COFERMENT A, CoA, a coenzyme consisting of the nucleotide adenosine-3",5"-diphosphate and ß-mercaptoethylamide pantothenic acid; participates in the transfer of acyl groups (acidic residues) that bind to the high-energy sulfhydryl group of CoA. acylthioether bond. The formation of acyl derivatives of CoA requires energy and is associated with the breakdown of ATP or oxidative processes (for example, oxidation of keto acids). Participates in more than 60 enzymatic reactions: oxidation and synthesis of fatty acids, synthesis of acetylcholine, lipids, porphyrins and many others. other compounds, oxidation of carbohydrate breakdown products, amino acid metabolism, etc. The most important acyl derivative of CoA, acetyl-CoA, which occupies the center, oxidizes the place at the intersection of pathways. decomposition and synthesis of various substances.
Scheme
Coenzyme A: 1 adenylic acid residue; 2 pyrophosphate group; 3 pantothenic acid residue; 4 β-mercaptoethanolamine residue
- - CoA, a coenzyme consisting of the nucleotide adenosine-3",5"-diphosphate and pantothenic acid p-mercaptoethylamide; participates in the transfer of acyl groups that bind to the high-energy sulfhydryl group of CoA...
- - COFERMENT A, CoA, a coenzyme consisting of the nucleotide adenosine-3",5"-diphosphate and ß-mercaptoethylamide of pantothenic acid...
Biological encyclopedic dictionary
- - COENZYME A, coenzyme A, acylation coenzyme, KoA, KoA-SH, a coenzyme that transfers acyl groups in many enzymatic reactions...
- - coenzyme A, acylation coenzyme, K o A, K o A - S H, coenzyme that transfers acyl groups with plurality. enzymatic reactions. It is a derivative of b...
Veterinary encyclopedic dictionary
- - coenzyme A, coenzyme A, acylation coenzyme, KoA, KoA-SH, coenzyme that transfers acyl groups in many enzymatic reactions...
Veterinary encyclopedic dictionary
- - See ubiquinone...
- - coenzyme A - . A coenzyme involved in more than 60 enzymatic reactions as a carrier of acyl groups, consists of the nucleotide adenosine-3’,5’-phosphate and β-mercaptoethylamide of pantothenic acid...
Molecular biology and genetics. Dictionary
- - a thermostable, relatively low-molecular organic compound necessary for the body as an additional factor in the activity of the enzyme, usually part of the enzyme and forming with its protein part...
Large medical dictionary
- - K., which activates and transfers acid residues in condensation reactions, oxidoreduction and reversible hydration of unsaturated acids: participates in cellular respiration, in the biosynthesis of steroids, acetylcholine,...
Large medical dictionary
- - see Ubiquinone...
Large medical dictionary
- - a nucleotide that contains pantothenic acid, which is the most important coenzyme in the Krebs cycle, as well as in the reactions of fatty acid metabolism...
Medical terms
- - CoA, acetylation coenzyme, the most important of the coenzymes that takes part in the reactions of transfer of acyl groups...
Great Soviet Encyclopedia
- - a complex natural compound, one of the most important coenzymes...
Large encyclopedic dictionary
- - ; pl. coffee farm/nty, R....
Spelling dictionary of the Russian language
- - coenzyme with, together) coenzyme - an organic substance of non-protein nature, more resistant to temperature influences, constituting, together with the protein component - the apoenzyme - an enzyme molecule...
It is convenient to classify coenzymes according to structural-physiological characteristics and functional (catalytic) properties. The structural and physiological classification simultaneously takes into account the origin and chemical structure of coenzymes:
Cofer cops
I. Vitamin noenzymes
1 Thiamine (TMF, TDF, THF)
2 "Flavinaceous (FMN. FAD)
3 Pantothenic (CoA, dephospho CoA. 4-$vogta ntothenate)
4 Nicotnamide (NAD. NADP) 5. Pyridoxine (PALF, PAMF) b. Folic or pteridine (THFA) 7. Cobamide (methyl cobalt a mi
d e n o z i l k o b a l m i h *
5 Biotin (carboxybiotin) () Other (reduced lipoamide)
10. Quinone (ubiquinone, plastoquinone)) I Carnitine (carnitium)
The starting materials for the formation of coenzymes of the first group are vitamins, therefore, insufficient intake of them from food immediately affects the synthesis of these coenzymes, and as a result, the function of the corresponding complex enzymes is disrupted. Coenzymes of the second group are formed from intermediate metabolic products, so there is no shortage of these coenzymes under physiological conditions, and the function of the enzymes with which they are associated is not impaired.
| . d). |
There is also a functional classification of coenzymes, according to which coenzymes are classified, like enzymes, into the corresponding six classes (the numbers in brackets indicate the enzyme class number):
Coenzymes
Coenzymes a
I Pyridoxine (PALP)
2. Pantothenic acids (CoA, dephospho-CoA)
3. Thiamine (TDP)
4. Cobamide (deoxyadenosylcobalamin) coenzymes (5)
1. Pyridoxine (PALF)
2. Cobamide (deoxyadenosylcobalamin)
3. Phosphates of monosaccharides (glucose-1,6-di- Coenzyme transferases (2) phosphate. 2.3-diphosphoglyierate)
1 Pyridoxine (PALF, PAMF) 4 Peptide (glutate)
2 Pantothenic (CoA, dephospho-CoA, 4-phos Coenzymes lmgaa (6) fopantothenate) I Nucleotide (UDP-glucose, CDP-holpn A. Nucleotide coenzymes (UDP-glucose, etc.)
CDP-holnn, etc.) 2. Biotin (carboxybiotics)
4 Pteridine or folic acids (THFA) 3. Folnic acids (5,10-methenyl THFA)
5 Cobamide (methylcobalamin)
Two features of coenzymes can be noted. The first is the absence of coenzymes of the third class - hydrolases and the polyfunctionality of a number of coenzymes (pyridoxin, cobamide), i.e. the ability of the same coenzyme to catalyze different reactions, depending on the active center of which enzyme it is part of. This serves as a clear example of the importance of the apoenzyme in the manifestation of the specific participation of the coenzyme in catalysis.
Vitamin coenzymes
Thiamine coenzymes. The source of their formation is thiamine (vitamin B), which, according to its chemical structure, belongs to the pyrimidine derivatives of thiazole. Its most active coenzyme form is thiamine diphosphate (TDP). The remaining thiamine derivatives are thiamine monophosphate (TMP), thiamine
triphosphate (TTP) are also considered coenzymes, but their significance is not clear. TDP is part of enzymes that catalyze the oxidative decarboxylation of α-keto acids - pyruvate and 2-oxoglutarate, and is also a coenzyme of transketolase, which converts substrates of the pentose phosphate cycle. Moreover, the “active” site in the TDP molecule, which serves as the site of attachment of the substrate, is the carbon atom in the thiazole ring (framed).
Flavin coenzymes The source of their formation is riboflavin (vitamin B 2), which in chemical structure belongs to the derivatives of nzoalloxazine. The coenzymes flavin (4ononucleotide (FMN) and flavin adenine dinucleotide (FAD) are synthesized from riboflavin):
n-s-on nhon
riboflavin
(here R are the corresponding radicals enclosed in frames in the previous formulas).
Oxidized riboflavin and both coenzymes are yellow in color. When reduced, they transform into the leuco form, and the color of the solution disappears. The reduced coenzymes FMN H 2 and FAD ♦ H 2 are formed as a result of the addition of hydrogen atoms to N-I and N-5 of the isoalloxase ring. The ability to easily accept and donate protons and electrons determines the participation of these coenzymes in redox reactions.
Pantothenic coenzymes. Pantothenic acid (vitamin B 3) serves as the starting material for the formation of the following coenzymes: coenzyme A (KoASH), dephosphocoenzyme A (dephospho-CoA5H), panthetheine-4-phosphate (Pf), which are found in the cell in free form or bound to enzyme proteins. Coenzymes are involved in acyl group transfer reactions. Hence the name - acylation coenzyme (A). Coenzyme A
H ,0-P-o-p-o-s n,-
abbreviated to either KoASH or simply KoA. The SH group of all pantothenic acid coenzymes is the working, anchoring part of the molecule. Acyls are added to it, and the metabolic form of CoA is formed - acyl~CoA (the thioester bond is macroergic, therefore indicated by a wavy line). Dephospho-CoA and PF as coenzymes are used less than KoASH. It is believed that dephospho-CoA is a coenzyme that catalyzes the breakdown of citrate, and Pf is a coenzyme of the acyl-transfer protein of fatty acid synthetase.
menta - dinucleotides in which mononucleotides are connected by a phosphodiester bond. One of the mononucleotides of these coenzymes contains nicotinamide; the other is represented by adenylic acid. NADP has an additional phosphoric acid residue attached to the ribose hydroxyl.
Both coenzymes are capable of reversibly accepting electrons and protons, so they are part of dehydrogenases. In reactions catalyzed by nicotinamide enzymes, two hydrogen atoms are removed from the substrate. One hydrogen atom is added to the C-4 of the nicotinamide ring; the electron of the second hydrogen atom goes to the quaternary nitrogen of the same ring, and the remaining free proton goes into the medium. The oxidized forms of coenzymes are abbreviated in reactions as NAD + and NADP +, and the reduced forms are NAD ■ H + H" and NADP H + H; (or simplified NAD H 2 and NADP ■ H a):
In the cells of the body, the biologically active coenzymes pyridoxal phosphate (PALP) and pyridoxamine phosphate (PAMP) are formed from them:
n-s=o ch 2 nh 2
palf pamph
The first of them is the main coenzyme that is part of numerous enzymes. However, in some reactions PAMP acts as an independent coenzyme, for example, in the reaction of the formation of 3,6-dideoxyhexoses necessary for the synthesis of glycoproteins in bacterial membranes.
