The tricarboxylic acid cycle was discovered in 1937 by G. Krebs. In this regard, he received the name "Krebs cycle". This process is the central pathway of metabolism. It occurs in the cells of organisms at different stages of evolutionary development (microorganisms, plants, animals).

The initial substrate of the tricarboxylic acid cycle is acetyl-coenzyme A. This metabolite is the active form of acetic acid. Acetic acid acts as a common intermediate product of the breakdown of almost all organic substances contained in the cells of living organisms. This is due to the fact that organic molecules are compounds of carbon that can naturally decompose into two-carbon fragments of acetic acid.

Free acetic acid has a relatively weak reactivity. Its transformations take place under rather harsh conditions, which are unrealistic in a living cell. Therefore, acetic acid is activated in cells by combining it with coenzyme A. As a result, a metabolically active form of acetic acid, acetyl coenzyme A, is formed.

Coenzyme A is a low molecular weight compound that consists of phosphoadenosine, pantothenic acid residue (vitamin B3) and thioethanolamine. The acetic acid residue is added to the sulfhydryl group of thioethanolamine. In this case, a thioester is formed - acetyl-coenzyme A, which is the initial substrate of the Krebs cycle.

Acetyl coenzyme A

The scheme of transformation of intermediate products in the Krebs cycle is shown in fig. 67. The process begins with the condensation of acetyl coenzyme A with oxaloacetate (oxaloacetic acid, PAA), which results in the formation of citric acid (citrate). The reaction is catalyzed by the enzyme citrate synthase.

Figure 67 - Scheme of the transformation of intermediate products in the cycle

tricarboxylic acids

Further, under the action of the enzyme aconitase, citric acid is converted into isocitric acid. Isocitric acid undergoes oxidation and decarboxylation processes. In this reaction, catalyzed by the enzyme NAD-dependent isocitrate dehydrogenase, carbon dioxide reduced by NAD and α-ketoglutaric acid are formed as products, which is then involved in the process of oxidative decarboxylation (Fig. 68).

Figure 68 - The formation of a-ketoglutaric acid in the Krebs cycle

The process of oxidative decarboxylation of α-ketoglutarate is catalyzed by enzymes of the α-ketoglutarate dehydrogenase multienzyme complex. This complex is made up of three different enzymes. It requires coenzymes to function. The a-keto-glutarate dehydrogenase complex coenzymes include the following water-soluble vitamins:

Vitamin B 1 (thiamine) - thiamine pyrophosphate;

vitamin B 2 (riboflavin) - FAD;

Vitamin B 3 (pantothenic acid) - coenzyme A;

Vitamin B 5 (nicotinamide) - OVER;

vitamin-like substance - lipoic acid.

Schematically, the process of oxidative decarboxylation of a-keto-glutaric acid can be represented as the following balance reaction equation:


The product of this process is the thioester of the succinic acid residue (succinate) with coenzyme A - succinyl-coenzyme A. The thioether bond of succinyl-coenzyme A is macroergic.

The next reaction of the Krebs cycle is the process of substrate phosphorylation. In it, the hydrolysis of the thioether bond of succinyl-coenzyme A occurs under the action of the enzyme succinyl-CoA synthetase with the formation of succinic acid (succinate) and free coenzyme A. This process is accompanied by the release of energy, which is immediately used to phosphorylate GDP, which results in the formation of a macroergic molecule GTP phosphate. Substrate phosphorylation in the Krebs cycle:

where F n is orthophosphoric acid.

The GTP formed during oxidative phosphorylation can be used as an energy source in various energy-dependent reactions (in the process of protein biosynthesis, activation of fatty acids, etc.). In addition, GTP can be used to form ATP in the nucleoside diphosphate kinase reaction.

The product of the succinyl-CoA synthetase reaction, succinate, is further oxidized with the participation of the enzyme succinate dehydrogenase. This enzyme is a flavin dehydrogenase, which contains a FAD molecule as a coenzyme (prosthetic group). As a result of the reaction, succinic acid is oxidized to fumaric acid. At the same time, FAD is restored.

where E is FAD, a prosthetic group associated with the polypeptide chain of the enzyme.

