- General idea. Characteristics of the stages of the cycle cycle.
- Final products of the TFC.
- Biological role of the TCA cycle.
- Regulation of the TCA cycle.
- Disturbances in the operation of the central heating system.
· GENERAL VIEW. CHARACTERISTICS OF CTC STAGES
The tricarboxylic acid cycle (TCA cycle) is main, cyclic, metabolic pathway, in which the oxidation of active acetic acid and some other compounds formed during the breakdown of carbohydrates, lipids, proteins occurs and which provides the respiratory chain with reduced coenzymes.
CTK was opened in 1937 G. Krebs. He summarized the experimental studies available at that time and constructed a complete diagram of the process.
TCA cycle reactions proceed in mitochondria under aerobic conditions.
At the beginning of the cycle (Fig. 6), active acetic acid (acetyl-CoA) condenses with oxaloacetic acid (oxaloacetate) to form citric acid (citrate). This reaction is catalyzed citrate synthase .
The citrate is then isomerized to isocitrate. Isomerization of citrate is carried out by dehydration to form cis-aconitate and its subsequent hydration. Catalysis of both reactions provides aconitase .
At the 4th stage of the cycle, oxidative decarboxylation of isocitrate occurs under the influence of isocitrate dehydrogenase (ICDG) with education a-ketoglutaric acid, NADH(H +) or NADPH(H +) and CO 2 . NAD-dependent IDH is localized in mitochondria, and NADP-dependent enzyme is present in mitochondria and cytoplasm.
During the 5th stage, oxidative decarboxylation of a-ketoglutarate occurs with the formation active succinic acid (succinyl-CoA), NADH(H) and CO2. This process is catalyzed a-ketoglutarate dehydrogenase complex , consisting of three enzymes and five coenzymes. Enzymes: 1) a-ketoglutarate dehydrogenase associated with the coenzyme TPP; 2) transsuccinylase, the coenzyme of which is lipoic acid;
3) dihydrolipoyl dehydrogenase associated with FAD. In the work of a-ketoglutarate dehydrogenases
This complex also involves coenzymes CoA-SH and NAD.
At the 6th stage, the high-energy thioester bond of succinyl-CoA is cleaved, coupled with the phosphorylation of GDP. Are formed succinic acid (succinate) And GTP (at the level of substrate phosphorylation). The reaction is catalyzed succinyl-CoA synthetase (succinylthiokinase) . The phosphoryl group of GTP can be transferred to ADP: GTP + ADP ® GDP + ATP. The reaction is catalyzed with the participation of the enzyme nucleoside diphosphokinase.
During the 7th stage, succinate is oxidized under the influence of succinate dehydrogenase with education fumarateand FADN 2.
At the 8th stage fumarate hydratase ensures the addition of water to fumaric acid to form L-malic acid (L-malate).
L-malate at the 9th stage under the influence malate dehydrogenase oxidizes to oxaloacetate, the reaction also produces NADH(H+). The metabolic pathway closes on oxaloacetate and again repeats itself, purchasing cyclical character.
Rice. 6. Scheme of reactions of the tricarboxylic acid cycle.
· FINAL PRODUCTS
The overall CTC equation has the following form:
// ABOUT
CH 3 – C~ S-CoA + 3 NAD + + FAD + ADP + H 3 PO 4 + 3 H 2 O ®
® 2 CO 2 + 3 NADH(H +) + FADH 2 + ATP + CoA-SH
Thus, the final products of the cycle (per 1 turnover) are reduced coenzymes - 3 NADH (H +) and 1 FADH 2, 2 molecules of carbon dioxide, 1 molecule of ATP and 1 molecule of CoA - SH.
· BIOLOGICAL ROLE OF THE TCA cycle
The Krebs cycle performs integration, amphibolic (i.e. catabolic and anabolic), energy and hydrogen donor role.
Integration role is that the TTC is final common oxidation pathway fuel molecules - carbohydrates, fatty acids and amino acids.
Happens at the TsTK oxidation of acetyl-CoA iscatabolicrole.