Folic, or pteridine, coenzymes. Folacin unites a group of related vitamins, the main representative of which is folic acid. The body produces the coenzyme tetrahydrofolic acid from it.
Cobamide coenzymes. The source of formation of cobamide coenzymes is vitamin B, 2. The main part of this vitamin is the Co-complex of the nitrogen macrocycle called corrin. Corrin contains four reduced pyrrole rings bearing various substituents. Cobalt, located in the center of the corrin ring, can have different oxidation states: from Co 3+ to Co 6+. It is linked by covalent and coordination bonds to the nitrogen atoms of the pyrrole rings of corrin. In vitamin B 12, the remaining bonds are occupied by the 5,6-dimethylbenzimidazolyl ribotide residue and the CN group. Therefore, vitamin B|g is called cyanocobalamin. Replacing the CN group with an hydroxy group or a nttro group leads to the formation of other vitamers B, 2 - oxocobalamin and nitritecobalamin, respectively. In the body it is found in the form of coenzyme forms - methylcobalamin and 5-deoxyadenosylcobalamin. Below is the schematic structure of the central part of cyanocobalamin (I) and its coenzyme forms - methylcobalamin (II) and 5-deoxyadenosylco
balamin (III):
i ■ II
(here R is 5,6-dimethylbenzimidazolyl ribotide, and R" is 5"-deoxyadenosyl). These coenzymes are involved in group transfer reactions, isomerization, etc.
Biotin coenzymes. Biotin (vitamin H) forms an active coenzyme form - carboxybiotin:
;НN__ _N Н" ;HN _ _ _N_~ CO 0_ J
H G ^NGGNO COOH H.C CH(CH,)„COOH
The key role in the biotn molecule is played by nitrogen atoms, to which CO 2 is added. Biotin is involved in the transfer of carboxyl groups.
Lipoic coenzymes. The starting compound for the formation of coenzymes is lipoic acid (vitamin N). Lipoic coenzymes participate in redox reactions during the conversion of α-keto acids in the pyruvate dehydrogenase complex. There are oxidized and reduced forms of lipoic acid, which is linked to the enzyme (E) by an amide bond.
Quinone coenzymes. Among natural quinoid compounds, ubiquinone, or coenzyme Q (KoQ), as well as its analogue plastoquinone, found in plant organisms, have coenzyme properties. Ubiquinone is classified as a lipophilic vitamin-like substance. According to its chemical structure, it is a quinone with a side isoprenoid chain. The number of isoprenoid units in the side chain of natural ubiquinones is different, therefore ubiquinopes are designated by the symbol Q„. Ubiquinones Q, - Q 12 are found in nature. The most common coenzymes are Q s -Q 10 - A lot of ubiquinone is found in mitochondrial membranes; It is also present in the membranes of the endoplasmic reticulum and cell nuclei. Ubiquinone is capable of reversible redox transformations;
HzSO^C n 2 -c n=c-C n^n
When reduced, it turns into ubiquinol, which, when oxidized, turns back into ubiquinone. Due to its redox properties, ubiquinone is involved in the transfer of electrons and protons in the respiratory chain of mitochondria, and its analogue plastoquinnon plays the same role in chloroplasts.
For other natural quinoid compounds - naphthoquinones (vitamin K) and tocopherols (vitamin E), which are similar in structure and redox properties to ubnquinone, coenzyme functions have not yet been proven.
Carnitium coenzymes. The bitamin-like substance carnitine (vitamin Bt), being a coenzyme of transferases, is involved in the transfer of acyl groups (acetic acid residues and higher fatty acids) through the lipid layer of mitochondrial and possibly other membranes. Carnitine can be in expanded and cyclic forms:
| R-C-0-HC< |
Acyls add to the OH group of the carnitine to form the corresponding acylcarnitine:
Since the “cyclic form is more fat-soluble (due to shielding of charges by methyl groups), it is in this cyclic form, as S.E. Severin et al. believe, that carnitine is able to diffuse through the lipid layer of the membrane and transfer acyls.
Non-vitamin coenzymes
Nucleotide coenzymes. To nucleotide coenzymes that are not derivatives of vitamins (in this they differ from the considered nucleotide coenzymes - NAD, NADP, FAD, CoA, in the construction of which they participate
vitamins), include nucleoside diphosphates and nucleoside monophosphates, connected through the terminal phosphate e by various substrates.
All nucleotide coenzymes are divided into five groups depending on the type of nucleoside: uridine, cytidine, thymidine, adenosine and guanosine. Individual nucleotide coenzymes within each group differ from each other by the substrate attached to them. Over 60 different nucleotide coenzymes are already known, containing residues of sugars, alcohols, amino acids, lipids, and inorganic substances. The most representative among them is the group of nucleoside diphosphate sugars. Below is the structure of some representatives of nucleotide coenzymes:
Most of the known nucleotide coenzymes are represented by nucleoside diphosphates; But there are n nucleoside monophosphates, for example, CM-sialic acid. Reactions in which nucleotide non-vitamin coenzymes participate can be divided into two types. The first includes reactions associated with the transformation of the substrate in the coenzyme molecule. In this case, the coenzyme is likened to a cosubstrate. Various transformations can occur with the substrate in the coenzyme: stereo isomerization (for example, UDP-glucose is converted into UDP-galactose), oxidation or reduction (for example, enzymatic oxidation of the C 6 atom of glucose occurs in the liver and the latter is converted into UDP-glucuronic acid) etc.
Reactions of the second type are associated with the participation of nucleotide coenzymes as substrate donors in group transfer reactions. In this case, the phosphoester bonds connecting the coenzyme and the substrate are broken. This type of reaction is used in the synthesis of various substances. Thus, UDP-glucose is a donor of glucose in the biosynthesis of glycogen, UDP-glucuronic acid is a donor of a glucuronic acid residue in conjugation reactions of natural (for example, bilirubin) and foreign substances, CDP-choline is a donor of choline in the biosynthesis of choline phosphatides, etc.
Carbohydrate phosphates as coenzymes. Some carbohydrate phosphates function as coenzymes. Thus, glucose-1,6-diphosphate acts as a coenzyme of the enzyme glucose phosphate isomerase, which catalyzes the reversible isomerization of glucose-6-phosphate and fructose-6-phosphate; 2,3-diphosphoglycerate - as a coenzyme of phosphoglycerate mutase, which is involved in the conversion of 2-phosphoglycerate into 3-phosphoglycerate and back. It should be noted that 2,3-diphosphoglycerate is also a regulator of hemoglobin functions.
Metal porphyrin coenzymes. These include the previously discussed hemes, which participate as coenzymes in redox reactions catalyzed by oxido-reductases (cytochromes, catalase, peroxidase, tryptophan oxygenase, etc.)> and chlorophylls involved in the oxidative decomposition of water during photosynthesis (see Chap. Energy formation in photosynthesizing organisms).
Peptide coenzymes. These include glutathione. According to its chemical structure, it is a tripeptide - glutaminsteinylglycine.
Its functional group is the SH group of cysteine, which is capable of reversible redox transformations. Therefore, there are two forms of glutathione: reduced (abbreviated G-SH) and oxidized, or disulfide (G-S-ST):
HOOC-CH-CH 2 -CH 2 - CO-NH-CH-CO-NH-CH*-COOH
HOOC-CH-CH,-CHj-CO-NH-CH-co-NH-CH 2 -COOH
HOOC-CH-CH 5 -CHj-CO-NH-CH-CO-NH-CH g -COOH
glutathione window ff-S-ST]
Glutathione is a coenzyme of a number of oxidoreductases, for example glutathione peroxidase.
5. Metal ions as enzyme cofactors
Metal ions can also be cofactors. Met allo enzymes are a very common group of enzymes that make up "/< от всех ферментов. Роль металлов в этих ферментах различна. Металлоферменты делятся на две группы:
I. Enzymes where metal ions act as an activator (these enzymes can catalyze without metal).
II. Enzymes where metal ions act as a cofactor (without metal ions these enzymes are inactive).
1) Dissociating metalloenzymes (the metal ion easily dissociates from the apoenzyme).
2) Non-dissociating metalloenzymes (the metal ion is tightly bound to the apoenzyme):
a) losing activity when binding the metal with a reagent;
b) do not lose activity when binding the metal with a reagent.
Metal ions as cofactors are part of enzymes belonging to different classes. Metalloenzymes that catalyze oxidation-reduction reactions are quite numerous. The metal ion may be located^
Table 21. Examples of metalloenzymes of various classes
|
in the active center or be part of a larger organic molecule (for example, heme), or can be directly associated with amino acid residues of the apoenzyme. Since electron transfer occurs under the action of oxidoreductases and the degree of oxidation of substrates changes, metals with variable valence act as cofactors: iron, copper, molybdenum, cobalt. v
If the metal is not directly involved in catalysis, but serves other purposes, for example, binds the substrate, then oxidoreductases include metals with a constant degree of oxidation.
