Tricarboxylic acid cycle (Krebs cycle)

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 carbohydrates. Later it was shown that the cycle of tricarboxylic acids is the center where almost all metabolic pathways converge. Thus, Krebs cycle- common end path oxidation acetyl groups (in the form of acetyl-CoA), into which it is converted in the process catabolism most of the organic molecules, playing the role of "cellular fuel»: carbohydrates, fatty acids and amino acids.

Formed as a result of oxidative decarboxylation pyruvate in mitochondria acetyl-CoA enters Krebs cycle. This cycle takes place in the matrix mitochondria and consists of eight successive reactions(Fig. 10.9). The cycle begins with the addition of acetyl-CoA to oxaloacetate and the formation citric acid (citrate). Then lemon acid(six-carbon compound) by a series dehydrogenation(taking away hydrogen) and two decarboxylations(cleavage of CO 2) loses two carbon atom and again in Krebs cycle turns into oxaloacetate (four-carbon compound), i.e. as a result of a full turn of the cycle one molecule acetyl-CoA burns to CO 2 and H 2 O, and molecule oxaloacetate is regenerated. Consider all eight successive reactions(stages) Krebs cycle.

Rice. 10.9.Tricarboxylic acid cycle (Krebs cycle).

First reaction catalyzed enzyme cit-rat-synthase, while acetyl the acetyl-CoA group condenses with oxaloacetate, resulting in the formation of lemon acid:

Apparently, in this reactions associated with enzyme citril-CoA. Then the latter spontaneously and irreversibly hydrolyzes to form citrate and HS-KoA.

As a result of the second reactions formed lemon acid undergoes dehydration with the formation of cis-aconitic acids, which, by adding molecule water, goes into isocitric acid(isocitrate). Catalyzes these reversible reactions hydration-dehydration enzyme aconitate hydratase (aconitase). As a result, there is a mutual movement of H and OH in molecule citrate:

Third reaction seems to limit the speed Krebs cycle. isocitric acid dehydrogenated in the presence of NAD-dependent iso-citrate dehydrogenase.

During isocitrate dehydrogenase reactions isocitric acid simultaneously decarboxylated. NAD-dependent isocitrate dehydrogenase is allosteric enzyme, which as a specific activator needed ADP. Besides, enzyme to express your activity needs to ions Mg 2+ or Mn 2+ .

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

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

As a result, the sixth reactions succinate dehydrated into fumaric acid. Oxidation succinate catalyzed succinate dehydrogenase, in molecule which since protein firmly (covalently) bound coenzyme FAD. In its turn succinate dehydrogenase strongly associated with the internal mitochondrial membrane:

seventh reaction carried out under the influence enzyme fumarate hydratase ( fumarases). Formed at the same time fumaric acid hydrated, product reactions is an Apple acid(malate). It should be noted that fumarate hydratase has stereospecificity(see chapter 4) – during reactions L-apple is formed acid:

Finally, during the eighth reactions tricarboxylic acid cycle under the influence of mitochondrial NAD-dependent malate dehydrogenase going on oxidation L-malate to oxaloacetate:

As can be seen, in one turn of the cycle, consisting of eight enzymatic reactions, complete oxidation("combustion") of one molecules acetyl-CoA. For continuous operation of the cycle, a constant supply of acetyl-CoA to the system is necessary, and coenzymes(NAD + and FAD), which have passed into the reduced state, must be oxidized again and again. This is oxidation carried out in the carrier system electrons in respiratory chain(in respiratory chain enzymes) localized in membrane mitochondria. The resulting FADH 2 is strongly associated with SDH, so it transmits atoms hydrogen via KoQ. released as a result oxidation acetyl-CoA energy is largely concentrated in macroergic phosphate bonds ATP. From 4 steam atoms hydrogen 3 couples transfer NADH to the transport system electrons; while counting on each couple in the system of biological oxidation formed 3 molecules ATP(during conjugated ), and in total, therefore, 9 molecules ATP(see chapter 9). One pair atoms from succinate dehydrogenase-FADH 2 enters the transport system electrons through KoQ, resulting in only 2 molecules ATP. During Krebs cycle one is also synthesized molecule GTP (substrate phosphorylation), which is equivalent to one molecule ATP. So, at oxidation one molecules acetyl-CoA in Krebs cycle and system oxidative phosphorylation may form 12 molecules ATP.

