(citric acid cycle or Krebs cycle)

Under aerobic conditions, the resulting acetyl-CoA enters the Krebs cycle. In the Krebs cycle, after the reactions of withdrawal and addition of water, decarboxylation and dehydrogenation, the acetyl residue that entered the cycle in the form of acetyl-CoA is completely cleaved. The overall reaction is written as follows:

CH 3 CO ~ S-CoA + 3H 2 O + ADP + H 3 RO 4 →

HS-CoA + 2CO 2 + 4[H 2] + ATP

The Krebs cycle is the same in animals and plants. This is another proof of the unity of origin. The cycle occurs in the stroma of mitochondria. Let's consider it in more detail:

The first reaction of the cycle is the transfer of an acetyl residue from acetyl-CoA to oxaloacetic acid (OAA) with the formation of citric acid (citrate) (Fig. 3.2).

During the reaction catalyzed by citrate synthase, the macroergic bond of acetyl-CoA is wasted, i.e., the energy that was stored in the process of pyruvate oxidation before the start of the cycle. This means, like glycolysis, the Krebs cycle does not begin with the storage of energy in the cell, but with expenditure.

We emphasize that the chain of transformations that form this cycle and are ultimately aimed at destroying the carbon composition of a number of acids begins with their increase: the two-carbon fragment (acetic acid) is added to the tetragonal fragment of AAA with the formation of six-carbon tricarboxylic acid citrate, which can be stored in cells in large quantities.

Thus, the Krebs cycle is a catalytic process and begins not with catabolism (destruction), but with the synthesis of citrate. Citrate synthetase, which catalyzes this reaction, belongs to the regulatory enzymes: it is inhibited by NADH and ATP. NADH is the end product in the form of which the energy released during respiration is stored. The more active citrate synthetase, the faster other reactions of the cycle will go, the faster the dehydrogenation of substances with the formation of NADH will go. However, an increase in the amount of the latter causes inhibition of the enzyme, and the cycle will slow down. This is an example of a feedback loop.

The next series of reactions is the conversion of citrate into active isocitric acid (isocitrate). It proceeds with the participation of water and, in fact, comes down to the intramolecular transformation of citric acid. The intermediate product of this transformation is cis-aconitic acid:

Both reactions are catalyzed by aconitase. The isocitrate is then dehydrated by isocitrate dehydrogenase, whose coenzyme is NAD+. As a result of oxidation, oxalo-succinic acid (oxalosuccinate) is formed.

The latter acid is decarboxylated. The detached CO 2 belongs to the acetyl residue, which entered the cycle in the form of acetyl-CoA. As a result of decarboxylation, a very active α-ketoglutaric acid (ketoglutarate) is formed.

α-Ketoglutarate, in turn, undergoes the same change that occurs before the start of the cycle with pyruvate: simultaneous oxidation and decarboxylation.

The α-ketoglutarate dehydrogenase complex takes part in the reaction:

α-ketoglutarate + NAD + + CoA–SH →

succinyl-S-CoA + CO 2 + NADH + H + →

succinyl-S-SOA + ADP + H 3 RO 4 →

succinic acid + ATP + CoA–SH

The released CO 2 is another particle that is split off from the acetyl residue. The succinic acid (succinate) formed as a result of these complex transformations is dehydrogenated again, and fumaric acid (fumarate) is formed. The reaction is mediated by succinate dehydrogenase. Fumarate, after the addition of a water molecule, is easily converted into malic acid (malate). Fumarate hydrotase takes part in the reaction.

Malic acid, being oxidized, is converted into PAA with the participation of NAD + - specific malate dehydrogenase.

Recall that PAA is the end product of the Krebs cycle – it is also formed during photosynthesis of C 4 plants (the Hatch–Sleck cycle) during the carboxylation of PEP in the light and in the dark in plants of the CAM type.

Thus, the Krebs cycle ends and can start over. One condition is the supply of new acetyl-CoA molecules.

The main significance of the Krebs cycle is the storage of energy, which is released as a result of the destruction of pyruvate, in the macroergic bonds of ATP. By supplying ATP to the cell, the Krebs cycle can be a regulator of other processes that require energy, such as the transport of water and salts, the synthesis and transport of organic substances. The faster the transformation of substances in the cycle takes place, the more ATP can be synthesized, the faster these processes will go.

Intermediate substances formed in the cycle can be used for the synthesis of proteins, fats, carbohydrates. For example, acetyl-CoA is a necessary product for the synthesis of fatty acids, ketoglutarate can be converted into glutamic acid as a result of reductive amination, and fumarate or PAA can be converted into aspartic acid.

The overall result of the Krebs cycle is thus reduced to the fact that each acetyl group (two-carbon fragment) that is formed from pyruvate (three-carbon fragment) is cleaved to CO 2 . During this process, NAD +, FAD + are restored and ATP is synthesized.