Metal enzymes that catalyze the hydrolysis reactions of substrates contain metals with constant valency: zinc, calcium, magnesium. Metals with variable oxidation states, such as manganese, are rarely found in hydrolases (see Table 21).
What is the role of metals in the catalytic action of the enzyme? Several possible options for the participation of metal ions in the operation of the enzyme have been proven. Firstly, the metal, being a kind of electrophilic group of the active center, is capable of interacting with negatively charged groups of the substrate. Such a metal-substrate complex is more easily attacked by the enzyme. For example, Mg 2+ (or Mn 2+ ) ions form a complex with ATP or ADP in reactions catalyzed by creatine phosphokinase and ATPase. As a result, the enzyme activity is fully manifested, and in the absence of metals the enzymes are inactive or inactive.
Secondly, a metal with variable valency can itself participate in electron transport, i.e., perform the function of a catalytic site.
Thirdly, the metal promotes the formation of a catalytically active conformation of the tertiary and quaternary structure of the apoenzyme. Stabilization is possible by the formation of salt bridges between the metal ion and the carboxyl groups of acidic amino acids during the formation of the tertiary structure of the enzyme protein molecule or between subunits during the formation of the quaternary structure. For example, calcium ions stabilize a-amylase, and zinc ions stabilize alcohol dehydrogenase. Alcohol dehydrogenase deprived of zinc dissociates into subunits and loses activity.
Fourthly, metals sometimes serve as a kind of bridge between the apoenzyme and the coenzyme. For example, in alcohol dehydrogenase, the zinc ion binds NAD+.
It should be remembered that, just like vitamin coenzymes, metals enter the body with food. Therefore, the normal function of a large family of metalloenzymes depends on the normal supply of metals, most of them belonging to the group of trace elements. Hence the high biological activity of these metals: insufficient intake from food can cause serious metabolic disorders in the body.
6. Mechanism of action of enzymes
The complex structural and functional organization of enzymes is partly the key to understanding the characteristic properties of enzymes - high specificity and speed of catalysis, which is not achievable for non-enzymatic catalysts. One of the first hypotheses explaining the action of enzymes was the adsorption hypothesis, proposed at the beginning of the 20th century. English physiologist Baylis and German biochemist Warburg. When substantiating the main provisions of this hypothesis, we proceeded from the mechanism of action of non-biological catalysts. According to the adsorption hypothesis, the surface of the enzyme, like spongy platinum, is the site of adsorption of reagent molecules. This facilitates their interaction and speeds up the reaction. However, this hypothesis did not explain the specificity of enzyme action and now has only historical significance.
A major role in the development of ideas about the mechanism of action of enzymes was played by the classical works of Michaelis and Menten, who developed the concept of enzyme-substrate complexes. According to the ideas of Michaelis-Menten, the entire process of enzymatic catalysis is described by a simple equation (rns. 21).
The process of enzymatic catalysis can be divided into three stages, each of which has its own characteristics.
Part 1. Diffusion of the substrate to the enzyme and its steric binding to the active center of the enzyme (formation of an enzyme-substrate complex
2. Conversion of the primary enzyme-substrate complex into one or more activated enzyme-substrate complexes (denoted ES* and ES** in the equation).
3. Separation of reaction products from the active center of the enzyme and their diffusion into the environment (the EP complex dissociates into E and P).
The first stage, usually short in time, depends on the concentration of the substrate in the medium and the rate of its diffusion to the active center of the enzyme. The formation of the ES complex occurs almost instantly. At this stage, the change in activation energy is insignificant. The orientation of substrates in the active site of the enzyme favors their approach and the passage of the reaction.
The second stage is the slowest, and its duration depends on the activation energy of a given chemical reaction. At this stage, the substrate bonds are loosened, broken, or new bonds are formed as a result of the interaction of the catalytic groups of the enzyme. It is precisely due to the formation of activated transition complexes that the activation energy of the substrate decreases. The second stage limits the rate of the entire catalysis.
The third stage is short-lived, like the first. It is determined by the rate of diffusion of reaction products into the environment.
The molecular mechanisms of enzyme action are still largely unclear. Among the studied mechanisms of enzyme action, the following can be noted:
1) the effect of orientation of reagents (approximation);
2) the effect of substrate deformation (tension, bending, tension);
3) acid-base catalysis;
4) covaleite catalysis.
The effect of orientation of reagents is a very characteristic property of enzymes, which makes it possible to accelerate the transformation (increase the reactivity of substrates) by thousands or tens of thousands of times. The contact areas of the enzyme's actin center specifically bind substrates and ensure their mutual orientation and approach in a manner that is beneficial for the action of the catalytic groups. This mutual orientation of two or more molecules, which is impossible during random collisions in an aqueous medium and on the surface of an inorganic catalyst, helps to increase the reaction rate. The ordered arrangement of substrates leads to a decrease in entropy, and therefore helps to reduce the activation energy.
The effect of substrate deformation (or the so-called “rack” theory) well explains the action of hydrolases, lyases and some transferases. Before joining the enzyme, the substrate has a “relaxed” configuration. After binding to the active center, the substrate molecule seems to stretch (“stressed” or “deformed” configuration). The greater the length of the interatomic bond in the substrate, the lower the energy of its rupture (i.e., the activation energy decreases). Places of deformation (stretching) are more easily attacked, for example by water molecules.
Acid-base catalysis. The peculiarity of the active center of the enzyme, unlike other catalysts, is that it contains functional groups of amino acid residues that exhibit the properties of both an acid and a base. Therefore, the enzyme exhibits acid-base properties during the catalytic act, i.e., it plays the role of both an acceptor and a donor of protons, which is impossible for conventional catalysts.
Table 22. Some enzymes capable of covalent catalysis
e product
Chymotryapsin, trypsin, thrombin, esterase
2. Phosphoglucomutase, alkaline phosphatase
O-R-O-CHj-CH-
When a substrate is fixed in the active site, its molecule is influenced by electrophilic and nucleophilic groups of the catalytic site, which causes a redistribution of electron density in areas of the substrate attacked by acid-base groups. This facilitates the rearrangement and breaking of bonds in the substrate molecule. Enzymes that have histidine in their catalytic center have a pronounced ability for acid-base catalysis. Histidine has distinct acid-base properties. When histidine is blocked, the enzyme is inactivated. Acid-base catalysis is characteristic of hydrolases, lyases, and isomerases. It is often combined with covalent catalysis.
Covalent catalysis is observed in enzymes that form covalent bonds between the catalytic groups of the active site and the substrate. Covadent enzyme-substrate intermediates are very unstable and easily break down, releasing reaction products. In table 22 shows some enzymes that have the ability to covalent catalysis. Most enzymes are characterized by a combination of the described mechanisms, which ensures their high catalytic activity.
7. Specificity of enzyme action
Enzymes have different specificities towards substrates. Based on the degree of specificity, enzymes are divided into the following main types, mentioned in order of decreasing specificity.
1. Stereochemical substrate specificity - the enzyme catalyzes the conversion of only one of the possible stereoisomers of the substrate. This is an extreme case of specificity. For example, fumarate hydrate catalyzes the conversion of only fumaric acid (the addition of a water molecule to it), and not its stereoisomer, maleic acid.
2,
Absolute substrate specificity - the enzyme catalyzes the conversion of only one substrate. For example, urease catalyzes the conversion of only urea.
3. Absolute group substrate specificity - the enzyme catalyzes the transformation of a similar group of substrates. For example, alcohol dehydrogenase catalyzes the transformation of not only ethanol, but also other aliphatic alcohols, although at different rates.
4. Relative group substrate specificity - the enzyme specifically acts not on a group of substrate molecules, but on individual bonds of a certain group of substrates. For example, digestive enzymes - pepsin, trypsin - are specific to peptide bonds formed by certain amino acids in different proteins.
5. Relative substrate specificity - the enzyme catalyzes the transformation of substrates belonging to different groups of chemical compounds. For example, the enzyme cntochrome is involved in the hydroxylation of various compounds (about 7000 names). These are the least specific enzyme system involved in the transformation of natural substances, drugs and poisons.
What explains the specificity of enzyme action? There are two points of view on this matter. One of them, the hypothesis of E. Fischer, or, as it is called, the “key and lock” or “template” hypothesis, stipulates that specificity is based on the strict steric correspondence of the substrate and the active center of the enzyme.
According to Fischer, an enzyme is a rigid structure, the active center of which is a cast of the substrate. If the substrate approaches the active
center, like a key to a lock, then the reaction will occur. If the substrate (“key”) is slightly changed, then it does not correspond to the active center (“lock”), and the reaction becomes impossible. Fisher's hypothesis is attractive for its simplicity in explaining the specificity of enzyme action. However, from the standpoint of the “template” hypothesis, it is difficult to explain, say, absolute and relative group substrate specificity, since the configuration of “keys” (substrates) that fit the same “lock” is too diverse.
Another hypothesis proposed by Koshland explains these external contradictions. It is called the “forced correspondence” hypothesis. According to Koshland, the enzyme molecule is not rigid, but flexible, elastic (which is confirmed by modern research methods); the conformation of the enzyme and its active center changes when a substrate or other ligands are attached; And. finally, the active center is not a rigid cast of the substrate, but the substrate forces it to take the appropriate shape at the moment of attachment (hence the name of the “forced conformity” hypothesis).