If we calculate the total energy effect of glycolytic cleavage glucose and subsequent oxidation two emerging molecules pyruvate to CO 2 and H 2 O, then it will be much larger.

As noted, one molecule NADH (3 molecules ATP) is formed during oxidative decarboxylation pyruvate to acetyl-CoA. When splitting one molecules glucose formed 2 molecules pyruvate, and oxidation up to 2 molecules acetyl-CoA and subsequent 2 turns tricarboxylic acid cycle synthesized 30 molecules ATP(hence, oxidation molecules pyruvate to CO 2 and H 2 O gives 15 molecules ATP). To this number must be added 2 molecules ATP formed during aerobic glycolysis, and 6 molecules ATP, synthesized by oxidation 2 molecules extramitochondrial NADH, which are formed during oxidation 2 molecules glyceraldehyde-3-phosphate in dehydrogenase reactions glycolysis. Therefore, when splitting into tissues one molecules glucose according to the equation C 6 H 12 O 6 + 6O 2 -> 6CO 2 + 6H 2 O, 38 is synthesized molecules ATP. Undoubtedly, in terms of energy, the complete splitting glucose is a more efficient process than anaerobic glycolysis.

It should be noted that the 2 molecules NADH in the future with oxidation can give not 6 molecules ATP, but only 4. The fact is that they themselves molecules extramitochondrial NADH are not able to penetrate through membrane inside mitochondria. However, they give electrons can be included in the mitochondrial chain of biological oxidation using the so-called glycerol phosphate shuttle mechanism (Fig. 10.10). Cytoplasmic NADH first reacts with cytoplasmic dihydroxyacetone phosphate to form glycerol-3-phosphate. Reaction catalysis

Rice. 10.10. Glycerol phosphate shuttle mechanism. Explanation in the text.

is controlled by NAD-dependent cytoplasmic glycerol-3-phosphate dehydrogenase:

Dihydroxyacetone phosphate + NADH + H +<=>Glycerol-3-phosphate + NAD +.

The resulting glycerol-3-phosphate easily penetrates through the mitochondrial membrane. Inside mitochondria another (mitochondrial) glycerol-3-phosphate dehydrogenase (flavin enzyme) oxidizes glycerol-3-phosphate again to dihydroxyacetone phosphate:

Glycerol-3-phosphate + FAD<=>Dihydroxyacetone phosphate + FADH 2.

restored flavoprotein(enzyme-FADH 2) introduces at the level of KoQ acquired by him electrons into the chain of biological oxidation and associated with it oxidative phosphorylation, and dihydroxyacetone phosphate comes out of mitochondria in cytoplasm and can again interact with cytoplasmic NADH + H + . Thus, pair electrons(from one molecules cytoplasmic NADH + H +), introduced into respiratory chain using a glycerol phosphate shuttle mechanism, gives not 3, but 2 ATP.

Rice. 10.11. Malate-aspartate shuttle system for the transfer of reducing equivalents from cytosolic NADH to the mitochondrial matrix. Explanation in the text.

Subsequently, it was shown that this shuttle mechanism is used only in skeletal muscles and the brain to transfer reduced equivalents from cytosolic NADH + H + to mitochondria.

AT cells liver, kidneys and heart, a more complex malate-as-partate shuttle system operates. The operation of such a shuttle mechanism becomes possible due to the presence malate dehydrogenase and aspartate aminotransferases both in the cytosol and in mitochondria.

It was found that from cytosolic NADH + H + reduced equivalents, first with the participation enzyme malate dehydrogenase(Fig. 10.11) are transferred to cytosolic oxaloacetate. As a result, malate is formed, which, with the help of a system that transports dicarboxylic acids, passes through the inner membrane mitochondria into the matrix. Here, malate is oxidized to oxaloacetate, and matrix NAD + is reduced to NADH + H + , which can now transfer its electrons in respiratory chain enzymes, localized on the inner membrane mitochondria. In turn, the resulting oxaloacetate in the presence of glutamate and enzyme ASAT enters into reaction transamination. The resulting aspartate and α-ketoglutarate, with the help of special transport systems, are able to pass through membrane mitochondria.

