The metabolism of animals and other organisms is based on the chemical processes of extracting energy stored in carbohydrates.

During photosynthesis, solar energy is stored in chemical bonds carbohydrate molecules, of which the six-carbon sugar glucose plays the most important role. After other living organisms use these molecules for food, the stored energy is released and used for metabolism. This occurs during the processes of glycolysis and respiration. Whole chemical process can be briefly described as:

Glucose + oxygen → carbon dioxide + water + energy

To better understand these processes, imagine that the body is "burning" carbohydrates for energy.

The term "glycolysis" is formed by combining the word lysis, meaning "cleavage", with the word glucose. As the name implies, the process begins with the chemical extraction of energy by splitting a glucose molecule into two parts, each containing three carbon atoms. In the process of glycolysis, each molecule of glucose produces two three-carbon molecules of pyruvic acid. In addition, the energy of glucose is stored in molecules (see Biological molecules) that we call the "energy currency" of the cell - two ATP molecules and two NADP molecules. Thus, already at the first stage of glycolysis, energy is released in a form that can be used by the cells of the body.

The further course of events depends on the presence or absence of oxygen in the medium. In the absence of oxygen, pyruvic acid is converted into other organic molecules in the course of the so-called anaerobic processes. For example, in yeast cells, pyruvic acid is converted to ethanol. In animals, including humans, when the oxygen supply in the muscles is depleted, pyruvic acid turns into lactic acid - it is this that causes the feeling of muscle stiffness so familiar to all of us after heavy physical exertion.

In the presence of oxygen, energy is released during aerobic respiration, when pyruvic acid is split into carbon dioxide and water molecules with the simultaneous release of the remaining energy stored in the carbohydrate molecule. Respiration takes place in a specialized cell organelle, the mitochondria. First, one carbon atom of pyruvic acid is cleaved off. This produces carbon dioxide, energy (it is stored in one NADP molecule) and a two-carbon molecule - an acetyl group. Then the reaction chain enters the metabolic coordination center of the cell - the Krebs cycle.

The Krebs cycle (also called the citric acid cycle or tricarboxylic acid cycle) is an example of a well-known phenomenon in biology - a chemical reaction that begins when a certain incoming molecule combines with another molecule that acts as a "helper". This combination initiates a series of other chemical reactions, in which product molecules are formed and at the end a helper molecule is recreated, which can start the whole process again. In the Krebs cycle, the role of the incoming molecule is played by the acetyl group formed during the breakdown of pyruvic acid, and the role of the helper molecule is played by the four-carbon oxaloacetic acid molecule. During the first chemical reaction of the cycle, these two molecules combine to form six-carbon molecules of citric acid (the cycle owes one of its names to this acid). Then eight chemical reactions take place, in which first energy carrier molecules and carbon dioxide are formed, and then a new oxaloacetic acid molecule. To process the energy stored in one glucose molecule, the Krebs cycle must be completed twice. The net profit turns out to be equal to two ATP molecules, four carbon dioxide molecules and ten other energy carrier molecules (more on them a little later). Carbon dioxide eventually diffuses out of the mitochondria and is released during exhalation.

(Note by Wild_Katze: The picture in the article was small and illegible, so I'm replacing it with a more descriptive picture of the Krebs cycle from here http://www.bsu.ru/content/hecadem/bahanova_mv/cl_718/files/mzip_618_14707/index.htm)

The Krebs cycle is a repetitive succession of biochemical reactions that occur during the respiration of animals, plants, and many microorganisms. Here is a simplified version of it. Numbers in parentheses indicate the number of carbon atoms in each organic molecule

The Krebs cycle is fundamentally important for life, not only because it generates energy. In addition to glucose, many other molecules that also form pyruvic acid can enter into it. For example, when you are on a diet, the body does not have enough glucose you consume to maintain metabolism, so lipids (fats) enter the Krebs cycle, after preliminary splitting. That's why you are losing weight. In addition, molecules can leave the Krebs cycle to take part in the construction of new proteins, carbohydrates and lipids. Thus the Krebs cycle can accept the energy stored in different form in many molecules, and create a variety of output molecules.