Anabolic the role of the cycle is that it supplies intermediate products For biosynthetic processes. For example, oxaloacetate is used to synthesize aspartate, a-ketoglutarate – for education glutamate, succinyl-CoA – for synthesis heme.
One molecule ATP is formed in the TCA at the level substrate phosphorylation is energy role.
Hydrogen donor The role is that the TCA cycle provides reduced coenzymes NADH(H+) and FADH 2 the respiratory chain, in which the oxidation of hydrogen from these coenzymes to water occurs, coupled with the synthesis of ATP. When one molecule of acetyl-CoA is oxidized in the TCA cycle, 3 NADH(H +) and 1 FADH are formed 2
The ATP yield during the oxidation of acetyl-CoA is 12 ATP molecules (1 ATP in the TCA cycle at the level of substrate phosphorylation and 11 ATP molecules during the oxidation of 3 molecules of NADH(H +) and 1 molecule of FADH 2 in the respiratory chain at the level of oxidative phosphorylation).
· REGULATION OF THE TCA CYCLE
The operating speed of the central heating system is precisely adjusted to needs cells in ATP, i.e. The Krebs cycle is associated with a respiratory chain that functions only under aerobic conditions. An important regulatory reaction of the cycle is the synthesis of citrate from acetyl-CoA and oxaloacetate, which occurs with the participation of citrate synthase. High ATP levels inhibit this enzyme. The second regulatory reaction of the cycle is isocitrate dehydrogenase. ADP and NAD + activate enzyme, NADH(H+) and ATP inhibit. The third regulatory reaction is oxidative decarboxylation of a-ketoglutarate. NADH(H+), succinyl-CoA and ATP inhibit a-ketoglutarate dehydrogenase.
· DISRUPTIONS OF THE CTK OPERATION
Violation The functioning of the central circulation system may be related to:
With a lack of acetyl-CoA;
With a lack of oxaloacetate (it is formed during the carboxylation of pyruvate, and the latter, in turn, during the breakdown of carbohydrates). An imbalance of the diet in carbohydrates entails the inclusion of acetyl-CoA in ketogenesis (the formation of ketone bodies), which leads to ketosis;
With a violation of the activity of enzymes due to a lack of vitamins that are part of the corresponding coenzymes (a lack of vitamin B 1 leads to a lack of TPP and disruption of the functioning of the a-ketoglutarate dehydrogenase complex; a lack of vitamin B 2 leads to a lack of FAD and a violation of the activity of succinate dehydrogenase; a lack of vitamin B 3 leads to is a deficiency of the coenzyme acylation CoA-SH and impaired activity of the a-ketoglutarate dehydrogenase complex; lack of vitamin B 5 leads to a lack of NAD and impaired activity of isocitrate dehydrogenase, a-ketoglutarate dehydrogenase complex and malate dehydrogenase; lack of lipoic acid also leads to impaired functioning of the a-ketoglutarate dehydrogenase complex);
With a lack of oxygen (hemoglobin synthesis and the functioning of the respiratory chain are impaired, and the accumulating NADH (H +) acts in this case as an allosteric inhibitor of isocitrate dehydrogenase and the a-ketoglutarate dehydrogenase complex)
· Control questions
Acetyl-SCoA formed in the PVK dehydrogenase reaction then enters tricarboxylic acid cycle(TCA cycle, citric acid cycle, Krebs cycle). In addition to pyruvate, keto acids coming from catabolism are involved in the cycle amino acids or any other substances.
Tricarboxylic acid cycle
The cycle proceeds in mitochondrial matrix and represents oxidation molecules acetyl-SCoA in eight consecutive reactions.
In the first reaction they bind acetyl And oxaloacetate(oxaloacetic acid) to form citrate(citric acid), then isomerization of citric acid occurs to isocitrate and two dehydrogenation reactions with concomitant release of CO 2 and reduction of NAD.
In the fifth reaction GTP is formed, this is the reaction substrate phosphorylation. Next, FAD-dependent dehydrogenation occurs sequentially succinate(succinic acid), hydration fumarova acid to malate(malic acid), then NAD-dependent dehydrogenation resulting in the formation oxaloacetate.
As a result, after eight reactions of the cycle again oxaloacetate is formed .