In the regulation of the cycle of di- and tricarboxylic acids, the ratio between NADH and NAD +, as well as the concentration of ATP, is important. A high content of ATP and NADH inhibits the activity of such enzymes of the Krebs cycle as pyruvate dehydrogenase, citrate synthetase, isocitrate dehydrogenase, malate dehydrogenase. An increase in the concentration of oxaloacetate inhibits the enzymes whose activity is associated with its synthesis - succinate dehydrogenase and malate dehydrogenase. The oxidation of 2-hydroxyglutaric acid is accelerated by adenylates, while that of succinate is accelerated by ATP, ADP, and ubiquinone. There are a number of other regulation points in the Krebs cycle.

Glyoxylate pathway

With the germination of fat-rich seeds, the course of the Krebs cycle changes slightly. This kind of Krebs cycle, in which glyoxylic acid participates, is called the glyoxylate cycle (Fig. 3.3).

The first stages of transformations before the formation of isocitrate (isocitric acid) are similar to the Krebs cycle. Then the course of reactions changes. Isocitrate, with the participation of isocitrate lyase, is cleaved into succinic and glyoxylic acids:

Succinate (succinic acid) leaves the cycle, and glyoxylate binds to acetyl-CoA and malate is formed. The reaction is catalyzed by malate synthase. The malate is oxidized to PI and the cycle ends. In addition to two enzymes - isocitratase (isocitrate lyase) and malate synthase, all the rest are the same as in the Krebs cycle. When malate is oxidized, the NAD + molecule is restored. The source of acetyl-CoA for this cycle are fatty acids formed during the destruction of fats. The total cycle equation can be written as:

2CH 3 CO-S-CoA + 2H 2 O + OVER + →

2HS-CoA + COOH-CH 2 -CH 2 -COOH + NADH + H +

The glyoxylate cycle occurs in special organelles - glyoxisomes.

What is the significance of this cycle? Reduced NADH can be oxidized to form three ATP molecules. Succinate (succinic acid) leaves the glyoxisome and enters the mitochondria, where it is included in the Krebs cycle. Here it is converted to PIE, then to pyruvate, phosphoenolpyruvate and further to sugar.

Thus, with the help of the glyoxylate cycle, fats can be converted into carbohydrates. This is very important especially during seed germination, since sugars can be transported from one part of the plant to another, while fats cannot. Glyoxylate can serve as a material for the synthesis of porphyrins, and this means chlorophyll.

Tricarboxylic acid cycle (Krebs cycle)

Glycolysis converts glucose to pyruvate and produces two ATP molecules from a glucose molecule - this is a small fraction of the potential energy of this molecule.

Under aerobic conditions, pyruvate is converted from glycolysis to acetyl-CoA and oxidized to CO 2 in the tricarboxylic acid cycle (citric acid cycle). In this case, the electrons released in the reactions of this cycle pass NADH and FADH 2 to 0 2 - the final acceptor. Electronic transport is associated with the creation of a proton gradient of the mitochondrial membrane, the energy of which is then used for ATP synthesis as a result of oxidative phosphorylation. Let's take a look at these reactions.

Under aerobic conditions, pyruvic acid (stage 1) undergoes oxidative decarboxylation, which is more efficient than transformation into lactic acid, with the formation of acetyl-CoA (stage 2), which can be oxidized to the end products of glucose breakdown - CO 2 and H 2 0 (3rd stage). G. Krebs (1900-1981), a German biochemist, having studied the oxidation of individual organic acids, combined their reactions into a single cycle. Therefore, the tricarboxylic acid cycle is often called the Krebs cycle in his honor.

The oxidation of pyruvic acid to acetyl-CoA occurs in mitochondria with the participation of three enzymes (pyruvate dehydrogenase, lipoamide dehydrogenase, lipoylacetyltransferase) and five coenzymes (NAD, FAD, thiamine pyrophosphate, lipoic acid amide, coenzyme A). These four coenzymes contain B vitamins (B x, B 2 , B 3 , B 5), which indicates the need for these vitamins for the normal oxidation of carbohydrates. Under the influence of this complex enzyme system, pyruvate in the oxidative decarboxylation reaction is converted into the active form of acetic acid - acetyl coenzyme A:

Under physiological conditions, pyruvate dehydrogenase is an exclusively irreversible enzyme, which explains the impossibility of converting fatty acids into carbohydrates.

The presence of a macroergic bond in the acetyl-CoA molecule indicates the high reactivity of this compound. In particular, acetyl-CoA can act in mitochondria to generate energy; in the liver, excess acetyl-CoA is used for the synthesis of ketone bodies; in the cytosol, it is involved in the synthesis of complex molecules such as sterides and fatty acids.