From an energetic point of view, the net result of the Krebs cycle is to complete the extraction of energy stored in the chemical bonds of glucose, transfer a small part of this energy to ATP molecules, and store the rest of the energy in other energy-carrying molecules. (Speaking of the energy of chemical bonds, one should not forget that work must be done to separate the connected atoms.) final stage respiration, this remaining energy is released from the carrier molecules and is also stored in ATP. Molecules that store energy move within the mitochondria until they collide with specialized proteins embedded in the inner membranes of the mitochondria. These proteins take electrons from energy carriers and begin to pass them along a chain of molecules - like a chain of people passing buckets of water on a fire - extracting the energy stored in chemical bonds. The energy extracted at each stage is stored in the form of ATP. In the final step, electrons combine with oxygen atoms, which then combine with hydrogen ions (protons) to form water. In the electron transport chain, at least 32 ATP molecules are formed - 90% of the energy stored in the original glucose molecule.

The transformation of energy in the Krebs cycle involves a rather complex process of chemiosmotic conjugation. This term indicates that along with chemical reactions, osmosis is involved in the release of energy - the slow seepage of solutions through organic partitions. In fact, the electrons from the energy carriers that are the product of the Krebs cycle are transferred along the transport chain and enter the proteins immersed in the membrane that separates the inner and outer compartments (compartments) of the mitochondria. The energy of the electrons is used to move hydrogen ions (protons) into the outer compartment, which serves as an "energy store" - like a reservoir formed in front of a dam. When protons flow across the membrane, energy is used to form ATP, similar to how water in front of a dam is used to produce electricity when it falls on a generator. Finally, in the inner compartment of the mitochondria, hydrogen ions combine with oxygen molecules to form water, one of the end products of metabolism.

This story of glycolysis and respiration illustrates just how far modern understanding of living systems has come. A simple statement about a specific process—for example, that carbohydrates must be “burned” for metabolism—entails an incredibly detailed description of the complex processes that occur at the molecular level and involve a huge number of different molecules. Comprehension of modern molecular biology is somewhat akin to reading a classic Russian novel: it is easy for you to understand every interaction between the characters, but, having reached page 1423, you may well forget who Petr Petrovich Alexei Alekseevich is. In the same way, every chemical reaction in the chain just described seems understandable, but by the time you read to the end, you will be amazed at the incomprehensible complexity of the process. As a consolation, I note that I feel the same way.

The acetyl-SCoA formed in the PVC-dehydrogenase reaction then enters into tricarboxylic acid cycle(CTC, citric acid cycle, Krebs cycle). In addition to pyruvate, keto acids are involved in the cycle, coming from the catabolism of amino acids or any other substances.

Tricarboxylic acid cycle

The cycle runs 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 citric acid isomerizes 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 fumaric acid up malate(malic acid), then NAD-dependent dehydrogenation to form oxaloacetate.

As a result, after eight reactions of the cycle again oxaloacetate is formed .

The last three reactions make up 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 fatty acid β-oxidation reactions. In reverse order (reduction, de hydration and recovery) this motif is observed in fatty acid synthesis reactions.

DTC functions

1. Energy

• generation hydrogen atoms for the operation of the respiratory chain, namely three NADH molecules and one FADH2 molecule,
• single molecule synthesis GTP(equivalent to ATP).

2. Anabolic. In the CTC are formed

• heme precursor succinyl-SCoA,
• keto acids that can be converted into amino acids - α-ketoglutarate for glutamic acid, oxaloacetate for aspartic,
• lemon acid, used for the synthesis of fatty acids,
• oxaloacetate, used for glucose synthesis.

Anabolic reactions of the TCA

Regulation of the tricarboxylic acid cycle

Allosteric regulation

Enzymes catalyzing the 1st, 3rd and 4th reactions of TCA are sensitive to allosteric regulation metabolites:

Regulation of oxaloacetate availability

chief and main the regulator of the TCA is oxaloacetate, or rather its availability. The presence of oxaloacetate involves acetyl-SCoA in 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

1)pyruvic acid formed from glucose or alanine,

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 starts to work and oxaloacetate is formed, but, of course, only in the presence of pyruvate.

