What is the result of the Krebs cycle, the formation of ATP. Krebs cycle, biological role, main reactions

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 pool of TCA cycle metabolites 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 with 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.

Krebs cycle.

The pike "ate" the acetate,

The result is citrate.

Via cis-aconitate

It will be isocitrate.

Giving hydrogens to OVER,

It loses CO 2.

I'm extremely happy about this

Alpha-keto-glutarate.

Oxidation is coming:

NAD will steal hydrogen,

B 1 and lipoate

They are in a hurry with coenzyme A,

CO 2 is collected.

And the energy is barely

Appeared in succinyl,

Immediately ATF was born.

And what remained was succinate.

Now he got to FAD -

He needs hydrogen.

Having lost hydrogens,

It became just fumarate.

Fumarate drank water,

Yes, it turned into malate.

Here NAD came to malate,

Purchased hydrogens.

PIKE has appeared again

And quietly hid

Guard acetate...

Enzymes in this diagram there is.

Coenzymes- is it NAD, NADP, ATP, GTP? Then there is.

Scheme:

The resulting PCA molecules react with a new Acetyl-CoA molecule and the cycle repeats again.

Energy balance of one revolution: 3 NADH 2 + 1 FADH 2 (further sent to the respiratory chain of oxidative phosphorylation) + 1 GTP (NADH 2 -> 3 ATP, FADH 2 -> 2 ATP, GTP -> 1 ATP) = 12 ATP.

Regulation of the TCA cycle: 4 regulatory enzymes: citrate synthase, isocitrate DG, α-KG DG and SDH. The TCA cycle is inhibited mainly by NADH 2 and ATP, which are products of the TCA cycle and the oxidative phosphorylation chain. The TCA cycle is activated mainly by NAD + and ADP.

The oxidase pathway for the use of oxygen in the cell is mitochondrial oxidative phosphorylation. Composition of respiratory complexes of the redox chain, localization and functions, tissue characteristics in childhood. Regulation.

Oxidase pathway for oxygen utilization in the cell:

It occurs in mitochondria, consumes 90% of O2 and provides the process of oxidative phosphorylation.

Oxidative phosphorylation- synthesis of ATP from ADP and H 3 PO 4 due to the energy of electron movement along the respiratory chain.

It is the main source of ATP in aerobic cells

Oxidative phosphorylation consists of the processes oxidation And phosphorylation.

1) Oxidation process

The oxidation process occurs when electrons move along the respiratory chain from the substrates of tissue respiration to oxygen. The respiratory chain of oxidative phosphorylation consists of 4 protein complexes embedded in the inner membrane of mitochondria and small mobile molecules of ubiquinone and cytochrome C that circulate in the lipid layer of the membrane between the protein complexes.

a. Complex I – NADH 2 dehydrogenase complex – the largest of the respiratory enzyme complexes, it contains FMN and 5 iron-sulfur (Fe 2 S 2 and Fe 4 S 4) proteins as coenzymes.

b. Complex II – SDH. Contains FAD and iron-sulfur protein as coenzymes.

c. Complex III – Complex b-c 1 (enzyme QH 2 DG). Each monomer contains 3 hemes associated with cytochromes b 562, b 566, c 1, and iron-sulfur protein.

d. Complex IV – Cytochrome oxidase complex. Each monomer contains 2 cytochromes (a and a 3) and 2 copper atoms.

e. Coenzyme Q (ubiquinone). Transfers 2H + and 2e -.

f. Cytochrome C. Peripheral water-soluble membrane protein. Contains a heme molecule.

Stages of movement e - along the respiratory chain

a. 2e - from NADH 2, pass through complex I (FMN→SFe protein) to CoQ, the energy released in this case ensures the pumping of H +.

b. CoQ with 2е - takes 2H+ from water from the matrix and turns into CoQH 2 (CoQ reduction also takes place with the participation of complex II).

c. CoQH 2 transfers 2e - to complex III, and 2H + into the intermembrane space.

d. Cytochrome C transfers e - c of complex III to complex IV.

e. Complex IV dumps e - onto O 2 , the energy released in this case ensures the pumping of H + .

The electrochemical potential formed on the inner mitochondrial membrane is used for:

a. phosphorylation of ADP to ATP;

b. transport of substances across the mitochondrial membrane;

c. heat production.

