Enzymes
Metabolism in the body can be defined as the totality of all chemical transformations undergone by compounds coming from outside. These transformations include all known types of chemical reactions: intermolecular transfer of functional groups, hydrolytic and non-hydrolytic cleavage of chemical bonds, intramolecular rearrangement, new formation of chemical bonds, and redox reactions. Such reactions proceed in the body at an extremely high rate only in the presence of catalysts. All biological catalysts are substances of a protein nature and are called enzymes (hereinafter F) or enzymes (E).
Enzymes are not components of reactions, but only accelerate the achievement of equilibrium by increasing the rate of both direct and reverse transformations. The acceleration of the reaction occurs due to a decrease in the activation energy - the energy barrier that separates one state of the system (the initial chemical compound) from another (the reaction product).
Enzymes speed up a wide variety of reactions in the body. So, quite simple from the point of view of traditional chemistry, the reaction of splitting off water from carbonic acid with the formation of CO 2 requires the participation of an enzyme, because without it, it proceeds too slowly to regulate the pH of the blood. Thanks to the catalytic action of enzymes in the body, it becomes possible to carry out such reactions that would go hundreds and thousands of times slower without a catalyst.
Enzyme Properties
1. Influence on the rate of a chemical reaction: enzymes increase the rate of a chemical reaction, but they themselves are not consumed.
The reaction rate is the change in the concentration of the reaction components per unit time. If it goes in the forward direction, then it is proportional to the concentration of the reactants; if it goes in the opposite direction, then it is proportional to the concentration of the reaction products. The ratio of the rates of forward and reverse reactions is called the equilibrium constant. Enzymes cannot change the values of the equilibrium constant, but the state of equilibrium in the presence of enzymes comes faster.
2. The specificity of the action of enzymes. In the cells of the body, 2-3 thousand reactions take place, each of which is catalyzed by a certain enzyme. The specificity of the action of an enzyme is the ability to accelerate the course of one particular reaction without affecting the rate of others, even very similar ones.
Distinguish:
Absolute- when F catalyzes only one specific reaction (arginase - splitting of arginine)
Relative(group special) - F catalyzes a certain class of reactions (eg hydrolytic cleavage) or reactions involving a certain class of substances.
The specificity of enzymes is due to their unique amino acid sequence, which determines the conformation of the active center interacting with the reaction components.
A substance whose chemical transformation is catalyzed by an enzyme is called substrate (S) .
3. The activity of enzymes is the ability to accelerate the reaction rate to varying degrees. Activity is expressed in:
1) International units of activity - (IU) the amount of the enzyme catalyzing the conversion of 1 μM of the substrate in 1 min.
2) Katalakh (cat) - the amount of catalyst (enzyme) capable of converting 1 mol of substrate in 1 s.
3) Specific activity - the number of units of activity (any of the above) in the test sample to the total mass of protein in this sample.
4) Less often, molar activity is used - the number of substrate molecules converted by one enzyme molecule per minute.
activity depends on temperature . This or that enzyme shows the greatest activity at an optimum temperature. For F of a living organism, this value is in the range of +37.0 - +39.0 °C, depending on the type of animal. With a decrease in temperature, Brownian motion slows down, the diffusion rate decreases and, consequently, the process of complex formation between the enzyme and the reaction components (substrates) slows down. If the temperature rises above +40 - +50 °C, the enzyme molecule, which is a protein, undergoes a denaturation process. At the same time, the rate of the chemical reaction drops noticeably (Fig. 4.3.1.).
Enzyme activity also depends on medium pH . For most of them, there is a certain optimal pH value at which their activity is maximum. Since the cell contains hundreds of enzymes and each of them has its own opt pH limits, the change in pH is one of the important factors in the regulation of enzymatic activity. So, as a result of one chemical reaction with the participation of a certain enzyme, the pH opt of which lies in the range of 7.0 - 7.2, a product is formed, which is an acid. In this case, the pH value shifts to the region of 5.5 - 6.0. The activity of the enzyme sharply decreases, the rate of product formation slows down, but another enzyme is activated, for which these pH values are optimal, and the product of the first reaction undergoes further chemical transformation. (Another example about pepsin and trypsin).