The last three reactions constitute the so-called biochemical motif (FAD-dependent dehydrogenation, hydration and NAD-dependent dehydrogenation, it is used to introduce a keto group into the succinate structure. This motif is also present in the β-oxidation reactions of fatty acids. In the reverse order (reduction, de hydration and reduction) this motif is observed in fatty acid synthesis reactions.
Functions of the TsTK
1. Energy
- generation hydrogen atoms for the functioning of the respiratory chain, namely three molecules of NADH and one molecule of FADH2,
- single molecule synthesis GTF(equivalent to ATP).
2. Anabolic. In the TCC are formed
- heme precursor succinyl-SCoA,
- keto acids that can be converted into amino acids - α-ketoglutarate for glutamic acid, oxaloacetate for aspartic acid,
- lemon acid, used for the synthesis of fatty acids,
- oxaloacetate, used for glucose synthesis.
Anabolic reactions of the TCA cycle
Regulation of the tricarboxylic acid cycle
Allosteric regulation
Enzymes catalyzing the 1st, 3rd and 4th reactions of the TCA cycle are sensitive to allosteric regulation metabolites:
Regulation of oxaloacetate availability
Main And basic The regulator of the TCA cycle is oxaloacetate, or rather its availability. The presence of oxaloacetate recruits acetyl-SCoA into the TCA cycle and starts the process.
Usually the cell has balance between the formation of acetyl-SCoA (from glucose, fatty acids or amino acids) and the amount of oxaloacetate. The source of oxaloacetate is pyruvate, (formed from glucose or alanine), obtained from aspartic acid as a result of transamination or the AMP-IMP cycle, and also from fruit acids cycle itself (succinic, α-ketoglutaric, malic, citric), which can be formed during the catabolism of amino acids or come from other processes.
Synthesis of oxaloacetate from pyruvate
Regulation of enzyme activity pyruvate carboxylase carried out with the participation acetyl-SCoA. It is allosteric activator enzyme, and without it pyruvate carboxylase is practically inactive. When acetyl-SCoA accumulates, the enzyme begins to work and oxaloacetate is formed, but, of course, only in the presence of pyruvate.
Also the majority amino acids during their catabolism, they are able to transform into metabolites of the TCA cycle, which then go into oxaloacetate, which also maintains the activity of the cycle.
Replenishment of the TCA cycle metabolite pool from amino acids
Reactions of replenishment of the cycle with new metabolites (oxaloacetate, citrate, α-ketoglutarate, etc.) are called anaplerotic.
The role of oxaloacetate in metabolism
An example of a significant role oxaloacetate serves to activate the synthesis of ketone bodies and ketoacidosis blood plasma at insufficient amount of oxaloacetate in the liver. This condition is observed during decompensation of insulin-dependent diabetes mellitus (type 1 diabetes) and during fasting. With these disorders, the process of gluconeogenesis is activated in the liver, i.e. the formation of glucose from oxaloacetate and other metabolites, which entails a decrease in the amount of oxaloacetate. The simultaneous activation of fatty acid oxidation and the accumulation of acetyl-SCoA triggers a backup pathway for the utilization of the acetyl group - synthesis of ketone bodies. In this case, blood acidification develops in the body ( ketoacidosis) with a characteristic clinical picture: weakness, headache, drowsiness, decreased muscle tone, body temperature and blood pressure.
Changes in the rate of TCA cycle reactions and the reasons for the accumulation of ketone bodies under certain conditions
The described method of regulation with the participation of oxaloacetate is an illustration of the beautiful formulation " Fats burn in the flames of carbohydrates"It implies that the "flame of combustion" of glucose leads to the appearance of pyruvate, and pyruvate is converted not only into acetyl-SCoA, but also into oxaloacetate. The presence of oxaloacetate ensures the inclusion of the acetyl group formed from fatty acids in the form of acetyl-SCoA, in the first reaction of the TCA cycle.
In the case of large-scale “combustion” of fatty acids, which is observed in muscles during physical work and in the liver fasting, the rate of entry of acetyl-SCoA into the TCA cycle reaction will directly depend on the amount of oxaloacetate (or oxidized glucose).