2) Getting from aspartic acid as a result of transamination or from the AMP-IMF cycle,

3) Receipt from fruit acids the cycle itself (amber, α-ketoglutaric, malic, citric) formed during the catabolism of amino acids or in other processes. Majority amino acids during their catabolism, they are able to turn into metabolites of TCA, which then go to oxaloacetate, which also maintains the activity of the cycle.

Replenishment of the pool of TCA metabolites from amino acids

Cycle replenishment reactions 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 inadequate the amount of oxaloacetate in the liver. This condition is observed during decompensation of insulin-dependent diabetes mellitus (type 1 diabetes) and during starvation. 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. Simultaneous activation of fatty acid oxidation and accumulation of acetyl-SCoA triggers a backup pathway for the utilization of the acetyl group - synthesis of ketone bodies. In this case, the body develops acidification of the blood ( ketoacidosis) with a characteristic clinical picture: weakness, headache, drowsiness, decreased muscle tone, body temperature and blood pressure.

Change in the rate of TCA 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 flame of carbohydrates". It implies that the "burning flame" 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 guarantees the inclusion of an acetyl group formed from fatty acids in the form of acetyl-SCoA, in the first reaction of the TCA.

In the case of a large-scale "burning" of fatty acids, which is observed in the muscles during physical work and in the liver fasting, the rate of entry of acetyl-SCoA in the TCA reaction will directly depend on the amount of oxaloacetate (or oxidized glucose).

If the amount of oxaloacetate in hepatocyte not enough (no glucose or it is not oxidized to pyruvate), then the acetyl group will go to the synthesis of ketone bodies. This happens when prolonged fasting and type 1 diabetes.

Not all of us are aware of such a phenomenon as the Krebs cycle. What it is? In simple terms, this phenomenon can be described as chemical reactions in the human body, as a result of which adenosine triphosphate is produced.

This phenomenon was investigated by Hans Krebs, a German scientist in the 30s of the 20th century. At this time, he and his assistant studied the circulation of urea. During the period when the Second World War, the scientist moved to live in England, where his research showed that certain acids can catalyze processes in the human body. Per this study the scientist was awarded Nobel Prize.

What is the Krebs cycle?

Energy in the human body depends on glucose, a substance found in the blood. To transform glucose into energy, the cells of the body contain mitochondria. When the entire transformation process takes place, the substance adenosine triphosphate, abbreviated as ATP, is obtained from glucose. ATP is the main source of energy in the human body.

The structure of the resulting substance gives it the ability to integrate into protein in order to provide the necessary amount of energy to human organs and systems. Glucose itself cannot be directly converted into ATP, so this process requires complex mechanisms. This mechanism is the Krebs cycle.

If a plain language To explain this process, we can say that the Krebs cycle is a chain of chemical reactions that occur in our body, more precisely in each of its cells. This process is a cycle, and it is called so because it goes on endlessly. When the Krebs cycle is complete, the result is the production of the substance adenosine triphosphate. This is the energy basis for the human body to function.

Otherwise, this cycle is called cell respiration. The second name of the process was due to the fact that all of its stages require the presence of oxygen. During this process, amino acids and carbohydrates are produced. By this we can judge that the cycle performs another function - construction.

In order for the above process to be realized, there must be enough trace elements in the human body, there must be at least a hundred of them. Vitamins are among the necessary components. If there are not enough trace elements, at least one of them is missing, then the cycle will not be so effective. And the inefficiency of the Krebs cycle leads to the fact that the metabolism in the body is disturbed.

Cycle regulation

The regulation of such a phenomenon as the Krebs cycle has a great influence on the functioning of the human body. It is important so that he can adapt to how conditions change. external environment and how physiological systems change. There are regulatory factors, which are divided into several groups:

• regulation that occurs with carbon-containing substrates, as well as products that are intermediate in the cycle itself;
• regulation with the help of adenyl nucleotides, which can be both coenzymes and products of the final process.