Transport in the cytosol regenerates the oxaloacetate, which triggers the next cycle. In general, the process includes easily reversible reactions, occurs without energy consumption, its "driving force" is a constant recovery NAD + in the cytosol by glyceraldehyde-3-phosphate, which is formed during catabolism glucose.

So, if the malate-aspartate mechanism functions, then as a result of the complete oxidation one molecules glucose may form not 36, but 38 molecules ATP(Table 10.1).

In table. 10.1 are given reactions, in which the formation of high-energy phosphate bonds occurs during catabolism glucose, indicating the efficiency of the process under aerobic and anaerobic conditions

Ministry of Education of the Russian Federation

Samara State Technical University

Department of Organic Chemistry

Abstract on the topic:

"THE CYCLE OF TRICABOXIC ACIDS (KREBS CYCLE)"

Completed by student: III - NTF - 11

Eroshkina N.V.

Checked.

The processes of anaerobic fermentation served as the main source of energy for all living things in those days when there was no oxygen in the Earth's atmosphere. Its appearance opened up fundamentally new possibilities for obtaining energy. Oxygen is a good oxidizing agent, and when organic substances are oxidized, ten times more energy is released than during fermentation. So, during the oxidation of glucose C 6 H 12 O 6 + 6O 2 → 6H 2 O + 6CO 2, 686 kcal per mol is released, while in the reaction of lactic acid fermentation only 47 kcal per mol.

Naturally, the cells began to use the opportunities that had opened up. ATP synthesis under aerobic conditions is much more efficient than anaerobic synthesis: if 2 ATP molecules are formed during the utilization of 1 glucose molecule during fermentation, then during oxidative phosphorylation - about 30 (according to old data - 38). We will talk more about energy balance in Lesson 12.

Various organic substances undergo oxidative transformations - intermediate metabolites of the metabolism of amino acids, sugars, fatty acids, etc. It would be illogical to create their own metabolic pathway for each of them. It is much more convenient to first oxidize all these substances with one, unified oxidizing agent, and then oxidize the resulting reduced form of such a “universal oxidizing agent” with oxygen. Nicotinamide adenine dinucleotide, NAD, is used as this universal redox intermediate in the cell; we already talked about this compound in lesson 10. As indicated in lesson 10, this substance can exist in two forms: oxidized NAD + and reduced NAD∙H. The transformation of the first form into the second requires the supply of two electrons and one H + ion.

System plays the role of a redox shuttle that transfers electrons from various organic substances to oxygen: at the first stage, NAD + takes electrons from organic substances, oxidizing them eventually to CO 2 and H 2 O (of course, not in one stage, but through numerous intermediate connections); in the second stage, oxygen oxidizes the NAD∙H formed during the first stage and returns it back to the oxidized state.

So, in the most general form, the set of decomposition reactions of various substances under aerobic conditions (that is, in the presence of oxygen) can be represented as follows:

1)	organic compounds +
2)

The reactions of the first stage take place either in the cytoplasm or in the mitochondria, while the reactions of the second stage take place only in the mitochondria. In this lesson, we will consider only the reactions of the first group, the reactions of the second group will be studied in the 12th lesson.

There is another coenzyme in the cell - FAD (flavin adenine dinucleotide) - which also serves as a redox shuttle, but is used in fewer reactions than NAD; it is synthesized from vitamin B 2 - riboflavin.

Let's look at specific metabolic pathways - the oxidative conversion of glucose and fatty acids. Aerobic glycolysis begins with the same reactions as the anaerobic glycolysis we have already considered (see lesson 10). However, the final stages of the process will proceed differently. When carrying out anaerobic glycolysis, the cell faced a problem: what to do with the reduced NAD∙H, which is formed during the glyceraldehyde-3-phosphate dehydrogenase reaction? If it is not oxidized back to NAD +, then the process will quickly stop, therefore, in anaerobic glycolysis, the last reaction - lactate dehydrogenase - just served to return this coenzyme to its original form. Under aerobic conditions, there is no such problem. On the contrary, NAD∙H serves as the most valuable source of energy in oxygen metabolism - a special carrier system delivers it from the cytosol to the mitochondria, where it is oxidized, and ATP is synthesized due to this energy.