2) Phosphorylation process

The phosphorylation process is carried out by ATP synthetase (H + -ATPase), which consumes 40-45% of the free energy released during oxidation. H + -ATPase is an integral protein of the inner membrane of mitochondria; it consists of 2 protein complexes F 0 and F 1.

a. Hydrophobic complex F 0 immersed in the membrane and serves as a base that fixes ATP synthase in the membrane. It consists of several subunits that form a channel through which protons are transported into the matrix.

b. Complex F 1 protrudes into the mitochondrial matrix. It consists of 9 subunits (3α, 3β, γ, δ, ε). The α and β subunits are stacked in pairs to form a “head”; between the a- and β-subunits there are 3 active centers in which ATP synthesis occurs; γ, δ, ε – subunits connect the F 1 complex with F 0.

ATP synthetase ensures the reversible interconversion of the energy of the electrochemical potential and the energy of chemical bonds.

The electrochemical potential of the inner membrane causes H+ to move from the intermembrane space through the ATP synthase channel into the mitochondrial matrix. With each transfer of protons through the F o channel, the energy of the electrochemical potential is spent on turning the rod, as a result of which the conformation of the a- and β-subunits changes cyclically and all 3 active centers formed by pairs of α- and β-subunits catalyze the next phase of the cycle: 1) binding of ADP and H 3 PO 4; 2) formation of phosphoanhydride bond of ATP; 3) release of the final product ATP.

TRICARBOXYLIC ACIDS CYCLE (KREBS CYCLE)

Glycolysis converts glucose into pyruvate and produces two ATP molecules from a glucose molecule—a small fraction of that molecule's potential energy.

Under aerobic conditions, pyruvate is converted from glycolysis to acetyl-CoA and oxidized to CO2 in the tricarboxylic acid cycle (citric acid cycle). In this case, the electrons released in the reactions of this cycle pass through NADH and FADH 2 to 0 2 - the final acceptor. Electron transport is associated with the creation of a proton gradient in the mitochondrial membrane, the energy of which is then used for the synthesis of ATP as a result of oxidative phosphorylation. Let's consider these reactions.

Under aerobic conditions, pyruvic acid (1st stage) undergoes oxidative decarboxylation, more efficient than transformation into lactic acid, with the formation of acetyl-CoA (2nd stage), which can be oxidized to the final 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, lipoyl acetyltransferase) 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 is converted in an oxidative decarboxylation reaction 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 high-energy 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 participates in the synthesis of complex molecules such as steroids 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, and amino acids, is essentially a “metabolic cauldron.” The reactions of the Krebs cycle, which occur exclusively in mitochondria, are also called the citric acid cycle or the tricarboxylic acid cycle (TCA cycle).

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 clear that the tricarboxylic acid cycle is aerobic, oxygen dependent.

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 enzyme citrate synthase to form citric acid.

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

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

3. The enzyme isocitrate dehydrogenase then catalyzes the first dehydrogenase reaction of the citric acid cycle, when isocitric acid is converted by oxidative decarboxylation to α-ketoglutaric acid:

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, a NADH + H + and CO 2 molecule is again formed.

5. The succinyl-CoA molecule has a high-energy 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-sulfur-containing enzyme, the coenzyme of which is FAD. Succinate dehydrogenase is the only enzyme anchored to 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 into 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 with the participation of the active enzyme mitochondrial NAD~-dependent malate dehydrogenase, in which the third molecule of reduced NADH + H + is formed:

The formation of oxaloacetic acid (oxaloacetate) completes one revolution of the tricarboxylic acid cycle. Oxalacetic acid can be used in the oxidation of a second molecule of acetyl-CoA, 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 in the TCA cycle as a substrate of the cycle leads to the production of one molecule of GTP, three molecules of NADP + H + and one molecule of FADH 2. Oxidation of these reducing agents in the biological oxidation chain

lenition 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 ensures 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 as 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 C0 2 and H 2 0, the formation of energy in the form of ATP is:

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

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

Under aerobic conditions, from 688 kcal of a gram molecule of glucose in 38 ATP molecules, 380 kcal are obtained. Thus, the efficiency of glucose use under aerobic conditions is approximately 19 times higher than in anaerobic glycolysis.

It should be noted 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 phosphorus (Pasteur effect). This means that the resulting molecule NADH + H + in oxidation reactions has a choice between the reactions of the respiratory system, transferring hydrogen to oxygen, and the enzyme LDH, transferring 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 disrupting the functioning of the cycle itself. Various factors are involved in the regulation of tricarboxylic acid cycle activity. Among them, primarily the supply of acetyl-CoA molecules, the activity of the pyruvate dehydrogenase complex, the activity of the components of the respiratory chain and associated oxidative phosphorylation, as well as the level of oxaloacetic acid should be mentioned.