Acetyl-CoA obtained in the reaction of oxidative decarboxylation of pyruvic acid enters the tricarboxylic acid cycle (Krebs cycle). The Krebs cycle - the final catabolic pathway for the oxidation of carbohydrates, fats, amino acids, is essentially a "metabolic boiler". The reactions of the Krebs cycle, which take place exclusively in the mitochondria, are also called the citric acid cycle or the tricarboxylic acid cycle (TCA).

One of the most important functions of the tricarboxylic acid cycle is the generation of reduced coenzymes (3 molecules of NADH + H + and 1 molecule of FADH 2) followed by the transfer of hydrogen atoms or their electrons to the final acceptor, molecular oxygen. This transport is accompanied by a large decrease in free energy, part of which is used in the process of oxidative phosphorylation for storage in the form of ATP. It is understood that the tricarboxylic acid cycle is aerobic, dependent on oxygen.

1. The initial reaction of the tricarboxylic acid cycle is the condensation of acetyl-CoA and oxaloacetic acid with the participation of the mitochondrial matrix citrate synthase enzyme to form citric acid.

2. Under the influence of the enzyme aconitase, which catalyzes the removal of a water molecule from citrate, the latter is converted

to cis-aconitic acid. Water combines with cis-aconitic acid, turning into isocitric acid.

3. Then the enzyme isocitrate dehydrogenase catalyzes the first dehydrogenase reaction of the citric acid cycle, when isocitric acid is converted into α-ketoglutaric acid in oxidative decarboxylation reactions:

In this reaction, the first molecule of CO 2 and the first molecule of NADH 4- H + cycle are formed.

4. Further conversion of α-ketoglutaric acid to succinyl-CoA is catalyzed by the multienzyme complex of α-ketoglutaric dehydrogenase. This reaction is chemically analogous to the pyruvate dehydrogenase reaction. It involves lipoic acid, thiamine pyrophosphate, HS-KoA, NAD +, FAD.

As a result of this reaction, the molecule of NADH + H + and CO 2 is again formed.

5. The succinyl-CoA molecule has a macroergic bond, the energy of which is stored in the next reaction in the form of GTP. Under the influence of the enzyme succinyl-CoA synthetase, succinyl-CoA is converted into free succinic acid. Note that succinic acid can also be obtained from methylmalonyl-CoA by oxidation of fatty acids with an odd number of carbon atoms.

This reaction is an example of substrate phosphorylation, since the high-energy GTP molecule in this case is formed without the participation of the electron and oxygen transport chain.

6. Succinic acid is oxidized to fumaric acid in the succinate dehydrogenase reaction. Succinate dehydrogenase, a typical iron-sulphur-containing enzyme whose coenzyme is FAD. Succinate dehydrogenase is the only enzyme fixed on the inner mitochondrial membrane, while all other cycle enzymes are located in the mitochondrial matrix.

7. This is followed by the hydration of fumaric acid to malic acid under the influence of the fumarase enzyme in a reversible reaction under physiological conditions:

8. The final reaction of the tricarboxylic acid cycle is the malate dehydrogenase reaction involving the active enzyme of the mitochondrial NAD~-dependent malate dehydrogenase, in which the third molecule of reduced NADH + H + is formed:

The formation of oxaloacetic acid (oxaloacetate) completes one turn of the tricarboxylic acid cycle. Oxaloacetic acid can be used in the oxidation of the second acetyl-CoA molecule, and this cycle of reactions can be repeated many times, constantly leading to the production of oxaloacetic acid.

Thus, the oxidation of one molecule of acetyl-CoA as a cycle substrate in the TCA cycle leads to the production of one GTP molecule, three NADP + H + molecules, and one FADH 2 molecule. The oxidation of these reducing agents in the biological oxidation chain

ion leads to the synthesis of 12 ATP molecules. This calculation is clear from the topic “Biological oxidation”: the inclusion of one NAD + molecule in the electron transport system is ultimately accompanied by the formation of 3 ATP molecules, the inclusion of a FADH 2 molecule provides the formation of 2 ATP molecules, and one GTP molecule is equivalent to 1 ATP molecule.

Note that two carbon atoms of adetyl-CoA enter the tricarboxylic acid cycle and two carbon atoms leave the cycle in the form of CO 2 in decarboxylation reactions catalyzed by isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase.