In other words, the “keyhole,” according to Koshland, is made of a malleable material and therefore takes the final shape of the “key” upon contact.
The “forced conformity” hypothesis received experimental confirmation after a change in the arrangement of the functional groups of the active site of a number of enzymes after the addition of a substrate was recorded. This hypothesis also allows us to explain why the transformation of close analogues of substrates occurs. If the “false” substrate (quasi-substrate) differs slightly from the natural one and the active center takes on a conformation. close to true, then the arrangement of catalytic groups in t
such an enzyme-substrate complex will allow the reaction to take place (Fig. 22). The enzyme does not seem to notice this “deception”. However, the enzymatic reaction will not proceed as quickly as with a true substrate, since there is no ideal arrangement of the catalytic groups in the active site of the enzyme.
Only if the configuration of the quasi-substrate does not allow the correct placement of the catalytic groups will the reaction not proceed (Fig. 22, c). Obviously, the unequal degree of specificity of different enzymes reflects, as it were, the range of conformational rearrangements of the active center. If it is limited to a single possible conformation, the enzyme is highly specific. If the possibilities of restructuring are great, then the enzyme also works on quasi-substrates.
8. Kinetics of enzymatic reactions
The kinetics of enzyme action is a branch of enzymology that studies the dependence of the reaction rate catalyzed by enzymes on the chemical nature and conditions of interaction of the substrate with the enzyme, as well as on environmental factors. In other words, enzyme kinetics allows us to understand the nature of the molecular mechanisms of action of factors affecting the rate of enzymatic catalysis.
The rate of an enzymatic reaction is determined by the amount of substance (or substances) that is converted per unit of time. The rate of these reactions depends on the influence of external conditions (temperature, pH of the environment, the influence of natural and foreign compounds, etc.).
The foundations of the kinetics of enzymatic reactions were laid in the works of Michaelis and Menten. The rate of an enzymatic reaction is a measure of the catalytic activity of the enzyme and is simply referred to as the activity of the enzyme. Enzyme activity can only be measured indirectly: by the amount of substrate converted or by the increase in product concentration per unit time.
Dependence of the rate of enzymatic reaction on the concentration of substrate and enzyme. The enzymatic reaction is schematically described by the equation where k are the rate constants of forward (+) and reverse reactions (-).
Using this equation, Briggs and Haldane derived a mathematical expression for the dependence of reaction rate on substrate concentration:
v=iw),
where v is the observed reaction rate; - maximum reaction speed; Kt - Michaelis constant. This equation is called equation
Michaelisa - Menten. When and = "/ 2 and tlx after appropriate transformations
ta, i.e. K m = [S]. Therefore, the Michaelis constant has the dimension of concentration." It is equal to the concentration of the substrate at which the reaction rate is half the maximum, n is expressed in moles per liter. K t at k-x^k+t is or the rate constant of the enzymatic reaction. The higher K t , the lower the rate of catalytic transformation of the substrate by this enzyme. According to the value of Kt, enzymes can be divided into “fast” (with low Kt) and “slow” (with high Kt). If any enzyme catalyzes a two-substrate reaction, then each substrate has its own Km, and they can vary significantly.In enzymes with group substrate specificity, each substrate has its own Km.
The affinity of the substrate for the enzyme is judged by the substrate constant, denoted by the symbol K s - It is the dissociation constant of the ES complex. The more tightly the substrate is bound, the slower the ES decomposes into E and S, which means that such a substrate has a high affinity (binding specificity) for the active center of the enzyme and vice versa.
Graphically, the dependence of the reaction rate on the substrate concentration is described by a hyperbola called the Michaelis curve (Fig. 23). The shape of the curve shows that with increasing substrate concentration, all active centers of the enzyme molecules are saturated. This corresponds to the maximum formation of enzyme-substrate complexes and the maximum reaction rate vta» Km is easily found on such a graph. Sometimes a graph of the reaction rate versus substrate concentration is plotted using the double reciprocal method (Lineweaver-Burk method) (Fig. 23.6). The value of the Michaelis constant is found as shown in the graph.
From how the reaction rate changes at different substrate concentrations, one can judge the order of the reaction, which must be known to work with enzymes and to correctly determine their activity in clinical laboratories. The order of the reaction can vary from zero and higher. At zero order, the reaction rate is constant and does not depend on the concentration of the substrate. In this case, the reaction rate is maximum (Ppi*) - In the first order, the reaction rate is directly proportional to the concentration of one of the substrates, etc. In order to correctly determine the activity of the enzyme, it is necessary to achieve a zero reaction rate, i.e., determine the rate of the enzymatic reaction at saturating substrate concentrations. In this case, all changes in the reaction rate will depend only on the amount of fermekg.
To assess the operating conditions of any enzyme in the cells of the body, it is necessary to know the actual concentrations of substrates present in them. Under physiological conditions, enzymes almost never work at full strength, because the concentrations of their substrates are far from saturating. Perhaps the only substrate required for hydrolases is water, which is present in cells in saturating concentrations, except in cases where the structural localization of the enzyme limits the access of water to the active center.
The dependence of the reaction rate on the amount of enzyme is linear, which, as already mentioned, distinguishes the enzyme from non-biological catalysts. From this we can draw a certain practical conclusion that the greater the number of molecules of a given enzyme in an organism cell compared to others, the higher the rate of chemical transformations catalyzed by this enzyme. If any enzyme is insufficient (synthesis is impaired), then the rate of the reaction catalyzed by it limits the course of associated biochemical processes.
An increase in the number of enzyme molecules, achieved by natural stimulation of their formation or with the help of drugs, allows either to restore the impaired reaction rate or to adapt the necessary biochemical reactions to new living conditions.
Dependence of the reaction rate on the pH of the medium. Typically, the curve of the dependence of the rate of an enzymatic reaction on the pH of the medium is bell-shaped (Fig. 24), since each enzyme has its own optimum pH, at which the rate of the reaction it catalyzes is maximum. A pH deviation in one direction or another leads to a decrease in the rate of the enzymatic reaction.
Optimal pH values for some enzymes
Enzyme.. Pepsin Acid Urease, Trypsin Arginase
Phosphataea pancreatic amylase
Optimum pH 1.5-2.5 4.5-5.0 6.4-7.2 7.8 9.5-9.9
From the data presented it is clear that the optimum pH is not the same for different enzymes. However, most cell enzymes have an optimum pH close to neutral, i.e., coinciding with physiological pH values.
The dependence of the rate of an enzymatic reaction on pH mainly indicates the state of the functional groups of the active center of the enzyme. A change in the pH of the medium affects the ionization of acidic and basic groups of amino acid residues of the active center, which are involved either in the binding of the substrate (in the contact site) or in its transformation (in the catalytic site). Therefore, the specific effect of pH may be caused
either by a change in the affinity of the substrate for the enzyme, or by a change in the catalytic activity of the enzyme, or both.
Most substrates have acidic or basic groups, so pH affects the degree of ionization of the substrate. The enzyme preferentially binds to either the ionized or non-ionized form of the substrate. Obviously, at optimal pH, the functional groups of the active site are in the most reactive state, and the substrate is in a form preferred for binding by these enzyme groups.
The dependence of the enzymatic reaction on the pH of the medium is of practical importance. First of all, determination of enzyme activity should be carried out at the optimal pH for a given enzyme. To do this, select the required buffer solution with the required pH value.
The range of pH fluctuations under physiological conditions is insignificant, but there may be changes in pH in a limited area of the cell. They influence the activity of enzymes. For example, during active muscle work, lactic acid accumulates, which briefly shifts the pH of the muscle cell environment to the acidic side, which changes the rate of enzymatic reactions. t
Knowledge of pH optima for individual enzymes is important for practical medicine. For example, pepsin requires a strongly acidic environment for active hydrolysis of proteins in the stomach, so to restore the impaired activity of endogenous pepsin, it is necessary to take acidic substances. Pepsin is taken with hydrochloric acid, which creates the desired pH.
Dependence of the rate of enzymatic reaction on temperature. As the temperature of the environment increases, the rate of the enzymatic reaction increases, reaching a maximum at some optimal temperature, and then drops to zero (Fig. 25). For chemical reactions, there is a rule that when the temperature increases by 10°C, the reaction rate increases two to three times. For enzymatic reactions, this temperature coefficient is lower: for every 10°C, the reaction rate increases by 2 times or even less. The subsequent decrease in the reaction rate to zero (descending branch in Fig. 25) indicates denaturation of the enzyme block. Optimal temperature values
For most enzymes, they are in the range of 20-40°C. The thermolability of enzymes is associated with their protein structure. Some enzymes are denatured already at a temperature of about 40°C, but the majority of them are not activated at temperatures above 40-50°C. Some enzymes are inactivated by cold, i.e., at temperatures close to 0°C, denaturation occurs.
However, some enzymes do not obey these patterns. Thus, the enzyme catalase is most active at temperatures approaching 0°C. There are also thermostable enzymes. For example, adenylate kinase can withstand temperatures of 100°C for a short time without inactivation. Microorganisms living in hot springs contain many proteins, including enzymes, which are characterized by high thermal stability. As mentioned earlier, such enzymes are glycoproteins, since the carbohydrate component gives the protein heat stability.