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

In addition to the formation of ATP, the tricarboxylic acid cycle has another important meaning: the cycle provides intermediary structures for various biosyntheses of the body. For example, most of the atoms of porphyrins come from succinyl-CoA, many amino acids are derivatives of α-ketoglutaric and oxaloacetic acids, and fumaric acid occurs in the process of urea synthesis. This demonstrates the integrity of the tricarboxylic acid cycle in the metabolism of carbohydrates, fats, and proteins.

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

Mitochondria include two membranes - outer and inner. The outer membrane is simpler, consisting of 50% fats and 50% proteins, 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 intermediaries 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 into the citrate synthase reaction of the tricarboxylic acid cycle and enters the cytosol in this form.

TRICARBOXYLIC ACIDS CYCLE– the citric acid cycle or Krebs cycle is a widely represented pathway in the organisms of animals, plants and microbes for the oxidative transformations of di- and tricarboxylic acids formed as intermediate products during the breakdown and synthesis of proteins, fats and carbohydrates. Discovered by H. Krebs and W. Johnson (1937). This cycle is the basis of metabolism and performs two important functions - supplying the body with energy and integrating all the main metabolic flows, both catabolic (biodegradation) and anabolic (biosynthesis).

The Krebs cycle consists of 8 stages (intermediate products are highlighted in two stages in the diagram), during which the following occurs:

1) complete oxidation of the acetyl residue to two CO 2 molecules,

2) three molecules of reduced nicotinamide adenine dinucleotide (NADH) and one reduced flavin adenine dinucleotide (FADH 2) are formed, which is the main source of energy produced in the cycle and

3) one molecule of guanosine triphosphate (GTP) is formed as a result of the so-called substrate oxidation.

In general, the path is energetically beneficial (DG 0 " = –14.8 kcal.)

The Krebs cycle, localized in mitochondria, begins with citric acid (citrate) and ends with the formation of oxaloacetic acid (oxaloacetate - OA). The substrates of the cycle include tricarboxylic acids - citric, cis-aconitic, isocitric, oxalosuccinate (oxalosuccinate) and dicarboxylic acids - 2-ketoglutaric (KG), succinic, fumaric, malic (malate) and oxaloacetic. Substrates of the Krebs cycle also include acetic acid, which in its active form (i.e. in the form of acetyl coenzyme A, acetyl-SCoA) participates in condensation with oxaloacetic acid, leading to the formation of citric acid. It is the acetyl residue included in the structure of citric acid that is oxidized; carbon atoms are oxidized to CO 2, hydrogen atoms are partially accepted by coenzymes of dehydrogenases, and partially pass into solution, that is, into the environment in protonated form.

Pyruvic acid (pyruvate), which is formed during glycolysis and occupies one of the central places in intersecting metabolic pathways, is usually indicated as the starting compound for the formation of acetyl-CoA. Under the influence of an enzyme with a complex structure - pyruvate dehydrogenase (CP1.2.4.1 - PDHase), pyruvate is oxidized to form CO 2 (first decarboxylation), acetyl-CoA and is reduced by NAD ( cm. diagram). However, the oxidation of pyruvate is far from the only way to form acetyl-CoA, which is also a characteristic product of the oxidation of fatty acids (thiolase enzyme or fatty acid synthetase) and other reactions of the decomposition of carbohydrates and amino acids. All enzymes involved in the reactions of the Krebs cycle are localized in mitochondria, most of them are soluble, and succinate dehydrogenase (KF1.3.99.1) is tightly associated with membrane structures.