1. change the free energy of the reaction
2. Inhibit back reaction
3. change the equilibrium constant of the reaction
4. direct the reaction along a bypass with lower activation energies of intermediate reactions
102. A change in the conformation of an enzyme molecule can occur:
2. only when the pH changes
103. A change in the degree of ionization of the functional groups of the enzyme occurs when:
1. only when the temperature changes
2. only when the pH changes
3. only when both conditions change
4. does not occur with any changes
104. Hydrolysis of peptide bonds occurs when:
1. only when the temperature changes
2. only when the pH changes
3. when both conditions change
4. does not occur under any temperature and pH changes
105. Violation of weak bonds in an enzyme molecule occurs when:
2. only when the pH changes
3. when both conditions change
4. does not occur with any changes
106 Pepsin exhibits optimal activity at a pH value of:
1. 1,5-2,5
107. Optimum pH for the work of most enzymes is:
1. pH< 4,0
3. 6,0 < pH < 8,0
108. Choose the correct statements from the following:
1. all enzymes show maximum activity at pH=7
2. most enzymes show maximum activity at a pH close to neutral
3. pepsin shows maximum activity at pH = 1.5-2.5
109. Using the Michaelis-Menten equation, you can calculate:
4. change in free energy during a chemical reaction
V = V max x [S] / K m + [S]
1. activation energy of a chemical reaction
2. the rate of the enzyme-catalyzed reaction
3. energy barrier of a chemical reaction
111. Choose the correct answers: The Michaelis constant (K m) is:
2. May have different meanings for isoenzymes
3. The value at which all enzyme molecules are in the form of ES
4. The greater its value, the greater the affinity of the enzyme for the substrate
112. Choose the correct answers: The Michaelis constant (K m) is:
1. Parameter of the kinetics of the enzymatic reaction
2. The value at which all enzyme molecules are in the form of ES
3. The greater its value, the greater the affinity of the enzyme for the substrate
4. Substrate concentration at which half of the maximum reaction of the reaction rate is reached (V max)
113. Name the features of the structure and functioning of allosteric enzymes:
3. when interacting with ligands, a cooperative change in the conformation of subunits is observed
4. when interacting with ligands, a cooperative change in the conformation of subunits is observed
114. Name the features of the structure and functioning of allosteric enzymes:
1. as a rule, are oligomeric proteins
2. as a rule, are not oligomeric proteins
3. exhibit regulatory properties during the dissociation of the molecule into protomers
4. when interacting with ligands, a cooperative change in the conformation of subunits is observed
Enzymes. Kinetics of enzymatic reactions
Biochemical reactions proceed only with the participation of enzymes, i.e., catalysts that are proteins in their composition and structure. Substances exhibiting a catalytic effect are known both from the course of inorganic chemistry and from the course of organic chemistry. Such substances, called catalysts, are found in all classes of substances - simple substances (both metals and non-metals), acids, bases, oxides, salts. Catalysts are especially widely used in organic chemistry, since organic substances are characterized by a relatively low reactivity. Moving on to a new stage of chemistry - biochemistry, we also meet with a new class of catalysts - enzymes. The infinite diversity of the structure of protein molecules turns out to be a prerequisite for the biosynthesis of special proteins suitable as catalysts for all biochemical processes occurring in nature.
Enzymatic catalysis has characteristic features of all catalytic processes, but there are also fundamentally important differences. The general rules include the following:
Enzymes increase the rate of a reaction but do not shift the chemical equilibrium;
Enzymes accelerate those reactions that can spontaneously proceed under given conditions;
A non-spontaneous reaction, coupled with a spontaneous one, also proceeds with the participation of enzymes
The rate of the enzymatic reaction depends on the temperature and the concentrations of the reactants (substrate and enzyme).
The specific features of enzymatic reactions include the following:
Enzymes are distinguished by a higher selectivity for substrates than conventional catalysts. Often an enzyme accelerates only one biochemical reaction or a rather narrow group of related reactions;
Enzymes act stereospecifically, accelerating the synthesis of only one of the possible spatial isomers.
Enzymes are active in a limited temperature range - below the denaturation temperature of a given protein;
The activity of the enzyme depends on the pH of the medium; each enzyme has an optimal pH value at which activity is maximum.
Many enzymes work only when activated by coenzymes - low molecular weight molecules and ions.
Enzymes can be dissolved or embedded in cell membranes.