If the amount of oxaloacetate in hepatocyte is not enough (there is no glucose or it is not oxidized to pyruvate), then the acetyl group will go to the synthesis of ketone bodies. This happens when long fasting And type 1 diabetes mellitus.
Citric acid cycle(tricarboxylic acid cycle - TCA cycle, Krebs cycle) is a series of reactions occurring in mitochondria, during which the catabolism of acetyl groups and the release of reducing equivalents occurs; during the oxidation of the latter, free energy is supplied to the ETC and accumulated in ATP. The cycle is started by oxaloacetate, which is synthesized from PVC under the influence of pyruvate carboxylase.
The acetyl-CoA molecule, obtained in the oxidative decarboxylation of PVK and β-oxidation of VFA, interacts with OA; as a result, a 6-carbon tricarboxylic acid is generated - lemon (citrate)(Fig. 3.8) . Next, in a series of reactions, two molecules of carbon dioxide are released and oxaloacetate is regenerated. Since the amount of the latter required to convert a large number of acetyl groups is very small, we can assume that this compound performs a catalytic function.
In the TCA cycle, due to the activity of a number of specific dehydrogenases, the formation of reducing equivalents occurs in the form of protons and electrons, inducing the respiratory chain, during the functioning of which ATP is synthesized
Formation of high-energy compounds in the TCA cycle
| Oxidizable substrate |
Enzyme, catalytic |
Place of formation of macroergs and the nature of the associated process | Number of ATP molecules synthesized |
| Isocitrate | IsocitrateDH | 3 | |
| α-Ketoglutarate | α-ketoglutarateDH | Oxidation of NADH in the respiratory chain | 3 |
| Succinyl phosphate | Succinate thiokinase | ATP synthesis at the substrate level | 1 |
| Succinate | SuccinateDH | Oxidation of FADH 2 in the respiratory chain | 2 |
| Malate | MalatDG | Oxidation of NADH in the respiratory chain | 3 |
| Total | 12 | ||
Thus, each cycle provides the synthesis of 12 macroerg molecules.
Biological functions of the Krebs cycle
The TCA cycle is the common final pathway for the oxidative breakdown of carbohydrates, lipids, and proteins, since during metabolism, glucose, FA, glycerol, amino acids and acyclic nitrogenous bases are converted either into acetyl-CoA or into metabolites of this process, which are sources of reducing equivalents that trigger ETC and oxidative phosphorylation, thereby ensuring the energy demands of various organs and tissues, and constant body temperature. Endogenous water is also formed, as is known, due to biological oxidation, the substrates of which are metabolites of the TCA cycle. Intermediate products of the TCA cycle can be used in anabolism: OA and its precursors serve as substrates in the GNG; It is easy to obtain amino acids from α-ketoglutarate and OA using transamination; succinyl-CoA is necessary for heme synthesis; Excess citrate, leaving the mitochondria, splits off acetyl-CoA, from which IVH, cholesterol, acetylcholine, and derivatives of monosaccharides (monomers of heteropolysaccharides) are generated.
In humans, genetically determined damage to the enzymes catalyzing its various stages has not been described, because the occurrence of such disorders is incompatible with the normal development of the body.
The tricarboxylic acid cycle was discovered in 1937 by G. Krebs. In this regard, it was called the “Krebs cycle”. This process is the central pathway of metabolism. It occurs in the cells of organisms at different stages of evolutionary development (microorganisms, plants, animals).
The initial substrate of the tricarboxylic acid cycle is acetyl coenzyme A. This metabolite is the active form of acetic acid. Acetic acid acts as a common intermediate breakdown product of almost all organic substances contained in the cells of living organisms. This is because organic molecules are carbon compounds that can naturally break down into two-carbon acetic acid units.
Free acetic acid has a relatively weak reactivity. Its transformations occur under rather harsh conditions, which are unrealistic in a living cell. Therefore, acetic acid is activated in cells by combining it with coenzyme A. As a result, a metabolically active form of acetic acid is formed - acetyl-coenzyme A.