With the complete oxidation of a glucose molecule under aerobic conditions to CO 2 and H 2 0, the formation of energy in the form of ATP is:

• 4 ATP molecules during the conversion of a glucose molecule into 2 molecules of pyruvic acid (glycolysis);
• 6 ATP molecules formed in the 3-phosphoglyceraldehyde dehydrogenase reaction (glycolysis);
• 30 ATP molecules formed during the oxidation of two pyruvic acid molecules in the pyruvate dehydrogenase reaction and in the subsequent transformations of two acetyl-CoA molecules to CO 2 and H 2 0 in the tricarboxylic acid cycle. Therefore, the total energy output during the complete oxidation of a glucose molecule can be 40 ATP molecules. However, it should be taken into account that during the oxidation of glucose at the stage of converting glucose into glucose-6-phosphate and at the stage of converting fructose-6-phosphate into fructose-1,6-diphosphate, two ATP molecules were consumed. Therefore, the "net" energy output during the oxidation of a glucose molecule is 38 ATP molecules.

You can compare the energy of anaerobic glycolysis and aerobic glucose catabolism. Of the 688 kcal of energy theoretically contained in 1 gram-molecule of glucose (180 g), 20 kcal are in two ATP molecules formed in the reactions of anaerobic glycolysis, and 628 kcal theoretically remain in the form of lactic acid.

Under aerobic conditions, out of 688 kcal of a gram-molecule of glucose in 38 ATP molecules, 380 kcal were obtained. Thus, the efficiency of glucose utilization under aerobic conditions is about 19 times higher than in anaerobic glycolysis.

It should be pointed out that all oxidation reactions (oxidation of triose phosphate, pyruvic acid, four oxidation reactions of the tricarboxylic acid cycle) compete in the synthesis of ATP from ADP and Phneor (the Pasteur effect). This means that the resulting NADH + H + molecule in the oxidation reactions has a choice between the reactions of the respiratory system, which transfer hydrogen to oxygen, and the LDH enzyme, which transfers hydrogen to pyruvic acid.

In the early stages of the tricarboxylic acid cycle, its acids can leave the cycle to participate in the synthesis of other cell compounds without disturbing the functioning of the cycle itself. Various factors are involved in the regulation of the activity of the tricarboxylic acid cycle. Among them, first of all, we should mention the intake of acetyl-CoA molecules, the activity of the pyruvate dehydrogenase complex, the activity of the components of the respiratory chain and the oxidative phosphorylation associated with it, as well as the level of oxaloacetic acid.

Molecular oxygen is not directly involved in the tricarboxylic acid cycle, however, its reactions are carried out only under aerobic conditions, since NAD ~ and FAD can be regenerated in mitochondria only when electrons are transferred to molecular oxygen. It should be emphasized that glycolysis, in contrast to the cycle of tricarboxylic acids, is also possible under anaerobic conditions, since NAD ~ is regenerated when pyruvic acid passes into lactic acid.

In addition to the formation of ATP, the tricarboxylic acid cycle has another important significance: the cycle provides intermediary structures for various biosynthesis of the body. For example, most porphyrin atoms originate from succinyl-CoA, many amino acids are derivatives of α-keto-glutaric and oxalo-acetic acids, and fumaric acid occurs during the synthesis of urea. This manifests the integrality of the tricarboxylic acid cycle in the metabolism of carbohydrates, fats, and proteins.

As shown by the reactions of glycolysis, the ability of most cells to generate energy lies in their mitochondria. The number of mitochondria in various tissues is related to the physiological functions of tissues and reflects their ability to participate in aerobic conditions. For example, red blood cells do not have mitochondria and therefore lack the ability to generate energy using oxygen as the final electron acceptor. However, in the cardiac muscle functioning under aerobic conditions, half of the cell cytoplasm volume is represented by mitochondria. The liver also depends on aerobic conditions for its various functions, and mammalian hepatocytes contain up to 2,000 mitochondria per cell.

Mitochondria include two membranes - outer and inner. The outer membrane is simpler, consisting of 50% fat and 50% protein, and has relatively few functions. The inner membrane is structurally and functionally more complex. Approximately 80% of its volume is proteins. It contains most of the enzymes involved in electron transport and oxidative phosphorylation, metabolic mediators, and adenine nucleotides between the cytosol and the mitochondrial matrix.

Various nucleotides involved in redox reactions, such as NAD + , NADH, NADP + , FAD and FADH 2 do not penetrate the inner mitochondrial membrane. Acetyl-CoA cannot move from the mitochondrial compartment to the cytosol, where it is required for the synthesis of fatty acids or sterols. Therefore, intramitochondrial acetyl-CoA is converted in the citrate-synthase reaction of the tricarboxylic acid cycle and enters the cytosol in this form.

The tricarboxylic acid cycle was first discovered by the English biochemist G. Krebs.

