are enzymes that catalyze the reaction of splitting C-O bonds.
c) Amidine-lyases - destroy C-N bonds. Example: arginine succinate lyase.
d) P-O lyase. Such enzymes, as a rule, cleave off the phosphate group from the 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. An 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 is attached to the substrate. Depending on the conformation of the protein molecule, the catalytic center can take a variety of forms, which must fit the substrate in the same way as a lock to a key. Such a complex structure explains why 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 that the adsorption center forms are very weak, which means that the catalytic reaction is reversible at this stage.
3) Allosteric centers can be located asin the active site, 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, binding to the enzyme molecule, speed up its work. Inhibitors, on the contrary, inhibit catalytic activity, and this can occur in two ways: either the molecule binds to the allosteric site in the region of the active site of the enzyme (competitive inhibition), or it attaches to another region of the protein (noncompetitive inhibition). Competitive inhibition is considered more effective. After all, this closes the place for binding the substrate to the enzyme, and this process is possible only in the case of 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, the apoenzyme is isolated - the protein part, the coenzyme - the organic part, and the cofactor - the inorganic part. The coenzyme can be represented by carbohydrates, fats, nucleic acids, vitamins. In turn, the cofactor is most often auxiliary metal ions. The activity of enzymes is determined by its structure: additional substances that make up the composition change the catalytic properties. Diverse types of enzymes are the result of a combination of all of the above complex formation factors.
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.
T. Since enzymes are of a protein nature, they are easily destroyed at high temperatures. The denaturation process is reversible, but it can significantly affect the functioning 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). There are also enzymes that work only in an acidic environment or only in an alkaline one. So, in cellular lysosomes, a low pH is maintained, 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. Such 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 of 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, according to the systematic nomenclature, not one, but two substrates are used in the name.
Examples of the names of some enzymes:
- Liver enzymes: lactate-dehydrogenase, glutamate dehydrogenase.
- Full systematic name of the enzyme: lactate-NAD+-oxidoreduct-ase.
There are also trivial names that do not adhere to the rules of nomenclature. Examples are digestive enzymes: 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. Messenger RNA codes for 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 a 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, 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. On the same ER, 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, however, to activate the catalytic activity, it is often necessaryaddition of 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 function without the presence of a coenzyme.
Cofactor plays a crucial 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, the presence of a quaternary structure is very important for them, in which the connecting link between several protein globules is a metal ion.
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 at once, catalyzing the same reaction, but operating under different conditions. There are two types of multiple forms of enzymes:
- Isoenzymes. Such proteins are encoded by different genes, consist of different amino acids, but catalyze the same reaction.
- True plural forms. These proteins are transcribed from the same gene, but peptides are modified on the ribosomes. The output is several forms of the same enzyme.
BAs 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.
Importance 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 widely available that can temporarily make up for their deficiency.
Different enzymes in the cell catalyze a wide variety of life-sustaining reactions. 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, removing damaged DNA and RNA.
Do not forget about such a phenomenon as blood clotting. Being an effective measure of protection, this process is under the control of a number of enzymes. The main one is thrombin, which converts the inactive protein fibrinogen 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, obtaining many fermented milk products. Yeast can be used to produce alcohol from glucose, but an extract from it is sufficient for the successful flow of this process.
Interesting facts you didn't know
- All enzymes of the body have a huge mass - from 5000 to1000000 Yes. 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 are not yet fully understood.
- Enzyme activity is used to produce effective laundry detergents. 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 a similar washing powder at a temperature not 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.
- The properties of enzymes have been known for a very long time, but only in 1897 people realized that not the yeast itself, but the extract from their cells can be used to ferment sugar into alcohol.