Enzyme activity may depend on the concentration of the reaction product.
Enzymes are present in cells in extremely low concentrations. Determining them in tissue extracts or liquids is a difficult task. Therefore, special approaches have been developed to determine the catalytic activity of enzymes. The rate of the reaction proceeding under the action of the existing enzyme is measured. The result is expressed in units of enzyme activity. The relative amounts of the enzyme in different extracts are then compared. Activity units are expressed in µmol (10–6), nmol (10–9), or pmol (10–12) of the consumed substrate or product formed per unit of time (minute). International activity units are denoted U, nU and pU.
The main provisions of the theory of rates of chemical reactions are applicable to enzymatic catalysis. For the reaction to proceed, it is necessary for the enzyme molecules (there are designations F, E, Enz) and the substrate (S) to approach (collision) enough to form bonds. In order for the collision to be productive (active), the molecules must have enough energy to overcome the energy barrier. As you know, this barrier is called the activation energy. At certain stages of the enzymatic reaction, the enzyme acts as a common reactant, reacting in a molar ratio of 1:1. Enzymatic processes are often represented by special schemes. For example, the group transfer reaction
A–B+D A–D+B
with the participation of the enzyme is depicted as follows:
A–B Enz A–D
As another example of writing an enzymatic reaction scheme, let's take the isomerization reaction
S iso-S
With the participation of the enzyme, the reaction is written as follows:
S Enz iso-S
The arrows depict a cyclic process in which substrate S molecules are involved and product molecules are released, often referred to as P.
An enzyme is a complex molecule composed of hundreds of amino acid residues and thousands of atoms. Only a small group of atoms in such a molecule can participate in binding to a substrate. This group is called the active center. E. Fischer proposed the Enz–S interaction model as a correspondence between a key and a lock. Only in the presence of such a correspondence can the transformation of the substrate take place. The selectivity of the enzyme action becomes clear. This model has not lost its significance, but later the induced fit model (Koshland) was proposed, which takes into account the flexibility of the enzyme molecule. When the molecules of the enzyme and the substrate approach each other, conformational changes in the enzyme occur, giving the final configuration to the reaction center. Molecules similar to the substrate can also cause conformational changes in the enzyme, but differences in conformations appear at which a working active center does not arise.
Temperature effect
In a limited temperature range before protein denaturation, the rate of the enzymatic reaction increases, obeying the usual law expressed by the Arrhenius equation. Many enzymatic reactions are characterized by a temperature coefficient of rate Q 10 close to two. This corresponds to the activation energy E a = 55 kJ/mol at 37.
When approaching the temperature of protein denaturation, the increase in the rate slows down, then the maximum rate is reached, and then a sharp drop in the rate begins, since the enzyme molecules capable of catalysis disappear. The temperature dependence of the catalytic reaction rate is shown in Figure 1.
pH dependence
When the pH changes, the proton transfer equilibria shift, and, accordingly, the charges on the enzyme molecules, and often also on the substrate molecules. At low pH values, the enzyme is protonated and acquires a positive charge. At high levels, it deprotonates and acquires a negative charge. This affects the rate of enzymatic reactions. If only one of the forms of the enzyme molecule with a certain charge value exhibits activity, then its concentration passes through a maximum at a certain value of pH M, and the activity will manifest itself within pH M 1. The dependence of activity on pH is obtained, shown in Fig. 2.
For each enzyme, there is an optimal pH value, at which the greatest activity is manifested. With large deviations of pH from the optimal value, denaturation of the enzyme may occur.
Concentration dependence
In mathematical form, the dependence of velocity on concentration is represented as a kinetic equation. The rate of the enzymatic reaction depends both on the concentration of the substrate and on the concentration of the enzyme, all other things being equal (T, pH). It must be taken into account that the enzyme is a macromolecular substance, and its concentration is many times lower than the concentration of the substrate. Let the solution contain a substrate with M r = 100 and enzyme c M r = 100000. Mass concentrations of both reactants 1 mg/l. Their molar concentrations will be:
с(S) = 110 –5 mol/l, с(E) = 110 –8 mol/l
There is one enzyme molecule per 1000 substrate molecules. The actual ratio could be much higher. This determines the form of the kinetic equations in enzymatic kinetics.