The formation of citric acid, with the synthesis of which the cycle proper begins, with the help of citrate synthase (EC4.1.3.7 - the condensing enzyme in the diagram), is an endergonic reaction (with energy absorption), and its implementation is possible due to the use of the energy-rich bond of the acetyl residue with KoA [CH 3 CO~SKoA]. This is the main stage of regulation of the entire cycle. This is followed by the isomerization of citric acid into isocitric acid through the intermediate stage of the formation of cis-aconitic acid (the enzyme aconitase KF4.2.1.3, has absolute stereospecificity - sensitivity to the location of hydrogen). The product of further transformation of isocitric acid under the influence of the corresponding dehydrogenase (isocitrate dehydrogenase KF1.1.1.41) is apparently oxalosuccinic acid, the decarboxylation of which (the second CO 2 molecule) leads to CG. This stage is also strictly regulated. In a number of characteristics (high molecular weight, complex multicomponent structure, stepwise reactions, partially the same coenzymes, etc.) KH dehydrogenase (EC1.2.4.2) resembles PDHase. The reaction products are CO 2 (third decarboxylation), H + and succinyl-CoA. At this stage, succinyl-CoA synthetase, otherwise called succinate thiokinase (EC6.2.1.4), is activated, catalyzing the reversible reaction of the formation of free succinate: Succinyl-CoA + P inorg + GDP = Succinate + KoA + GTP. During this reaction, so-called substrate phosphorylation occurs, i.e. formation of energy-rich guanosine triphosphate (GTP) at the expense of guanosine diphosphate (GDP) and mineral phosphate (P inorg) using the energy of succinyl-CoA. After the formation of succinate, succinate dehydrogenase (KF1.3.99.1), a flavoprotein, comes into action, leading to fumaric acid. FAD is connected to the protein part of the enzyme and is the metabolically active form of riboflavin (vitamin B 2). This enzyme is also characterized by absolute stereospecificity in hydrogen elimination. Fumarase (EC4.2.1.2) ensures the balance between fumaric acid and malic acid (also stereospecific), and malic acid dehydrogenase (malate dehydrogenase EC1.1.1.37, which requires the coenzyme NAD +, is also stereospecific) leads to the completion of the Krebs cycle, that is, to formation of oxaloacetic acid. After this, the condensation reaction of oxaloacetic acid with acetyl-CoA is repeated, leading to the formation of citric acid, and the cycle resumes.

Succinate dehydrogenase is part of the more complex succinate dehydrogenase complex (complex II) of the respiratory chain, supplying reducing equivalents (NAD-H 2), formed during the reaction, to the respiratory chain.

Using the example of PDHase, you can get acquainted with the principle of cascade regulation of metabolic activity due to phosphorylation-dephosphorylation of the corresponding enzyme by special kinase and phosphatase PDHase. Both of them are connected to PDGase.

It is assumed that the catalysis of individual enzymatic reactions is carried out as part of a supramolecular “supercomplex”, the so-called “metabolon”. The advantages of such an organization of enzymes are that there is no diffusion of cofactors (coenzymes and metal ions) and substrates, and this contributes to more efficient operation of the cycle.

The energy efficiency of the processes considered is low, however, 3 moles of NADH and 1 mole of FADH2 formed during the oxidation of pyruvate and subsequent reactions of the Krebs cycle are important products of oxidative transformations. Their further oxidation is carried out by enzymes of the respiratory chain also in mitochondria and is associated with phosphorylation, i.e. the formation of ATP due to esterification (formation of organophosphorus esters) of mineral phosphate. Glycolysis, the enzymatic action of PDHase and the Krebs cycle - a total of 19 reactions - determine the complete oxidation of one molecule of glucose to 6 molecules of CO 2 with the formation of 38 molecules of ATP - this bargaining chip "energy currency" of the cell. The process of oxidation of NADH and FADH 2 by enzymes of the respiratory chain is energetically very efficient, occurs using atmospheric oxygen, leads to the formation of water and serves as the main source of energy resources of the cell (more than 90%). However, enzymes of the Krebs cycle are not involved in its direct implementation. Each human cell has from 100 to 1000 mitochondria, which provide energy for life.

The basis of the integrating function of the Krebs cycle in metabolism is that carbohydrates, fats and amino acids from proteins can ultimately be converted into intermediates (intermediates) of this cycle or synthesized from them. The removal of intermediates from the cycle during anabolism must be combined with the continuation of the catabolic activity of the cycle for the constant formation of ATP necessary for biosynthesis. Thus, the loop must perform two functions simultaneously. At the same time, the concentration of intermediates (especially OA) may decrease, which can lead to a dangerous decrease in energy production. To prevent this, there are “safety valves” called anaplerotic reactions (from the Greek “to fill”). The most important reaction is the synthesis of OA from pyruvate, carried out by pyruvate carboxylase (EC6.4.1.1), also localized in mitochondria. As a result, a large amount of OA accumulates, which ensures the synthesis of citrate and other intermediates, which allows the Krebs cycle to function normally and, at the same time, ensure the removal of intermediates into the cytoplasm for subsequent biosynthesis. Thus, at the level of the Krebs cycle, an effectively coordinated integration of the processes of anabolism and catabolism occurs under the influence of numerous and subtle regulatory mechanisms, including hormonal ones.