Coenzyme A is a low molecular weight compound that consists of phosphoadenosine, a pantothenic acid residue (vitamin B3) and thioethanolamine. The acetic acid residue is added to the sulfhydryl group of thioethanolamine. In this case, a thioether is formed - acetyl-coenzyme A, which is the initial substrate of the Krebs cycle.
Acetyl coenzyme A
A diagram of the transformation of intermediate products in the Krebs cycle is shown in Fig. 67. The process begins with the condensation of acetyl coenzyme A with oxaloacetate (oxaloacetic acid, OCA), resulting in the formation of citric acid (citrate). The reaction is catalyzed by the enzyme citrate synthase.
Figure 67 – Scheme of the transformation of intermediate products in the cycle
tricarboxylic acids
Further, under the action of the enzyme aconitase, citric acid is converted into isocitric acid. Isocitric acid undergoes oxidation and decarboxylation processes. In this reaction, catalyzed by the enzyme NAD-dependent isocitrate dehydrogenase, the products are carbon dioxide, reduced NAD, and a-ketoglutaric acid, which is then involved in the process of oxidative decarboxylation (Fig. 68).
Figure 68 – Formation of a-ketoglutaric acid in the Krebs cycle
The process of oxidative decarboxylation of a-ketoglutarate is catalyzed by the enzymes of the a-ketoglutarate dehydrogenase multienzyme complex. This complex consists of three different enzymes. It requires coenzymes to function. Coenzymes of the a-keto-glutarate dehydrogenase complex include the following water-soluble vitamins:
· vitamin B 1 (thiamine) – thiamine pyrophosphate;
· vitamin B 2 (riboflavin) – FAD;
· vitamin B 3 (pantothenic acid) – coenzyme A;
· vitamin B 5 (nicotinamide) – NAD;
· vitamin-like substance – lipoic acid.
Schematically, the process of oxidative decarboxylation of a-keto-glutaric acid can be represented as the following balance reaction equation:
The product of this process is a thioester of the succinic acid residue (succinate) with coenzyme A - succinyl-coenzyme A. The thioester bond of succinyl-coenzyme A is macroergic.
The next reaction of the Krebs cycle is the process of substrate phosphorylation. In it, the thioester bond of succinyl-coenzyme A is hydrolyzed under the action of the enzyme succinyl-CoA synthetase with the formation of succinic acid (succinate) and free coenzyme A. This process is accompanied by the release of energy, which is immediately used for phosphorylation of HDP, which results in the formation of a high-energy molecule GTP phosphate. Substrate phosphorylation in the Krebs cycle:
where Fn is orthophosphoric acid.
GTP formed during oxidative phosphorylation can be used as an energy source in various energy-dependent reactions (in the process of protein biosynthesis, activation of fatty acids, etc.). In addition, GTP can be used to generate ATP in the nucleoside diphosphate kinase reaction
The product of the succinyl-CoA synthetase reaction, succinate, is further oxidized with the participation of the enzyme succinate dehydrogenase. This enzyme is a flavin dehydrogenase, which contains the FAD molecule as a coenzyme (prosthetic group). As a result of the reaction, succinic acid is oxidized to fumaric acid. At the same time, FAD is restored.
where E is the FAD prosthetic group associated with the polypeptide chain of the enzyme.
Fumaric acid formed in the succinate dehydrogenase reaction, under the action of the fumarase enzyme (Fig. 69), attaches a water molecule and is converted into malic acid, which is then oxidized in the malate dehydrogenase reaction into oxaloacetic acid (oxaloacetate). The latter can be used again in the citrate synthase reaction for the synthesis of citric acid (Fig. 67). Due to this, transformations in the Krebs cycle are cyclic in nature.
Figure 69 – Metabolism of malic acid in the Krebs cycle
The balance equation of the Krebs cycle can be presented as:
It shows that in the cycle there is complete oxidation of the acetyl radical of the residue from acetyl-coenzyme A to two molecules of CO 2. This process is accompanied by the formation of three molecules of reduced NAD, one molecule of reduced FAD and one molecule of high-energy phosphate - GTP.