A typical feature of the kinetics of enzymatic reactions turned out to be that the rate is proportional to the concentration of the substrate at its low concentration, and becomes independent of the concentration at a high concentration. These experimental results are graphically depicted by the curved line in Fig. 3.
To explain this dependence, a two-stage reaction scheme was proposed. At the beginning, by a reversible reaction, the enzyme-substrate complex S … E, in which the transformation of the substrate molecule takes place. At the second stage, the bond between the changed substrate molecule and the enzyme is broken, and a free product molecule P appears. Each transformation is characterized by its own rate constant.
k 1 k 2
S + E S .... E E + P
For a process with such a mechanism, L. Michaelis and Menten derived an equation for the dependence of the rate on the concentration S, which was called the Michaelis-Menten equation.
Let us write the kinetic equations for the formation of the final product and the enzyme-substrate complex:
v
=
= k 2
c(SE) (1)
= k 1
c(S) c(E)
k
1
c(SE)
k 2
c(SE) (2)
The total (initial) concentration of the enzyme is always much less than the concentration of the substrate, as noted above. During the reaction, the concentration of free enzyme c(E) decreases due to complex formation
c(E) = c o(E) c(SE) (3)
In the stationary state, the concentration of the complex remains constant:
= 0
From this condition we get
k 1 c(S) c(E) k 1 c(SE) k 2 c(SE) = 0 (4)
We substitute the expression (3) in (4)
k 1 c(S)[ c o(E) c(SE)] k 1 c(SE) k 2 c(SE) = 0 (5)
In equation (5), open the square brackets and transform it to find the concentration of the enzyme-substrate complex SE:
Dividing the numerator and denominator by k 1 , we get
(6)
An expression consisting of constants in the denominator of the equation is called Michaelis constantK M :
(7)
We substitute the resulting expression in eq. 1:
(8)
Received Lv. 8 is one of the forms of writing the Michaelis-Menten equation. Let's analyze this equation. In many enzymatic reactions, the second step constant k 2 is significantly less than the formation constants k 1 and decay k–1 enzyme-substrate complex. In such cases, the Michaelis constant is approximately equal to the equilibrium constant of the decomposition of the complex into the original molecules:
At a high substrate concentration, when c(S) K M , constant K M can be neglected, and then c(S) in ur. 8 is shrinking; while the speed takes the maximum value:
v max = k 2 c o(E)(9)
The maximum rate depends on the concentration of the enzyme and does not depend on the concentration of the substrate. This means that the reaction proceeds in zero order with respect to the substrate.
At low substrate concentrations, when c(S) K M, the reaction proceeds in first order with respect to the substrate:
v
=
Thus, as the substrate concentration increases, the reaction order changes from the first (region I in Fig. 4) to zero (region III).
1/2v max
The Michaelis-Menten equation can be written using the maximum velocity:
(10)
This form of the equation is convenient for presenting experimental results when the enzyme concentration is not known.
If the reaction rate is half the maximum rate, then from Eq. 10 it follows that the Michaelis constant is equal to the corresponding substrate concentration (Fig. 4):
, where K M= c"(S)
For a more accurate determination of the Michaelis constant by a graphical method, a transformation of eq. 10 through the reciprocals of the variables. Swap the numerator and denominator in eq. 10:
or 
Graphical representation of the Michaelis-Menten equation in reciprocal coordinates 1/ v – 1/c(S) is called the Lineweaver-Burk plot (Fig. 5). This is a graph of a straight line that cuts off on the 1/ v segment equal to the reciprocal of the maximum speed. The continuation of a straight line into the negative region until it intersects with the horizontal axis gives a segment whose absolute value is 1/ K M. Thus, from the graph, the inverse values of the parameters 1/ v max and 1/ K M , and then the parameters themselves.
There are enzymes, the action of which is not strictly subject to ur. Michaelis-Menten. At a high concentration of the substrate, the maximum speed is reached, but at a low concentration, the dependence plot v- S takes the so-called sigmoid form. This means that at first the speed increases with acceleration (the convexity of the curve is directed downwards, see Fig. 6), and then after the inflection point the speed increases with deceleration and approaches the maximum speed. This is explained by the cooperative effect of the substrate in the presence of several binding sites in the enzyme. The binding of one S molecule facilitates the binding of the second molecule on the other site.