At the beginning, it is necessary to understand what the functions of the products are during the passage of the cycle, which are intermediate. Let's pay attention to the role of oxaloacetate. This is a very important element, because when its tissue reserves decrease, the cycle stops repeating.

This depletes a very important energy source of the body, and the consequences for the cells are terrible. The consequences are also detrimental because there is not enough oxaloacetate, which is needed in order for acetyl-CoA to act. Acetyl-CoA is formed during the catabolism of carbohydrates and fats. In this case, two-carbon fragments accumulate. When they condense, an excess amount of acetoacetate accumulates in the tissues. In addition to it, other similar bodies accumulate. At the same time, ketosis develops in the human body, which is a pathological condition.

In each case, when acetyl-CoA is formed, and there is a lot of it, there is not enough oxaloacetate to condense it. With each of these cycles, ketosis occurs. Simply put, ketosis provokes a lack of oxaloacetate if its level is lower than the amount of acetyl-CoA.

When ketosis occurs in the body, there is a violation between the processes of fat oxidation and carbohydrate catabolism. This phenomenon is due to the fact that the latter can produce oxaloceate during the carboxylation of pyruvate. This reaction undergoes a process of catalysis. It is catalyzed in mitochondria by the biotin enzyme. This is the main mechanism by which carbohydrates are produced in the body. This is how CO2 is formed, which further takes part in the Krebs cycle. It also provides the gluconeogenesis process with fragments that contain carbohydrates.

The reactions of this cycle lead to the formation of oxaloacetate. Its regulation occurs as a feedback, and this is ensured by the fact that oxaloacetate acts as a competitive inhibitor of succinate dehydrogenase. At the same time, the enzyme has the role of a regulator in this cycle.

Summing up, it should be said that the Krebs cycle is a process in the cells of the body that can produce energy for its normal functioning. If this process occurs incorrectly, then this leads to a pathological condition and impaired metabolism in the human body.

Video

This metabolic pathway is named after the author who discovered it - G. Krebs, who received (together with F. Lipman) for this discovery in 1953 the Nobel Prize. The citric acid cycle captures most of the free energy from the breakdown of proteins, fats, and carbohydrates in food. The Krebs cycle is the central metabolic pathway.

The acetyl-CoA formed as a result of oxidative decarboxylation of pyruvate in the mitochondrial matrix is ​​included in the chain of successive oxidation reactions. There are eight such reactions.

1st reaction - the formation of citric acid. The formation of citrate occurs by condensation of the acetyl residue of acetyl-CoA with oxalacetate (OA) using the enzyme citrate synthase (with the participation of water):

This reaction is practically irreversible, since the energy-rich thioether bond of acetyl~S-CoA decomposes.

2nd reaction - the formation of isocitric acid. This reaction is catalyzed by an iron-containing (Fe - non-heme) enzyme - aconitase. The reaction proceeds through the formation stage cis-aconitic acid (citric acid undergoes dehydration to form cis-aconitic acid, which, by attaching a water molecule, turns into isocitric acid).

3rd reaction - dehydrogenation and direct decarboxylation of isocitric acid. The reaction is catalyzed by the NAD+-dependent enzyme isocitrate dehydrogenase. The enzyme needs the presence of manganese (or magnesium) ions. Being by nature an allosteric protein, isocitrate dehydrogenase needs a specific activator - ADP.

4th reaction - oxidative decarboxylation of α-ketoglutaric acid. The process is catalyzed by α-ketoglutarate dehydrogenase - an enzyme complex similar in structure and mechanism of action to the pyruvate dehydrogenase complex. It consists of the same coenzymes: TPP, LA and FAD - the complex's own coenzymes; KoA-SH and NAD+ are external coenzymes.

5th reaction - substrate phosphorylation. The essence of the reaction is the transfer of a rich bond energy of succinyl-CoA (macroergic compound) to GDP with the participation of phosphoric acid - in this case, GTP is formed, the molecule of which reacts rephosphorylation with ADP, ATP is formed.