The Krebs cycle occurs in the mitochondrial matrix. This is due to the fact that this is where most of its enzymes are located. And only a single enzyme, succinate dehydrogenase, is built into the inner mitochondrial membrane. The individual enzymes of the tricarboxylic acid cycle are combined into a functional multienzyme complex (metabolon) associated with the inner surface of the inner mitochondrial membrane. By combining enzymes into a metabolon, the efficiency of functioning of this metabolic pathway is significantly increased and additional opportunities for its fine regulation appear.
Features of the regulation of the tricarboxylic acid cycle are largely determined by its significance. This process performs the following functions:
1) energy. The Krebs cycle is the most powerful source of substrates (reduced coenzymes - NAD and FAD) for tissue respiration. In addition, energy is stored in it in the form of high-energy phosphate - GTP;
2) plastic. Intermediate products of the Krebs cycle are precursors for the synthesis of various classes of organic substances - amino acids, monosaccharides, fatty acids, etc.
Thus, the Krebs cycle performs a dual function: on the one hand, it is a general pathway of catabolism, playing a central role in the energy supply of the cell, and on the other, it provides biosynthetic processes with substrates. Such metabolic processes are called amphibolic. The Krebs cycle is a typical amphibolic cycle.
The regulation of metabolic processes in the cell is closely related to the existence of “key” enzymes. The key enzymes in the process are those that determine its speed. Typically, one of the “key” enzymes in a process is the enzyme that catalyzes its initial reaction.
The “key” enzymes are characterized by the following features. These enzymes
· catalyze irreversible reactions;
· have the least activity compared to other enzymes involved in the process;
· are allosteric enzymes.
The key enzymes of the Krebs cycle are citrate synthase and isocitrate dehydrogenase. Like key enzymes in other metabolic pathways, their activity is regulated by negative feedback: it decreases as the concentration of Krebs cycle intermediates in mitochondria increases. Thus, citric acid and succinyl-coenzyme A act as citrate synthase inhibitors, and reduced NAD acts as isocitrate dehydrogenase.
ADP is an activator of isocitrate dehydrogenase. Under conditions of increasing cell need for ATP as an energy source, when the content of breakdown products (ADP) increases in it, prerequisites arise for increasing the rate of redox transformations in the Krebs cycle and, consequently, increasing the level of its energy supply.
The tricarboxylic acid cycle (TCA cycle, Krebs cycle, citric acid cycle) is the most important supplier to the respiratory chain of reduced forms of coenzymes and prosthetic groups formed during the utilization of acetyl-CoA (1), keto acids, oxidation products of monosaccharides, higher fatty acids (HFAs) and amino acids (see Fig. 28).
All enzymes of the process are localized in the mitochondrial matrix, with the exception of succinate dehydrogenase (6*, Fig. 28). The rate of flow of the TCA cycle depends primarily on the rate of formation of acetyl-CoA in the mitochondrial matrix (Fig. 28, (1)), the supply of its precursors (pyruvate, IVF) and a number of other factors that need to be considered in relation to each of the eight reactions of the cycle Krebs:
1) Condensation of acetyl-CoA (1) with oxaloacetate (oxaloacetic acid (OA), 2) is carried out by the enzyme citrate synthase (1*). The activity of citrate synthase is inhibited by the accumulation of ATP, NADH, succinyl-CoA and IVF acyls in the matrix;
2) Isomerization of citrate (3) into isocitrate (5) is carried out by the enzyme aconitase (Fe 2+ -containing protein, 2*) in two stages:
Stage 1 - dehydration of citrate with the formation of cis-aconitic acid (4);
Stage 2 – hydration of cis-aconitic acid at the double bond to form isocitrate (5).
The enzyme is inhibited by arsenic acid derivatives.
Figure 28. Krebs cycle. In the process diagram, all enzymes are marked with a number with an asterisk, metabolites are marked with a number in parentheses (see names in the text).