When glycolysis occurs under aerobic conditions, pyruvic acid will not be reduced, but will be transported to the mitochondria and oxidized. First, it will turn into an acetic acid residue, acetyl, covalently attached to a special coenzyme - the so-called coenzyme A.

This irreversible reaction is carried out by the mitochondrial enzyme pyruvate dehydrogenase, which oxidizes pyruvic acid to acetyl-coenzyme A with the release of carbon dioxide. This enzyme contains several coenzymes necessary for its work: thiamine pyrophosphate (formed from vitamin B 1 - thiamine), lipoic acid (it is sometimes used as a health-promoting dietary supplement) and FAD (we already wrote about it above). It is a very complex protein, consisting of many subunits, its molecular weight is several million daltons.

Coenzyme A, to which an acetyl residue is attached, is synthesized from pantothenic acid, which is also a vitamin (vitamin B 5). Acetyl-coenzyme A is a macroerg that is as rich in energy as ATP (see lesson 9).

Pyruvate dehydrogenase plays an important role in the regulation of aerobic glucose catabolism. This enzyme is inhibited by NAD∙H and acetyl-CoA, its end products, in a negative feedback manner. Regulation is carried out using a complex mechanism, including both allostery and covalent modification of this protein. This enzyme is also inhibited by fatty acids. Fatty acids are a more caloric source of energy, and besides, they are less valuable for carrying out synthetic processes in the cell, therefore, in the presence of both glucose (after all, pyruvate is formed from it) and fatty acids, it is advisable to oxidize fatty acids first.

Then acetyl-coenzyme A will be oxidized to CO 2 and H 2 O in a process called the Krebs cycle (in honor of G. Krebs, who first described it in 1937).

The main role of the Krebs cycle in the energy metabolism of the cell is to obtain reduced coenzymes NAD∙H and FAD∙H 2, which will then be oxidized by oxygen to synthesize ATP from ADP and phosphate (we will consider this process in lesson 12). Restoration of coenzymes is achieved by complete oxidation of the acetic acid residue to CO 2 and H 2 O.

The cycle begins with the transfer of the acetic acid residue from acetyl-CoA to oxaloacetic acid (in a neutral environment, this is the oxaloacetate ion), as a result of which citric acid (more precisely, the citrate ion) is formed, and coenzyme A is released. This reaction is catalyzed by the enzyme citrate synthase and is irreversible.

The organic acids involved at this stage have three carboxyl groups, sometimes the whole cycle is called the “tricarboxylic acid cycle”, but this name is unfortunate - already at the next stage one carboxyl group is lost. Therefore, the cycle is often referred to as the "tricarboxylic and dicarboxylic acid cycle".

In both cases, carbon dioxide is released, the oxidizing agent NAD + is reduced to NAD ∙ H, and the shortened acid residue is added to coenzyme A during the reaction. A. The α-ketoglutarate dehydrogenase reaction is as irreversible as the pyruvate dehydrogenase reaction, and the enzyme catalyzing it contains the same coenzymes.

The reaction product succinyl-coenzyme A is as rich in energy as acetyl-coenzyme A. It would be foolish to dissipate this energy into heat, and the cell does not allow such waste. Succinyl-CoA is not simply hydrolyzed to succinic acid (more precisely, succinate ion) and coenzyme A; during this reaction, GTP is synthesized from GDP and phosphate, and GTP is as macroergic as ATP.

Succinic acid undergoes further oxidation. However, its oxidizing agent is not the usual NAD +, but another coenzyme - FAD. Nature used this particular coenzyme not at all to poison the life of students and schoolchildren studying the Krebs cycle. The fact is that in succinic acid, a very inert group -CH 2 -CH 2 - is subjected to oxidation. Remember the course of organic chemistry - alkanes are generally slightly reactive compared to alcohols and aldehydes, it is much more difficult to oxidize them. Here, too, the cell is forced to use a stronger flavin oxidant, and not the usual nicotinamide. At the same time, succinic acid turns into fumaric acid, the reaction is accelerated by the enzyme succinate dehydrogenase.

The last reaction of the cycle is the oxidation of malic acid to oxaloacetic acid, the well-known NAD + serves as an oxidizing agent, and the reaction is catalyzed by the enzyme malate dehydrogenase.