He was the first to postulate the significance of this cycle for the complete combustion of pyruvate, the main source of which is the glycolytic conversion of carbohydrates. Subsequently, it was proved that the tricarboxylic acid cycle is the center in which almost all metabolic pathways converge. Thus, the Krebs cycle is a common final pathway for the oxidation of acetyl groups (in the form of acetyl-CoA), into which most of the organic molecules that play the role of “cellular fuel” are converted during catabolism: carbohydrates, fatty acids and amino acids.

Acetyl-CoA, formed as a result of oxidative decarboxylation of pyruvate in mitochondria, enters the Krebs cycle. This cycle occurs in the mitochondrial matrix and consists of eight successive reactions. The cycle begins with the condensation of acetyl-CoA with oxaloacetate and the formation of citric acid (citrate). Then citric acid (a six-carbon compound), by a series of dehydrogenations (abstraction of hydrogen) and two decarboxylations (elimination of CO 2), loses two carbon atoms and again turns into oxaloacetate (a four-carbon compound) in the Krebs cycle, i.e. as a result of a complete turn of the cycle, one molecule of acetyl-CoA burns to CO 2 and H 2 O, and the oxaloacetate molecule is regenerated. Consider all eight consecutive reactions (stages) of the Krebs cycle.

The first reaction is catalyzed by the enzyme citrate synthase; in this case, the acetyl group of acetyl-CoA condenses with oxaloacetate, resulting in the formation of citric acid:

Apparently, in this reaction, citryl-CoA bound to the enzyme is formed as an intermediate product, which is then spontaneously and irreversibly hydrolyzed to form citrate and HS-CoA.

As a result of the second reaction, the formed citric acid undergoes dehydration with the formation of cis - aconitic acid, which, by attaching a water molecule, passes into isocitric acid (isocitrate). These reversible hydration-dehydration reactions are catalyzed by the enzyme aconitate hydratase (aconitase). As a result, H and OH move in the citrate molecule:

The third reaction seems to limit the rate of the Krebs cycle. Isocitric acid is dehydrogenated in the presence of NAD-dependent iso-citrate dehydrogenase.

During the isocitrate dehydrogenase reaction, isocitric acid is simultaneously decarboxylated. NAD + -dependent isocitrate dehydrogenase is an allosteric enzyme that requires ADP as a specific activator. In addition, the enzyme needs Mg 2+ or Mn 2+ ions to manifest its activity.

During the fourth reaction, the oxidative decarboxylation of α-ketoglutaric acid occurs with the formation of a high-energy compound succinyl-CoA. The mechanism of this reaction is similar to that of the oxidative decarboxylation of pyruvate to acetyl-CoA; the α-ketoglutarate dehydrogenase complex resembles the pyruvate dehydrogenase complex in its structure. In both cases, 5 coenzymes take part in the reaction: TPP, lipoic acid amide, HS-CoA, FAD and NAD +.

The fifth reaction is catalyzed by the enzyme succinyl-CoA synthetase. During this reaction, succinyl-CoA, with the participation of GTP and inorganic phosphate, is converted to succinic acid (succinate). At the same time, the formation of a high-energy GTP phosphate bond occurs due to the high-energy thioether bond of succinyl-CoA:

As a result of the sixth reaction, succinate is dehydrogenated to fumaric acid. The oxidation of succinate is catalyzed by succinate dehydrogenase, in the molecule of which the FAD coenzyme is firmly (covalently) bound to the protein. In turn, succinate dehydrogenase is strongly associated with the inner mitochondrial membrane:

The seventh reaction is carried out under the influence of the enzyme fumarate hydratase (fumarase). The resulting fumaric acid is hydrated, the reaction product is malic acid (malate). It should be noted that fumarate hydratase is stereospecific; during the reaction, L-malic acid is formed:

Finally, during the eighth reaction of the tricarboxylic acid cycle, under the influence of mitochondrial NAD-dependent malate dehydrogenase, L-malate is oxidized to oxaloacetate:

As can be seen, in one turn of the cycle, consisting of eight enzymatic reactions, complete oxidation (“combustion”) of one acetyl-CoA molecule occurs. For the continuous operation of the cycle, a constant supply of acetyl-CoA to the system is necessary, and the coenzymes (NAD + and FAD), which have passed into a reduced state, must be oxidized again and again. This oxidation is carried out in the electron carrier system in the respiratory chain (in the chain of respiratory enzymes) localized in the mitochondrial membrane. The resulting FADH 2 is strongly associated with succinate dehydrogenase, so it transfers hydrogen atoms through CoQ.

The energy released as a result of acetyl-CoA oxidation is largely concentrated in high-energy phosphate bonds of ATP. Of the four pairs of hydrogen atoms, three pairs carry NADH to the electron transport system; in this case, for each pair in the biological oxidation system, three ATP molecules are formed (in the process of conjugated oxidative phosphorylation), and therefore a total of nine ATP molecules. One pair of atoms from succinate dehydrogenase-FADH 2 enters the electron transport system through CoQ, resulting in the formation of only two ATP molecules. During the Krebs cycle, one GTP molecule (substrate phosphorylation) is also synthesized, which is equivalent to one ATP molecule. So, when one molecule of acetyl-CoA is oxidized in the Krebs cycle and the system of oxidative phosphorylation, twelve ATP molecules can be formed.