6th reaction - dehydrogenation of succinic acid with succinate dehydrogenase. The enzyme directly transfers hydrogen from the substrate (succinate) to the ubiquinone of the inner mitochondrial membrane. Succinate dehydrogenase is the II complex of the mitochondrial respiratory chain. The coenzyme in this reaction is FAD.

7th reaction - the formation of malic acid by the enzyme fumarase. Fumarase (fumarate hydratase) hydrates fumaric acid - this forms malic acid, and its L-form, since the enzyme is stereospecific.

8th reaction - the formation of oxalacetate. The reaction is catalyzed malate dehydrogenase , whose coenzyme is OVER + . The oxalacetate formed under the action of the enzyme is again included in the Krebs cycle and the entire cyclic process is repeated.

The last three reactions are reversible, but since NADH?H+ is taken up by the respiratory chain, the equilibrium of the reaction shifts to the right, i.e. towards the formation of oxalacetate. As can be seen, complete oxidation, “combustion”, of acetyl-CoA molecules occurs in one turn of the cycle. During the cycle, reduced forms of nicotinamide and flavin coenzymes are formed, which are oxidized in the respiratory chain of mitochondria. Thus, the Krebs cycle is closely related to the process of cellular respiration.

The functions of the tricarboxylic acid cycle are diverse:

· Integrative - the Krebs cycle is the central metabolic pathway that combines the processes of decay and synthesis of the most important components of the cell.

· Anabolic - substrates of the cycle are used for the synthesis of many other compounds: oxalacetate is used for the synthesis of glucose (gluconeogenesis) and the synthesis of aspartic acid, acetyl-CoA - for the synthesis of heme, α-ketoglutarate - for the synthesis of glutamic acid, acetyl-CoA - for the synthesis of fatty acids, cholesterol , steroid hormones, acetone bodies, etc.

· catabolic - in this cycle, the decay products of glucose, fatty acids, ketogenic amino acids complete their journey - they all turn into acetyl-CoA; glutamic acid - to α-ketoglutaric; aspartic - to oxaloacetate, etc.

· Actually energy - one of the cycle reactions (decay of succinyl-CoA) is a substrate phosphorylation reaction. During this reaction, one molecule of GTP is formed (the rephosphorylation reaction leads to the formation of ATP).

· Hydrogen donor - with the participation of three NAD + -dependent dehydrogenases (isocitrate, α-ketoglutarate and malate dehydrogenases) and FAD-dependent succinate dehydrogenase, 3 NADH?H + and 1 FADH 2 are formed. These reduced coenzymes are hydrogen donors for the mitochondrial respiratory chain, the energy of hydrogen transfer is used for ATP synthesis.

· Anaplerotic - replenishing. Significant amounts of Krebs cycle substrates are used for the synthesis of various compounds and leave the cycle. One of the reactions that make up for these losses is the reaction catalyzed by pyruvate carboxylase.

The reaction rate of the Krebs cycle is determined by the energy needs of the cell

The rate of reactions of the Krebs cycle correlates with the intensity of the process of tissue respiration and the associated oxidative phosphorylation - respiratory control. All metabolites that reflect sufficient supply of energy to the cell are inhibitors of the Krebs cycle. An increase in the ratio of ATP / ADP is an indicator of sufficient energy supply to the cell and reduces the activity of the cycle. An increase in the ratio of NAD + / NADH, FAD / FADH 2 indicates energy deficiency and is a signal of the acceleration of oxidation processes in the Krebs cycle.

The main action of the regulators is aimed at the activity of three key enzymes: citrate synthase, isocitrate dehydrogenase, and a-ketoglutarate dehydrogenase. Allosteric inhibitors of citrate synthase are ATP, fatty acids. In some cells, citrate and NADH play the role of its inhibitors. Isocitrate dehydrogenase is allosterically activated by ADP and inhibited by elevated levels of NADH+H+.