The resulting NAD∙H and FAD∙H 2 are then oxidized in mitochondria, providing energy for ATP synthesis. The Krebs cycle also produces 1 molecule of GTP, an energy-rich compound capable of transferring a phosphate residue to ADP and forming ATP. The oxaloacetic acid molecule leaves the cycle without any changes - it serves as a catalyst for the oxidation of acetyl coenzyme A, and itself returns to its original state at the end of each turn of the cycle. The Krebs cycle enzymes are located in the mitochondrial matrix (except for succinate dehydrogenase, it is located on the inner mitochondrial membrane).

In the Krebs cycle, several enzymes are regulated at once. Isocitrate dehydrogenase is inhibited by NAD∙H, the end product of the cycle, and activated by ADP, a substance formed during energy expenditure. The reversibility of the malate dehydrogenase reaction also plays an important role in cycle regulation. At high concentrations of NAD∙H, this reaction proceeds from right to left, towards the formation of malate. As a result, the concentration of oxaloacetate falls, and the rate of the citrate synthase reaction decreases. The resulting malate can be used in other metabolic processes. Citrate synthase is also allosterically inhibited by ATP. The activity of α-ketoglutarate dehydrogenase is also regulated.

The Krebs cycle is involved in the oxidative transformations of not only glucose, but also fatty acids and amino acids. After penetration through the outer membrane, fatty acids are first activated in the cytoplasm by the addition of coenzyme A, while two macroergic bonds of ATP are consumed:

R–COOH + HS–KoA + ATP = R–CO–S–KoA + AMP + P–P.

Pyrophosphate is immediately cleaved by the enzyme pyrophosphatase, shifting the equilibrium of the reaction to the right.

Acyl-coenzyme A is then transferred to the mitochondria.

In these organelles, an enzymatic system of the so-called β-oxidation of fatty acids operates. The process of β-oxidation proceeds in stages. At each stage, a two-carbon fragment in the form of acetyl coenzyme A is cleaved from the fatty acid, and NAD + is reduced to NAD ∙ H and FAD to FAD ∙ H 2 .

During the first reaction, the -CH 2 -CH 2 - group, located near the carbonyl carbon atom, is oxidized. As in the case of succinate oxidation in the Krebs cycle, FAD serves as the oxidizing agent. Then (the second reaction) the double bond of the formed unsaturated compound is hydrated, while the third carbon atom becomes hydroxylated - a β-hydroxy acid is formed attached to coenzyme A. During the third reaction, this alcohol group is oxidized to a keto group, NAD + is used as an oxidizing agent. Finally, another molecule of coenzyme A reacts with the resulting β-ketoacyl coenzyme A. As a result, acetyl coenzyme A is cleaved off, and acyl-CoA is shortened by two carbon atoms. Now the cyclic process will proceed in the second run, the fatty acid residue will be shortened by one more acetyl-CoA, and so on until the fatty acid is completely cleaved. Of the four reactions of β-oxidation, only the first is irreversible, the rest are reversible, their passage from left to right is ensured by the constant output of end products.

In total, β-oxidation of palmitoyl-coenzyme A proceeds according to the equation:

Acetyl-CoA then enters the Krebs cycle. NAD∙H and FAD∙H 2 are oxidized in mitochondria, providing energy for ATP synthesis.

Amino acid catabolism also proceeds through the Krebs cycle. Different amino acids enter the cycle through different metabolic pathways, and their consideration is too complicated for this course.

The Krebs cycle is used by the cell not only for energy needs, but also for the synthesis of a number of substances it needs. It is the central metabolic pathway in both the catabolic and anabolic processes of the cell.

Hans Krebs himself first theoretically suggested that the transformations of di- and tricarboxylic acids proceed cyclically, and then did a series of experiments in which he showed the interconversions of these acids and their ability to stimulate aerobic glycolysis. However, strong evidence for the flow of this metabolic pathway in this way, and not otherwise, was obtained using experiments with isotopic labeling.