As noted, one NADH molecule (three ATP molecules) is formed by the oxidative decarboxylation of pyruvate to acetyl-CoA. When one glucose molecule is cleaved, two pyruvate molecules are formed, and when they are oxidized to two acetyl-CoA molecules and during two turns of the tricarboxylic acid cycle, thirty ATP molecules are synthesized (hence, the oxidation of a pyruvate molecule to CO 2 and H 2 O gives fifteen ATP molecules) . To this amount must be added two ATP molecules formed during aerobic glycolysis, and six ATP molecules synthesized due to the oxidation of two extramitochondrial NADH molecules, which are formed during the oxidation of two molecules of glyceraldehyde-3-phosphate in the dehydrogenase reaction of glycolysis. Therefore, when one glucose molecule is broken down in tissues according to the equation C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O, thirty-eight ATP molecules are synthesized. Undoubtedly, in terms of energy, the complete breakdown of glucose is a more efficient process than anaerobic glycolysis.

It should be noted that two NADH molecules formed in the course of the conversion of glyceraldehyde-3-phosphate can later, upon oxidation, give not six ATP molecules, but only four. The fact is that extramitochondrial NADH molecules themselves are not able to penetrate through the membrane into mitochondria. However, the electrons they donate can be included in the mitochondrial chain of biological oxidation using the so-called glycerol phosphate shuttle mechanism. Cytoplasmic NADH first reacts with cytoplasmic dihydroxyacetone phosphate to form glycerol-3-phosphate. The reaction is catalyzed by NADH-dependent cytoplasmic glycerol-3-phosphate dehydrogenase:

Dihydroxyacetone phosphate + NADH + H + ↔ Glycerol-3-phosphate + NAD +.

The resulting glycerol-3-phosphate easily penetrates the mitochondrial membrane. Inside the mitochondria, another (mitochondrial) glycerol-3-phosphate dehydrogenase (a flavin enzyme) re-oxidizes glycerol-3-phosphate to dihydroxyacetone phosphate.

The tricarboxylic acid cycle (CTC) or the citric acid cycle or the Krebs cycle is a path of oxidative transformations of di- and tricarboxylic acids formed as intermediate products during the breakdown and synthesis of proteins, fats and carbohydrates.

The tricarboxylic acid cycle is present in the cells of all organisms: plants, animals and microorganisms.

This cycle is the basis of metabolism and performs two important functions:

Supplying the body with energy;

Integration of all major metabolic flows, both catabolic (biodegradation) and anabolic (biosynthesis).

Let me remind you that the reactions of aerobic glycolysis are localized in the cytoplasm of the cell and lead to the formation of pyruvate (PVC).

Subsequent transformations pyruvate take place in the mitochondrial matrix.

In the matrix, pyruvate is converted to acetyl-CoA- macroergic compound. The reaction is catalyzed by the enzyme NAD-dependent pyruvate decarboxylase:

The reduced form of NADH∙H + , formed as a result of this reaction, enters the respiratory chain and generates 6 ATP molecules (in terms of 1 glucose molecule).

CTC is a sequence of eight reactions that take place in matrix mitochondria(Fig. 1):

Rice. 1. Scheme of the tricarboxylic acid cycle

1) Irreversible condensation reaction acetyl-CoA co oxaloacetic acid (oxaloacetate), catalyzed by the enzyme citrate synthetase, to form citric acid (citrate).

2) Reversible isomerization reaction citric acid (citrate) V isocitric acid (isocitrate), during which the transfer of the hydroxy group to another carbon atom occurs, is catalyzed by the enzyme aconitase.

The reaction proceeds through the formation of an intermediate product
cis-acanitic acid ( cis aconitate).

3) Irreversible oxidative decarboxylation reaction isocitric acid (isocitrate): hydroxy group isocitric acid oxidized to a carbonyl group via the oxidized form OVER+ and at the same time the carboxyl group is cleaved off
β-position to form α-ketoglutaric acid (α-ketoglutarate). The intermediate product of this reaction oxalosuccinic acid (oxalosuccinate).

This is the first reaction of the cycle in which the oxidized form of NAD + -coenzyme is reduced to NADH ∙ H + , the enzyme isocitrate dehydrogenase.

The reduced form of NADH∙H enters the respiratory chain, where it is oxidized to NAD +, which leads to the formation of 2 molecules ATP.