Enzymes are highly specialized protein catalysts that speed up chemical reactions in animals and plants. Almost all chemical transformations in living matter are carried out with the help of enzymes. We can say that enzymes are the driving force of biochemical processes in living organisms.
Depending on the nature and purpose, enzymes can be released into the environment or retained inside the cell. They do not lose their catalytic ability even after excretion from the body (but outside the cells, enzymes only break down substances). This is the basis for their use in the food, light and medical industries, agriculture and other sectors of the national economy.
Academician I.P. Pavlov wrote: “Enzymes are, so to speak, the first act of vital activity. All chemical processes in the body are directed precisely by these substances; they are the causative agents of all chemical transformations. All these substances play an enormous role, they determine the processes due to which life manifests itself, they are in the full sense the activators of life.
All enzymatic reactions proceed easily and quickly. Enzymatic reactions catalyzed in the body are not accompanied by the formation of by-products, while organic reactions carried out with the help of artificial catalysts always form at least one or more of these products.
In living organisms, enzymes are in an ordered state. In individual structural formations of the cell, enzymatic reactions proceed in a strictly defined order. Being precisely coordinated with each other, individual cycles of reactions ensure the vital activity of cells, organs, tissues and the organism as a whole. Strictly defined biochemical processes are carried out in certain parts of the cell.
Along with the fact that enzymes play a decisive role in living organisms, they have a prominent place in the production of food products and many other industries, as well as in agriculture. Production ethyl alcohol, beer, wine, tea, bread, fermented milk and many other products based on the action of enzymes . Enzymes are involved in maturation and overripe fruits and vegetables, maturation and deterioration meat and fish, persistence grains, flour, cereals and other products .
In some cases, the presence of enzymes in the processing of products is undesirable. An example of this is the reaction of enzymatic browning of fruits and vegetables as a result of the action of the polyphenol oxidase enzyme or the rancidity of flour fats as a result of the action of the lipase and lipoxidase enzymes present in the grain germ.
At present, about 3500 enzymes have been isolated from biological objects and several hundred enzymes have been studied. It is believed that a living cell can contain more than 1000 different enzymes. Each enzyme typically catalyzes only one type of chemical reaction. Since an enzyme can accelerate only one reaction, or rarely a group of reactions of one type, without affecting others, many different reactions can occur simultaneously in living organisms. Although the reactions of individual enzymes proceed independently of each other, nevertheless, most often they are interconnected by a complex sequence of formation of intermediate products. In this case, the product of one reaction can serve as a substrate or reagent for another. Therefore, in the same cell, hundreds and thousands of enzymatic reactions occur simultaneously, proceeding in a certain sequence and in such quantities that ensure the normal state of the cell.
Every living organism continuously synthesizes enzymes. In the process of body growth, the number of necessary enzymes also increases. A disproportionate increase or decrease in the number of enzymes could lead to a violation of the nature of metabolism that has developed in the body.
In a living cell, enzymes can be synthesized in various structural formations - the nucleus, cytoplasm, chloroplasts, mitochondria, cytoplasmic membrane, etc.
as biological catalysts enzymes, being in small quantities, capable of transforming huge amounts of the substrate on which they act. Thus, the saliva enzyme amylase exhibits noticeable catalytic activity at a dilution of 1: 1,000,000, and the peroxidase enzyme is active at a dilution of 1: 5,000,000. One catalase molecule breaks down 5 million molecules of hydrogen peroxide in one minute.
The catalytic activity of enzymes is many times greater than the activity of inorganic catalysts.. Thus, protein hydrolysis to amino acids in the presence of inorganic catalysts at a temperature of 100 °C and above is carried out in several tens of hours. The same hydrolysis with the participation of specific enzymes ends in less than an hour and proceeds at a temperature of 30-40 °C. Complete hydrolysis of starch with acid occurs in a few hours, while enzymatic hydrolysis at room temperature takes several minutes. Iron ions are known to catalytically accelerate the splitting of hydrogen peroxide into hydrogen and oxygen. But the iron atoms that make up the catalase enzyme act on hydrogen peroxide 10 billion times more energetically than ordinary iron: 1 mg of iron in the enzyme is able to replace 10 tons of inorganic iron during the catalytic breakdown of hydrogen peroxide.