Imagine that you have replaced an ordinary natural isotope with a radioactive one in a certain intermediate metabolite of the Krebs cycle. Now this substance, as it were, bears a radioactive label, and this makes it possible to trace its further fate. Such a labeled compound can be added to the cell extract and, after a while, see what it turns into. To do this, you can separate small molecules from macromolecules (for example, by precipitation of the latter) and separate their mixture using a chromatographic method (see lesson 8). Then it remains only to determine which substances contain radioactivity. For example, if you add radioactively labeled citric acid to the extract, then very soon the label will be found in cis-aconitic and isocitric acids, and after some time in α-ketoglutaric. If labeled α-ketoglutaric acid is added, then the label will first of all go into succinyl-coenzyme A and succinic acid, then into fumaric acid. Thus, by adding various radioactively labeled substances and determining where the radioactive label went, it is possible to find out the sequence of reactions at any stage of the metabolic pathway.

Radioactivity can be determined in various ways. The easiest way is to illuminate a photographic emulsion, because radioactivity itself was discovered by A. Becquerel precisely because of the ability of radioactive radiation to illuminate a photographic plate. For example, if we separated a mixture of substances by thin-layer chromatography and we know where the spot of a particular substance is located, then we can simply attach a photographic plate to our chromatogram. Then the section of the photographic plate that was in contact with the spot containing radioactivity will be illuminated. It remains only to see which substances the emulsion lit up near the spots of which substances, and one can immediately say that it was into these substances that the radioactive label passed.

This method is called autoradiography . It can be used to study not only small molecules, but also large ones - for example, by adding radioactively labeled uridine to a living cell. As we said in lesson 7, uridine nucleotides are part of RNA, so this macromolecule will soon be radioactively labeled. It is now possible to track the location and transport of RNA in the cell. To do this, you need to fix the cells so that the macromolecules precipitate and do not float away during further procedures, fill them with photographic emulsion and after a while look through the microscope, where the illuminated areas appear.

Autoradiography makes it possible to directly observe the fate of molecules in a cell. However, the method also has a drawback - it gives only a qualitative characteristic of the presence of a radioactive label and does not allow to measure it quantitatively. For precise quantitative measurements, a different method is used. β-particles emitted from radioactive isotopes cause the glow of special substances - scintillators. The intensity of this glow can be accurately measured using a special device - a scintillation counter. By accurately measuring the glow, we can accurately determine the amount of radioactive isotope. However, the use of a scintillation counter only measures the total amount of radioactive isotope in the sample. If we flood a cell suspension with a scintillator solution, we can determine the total amount of a radioactive compound, but not its distribution over organelles. To do this, we will have to isolate individual cell organelles and measure the radioactivity in them.

Usually in biochemical research such isotopes as tritium 3 H, carbon 14 C, phosphorus 32 P and sulfur 35 S are used.

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" - acetyl coenzyme 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 to 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). The efficiency of 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.

The tricarboxylic acid cycle is also known as the Krebs cycle, since the existence of such a cycle was proposed by Hans Krebs in 1937.
For this, 16 years later, he was awarded the Nobel Prize in Physiology or Medicine. So, the discovery is very significant. What is the meaning of this cycle and why is it so important?

Whatever one may say, you still have to start quite afar. If you undertook to read this article, then at least by hearsay you know that the main source of energy for cells is glucose. It is constantly present in the blood in an almost unchanged concentration - for this there are special mechanisms that store or release glucose.

Inside each cell are mitochondria - separate organelles ("organs" of the cell) that process glucose to obtain an intracellular energy source - ATP. ATP (adenosine triphosphoric acid) is versatile and very convenient to use as an energy source: it is directly integrated into proteins, providing them with energy. The simplest example is the protein myosin, thanks to which muscles are able to contract.

Glucose cannot be converted into ATP, despite the fact that it contains a large amount of energy. How to extract this energy and direct it in the right direction without resorting to barbaric (by cellular standards) means such as incineration? It is necessary to use workarounds, since enzymes (protein catalysts) allow some reactions to proceed much faster and more efficiently.

The first step is the conversion of a glucose molecule into two molecules of pyruvate (pyruvic acid) or lactate (lactic acid). In this case, a small part (about 5%) of the energy stored in the glucose molecule is released. Lactate is produced by anaerobic oxidation - that is, in the absence of oxygen. There is also a way to convert glucose under anaerobic conditions into two molecules of ethanol and carbon dioxide. This is called fermentation, and we will not consider this method.