Rice. 5.15. Tricarboxylic acid cycle (Krebs cycle)

The latter is also an inhibitor of α-ketoglutarate dehydrogenase, the activity of which also decreases with an increase in the level of succinyl-CoA.

The activity of the Krebs cycle largely depends on the availability of substrates. Constant “leakage” of substrates from the cycle (for example, in case of ammonia poisoning) can cause significant disturbances in the energy supply of cells.

The pentose phosphate pathway of glucose oxidation serves reductive syntheses in the cell.

As the name implies, much-needed pentose phosphates are produced in this pathway. Since the formation of pentoses is accompanied by the oxidation and elimination of the first carbon atom of glucose, this pathway is also called apotomous (apex- top).

The pentose phosphate pathway can be divided into two parts: oxidative and non-oxidative. In the oxidative part, which includes three reactions, NADPH?H + and ribulose-5-phosphate are formed. In the non-oxidative part, ribulose-5-phosphate is converted to various monosaccharides with 3, 4, 5, 6, 7 and 8 carbon atoms; end products are fructose-6-phosphate and 3-PHA.

· Oxidizing part . First reaction-dehydrogenation of glucose-6-phosphate by glucose-6-phosphate dehydrogenase with the formation of δ-lactone 6-phosphogluconic acid and NADPH?H + (NADP + - coenzyme glucose-6-phosphate dehydrogenase).

Second reaction- hydrolysis of 6-phosphogluconolactone by gluconolactone hydrolase. The reaction product is 6-phosphogluconate.

Third reaction- dehydrogenation and decarboxylation of 6-phosphogluconolactone by the enzyme 6-phosphogluconate dehydrogenase, the coenzyme of which is NADP + . During the reaction, the coenzyme is reduced and C-1 glucose is cleaved off to form ribulose-5-phosphate.

· Non-oxidizing part . Unlike the first, oxidative, all reactions of this part of the pentose phosphate pathway are reversible (Fig. 5.16)

Fig. 5.16. Oxidative part of the pentose phosphate pathway (F-variant)

Ribulose-5-phosphate can isomerize (enzyme - ketoisomerase ) into ribose-5-phosphate and epimerize (enzyme - epimerase ) to xylulose-5-phosphate. Two types of reactions follow: transketolase and transaldolase.

Transketolase(coenzyme - thiamine pyrophosphate) splits off a two-carbon fragment and transfers it to other sugars (see diagram). Transaldolase carries three-carbon fragments.

Ribose-5-phosphate and xylulose-5-phosphate enter the reaction first. This is a transketolase reaction: the 2C fragment is transferred from xylulose-5-phosphate to ribose-5-phosphate.

The two resulting compounds then react with each other in a transaldolase reaction; in this case, as a result of the transfer of the 3C fragment from sedoheptulose-7-phosphate to 3-PHA, erythrose-4-phosphate and fructose-6-phosphate are formed. This is the F-variant of the pentose phosphate pathway. It is characteristic of adipose tissue.

However, reactions can also go along a different path (Fig. 5.17). This path is designated as the L-variant. It occurs in the liver and other organs. In this case, octulose-1,8-diphosphate is formed in the transaldolase reaction.

Fig.5.17. Pentose phosphate (apotomic) pathway of glucose metabolism (octulose, or L-variant)

Erythrose-4-phosphate and fructose-6-phosphate can enter into a transketolase reaction, which results in the formation of fructose-6-phosphate and 3-PHA.

The general equation for the oxidative and non-oxidative parts of the pentose phosphate pathway can be represented as follows:

Glucose-6-P + 7H 2 O + 12NADP + 5 Pentose-5-P + 6CO 2 + 12 NADPH?N + + Fn.

The bulk of the chemical energy of carbon is released under aerobic conditions with the participation of oxygen. The Krebs cycle is also called the citric acid cycle, or cellular respiration. Many scientists took part in deciphering the individual reactions of this process: A. Szent-Gyorgyi, A. Lehninger, X. Krebs, after whom the cycle is named, S. E. Severin and others.