4) Reversible oxidative decarboxylation reaction
α-ketoglutaric acid to macroergic compound succinyl-CoA. The reaction is catalyzed by the enzyme 2-oxoglutarate dehydrogenase complex.

5) The reaction is the only reaction of substrate phosphorylation in the cycle; catalyzed by the enzyme succinyl-CoA synthetase. In this reaction, succinyl-CoA with the participation guanodine diphosphate (GDP) And inorganic phosphate (H 3 PO 4 ) turns into succinic acid (succinate).

At the same time, the synthesis of a macroergic compound occurs GTP at the expense macroergic connection thioether bond succinyl-CoA.

6) Dehydrogenation reaction succinic acid (succinate) with education fumaric acid(fumarate).

The reaction is catalyzed by the complex enzyme succinate dehydrogenase, in the molecule of which the coenzyme FAD + is covalently bound, and the protein part of the enzyme. The oxidized form of FAD + as a result of the reaction is reduced to FAD∙H 2 .

The reduced form of FAD ∙ H 2 enters the respiratory chain, where it regenerates to the oxidized form of FAD +, which leads to the formation of two ATP molecules.

7) Hydration reaction fumaric acid (fumarate) before malic acid (malate). The reaction is catalyzed by the enzyme fumarase.

8) Dehydrogenation reaction malic acid before oxalacetic acid (oxaloacetate). The reaction is catalyzed by the enzyme NAD+-dependent malate dehydrogenase.

As a result of the reaction, the oxidized form of NAD is reduced to the reduced form of NADH∙H + .

The reduced form of NADH∙H enters the respiratory chain, where it is oxidized to NAD +, which leads to the formation of 2 ATP molecules.

The overall CTC equation can be written as follows:

Acetyl-CoA + 3NAD + + FAD + + GDP + H 3 PO 4 =

2 CO 2 + H 2 O + HS -CoA + 3NADH ∙ H + FAD ∙ H 2 + GTP

As can be seen from the scheme of the total equation of the CTC in this process, the following are restored:

Three NADH∙H molecules (reactions 3, 4, 8);

One FAD∙H2 molecule (reaction 6).

During aerobic oxidation of these molecules in the electron transport chain in the process of oxidative phosphorylation, it is formed during oxidation:

One molecule of NADH∙H - 3 molecules ATP;

Brief historical information

Our favorite cycle is the CTC, or the Cycle of tricarboxylic acids - life on Earth and under the Earth and in the Earth ... Stop, but in general this is the most amazing mechanism - it is universal, it is by oxidizing the decay products of carbohydrates, fats, proteins in the cells of living organisms, as a result we get energy for the activity of our body.

This process was discovered by Hans Krebs himself, for which he received the Nobel Prize!

He was born in August 25 - 1900 in the German city of Hildesheim. He received a medical education from the University of Hamburg, continued biochemical research under the guidance of Otto Warburg in Berlin.

In 1930, together with a student, he discovered the process of neutralizing ammonia in the body, which was in many representatives of the living world, including humans. This cycle is the urea cycle, also known as the Krebs cycle #1.

When Hitler came to power, Hans emigrated to the UK, where he continues to study science at Cambridge and Sheffield universities. Developing the research of the Hungarian biochemist Albert Szent-Györgyi, he gets an insight and makes the most famous Krebs cycle No. 2, or in other words the "Szent-Györgyi-Krebs cycle" - 1937.

The research results are sent to the journal "Nature", which refuses to publish the article. Then the text flies to the magazine "Enzymologia" in Holland. Krebs receives the 1953 Nobel Prize in Physiology or Medicine.

The discovery was amazing: in 1935, Szent-Györgyi found that succinic, oxaloacetic, fumaric and malic acids (all 4 acids are natural chemical components of animal cells) enhance the oxidation process in the pectoral muscle of a pigeon. Which has been shredded.

It is in it that metabolic processes proceed at the highest speed.

F. Knoop and K. Martius in 1937 found that citric acid is converted into isocitric acid through an intermediate product, cis - aconitic acid. In addition, isocitric acid could be converted into a-ketoglutaric acid, and that acid into succinic acid.

Krebs noticed the effect of acids on the absorption of O2 by the pectoral muscle of the pigeon and revealed their activating effect on the oxidation of PVC and the formation of Acetyl-Coenzyme A. In addition, the processes in the muscle were inhibited by malonic acid, which is similar to succinic acid and could competitively inhibit enzymes whose substrate is succinic acid .

When Krebs added malonic acid to the reaction medium, the accumulation of a-ketoglutaric, citric and succinic acids began. Thus, it is clear that the joint action of a-ketoglutaric, citric acids leads to the formation of succinic.