...Just as we will not consider in detail the mechanism of glycolysis itself, that is, the breakdown of glucose into pyruvate. Because, to quote Leinger, "The conversion of glucose to pyruvate is catalyzed by ten enzymes acting in sequence." Those who wish can open a textbook on biochemistry and get acquainted in detail with all the stages of the process - it has been studied very well.

It would seem that the path from pyruvate to carbon dioxide should be quite simple. But it turned out that it is carried out through a nine-stage process, which is called the tricarboxylic acid cycle. This apparent contradiction with the principle of economy (couldn't it be simpler?) is partly due to the fact that the cycle connects several metabolic pathways: the substances formed in the cycle are precursors of other molecules that are no longer related to respiration (for example, amino acids), and any other compounds to be disposed of end up in the cycle and are either "burned" for energy or recycled into those that are in short supply.

The first step traditionally considered in relation to the Krebs cycle is the oxidative decarboxylation of pyruvate to an acetyl residue (Acetyl-CoA). CoA, if anyone does not know, is coenzyme A, which has a thiol group in its composition, on which it can carry an acetyl residue.


The breakdown of fats also leads to acetyls, which also enter the Krebs cycle. (They are synthesized similarly - from Acetyl-CoA, which explains the fact that only acids with an even number of carbon atoms are almost always present in fats).

Acetyl-CoA condenses with oxaloacetate to give citrate. This releases coenzyme A and a water molecule. This stage is irreversible.

Citrate is dehydrogenated to cis-aconitate, the second tricarboxylic acid in the cycle.

Cis-aconitate attaches back a water molecule, turning already into isocitric acid. This and the previous stages are reversible. (Enzymes catalyze both forward and reverse reactions - you know, right?)

Isocitric acid is decarboxylated (irreversibly) and simultaneously oxidized to give ketoglutaric acid. At the same time, NAD +, recovering, turns into NADH.

The next step is oxidative decarboxylation. But in this case, not succinate is formed, but succinyl-CoA, which is hydrolyzed at the next stage, directing the released energy to ATP synthesis.

This produces another NADH molecule and a FADH2 molecule (a coenzyme other than NAD, which, however, can also be oxidized and reduced, storing and releasing energy).

It turns out that oxaloacetate works as a catalyst - it does not accumulate and is not consumed in the process. So it is - the concentration of oxaloacetate in the mitochondria is maintained quite low. But how to avoid the accumulation of other products, how to coordinate all eight stages of the cycle?

For this, as it turned out, there are special mechanisms - a kind of negative feedback. As soon as the concentration of a certain product rises above the norm, this blocks the work of the enzyme responsible for its synthesis. And for reversible reactions, it is even simpler: when the concentration of the product is exceeded, the reaction simply starts to go in the opposite direction.

And a couple of minor remarks

Everyone knows that in order to function normally, the body needs a regular intake of a number of nutrients that are needed for a healthy metabolism and, accordingly, the balance of energy production and expenditure processes. The process of energy production, as you know, takes place in the mitochondria, which, thanks to this feature, are called the energy centers of cells. And the sequence of chemical reactions that allows you to get energy for the work of each cell of the body is called the Krebs cycle.

The Krebs cycle - miracles that happen in the mitochondria

The energy received through the Krebs cycle (also TCA - the cycle of tricarboxylic acids) 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) - it is this compound that is a universal source of energy necessary for the flow of all biochemical processes in our body.

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

• fatty acids;
• amino acids;
• monosaccharides.

In the process of aerobic decay, these biomolecules are broken down into smaller molecules that are used for energy or the synthesis of 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 by 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 FADH 2 ;
• one molecule of GTP (equivalent to ATP) is produced.

NADH and FADH 2 molecules act as electron carriers and are used to generate ATP in the next step in glucose metabolism, oxidative phosphorylation.

Functions of the Krebs cycle:

• catabolic (oxidation of acetyl residues of fuel molecules to end products of metabolism);
• anabolic (substrates of the Krebs cycle - the basis for the synthesis of molecules, including amino acids and glucose);
• integrative (CTK - a link between anabolic and catabolic reactions);
• hydrogen donor (delivery of 3 NADH.H + and 1 FADH 2 to the respiratory chain of mitochondria);
• energy.

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

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