An important characteristic feature enzymes is the specificity of their action . The specificity of enzymes is much higher than that of inorganic catalysts. Sometimes slight changes in the chemical structure of a substance exclude the manifestation of the action of a specific enzyme on this substance. The specificity of the action of enzymes is also manifested in cases where the substance differs in chemical structure. Thus, enzymes that accelerate the hydrolysis of proteins do not have any effect on the hydrolysis of starch, and vice versa.
Enzymes differ in specificity. Some enzymes catalyze only a single reaction, while others catalyze a large number of reactions. Thus, the glycooxidase enzyme catalyzes the oxidation of glucose, and trypsin hydrolyzes specific peptide bonds in proteins and amino acid ethers.
The specificity of the action of enzymes sometimes leads to the fact that an organic compound is affected by not one, but two enzymes.
group specificity represent all enzymes, that is, they catalyze only a special type of reaction, such as the oxidation of monosaccharides or the hydrolysis of oligosaccharides.
Enzymes, being specific catalysts, speed up both the forward and reverse reactions , i.e., hydrolysis and synthesis of the substance on which they act. The direction of this process depends on the concentration of the initial and final products and the conditions under which the reaction proceeds. At the same time, it has been proven that most of the syntheses in a living cell occur under the action of enzymes other than those that catalyze the cleavage of one or another compound.
Chemical nature of enzymes
Enzymes are divided into two major classes - one-component, composed only of protein, and two-component, composed of a protein and a non-protein part called prosthetic group. Enzyme proteins can be simple (proteins) or complex (proteins). Active prosthetic group (active site) of an enzyme is called agony , A protein carrier – feron . The prosthetic group in the composition of the enzyme occupies up to about 1% of its mass.
The strength of the bond between the prosthetic group (agon) and feron is not the same for different enzymes. With a weak bond, the enzyme dissociates into a protein and prosthetic part, which is called coenzyme . Each of the resulting groups exhibits catalytic activity. The role of coenzymes is played by most vitamins - C, B 1, B 2, B 6, B 12, H, E, K, etc., as well as nucleotides, RNA, sulfhydryl groups, glutathione, iron, copper, magnesium atoms, etc. Many enzymes have a high catalytic ability only if the enzyme does not decompose into feron and agon.
TO one-component include many enzymes that break down proteins or carbohydrates (pepsin, trypsin, papin, amylase).
Typical two-component the enzyme is α-carboxylase, which catalyzes the breakdown of pyruvic acid into carbon dioxide and acetaldehyde:
α-carboxylase
CH3COCOOH ----→CH3CHO + CO 2 .
The chemical nature of α-carboxylase is fully established; the active group of this enzyme contains vitamin B 1 .
Often, coenzymes act as intermediates in the enzymatic reactions involved in hydrogen transfer. These include nicotinamide adenine dinucleotide (NAD), glutathione, L-ascorbic acid, quinones, and cytochromes. Other coenzymes act as carriers or transmitters for phosphate, amine, and methyl groups.
Molecular weight enzymes varies widely. from a few thousand to a million , but most enzymes have high molecular weight .
Many enzymes contain metals that take part in the catalytic action. So, iron is part of the prosthetic group of catalase and peroxidase enzymes, as well as cytochrome oxidase, which is involved in the processes of respiration. Copper is part of the oxidative enzymes polyphenol oxidase and ascorbate oxidase, which play an important role in plant metabolism.
In their pure form, all enzymes are crystals..
Catalytic reactions are carried out on the surface of enzyme molecules. The enzyme-protein forms a dispersed phase, on the surface of which reactions occur between substances dissolved in the dispersion medium. The surface of the enzyme-protein molecule is heterogeneous; on the surface of the molecule there are various chemically active groups that easily bind other compounds.
The properties of enzymes are due Firstly the presence of especially active centers on the surface of the protein molecule - radicals of amino acids or special chemical groups firmly attached to the protein. The active center is that part of the enzyme that combines with the substance (substrate) in the process of catalytic action. .
Enzymes, Enzyme-Substrate Complex and Activation Energy
The most important function of proteins is catalytic, it is performed by a certain class of proteins - enzymes. More than 2000 enzymes have been identified in the body. Enzymes are biological catalysts of a protein nature that significantly speed up biochemical reactions. Thus, the enzymatic reaction occurs 100-1000 times faster than without enzymes. They differ in many properties from the catalysts used in chemistry. Enzymes speed up reactions under normal conditions, unlike chemical catalysts.