The effect of temperature on enzyme activity is important for understanding life processes. When the temperature drops, some animals enter a state of hibernation or suspended animation. The rate of enzymatic reactions in this state slows down, which ensures low consumption of nutrients accumulated by the body and a decrease in the activity of cellular functions. Warming the body accelerates the course of enzymatic reactions and returns the animal's body to active activity.
Artificial cooling of the body, so-called hibernation, is used in the clinic for surgical operations. Cooling the body also slows down the rate of enzymatic reactions, which reduces the consumption of substances and preserves the viability of body cells for a longer period.
An increase in body temperature (fever), for example during infections, accelerates biochemical reactions catalyzed by enzymes. It is easy to calculate that every degree increase in body temperature increases the reaction rate by about 20%. At high temperatures of about 39-40°C, the wasteful use of endogenous substrates in the cells of a sick organism must be replenished with food. In addition, at a temperature of about 40°C, some very thermolabile enzymes can be denatured, which disrupts the natural course of biochemical processes. So, knowledge of the temperature dependence of enzymatic reactions allows them to be used in the practical work of a doctor.
To determine the activity of enzymes in laboratory practice, certain standard or optimal temperature conditions are always selected, taking into account the thermolability of a particular enzyme. Comparing changes in the activity of an enzyme determined, say, to identify disorders in the body, is possible only under the same temperature conditions.
Thermal dependence of enzymes is used in practice to develop temperature conditions for food storage. Their preservation at low temperatures is the result of the low activity of their own enzymes, which do not “eat” their substrates (for example, in vegetables, fruits, etc.), or enzymes of microorganisms that can spoil products.
9. Determination methods and units of enzyme activity
Enzymes contained in cells, tissues and organs are pre-extracted using special methodological techniques. During extraction, the necessary enzyme stabilizers are added to protect them from inactivation. An enzyme solution (extract from biological material) is used to determine enzymes. Serum or blood plasma, other biological fluids are ready-made solutions of enzymes, so they are immediately used for determination. If the goal of the study is to obtain a purified or crystalline enzyme, then the activity is determined after each stage of purification.
Qualitative and quantitative tests for the enzyme are carried out indirectly by the loss of the substrate or the accumulation of reaction products in the medium. Direct measurement of the amount of enzyme is in principle possible only for a homogeneous, crystalline enzyme. The amount of protein measured by direct chemical methods must correspond to the amount of enzyme. In practice, in this case too, an indirect method is used, since the amount of protein in a solution of a homogeneous enzyme is not yet a criterion for the activity of the enzyme (some of the molecules may be in an inactive or denatured state).
The rate at which the substrate disappears or the amount of reaction products increases per unit time serves as a measure of enzyme activity.
Standard conditions. To correctly determine the activity of an enzyme, it is necessary to carry it out under standard conditions, which are established for each enzyme from preliminary kinetic studies, and to accurately measure the change in the content of the substrate or reaction product over a certain period of time.
It is necessary to maintain the optimal pH value for the enzyme being determined (4eHHt) (use a suitable buffer). The substrate concentration must be greater than the saturating one, at which the maximum reaction rate is maintained (super-saturating substrate concentrations are specially established for enzymes subject to substrate inhibition). For complex enzymes that require cofactors (metal ions, coenzymes), the concentration of cofactors must also exceed the saturating one. The standard temperature is assumed to be 25°C (measurement at other temperatures is specifically specified in the experiment).These standard conditions provide a zero-order reaction, in which the change in the concentration of the substrate or reaction product depends only on the amount of enzyme added to the medium.
To correctly measure the activity of a enzyme, it is necessary to determine the initial reaction rate, i.e. at the beginning of the reaction, when the concentration of the substrate or product changes proportionally over equal periods of time.
Methods for determining the content of substrate or reaction product. The determination is carried out by any method (colorimetric, spectrophotometric, fluorimetric, polarographic, etc.) after stopping the reaction after a certain period of time or is recorded continuously during the reaction. The last method is more convenient. It is possible if the substrate or product absorbs in a certain region of the spectrum (the change in their absorption during the reaction is recorded on a spectrophotometer) or fluoresces (the change in fluorescence over a certain time is continuously recorded on spectrofluorine meters), etc. In other words, the choice of determination method Enzyme activity is limited by the ability to determine the substrate or reaction products.
Units of enzyme activity. The international unit of enzyme activity is the amount of enzyme capable of converting one micromole (µmol) of substrate in 1 minute under standard conditions. International units of enzyme quantity are designated by the symbol E or U.
The specific activity of an enzyme is equal to the mass of the enzyme (r milligrams) capable of converting 1 µmol of substrate in 1 min under standard conditions, expressed in µmol/(min mg protein"). A new unit of catalytic activity, katal (symbol - kat), is also recommended, which is the amount of enzyme capable of converting 1 mole of substrate in 1 second under standard conditions.
10. Regulation of enzyme activity
Enzymes, as already mentioned, are catalysts with controlled activity. Therefore, through enzymes it is possible to control the speed of chemical reactions in the body. Regulation of enzyme activity can be carried out by interaction with them of various biological components or foreign compounds (for example, drugs and poisons), which are usually called enzyme modifiers or regulators. Under the influence of modifiers on the enzyme, the reaction can accelerate (in this case they are called activators) or slow down (in this case they are called inhibitors).
Enzyme activation
Enzyme activation is determined by the acceleration of biochemical reactions that occurs after the action of the modifier. One group of activators consists of substances that affect the region of the active center of the enzyme. These include enzyme cofactors and substrates. Cofactors (metal ions and coenzymes) are not only obligatory structural elements of complex enzymes, but also essentially their activators.
Metal ions are quite specific activators. Often, some enzymes require ions of not one, but several metals. For example, Na^K^-ATPase, which transports monovalent cations across the cell membrane, requires magnesium, sodium and potassium ions as activators.
Activation with metal ions occurs through different mechanisms. In some enzymes they are part of the catalytic site. In some cases, metal nonons facilitate the binding of the substrate to the active center of the enzyme, forming a kind of bridge. Often the metal combines not with the enzyme, but with the substrate, forming a metal-substrate complex, which is preferable for the action of the enzyme.
The specificity of the participation of coenzymes in the binding and catalysis of the substrate explains their activation of enzymatic reactions. The activating effect of cofactors is especially noticeable when acting on an enzyme that is not saturated with cofactors.
The substrate is also an activator within certain concentration limits. After reaching saturating concentrations of the substrate, enzyme activity does not increase. The substrate increases the stability of the enzyme and facilitates the formation of the desired conformation of the active center of the enzyme. ,
Metal ions, coenzymes and their precursors and active analogues, substrates can be used in practice as drugs that activate enzymes.
Activation of some enzymes can be carried out by modifications that do not affect the active center of their molecules. Several options for such modification are possible: 1) activation of an inactive precursor - proenzyme, or zymogen; 2) activation by attaching any specific modifying group to the enzyme molecule; 3) activation by dissociation of the inactive protein-active enzyme complex.
Enzyme inhibition
Inhibitors are of great interest for understanding the mechanism of enzymatic catalysis. The use of various substances that bind the functional groups of the contact and catalytic sites of the active center of the enzyme can clarify the significance of certain groups involved in catalysis. Inhibitors allow us to understand not only the essence of enzymatic catalysis, but are also a unique tool for studying the role of individual chemical reactions that can be specifically turned off using an inhibitor of a given enzyme. The study of inhibition of enzymatic reactions is of practical importance for researching and deciphering the mechanism of action of drugs, pesticides, etc.
You should be careful when using the term inhibitor, meaning only that substance that causes a specific decrease in enzyme activity. Just the fact that a reaction is inhibited does not mean that we are dealing with an inhibitor. Any denaturing agents also inhibit the enzymatic reaction. Therefore, in the case of the action of denaturing substances, it is more correct to speak not of “inhibition”, but of “inactivation”. Often a substance in small concentrations is an inhibitor, and in large concentrations it is an inactivator, so this division is to some extent arbitrary.
Inhibitors are characterized primarily by such a common feature as the strength of binding to the enzyme. On this basis, inhibitors are divided into two groups: reversible and irreversible. The criterion for restoring enzyme activity after dialysis or a strong dilution of a solution of the enzyme with the inhibitor allows one to classify an inhibitor into one of two groups. Irreversible inhibitors bind tightly to the enzyme, and after these procedures, enzyme activity is not restored. On the contrary, the enzyme-reversible inhibitor complex is fragile and quickly dissociates. The activity of the enzyme is restored.
According to the mechanism of action, enzyme inhibitors are divided into main types: 1) competitive: 2) non-competitive; 3) non-competitive; 4) substrate; 5) allosteric
Competitive inhibition is the inhibition of an enzymatic reaction caused by the binding to the active center of an enzyme of an inhibitor that is similar in structure to the substrate and prevents the formation of an enzyme-substrate complex. In competitive inhibition, the inhibitor and substrate, being similar in structure, compete for the active center of the enzyme. The compound with more molecules binds to the active center. Either a substrate or an inhibitor is associated with the enzyme, so for this type of inhibition the following equation holds:
where I is an inhibitor; EI - enzyme-inhibitor complex. But during competitive inhibition, a ternary complex ESI (enzyme - substrate - inhibitor) is never formed, which is how this type of inhibition differs from others.