Under anaerobic conditions, instead of the Krebs cycle, its oxidative branch functions to KG (reactions 1, 2, 3) and its reductive branch functions from OA to succinate (reactions 8®7®6). In this case, much energy is not stored and the cycle supplies only intermediates for cellular synthesis.

When the body transitions from rest to activity, the need arises to mobilize energy and metabolic processes. This, in particular, is achieved in animals by shunting the slowest reactions (1–3) and predominant oxidation of succinate. In this case, KG, the initial substrate of the shortened Krebs cycle, is formed in the rapid transamination reaction (amine group transfer)

Glutamate + OA = CG + aspartate

Another modification of the Krebs cycle (the so-called 4-aminobutyrate shunt) is the conversion of KG to succinate through glutamate, 4-aminobutyrate and succinic semialdehyde (3-formylpropionic acid). This modification is important in brain tissue, where about 10% of glucose is broken down through this pathway.

The close coupling of the Krebs cycle with the respiratory chain, especially in animal mitochondria, as well as the inhibition of most enzymes of the cycle under the influence of ATP, determine a decrease in the activity of the cycle at a high phosphoryl potential of the cell, i.e. at a high ATP/ADP concentration ratio. In most plants, bacteria and many fungi, the tight coupling is overcome by the development of uncoupled alternative oxidation pathways, which allow simultaneous respiration and cycle activity to be maintained at a high level even at a high phosphoryl potential.

Igor Rapanovich

Everyone knows that for normal functioning the body needs a regular supply of a number of nutrients that are necessary for healthy metabolism and, accordingly, the balance of the processes of energy production and expenditure. The process of energy production, as is known, occurs in mitochondria, which, thanks to this feature, are called the energy centers of cells. And the sequence of chemical reactions that allows you to obtain energy for the work of each cell of the body is called the Krebs cycle.

Krebs cycle - miracles that happen in mitochondria

The energy obtained through the Krebs cycle (also the TCA cycle - the tricarboxylic acid cycle) goes to the needs of individual cells, which in turn make up various tissues and, accordingly, organs and systems of our body. Since the body simply cannot exist without energy, mitochondria are constantly working to continuously supply the cells with the energy they need.

Adenosine triphosphate (ATP) - this compound is a universal source of energy necessary for all biochemical processes in our body.

The TCA cycle is the central metabolic pathway, as a result of which the oxidation of metabolites is completed:

• fatty acids;
• amino acids;
• monosaccharides.

During the process of aerobic breakdown, these biomolecules are broken down into smaller molecules that are used to produce energy or synthesize new molecules.

The tricarboxylic acid cycle consists of 8 stages, i.e. reactions:

1. Formation of citric acid:

2. Formation of isocitric acid:

3. Dehydrogenation and direct decarboxylation of isocitric acid.

4. Oxidative decarboxylation of α-ketoglutaric acid

5. Substrate phosphorylation

6. Dehydrogenation of succinic acid with succinate dehydrogenase

7. Formation of malic acid by the enzyme fumarase

8. Formation of oxalacetate

Thus, after the completion of the reactions that make up the Krebs cycle:

• one molecule of acetyl-CoA (formed as a result of the breakdown of glucose) is oxidized to two molecules of carbon dioxide;
• three NAD molecules are reduced to NADH;
• one FAD molecule is reduced to FADN 2;
• one molecule of GTP (equivalent to ATP) is formed.

The molecules NADH and FADH 2 act as electron carriers and are used to produce ATP in the next step of glucose metabolism - oxidative phosphorylation.

Functions of the Krebs cycle:

• catabolic (oxidation of acetyl residues of fuel molecules to final metabolic products);
• anabolic (substrates of the Krebs cycle - the basis for the synthesis of molecules, including amino acids and glucose);
• integrative (TCC is the link between anabolic and catabolic reactions);
• hydrogen donor (supply of 3 NADH.H + and 1 FADH 2 to the mitochondrial respiratory chain);
• energy.

A lack of elements necessary for the normal functioning of the Krebs cycle can lead to serious problems in the body associated with a lack of energy.

Thanks to metabolic flexibility, the body is able to use not only glucose as an energy source, but also fats, the breakdown of which also produces molecules that form pyruvic acid (involved in the Krebs cycle). Thus, a properly flowing TCA cycle provides energy and building blocks for the formation of new molecules.