In humans and animals, a complex sequence of reactions takes place in a few seconds, which requires a long time (days, weeks or even months) with the use of conventional chemical catalysts. Unlike reactions without enzymes, by-products are not formed in enzymatic reactions (the yield of the final product is almost 100%). In the process of transformation, enzymes are not destroyed, therefore a small amount of them is able to catalyze the chemical reactions of a large number of substances. All enzymes are proteins and have their characteristic properties (sensitivity to changes in the pH of the medium, denaturation at high temperatures, etc.).
Enzymes are chemically classified into one-component (simple) And two-component (complex) .
One-component (simple)
Single-component enzymes consist only of proteins. The simple ones mainly include enzymes that carry out hydrolysis reactions (pepsin, trypsin, amylase, papain, etc.).
Two-component (complex)
Unlike simple enzymes, complex enzymes contain a non-protein part - a low molecular weight component. The protein part is called apoenzyme (enzyme carrier), non-protein - coenzyme (active or prosthetic group). The non-protein part of enzymes can be represented either by organic substances (for example, derivatives of vitamins, NAD, NADP, uridine, cytidyl nucleotides, flavins), or inorganic (for example, metal atoms - iron, magnesium, cobalt, copper, zinc, molybdenum, etc.).
Not all necessary coenzymes can be synthesized by organisms and therefore must be supplied with food. The lack of vitamins in the food of humans and animals causes the loss or decrease in the activity of those enzymes in which they are included. Unlike the protein part, organic and inorganic coenzymes are very resistant to adverse conditions (high or low temperatures, radiation, etc.) and can be separated from the apoenzyme.
Enzymes are characterized by high specificity: they can only convert the appropriate substrates and catalyze only certain reactions of the same type. It determines its protein component, but not its entire molecule, but only its small section - active center . Its structure corresponds to the chemical structure of the substances that react. Enzymes are characterized by a spatial correspondence between the substrate and the active center. They fit together like a key to a lock. There can be several active centers in one enzyme molecule. The active center, that is, the junction with other molecules, is not only in enzymes, but also in some other proteins (heme in the active centers of myoglobin and hemoglobin). Enzymatic reactions proceed in the form of successive stages - from several to tens.
The activity of complex enzymes is manifested only when the protein part is combined with the non-protein part. Also, their activity is manifested only under certain conditions: temperature, pressure, pH of the environment, etc. Enzymes of different organisms are most active at the temperature to which these creatures are adapted.
Enzyme-substrate complex
Substrate-enzyme bonds form enzyme-substrate complex.
At the same time, it changes not only its own conformation, but also the conformation of the substrate. Enzymatic reactions can be inhibited by their own reaction products - with the accumulation of products, the reaction rate decreases. If there are few reaction products, then the enzyme is activated.
Substances that penetrate the region of the active center and block the catalytic groups of enzymes are called inhibitors (from lat. inhibere- restrain, stop). The activity of enzymes is reduced by heavy metal ions (lead, mercury, etc.).
Enzymes reduce the activation energy, that is, the level of energy required to make molecules reactive.
Activation energy
Activation energy - this is the energy that is spent on breaking a certain bond for the chemical interaction of two compounds. Enzymes have a specific location in the cell and the body as a whole. In a cell, enzymes are found in certain parts of it. Many of them are associated with cell membranes or individual organelles: mitochondria, plastids, etc.
Biosynthesis of enzymes organisms are able to regulate. This makes it possible to maintain their relatively constant composition under significant changes in environmental conditions and to partially modify enzymes in response to such changes. The effect of various biologically active substances - hormones, drugs, plant growth stimulants, poisons, etc. - is that they can stimulate or suppress one or another enzymatic process.
Some enzymes are involved in the active transport of substances across membranes.
The suffix for the names of most enzymes is -az-. It is added to the name of the substrate with which the enzyme interacts. For example, hydrolases - catalyze the reactions of splitting complex compounds into monomers due to the addition of a water molecule at the site of a chemical bond break in the molecules of proteins, polysaccharides, fats; oxidoreductase - accelerate redox reactions (transfer of electrons or protons); isomerase- contribute to internal molecular rearrangement (isomerization), transformation of isomers, etc.