Inhibition occurs due to the fact that the substrate-like inhibitor binds some enzyme molecules that are no longer able to form an enzyme-substrate complex. Inhibition can be removed by using an excess of substrate, which displaces the inhibitor from the active centers of enzyme molecules, thereby restoring their ability to catalyze.
Due to the similarity of the competitive inhibitor with the substrate, such inhibition is also called isosteric. Competitive (isosteric) inhibitors can be metabolites, the accumulation of which regulates the activity of enzymes, and foreign substances.
An example of competitive inhibition is the effect of various substances on the activity of succinate dehydrogenase. This enzyme is part of the cyclic enzyme system - the Krebs cycle. Its natural substrate is succinate, and a similar competitive inhibitor is oxaloacetate, an intermediate product of the same Krebs cycle:
OOCt-CHJ- CH*-SOSG -OOC-C-CHJ-COO-
A similar competitive inhibitor of succinate dehydrogenase is malonic acid, often used in biochemical studies. In Fig. 26 schematically shows the mechanism of competition between succinate and malonate for the enzyme.
A clear example of competitive inhibition is the effect of a group of substrates on enzymes that have group substrate specificity. Quasi-substrates are competitive inhibitors of enzymes with respect to true substrates.
The principle of competitive inhibition is the basis for the action of many pharmacological drugs, pesticides used to destroy agricultural pests, and chemical warfare agents.
For example, a group of anticholinesterase drugs, which include derivatives of quaternary ammonium bases and organophosphorus
Drugs such as proserin, physostigmine, sevin inhibit the enzyme reversibly, and phosphorus
organoformic preparations such as armin, nibufin, chlorophos. 4arina and zoma act irreversibly, phosphorylating the catalytic group of the enzyme. As a result of their action, acetylcholine accumulates in those synapses where it is a mediator of nervous excitation, i.e., the body is poisoned by the accumulated acetylcholine. The effect of reversible inhibitors gradually wears off, since the more acetylcholine accumulates, the faster it displaces the inhibitor from the active center of cholinesterase. The toxicity of irreversible inhibitors is incomparably higher, so they are used to control agricultural pests, household insects and rodents (for example, chlorophos) and as chemical warfare agents (for example, sarin, soman, etc.).
By selectively turning off one or another enzyme, it is possible to conduct a unique analysis of the participation of a particular enzyme in metabolism. The phenomenon of competitive inhibition opens up the possibility of searching for antimetabolites that, having a similar configuration to the true substrate, may fall into the category of competitive inhibitors. Antimetabolites are promising as specific pharmacological agents.
However, we must not forget that competitive relationships are possible not only between the substrate and the inhibitor, but also between the inhibitor and the coenzyme.
Anticoenzymes (analogs of coenzymes that are not capable of performing their function) also act as competitive inhibitors, disabling the enzyme molecules with which they combine. Anticoenzymes (or their precursors, antivitamins) are widely used both in biochemical research and in medical practice as effective drugs.
Non-competitive inhibition of enzymes is inhibition associated with the influence of the inhibitor on the catalytic transformation, but not on the binding of the substrate to the enzyme. A non-competitive inhibitor either binds directly to the catalytic groups of the active site of the enzyme, "or, by binding to the enzyme outside the active center, changes the conformation
mation of the active center in such a way that it affects the structure of the catalytic site, interfering with the interaction of the substrate with it. Since a noncompetitive inhibitor does not affect substrate binding, in contrast to competitive inhibition, the formation of a ternary ESI complex is observed according to the equation
E + S + I - ESI
However, this complex is not converted into products.
Non-competitive inhibitors are, for example, cyanides, which bind tightly to ferric iron, which is part of the catalytic site of the heme enzyme, cytochrome oxidase. Blockade of this enzyme turns off the respiratory chain, and the cell dies. Non-competitive enzyme inhibitors include heavy metal ions and their organic compounds. Therefore, heavy metal ions of mercury, lead, cadmium, arsenic and others are very toxic. They block, for example, SH groups included in the catalytic site of the
Coenzyme A CoA, acetylation (or acylation) coenzyme, the most important of the coenzymes (See Coenzymes) ,
taking part in acyl group transfer reactions. The CoA molecule consists of an adenylic acid residue (1) linked by a pyrophosphate group (2) to a pantothenic acid residue (See Pantothenic acid) (3), which, in turn, is connected by a peptide bond to a β-mercaptoethanolamine residue (4); see formula. CoA is associated with a wide range of biochemical reactions underlying the oxidation and synthesis of fatty acids, lipid biosynthesis, oxidative transformations of carbohydrate breakdown products, etc. In all cases, CoA acts as an intermediate compound that binds (accepts) and transfers acidic residues to others. substances. In this case, the acidic residues either undergo one or another transformation as part of the compound with CoA, or are transferred without changes to certain metabolites. The “active” form of organic acids is represented by acyl residues attached to the sulfhydryl (SH) group of CoA by a high-energy acylthioether bond. Much credit for the study of the chemical structure and biological role of CoA belongs to F. Lipman ,
who isolated CoA from the liver of a pigeon (1947), and F. Linen ,
complete synthesis of CoA was carried out by X. Koran
(1961). Yu. N. Leikin.
Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .
See what “Coenzyme A” is in other dictionaries:
- (Coenzyme Q10) Chemical compound ... Wikipedia
Coenzyme A ... Wikipedia
General... Wikipedia
Coenzyme M, HSCH2CH2SO3 Coenzyme M, Coenzyme M (2 mercaptoethane sulfonate, HS CoM) is a coenzyme involved in methyl group transfer reactions in methanogens. This coenzyme is the anion HSCH2CH2SO3. Mercaptoethanesulfonate ... Wikipedia
Coenzyme B, Coenzyme B (2 [(7 mercapto 1 oxoheptyl)amino] 3 phosphonooxybutanoic acid) is a coenzyme involved in redox reactions in methanogens. The coenzyme B molecule contains a thiol, which takes ... ... Wikipedia
General... Wikipedia
CoA, a coenzyme consisting of the nucleotide adenosine 3,5 diphosphate and pantothenic β mercaptoethylamide; participates in the transfer of acyl groups (acidic residues) that bind to the high-energy sulfhydryl group of CoA. acyl thioether linkage... Biological encyclopedic dictionary
COENZYMS(syn. coenzymes) - low molecular weight organic compounds of biological origin, necessary as additional specific components (cofactors) for the catalytic action of a number of enzymes. Many vitamins are derivatives of vitamins. Biol, the effect of a significant group of vitamins (group B) is determined by their transformation into vitamins and enzymes in the cells of the body. Attempts (and not unsuccessful ones) were made to directly use some K. to treat. goals. The difficulties that arise in this case are that quantitative determinations of K. content in the blood and organs are not always made, and even less often the activity of enzymes that synthesize or destroy the K. being studied is determined in normal and pathological conditions. When a deficiency of one or another vitamin is discovered in any disease, they usually try to eliminate it by introducing the corresponding vitamin into the body. But if the systems for the synthesis of the missing coenzyme are disrupted, which is often the case, then the introduction of such a vitamin loses its meaning: the therapeutic effect can only be obtained by introducing the missing coenzyme. With lech. purposes, cocarboxylase (see Thiamine), FAD, coenzyme forms of vitamin B 12 (see Cyanocobalamin) and some other K are used. For this purpose, K. is administered parenterally, but even under this condition there is not always confidence that they can penetrate to the site of their action (into the intracellular environment) without splitting.
Possessing a small pier. weight, K., in contrast to protein biocatalysts (enzymes), are characterized by thermal stability and accessibility to dialysis. Respiratory chromogens of plants (polyphenols), glutamine acid, ornithine, bisphosphates (diphosphates) of glucose and glycerol acid and other metabolites that act under certain circumstances as cofactors of enzymatic transfer processes are often referred to as K. of the corresponding processes. It is more correct to apply the term “coenzyme” only to compounds, biol, whose function is reduced entirely or predominantly to their specific participation in the action of enzymes (see).
The term “coenzyme” was proposed by G. Bertrand in 1897 to designate the function of manganese salts, which he considered a specific cofactor of phenolase (laccase); however, now it is not customary to classify inorganic components of enzyme systems as K. The existence of true (organic) K. was first established by the English. biochemists A. Harden and W. Young in 1904, who showed that dialysis removes the thermostable organic substance necessary for the action of the enzyme complex that catalyzes alcoholic fermentation from enzyme extracts of yeast cells (see). This auxiliary fermentation catalyst was named cosimase by Harden and Young; its structure was established in 1936 in the laboratories of H. Euler-Helpin and O. Warburg almost simultaneously.
The mechanism of action of K. is not the same. In many cases, they act as intermediate acceptors (transporters) of certain chemicals. groups (phosphate, acyl, amine, etc.), hydrogen atoms or electrons. In other cases, K. participate in the activation of molecules of substrates of enzymatic reactions, forming reactive intermediate compounds with these molecules. In the form of such compounds, substrates undergo certain enzymatic transformations; These are the functions of glutathione (see) as a coenzyme of glyoxalase and formaldehyde dehydrogenase, CoA - in a number of transformations of fatty acids (see) and other organic compounds, etc.