Hans investigated more than 20 substances, but they did not affect the oxidation. Comparing the data obtained, Krebs received a cycle. At the very beginning, the researcher could not say exactly whether the process begins with citric or isocitric acid, so he called it the "tricarboxylic acid cycle".

Now we know that the first is citric acid, so the correct one is the citrate cycle or the citric acid cycle.

In eukaryotes, TCA reactions take place in mitochondria, while all the enzymes for catalysis, except for 1, are contained in the free state in the mitochondrial matrix, with the exception of succinate dehydrogenase, which is localized on the inner mitochondrial membrane and is incorporated into the lipid bilayer. In prokaryotes, the reactions of the cycle take place in the cytoplasm.

Let's meet the participants of the cycle:

1) Acetyl-Coenzyme A:
- Acetyl group
- Coenzyme A - Coenzyme A:

2) PIE - Oxaloacetate - Oxalic-Acetic acid:
as it consists of two parts: oxalic and acetic acid.

3-4) Citric and Isocitric acids:

5) a-Ketoglutaric acid:

6) Succinyl-Coenzyme A:

7) Succinic acid:

8) Fumaric acid:

9) Malic acid:

How do reactions take place? In general, we are all used to the appearance of the ring, which is shown below in the picture. Everything is listed in stages below:

1. Condensation of Acetyl-Coenzyme A and Oxal-Acetic acid ➙ citric acid.

The transformation of Acetyl-Coenzyme A originates from the condensation with Oxalo-Acetic acid, resulting in the formation of citric acid.

The reaction does not require the consumption of ATP, since the energy for this process is provided as a result of the hydrolysis of the thioether bond with Acetyl-Coenzyme A, which is macroergic:

2. Citric acid passes through cis-aconitic acid into isocitric acid.

Citric acid is isomerized to isocitric acid. The conversion enzyme - aconitase - first dehydrates citric acid to form cis-aconitic acid, then combines water to the double bond of the metabolite, forming isocitric acid:

3. Isolicitric acid is dehydrogenated to form a-ketoglutaric acid and CO2.

Isolicitric acid is oxidized by a specific dehydrogenase, the coenzyme of which is NAD.

Simultaneously with the oxidation, isocitric acid is decarboxylated. As a result of transformations, α-ketoglutaric acid is formed.

4. Alpha-ketoglutaric acid is dehydrated ➙ succinyl-coenzyme A and CO2.

The next step is the oxidative decarboxylation of α-ketoglutaric acid.

It is catalyzed by the α-ketoglutarate dehydrogenase complex, which is similar in mechanism, structure and action to the pyruvate dehydrogenase complex. As a result, succinyl-CoA is formed.

5. Succinyl-coenzyme A ➙ succinic acid.

Succinyl-CoA is hydrolyzed to free succinic acid, the released energy is stored by the formation of guanosine triphosphate. This stage is the only one in the cycle where energy is directly released.

6. Succinic acid is dehydrated ➙ fumaric.

Dehydrogenation of succinic acid is accelerated by succinate dehydrogenase, its coenzyme is FAD.

7. Fumaric hydrated ➙ malic.

Fumaric acid, which is formed during the dehydrogenation of succinic acid, is hydrated and malic acid is formed.

8. Malic acid is dehydrogenated ➙ Oxalic-Acetic - the cycle is closed.

The final process is the dehydrogenation of malic acid catalyzed by malate dehydrogenase;

The result of the stage is a metabolite from which the cycle of tricarboxylic acids begins - Oxalic Acetic Acid.

In 1 reaction of the next cycle, another ml of Acetyl-Coenzyme A will enter.

How to remember this cycle? Just!

1) Very figurative expression:
A Whole Pineapple And A Slice Of Soufflé Today Is Actually My Lunch, which corresponds to citrate, cis-aconitate, isocitrate, (alpha-)ketoglutarate, succinyl-CoA, succinate, fumarate, malate, oxaloacetate.

2) Another long poem:

Pike ate acetate, it turns out citrate,
Through cisaconite it will be isocitrate.
Having given up hydrogen OVER, it loses CO2,
Alpha-ketoglutarate is immensely happy about this.
Oxidation is coming - NAD has stolen hydrogen,
TDP, coenzyme A take CO2.
And the energy barely appeared in succinyl,
Immediately ATP was born and succinate remained.
So he got to FAD - he needs hydrogen,
Fumarate drank water, and turned into malate.
Then OVER came to malate, acquired hydrogen,
The PIKE reappeared and quietly hid.

3) The original poem is shorter:

PIKE ACETYL LIMONIL,
But Narcissus Horse was afraid
He is above him ISOLIMONO
ALPHA - KETOGLUTARAL.
SUCCINATED WITH COENZYME,
AMBER FUMAROVO,
APPLES in store for the winter,
Turned into a PIKE again.