There is a close correlation between anaerobic and aerobic digestion of carbohydrates. First of all, it is expressed in the presence of pyruvic acid, which completes the anaerobic breakdown of carbohydrates and begins cellular respiration (the Krebs cycle). Both phases are catalyzed by the same enzymes. Chemical energy is released during phosphorylation and is reserved in the form of ATP macroergs. The same coenzymes (NAD, NADP) and cations participate in chemical reactions. The differences are as follows: if the anaerobic digestion of carbohydrates is predominantly localized in the hyaloplasm, then the reactions of cellular respiration take place mainly in the mitochondria.

Under certain conditions, antagonism is observed between the two phases. So, in the presence of oxygen, glycolysis decreases sharply (Pasteur effect). Glycolysis products can inhibit the aerobic metabolism of carbohydrates (the Crabtree effect).

The Krebs cycle has a number of chemical reactions, as a result of which the breakdown products of carbohydrates are oxidized to carbon dioxide and water, and chemical energy is accumulated in macroergic compounds. During the formation of a "carrier" - oxaloacetic acid (SOC). Subsequently, condensation occurs with the "carrier" of the activated acetic acid residue. There is tricarboxylic acid - citric. During chemical reactions, there is a "turnover" of the acetic acid residue in the cycle. From each molecule of pyruvic acid, eighteen molecules of adenosine triphosphate are formed. At the end of the cycle, a "carrier" is released, which reacts with new molecules of the activated acetic acid residue.

Krebs cycle: reactions

If the end product of anaerobic digestion of carbohydrates is lactic acid, then under the influence of lactate dehydrogenase it is oxidized to pyruvic acid. Part of the pyruvic acid molecules is used for the synthesis of the “carrier” of BJC under the influence of the pyruvate carboxylase enzyme and in the presence of Mg2 + ions. Part of the molecules of pyruvic acid is the source of the formation of "active acetate" - acetylcoenzyme A (acetyl-CoA). The reaction is carried out under the influence of pyruvate dehydrogenase. Acetyl-CoA contains which accumulates about 5-7% of energy. The main mass of chemical energy is formed as a result of the oxidation of "active acetate".

Under the influence of citrate synthetase, the Krebs cycle itself begins to function, which leads to the formation of citrate acid. This acid, under the influence of aconitate hydratase, dehydrates and turns into cis-aconitic acid, which, after the addition of a water molecule, becomes isocitric. A dynamic equilibrium is established between the three tricarboxylic acids.

Isocitric acid is oxidized to oxalosuccinic acid, which is decarboxylated and converted to alpha-ketoglutaric acid. The reaction is catalyzed by the enzyme isocitrate dehydrogenase. Alpha-ketoglutaric acid, under the influence of the enzyme 2-oxo-(alpha-keto)-glutarate dehydrogenase, is decarboxylated, resulting in the formation of succinyl-CoA containing a macroergic bond.

At the next stage, succinyl-CoA, under the action of the enzyme succinyl-CoA synthetase, transfers the macroergic bond to GDP (guanosine diphosphate acid). GTP (guanosine triphosphate acid) under the influence of the enzyme GTP-adenylate kinase gives a macroergic bond to AMP (adenosine monophosphate acid). Krebs cycle: formulas - GTP + AMP - GDP + ADP.

Under the influence of the enzyme succinate dehydrogenase (SDH) is oxidized to fumaric. The coenzyme of SDH is flavin adenine dinucleotide. Fumarate, under the influence of the enzyme fumarate hydratase, is converted into malic acid, which in turn is oxidized, forming BOC. In the presence of acetyl-CoA in the reacting system, BFA is again included in the tricarboxylic acid cycle.

So, up to 38 ATP molecules are formed from one glucose molecule (two - due to anaerobic glycolysis, six - as a result of the oxidation of two NAD H + H + molecules, which were formed during glycolytic oxidization, and 30 - due to TCA). Coefficient useful action The CTC is 0.5. The rest of the energy is dissipated as heat. In the TCA, 16-33% of lactic acid is oxidized, the rest of its mass is used for glycogen resynthesis.