Typical K. form fragile, strongly dissociated compounds with specific proteins (apoenzymes) of soluble enzymes, from which they can easily be separated by dialysis (see) or gel filtration (see). In many group transfer reactions that occur under the conjugate action of two enzyme proteins, the alternating reversible addition of K particles to the molecules of these proteins occurs in two forms - acceptor and donor (for example, oxidized and reduced, phosphorylated and non-phosphorylated). The diagram below shows (in a somewhat simplified form) the mechanism of reversible hydrogen transfer between a hydrogen donor molecule (AH2) and an acceptor molecule (B) under the action of two dehydrogenases (Pha and Pb) and a coenzyme (Co):
Total reaction:
In the full cycle of the redox process (reactions 1-6), the coenzyme codedehydrogenase does not change and is not included in the balance of reaction products, i.e., it serves as a catalyst. If successive phases of the cycle are considered, each occurring with the participation of one enzyme (reactions 1-3 and 4-6), then Co and CoH2 act on a par with the molecules AN2, A, B, BN2 as the second substrate. In the same sense, the difference between substrates and dissociating compounds involved in coupled reactions of transfer of phosphate, acyl, glycosyl and other groups is relative.
In many two-component enzymes, built like proteids, the apoenzyme forms a strong, difficult-to-dissociate compound with a non-protein thermostable component. The non-protein components of protein enzymes, usually called prosthetic groups (eg, flavin nucleotides, pyridoxal phosphate, metalloporphyrins), interact with the substrate, remaining throughout the enzymatic reaction as part of an unresolved molecule of one protein. The term “coenzyme” is usually extended to chemically interact with substrate molecules, tightly bound organic prosthetic groups of enzymes, which are difficult to distinguish from easily dissociating enzymes, since there are gradual transitions between both types of cofactors.
In the same way, it is impossible to draw a sharp line between K. and certain intermediate metabolic products (metabolites), which in enzymatic processes act either as ordinary substrates that undergo a basically irreversible change in this process, or as necessary auxiliary catalysts for associated enzymatic transformations, from which these metabolites come out unchanged. Metabolites of this kind can serve as intermediate acceptors of certain groups in enzymatic transfer processes that proceed similarly to the process schematically depicted above (for example, the role of polyphenols as hydrogen carriers in the respiration of plant cells, the role of glutamic acid in the transfer of amine groups through transamination reactions and etc.), or in more complex cyclic transformations involving several enzymes (an example is the function of ornithine in the urea formation cycle). The coenzyme-like effect of 1,6-bisphosphoglucose is of a somewhat different nature; it serves as a necessary cofactor and at the same time an intermediate step in the process of intermolecular transfer of phosphate residues during the interconversion of 1-phosphoglucose and 6-phosphoglucose under the action of phosphoglucomutase, when the cofactor molecule transforms into a molecule the final product, giving one phosphate residue to the original product, from which a new cofactor molecule is formed. Exactly the same function is performed by 2,3-bisphosphoglycerol acid during the interconversion of 2-phosphoglycerol and 3-phosphoglycerol acid catalyzed by another phosphomutase.
K. are very diverse in chemistry. structure. However, most often among them there are compounds of two types: a) nucleotides and some other organic derivatives of phosphorus compounds; b) peptides and their derivatives (eg, folic acid, CoA, glutathione). In animals and in many microorganisms, the construction of molecules of the K series requires compounds that are not synthesized by these organisms and must be supplied with food, i.e., vitamins (see). Water-soluble B vitamins are mostly part of vitamins, the structure and functions of which are known (this applies to thiamine, riboflavin, pyridoxal, nicotinamide, pantothenic acid), or they can themselves act as active molecules of vitamins (vitamin B 12 , folic acid). The same probably applies to other water- and fat-soluble vitamins, the role of which in the processes of biol catalysis has not yet been fully elucidated.
The most important enzymes are listed below, indicating their type of structure and the main types of enzymatic transformations in which they participate. Articles about individual K. provide more detailed information about their structure and mechanism of action.
Coenzymes of nucleotide nature. Adenyl ribonucleotides (adenosine-5"-mono-, di- and triphosphoric acids) are involved in numerous reactions of activation and transfer of ortho- and pyrophosphate residues, amino acid residues (aminoacyls), carbonic and sulfuric acids, as well as in a number of others enzymatic transformations. In certain cases, similar functions are performed by derivatives of inosine-5"-phosphoric and guanosine-5"-phosphoric acid.
Guanylic ribonucleotides (guanosine-5"-mono-, di- and triphosphoric acids) play the role of K. in reactions of transfer of the succinic acid residue (succinyl), the biosynthesis of ribonucleoproteins in microsomes, the biosynthesis of adenyl acid from inosine and, possibly, , during the transfer of mannose residues.
In the biosynthesis of phosphatides, cytidyl ribonucleotides (cytidine-5"-phosphorus compounds) play the role of transport of O-phosphoethanol choline, O-phosphoethanolamine, etc. residues.
Uridyl ribonucleotides (uridine-5"-phosphorus compounds) perform K. functions in the processes of transglycosylation, that is, the transfer of monosacid residues (glucose, galactose, etc.) and their derivatives (hexosamine residues, glucuronic acid, etc.) etc.) during the biosynthesis of di- and polysaccharides, glucuronosides, hexosaminides (mucopolysaccharides), as well as during the activation of sugar residues and their derivatives in some other enzymatic processes (for example, the interconversion of glucose and galactose, etc.).
Nicotinamide adenine dinucleotide (NAD) participates in the most important hydrogen transfer reactions for cellular metabolism as a specific K. of numerous dehydrogenases (see).
Nicotinamide adenine dinucleotide phosphate (NADP) participates in hydrogen transfer reactions, which are important for cellular metabolism, as a specific enzyme for certain dehydrogenases.
Flavin mononucleotide (FMN) is involved in biol, hydrogen transfer as a K (prosthetic group) of some flavin (“yellow”) oxidative enzymes.
Flavin adenine dinucleotide (FAD) is involved in biol, hydrogen transfer as a K (prosthetic group) of most flavin (“yellow”) oxidative enzymes.
Coenzyme A (CoA, reduced form - KoA-SH, acylation coenzyme; compound of adenosine-3,5"-bisphosphoric acid with pantothenyl-aminoethanethiol or pantethene) forms with residues of acetic and other organic compounds thioesters of the R-CO type -S-CoA, where R is the residue of the organic acid, and plays the role of K. in the transfer and activation of acidic residues as in acylation reactions (synthesis of acetylcholine, hippuric acid, paired bile acids, etc.), and in many other enzymatic transformations of acidic residues (condensation reactions, oxidoreduction or reversible hydration of unsaturated compounds). With the participation of CoA, a number of intermediate reactions of cellular respiration, biosynthesis and oxidation of fatty acids, synthesis of steroids, terpenes, rubber, etc. occur.
Coenzyme B 12. It is possible that various biol, functions of vitamin B 12, chemical. the mechanism of which is not yet clear, for example, in the process of hematopoiesis, during the biosynthesis of methyl groups, transformations of sulfhydryl groups (SH groups), etc., are due to its role as K. in the process of biosynthesis of enzyme proteins.
Other coenzymes containing phosphate residues. Diphosphothiamin serves as a acid in the decarboxylation (simple and oxidative) of pyruvic, alpha-ketoglutaric, and other alpha-keto acids, as well as in the reactions of cleavage of the carbon chain of phosphorylated ketosaccharides under the action of a special group of enzymes (ketolase, transketolase, phosphoketolase).
Pyridoxal phosphate condenses with amino acids (and amines) into active intermediates such as Schiff bases (see Schiff bases); is a K (prosthetic group) of enzymes that catalyze transamination and decarboxylation reactions, as well as many other enzymes that carry out various transformations of amino acids (cleavage, substitution, condensation reactions) that play an important role in cellular metabolism.
Peptide coenzymes. Formylation coenzyme. Reduced folic acid and its derivatives, containing three or seven glutamic acid residues connected by gamma peptide bonds, play the role of K. in the intermediate metabolism of the so-called. one-carbon, or “C1”, residues (formyl, hydroxymethyl and methyl), participating both in the transfer reactions of these residues and in their redox interconversions. Formyl and oxymethyl derivatives of H4-folic acid are “active forms” of formic acid and formaldehyde in the processes of biosynthesis and oxidation of methyl groups, in the exchange of serine, glycine, histidine, methionine, purine bases, etc.
Glutathione. Reduced glutathione (G-SH) acts like K. during the conversion of methylglyoxal into lactic acid under the influence of glyoxalase, during enzymatic dehydrogenation of formaldehyde, in certain stages of biol, oxidation of tyrosine, etc. In addition, glutathione (see) plays a major role in protecting various thiol (sulfhydryl) enzymes from inactivation as a result of oxidation of SH groups or their binding by heavy metals and other SH poisons.
Other coenzymes. Lipoic acid is the second K. of pyruvic and alpha-ketoglutaric dehydrogenases (along with diphosphothiamin); Under the action of these enzymes, the lipoic acid residue, linked by an amide bond (CO - NH) with specific enzyme proteins, functions as an intermediate acceptor (carrier) of hydrogen and acyl residues (acetyl, succinyl). Other putative functions of this K. have not been sufficiently studied.
Vitamin E (tocopherol), vitamin K (phylloquinone) and the products of their redox transformations or closely related derivatives of n-benzoquinone (ubiquinone, coenzyme Q) are considered as K (hydrogen carriers) participating in certain intermediate reactions of the respiratory oxidative chain and in the associated with them respiratory phosphorylation (see). It has been established that phylloquinone (vitamin K) plays the role of vitamin K in the biosynthesis of alpha-carboxyglutamine residues that are part of the molecules of the protein components of the blood coagulation system.