3) Under the action of NAD + - dependent isocitrate dehydrogenase (3*), oxidative decarboxylation of isocitrate (5) occurs with the formation of products:
ά-ketoglutarate (7), CO 2 and NADH (electron donor to the respiratory chain). The reaction proceeds in two stages: 1) dehydrogenation with the formation of oxalic-succinic acid (6); 2) decarboxylation of this substance to ά-ketoglutaric acid. Isocitrate dehydrogenase is the rate limiting enzyme for the entire Krebs cycle. The enzyme is activated by ADP, Mg 2+ and Mn 2+ ions; inhibited by the accumulation of ATP and NADH in the matrix;
4) Oxidative decarboxylation of ά-ketoglutarate is carried out by the ά-ketoglutarate dehydrogenase complex (4*). This is a multienzyme system in composition (three enzymes) and vitamin content: vitamins B 1 (coenzyme TDP), B 2 (prosthetic group FAD), B 5 (coenzyme CoASH), B 3 (coenzyme NAD +), lipoic acid amide). As a result of the complex's operation, CO 2 and succinyl-CoA (macroergic substance, 8) are formed; NADH (electron donor to the respiratory chain);
5) Succinyl-CoA thiokinase (synthase, 5*), using the energy of breaking the high-energy bond in succinyl-CoA, phosphorylates GDP with the formation of GTP, while in parallel the formation of succinic acid occurs (at the anion - succinate, 9). This reaction is called substrate phosphorylation. The formed GTP can then be converted into ATP by the action of nucleoside diphosphate kinase according to the equation:
GTP + ADP → ATP + GDP
6) Succinate dehydrogenase (the only enzyme of the TCA cycle localized on the inner membrane of mitochondria, 6*), thanks to the prosthetic group FAD, oxidizes succinic acid (9) to trans-fumaric acid (10). Succinate dehydrogenase in the inner membrane of mitochondria forms a complex with iron-sulfur-containing proteins, which is called complex II of the respiratory chain. Malonic acid is a competitive enzyme inhibitor;
7) The enzyme fumarase (7*) hydrates at the double bond only the trans-form of fumaric acid with the formation of L-malic acid (at the anion - L-malate, 11). The reaction is reversible, fumarase is stereospecific only to L-malate.
8) At the last stage of the cycle, NAD + - dependent malate dehydrogenase (8*) catalyzes the oxidation of L-malate into oxaloacetic acid (OA) with the formation of NADH (electron donor to the respiratory chain). The reaction is reversible, but rapid use of PCA in the citrate synthase reaction shifts the equilibrium to the right.
Thus, during eight reactions of the Krebs cycle, through the formation of three tricarboxylic acids (citric, cis-aconitic, isocitric), during four dehydrogenase reactions, two of which were accompanied by decarboxylation (3*, 4*), 2 moles of CO 2 are formed, 3 NADH, 1 FADH 2 and 1 GTP equivalent to 1 ATP. These substances are called the end products of the Krebs cycle per cycle. PIKE constantly regenerates and is again included in the citrate synthase reaction, so this substance need not be called the end product of the cycle.
The main regulatory reactions of the TCA cycle are citrate synthase and isocitrate dehydrogenase. The regulation of the TCA cycle involves the principle of metabolic feedback. The intensity of oxidation of substrates in it increases under conditions of increasing concentrations of ADP and NAD +. Under conditions of increasing concentrations of ATP and NADH, the rate of oxidation of substrates in the Krebs cycle decreases. Such regulation makes it possible to adequately change the intensity of the functioning of the TCA cycle under conditions that require an urgent change in the level of energy supply to the cell.
The intensity of the TCA flow can be determined by the value of respiratory control, which is expressed by the concentration ratio [ATP]/[ADP]. At [ATP]/[ADP] values<1 увеличивается скорость включения в дыхательную цепь восстановленных форм коферментов НАДН, при этом скорость ЦТК увеличивается.
The Krebs cycle is an amphibolic process, since although it is a catabolic process, some of its metabolites can be used by the cell for synthetic purposes. Succinyl-CoA is used by the cell as the starting substrate for the first reaction of heme synthesis. Oxaloacetate and its precursors in the cycle can be used in the synthesis of glucose (the process of gluconeogenesis). Keto acids - oxaloacetate and alpha-ketoglutarate, thanks to transamination reactions, can be used to form non-essential amino acids: aspartic and glutamic acids, respectively.