Biotin is a water-soluble vitamin that acts as a vitamin or prosthetic group in a number of enzymes that catalyze carboxylation-decarboxylation reactions of certain organic compounds (pyruvic acid, propionic acid, etc.). These enzymes have the structure of biotinyl proteins, in which the acyl residue (biotinyl) corresponding to biotin is attached by an amide bond to the N6-amino group of one of the lysine residues of the protein molecule.
Ascorbic acid serves as an activator of the enzyme system for tyrosine oxidation in animal tissues and some other enzyme systems (hydroxylases), which act on the nucleus of aromatic and heterocyclic compounds, including peptide-linked proline residues during the biosynthesis of collagen, Tocopherols, Phylloquinones, Flavoproteins.
Bibliography: Baldwin E. Fundamentals of dynamic biochemistry, trans. from English, p. 55 and others, M., 1949; Vitamins, ed. M. I. Smirnova, M., 1974; Dixon M. and Webb E. Enzymes, trans. from English, M., 1966; Coenzymes, ed. V. A. Yakovleva, M., 1973; Kochetov G. A. Thiamine enzymes, M., 1978, bibliogr.; Enzymes, ed. A.E. Braunstein, p. 147, M., 1964, bibliogr.
A.E. Braunstein.
3O(n2cnc1c(ncnc12)N)(O)3OP(=O)(O)O]
Coenzyme A (coenzyme A, CoA, CoA, HSKoA)- acetylation coenzyme; one of the most important coenzymes that takes part in the transfer reactions of acyl groups during the synthesis and oxidation of fatty acids and the oxidation of pyruvate in the citric acid cycle.
Structure
600pxBiosynthesis
Coenzyme A is synthesized in five steps from pantothenic acid (vitamin B 5) and cysteine:
- Pantothenic acid is phosphorylated to 4'-phosphopantothenate by the enzyme pantothenate kinase
- Cysteine is added to 4"-phosphopantothenate by the enzyme phosphopantothenoylcysteine synthetase to form 4"-phospho-N-pantothenoylcysteine
- 4"-phospho-N-pantothenoylcysteine is decarboxylated to form 4"-phosphopantotheine by the enzyme phosphopantothenoylcysteine decarboxylase
- 4"-phosphopantotheine with adenylic acid forms dephospho-CoA under the action of the enzyme phosphopantotheine adenyltransferase
- Finally, dephospho-CoA is phosphorylated by ATP into coenzyme A by the enzyme dephosphocoenzyme kinase.
Biochemical role
A number of biochemical reactions are associated with CoA, underlying the oxidation and synthesis of fatty acids, the biosynthesis of fats, and the oxidative transformations of carbohydrate breakdown products. In all cases, CoA acts as an intermediate that binds and transfers acidic residues to other substances. In this case, the acidic residues in the composition of the compound with CoA undergo one or another transformation, or are transferred without changes to certain metabolites.
History of discovery
The coenzyme was first isolated from the liver of a pigeon in 1947 by F. Lipman. The structure of coenzyme A was determined in the early 1950s by F. Linen at the Lister Institute in London. The complete synthesis of CoA was carried out in 1961 by X. Koran.
List of acyl-CoAs
Various acyl derivatives of coenzyme A have been isolated and identified from natural compounds:
Acyl-CoA from carboxylic acids:
- Propionyl-CoA
- Acetoacetyl-CoA
- Kumarol-CoA
- Butyryl-CoA
Acyl-CoA from dicarboxylic acids:
- Malonil-CoA
- Succinyl-CoA
- Hydroxymethylglutaryl-CoA
- Pimenyl-CoA
Acyl-CoA from carbocyclic acids:
- Benzoyl-CoA
- Phenylacetyl-CoA
There are also a variety of fatty acid acyl-CoAs that are important as substrates for lipid synthesis reactions.
see also
Write a review about the article "Coenzyme A"
Notes
Literature
- Filippovich, Yu. B. Fundamentals of biochemistry: Textbook. for chem. and biol. specialist. ped. un-tov and in-tov / Yu. B. Filippovich. – 4th ed., revised. and additional – M.: “Agar”, 1999. – 512 p., ill.
- Berezov, T. T. Biological chemistry: Textbook / T. T. Berezov, B. F. Korovkin. – 3rd ed., revised. and additional – M.: Medicine, 1998. – 704 p., ill.
- Ovchinnikov, Yu. A. Bioorganic chemistry / Yu. A. Ovchinnikov. – M.: Education, 1987. – 815 p., ill.
- Plemenkov, V.V. Introduction to the chemistry of natural compounds / V.V. Plemenkov. – Kazan: KSU, 2001. – 376 p.
|
||||||||||||||
Excerpt describing Coenzyme A
Dad was furious... He hated it when people didn't break down. He hated it if they weren’t afraid of him... And therefore, for the “disobedient”, the torture continued much more persistently and angrily.Morone became white as death. Large drops of sweat rolled down his thin face and, breaking off, dripped to the ground. His endurance was amazing, but I understood that he couldn’t go on like this for long - every living body had a limit... I wanted to help him, to try to somehow relieve the pain. And then a funny idea suddenly occurred to me, which I immediately tried to implement - the stone hanging on the cardinal’s feet became weightless!.. Caraffa, fortunately, did not notice this. And Morone raised his eyes in surprise, and then hastily closed them so as not to give it away. But I managed to see - he understood. And she continued to “conjure” further in order to relieve his pain as much as possible.
- Go away, Madonna! – Dad exclaimed displeased. “You’re preventing me from enjoying the spectacle.” I have long wanted to see whether our dear friend will be so proud after the “work” of my executioner? You are disturbing me, Isidora!
This meant that he nevertheless understood...
Caraffa was not a seer, but he somehow caught a lot of things with his incredibly sharp sense. So now, sensing that something was happening and not wanting to lose control over the situation, he ordered me to leave.
But now I no longer wanted to leave. The unfortunate cardinal needed my help, and I sincerely wanted to help him. For I knew that if I left him alone with Caraffa, no one knew whether Morone would see the coming day. But Karaffa clearly didn’t care about my wishes... Without even allowing me to be indignant, the second executioner literally carried me out the door and, pushing me towards the corridor, returned to the room where Karaffa was left alone with Karaffa, albeit a very brave, but completely helpless, good man. ..
I stood in the corridor, confused, wondering how I could help him. But, unfortunately, there was no way out of his sad situation. In any case, I couldn’t find him so quickly... Although, to be honest, my situation was probably even sadder... Yes, while Caraffa had not yet tormented me. But the physical pain was not as terrible as the torment and death of loved ones were terrible... I did not know what was happening to Anna, and, afraid to somehow interfere, I waited helplessly... From my sad experience, I am too good I understood that I had offended Dad with some rash action, and the result would only be worse - Anna would probably have to suffer.
Days passed, and I didn’t know if my girl was still in Meteora? Did Caraffa appear behind her?.. And was everything okay with her?
My life was empty and strange, if not hopeless. I could not leave Karaffa, because I knew that if I just disappeared, he would immediately take out his anger on my poor Anna... Also, I was still not able to destroy him, because I did not find a way to the protection that he gave he was once a “stranger” person. Time flowed mercilessly, and I felt more and more my helplessness, which, coupled with inaction, began to slowly drive me crazy...
Almost a month has passed since my first visit to the cellars. There was no one nearby with whom I could even say a word. Loneliness oppressed more and more deeply, planting an emptiness in the heart, acutely seasoned with despair...
I really hoped that Morone still survived, despite the “talents” of the Pope. But she was afraid to return to the cellars, because she was not sure whether the unfortunate cardinal was still there. My return visit could bring upon him the real anger of Caraffa, and Morona would have to pay really dearly for this.
Remaining fenced off from any communication, I spent my days in complete “silence of loneliness.” Until, finally, unable to bear it any longer, she went down to the basement again...
The room in which I found Morone a month ago was empty this time. One could only hope that the brave cardinal was still alive. And I sincerely wished him good luck, which, unfortunately, the prisoners of Caraffa clearly lacked.
And since I was already in the basement anyway, after thinking a little, I decided to look further and carefully opened the next door...
And there, on some terrible torture “instrument” lay a completely naked, bloody young girl, whose body was a real mixture of living burnt meat, cuts and blood, covering her from head to toe... Neither the executioner nor the more - Caraffa, fortunately for me, there were no tortures in the torture room.
I quietly approached the unfortunate woman and carefully stroked her swollen, tender cheek. The girl moaned. Then, carefully taking her fragile fingers into my palm, I slowly began to “treat” her... Soon clear, gray eyes looked at me in surprise...
- Quiet, honey... Lie quietly. I will try to help you as much as possible. But I don’t know if I’ll have enough time... You’ve been hurt a lot, and I’m not sure if I’ll be able to “fix” it all quickly. Relax, my dear, and try to remember something kind... if you can.
The girl (she turned out to be just a child) groaned, trying to say something, but for some reason the words did not come out. She mumbled, unable to pronounce even the shortest word clearly. And then a terrible realization struck me - this unfortunate woman had no tongue!!! They tore it out... so as not to say too much! So that she wouldn’t scream the truth when they burn her at the stake... So that she wouldn’t be able to say what they did to her...
Loading...