The fumaric acid formed in the succinate dehydrogenase reaction, under the action of the fumarase enzyme (Fig. 69), attaches a water molecule and turns into malic acid, which is then oxidized in the malate dehydrogenase reaction to oxaloacetic acid (oxaloacetate). The latter can again be used in the citrate synthase reaction for the synthesis of citric acid (Fig. 67). Due to this, the transformations in the Krebs cycle are cyclical.

Figure 69 - Metabolism of malic acid in the Krebs cycle

The balance equation of the Krebs cycle can be represented as:

It can be seen from it that in the cycle the acetyl radical of the residue from acetyl coenzyme A is completely oxidized to two molecules of CO 2 . This process is accompanied by the formation of three molecules of reduced NAD, one molecule of reduced FAD, and one molecule of high-energy phosphate - GTP.

The Krebs cycle occurs in the mitochondrial matrix. This is due to the fact that this is where most of its enzymes are located. And only a single enzyme - succinate dehydrogenase - is built into the inner mitochondrial membrane. Individual enzymes of the tricarboxylic acid cycle are combined into a functional polyenzymatic complex (metabolone) associated with the inner surface of the inner mitochondrial membrane. Due to the association of enzymes into a metabolon, the efficiency of the functioning of this metabolic pathway is significantly increased and additional opportunities for its fine regulation appear.

Features of the regulation of the cycle of tricarboxylic acids are largely determined by its value. This process performs the following functions:

1) energy. The Krebs cycle is the most powerful source of substrates (reduced coenzymes - NAD and FAD) for tissue respiration. In addition, it stores energy in the form of high-energy phosphate - GTP;

2) plastic. The intermediate products of the Krebs cycle are precursors for the synthesis of various classes of organic substances - amino acids, monosaccharides, fatty acids, etc.

Thus, the Krebs cycle performs a dual function: on the one hand, it is a common pathway of catabolism, which plays a central role in the energy supply of the cell, and on the other hand, it provides biosynthetic processes with substrates. Such metabolic processes are called amphibolic. The Krebs cycle is a typical amphibolic cycle.

The regulation of metabolic processes in the cell is closely related to the existence of "key" enzymes. The key are those enzymes of the process that determine its speed. As a rule, one of the "key" enzymes of the process is the enzyme that catalyzes its initial reaction.

The “key” enzymes are characterized by the following features. These enzymes

Catalyze irreversible reactions

have the least activity compared to other enzymes involved in the process;

They are allosteric enzymes.

The key enzymes of the Krebs cycle are citrate synthase and isocitrate dehydrogenase. Like key enzymes of other metabolic pathways, their activity is regulated by the principle of negative feedback: it decreases with an increase in the concentration of intermediate products of the Krebs cycle in mitochondria. Thus, citric acid and succinyl-coenzyme A act as inhibitors of citrate synthase, and reduced NAD acts as isocitrate dehydrogenase inhibitors.

ADP is an activator of isocitrate dehydrogenase. Under conditions of an increase in the cell's need for ATP as an energy source, when the content of decay products (ADP) increases in it, prerequisites arise for increasing the rate of redox transformations in the Krebs cycle and, consequently, increasing the level of its energy supply.

Everyone knows that in order to function properly, 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.

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 coming from catabolism are involved in the cycle. 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 with the formation of 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 basic 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 pyruvate, (formed from glucose or alanine), derived from aspartic acid as a result of transamination or the AMP-IMF cycle, and also from fruit acids the 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 starts to work and oxaloacetate is formed, but, of course, only in the presence of pyruvate.

Also most 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 insufficient 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.

Hello! Summer is coming, which means that all sophomores of medical universities will take biochemistry. A difficult subject, really. To help a little those who repeat the material for exams, I decided to make an article in which I will tell you about the "golden ring" of biochemistry - the Krebs cycle. It is also called the tricarboxylic acid cycle and the citric acid cycle, which are all synonyms.

I will write the reactions themselves in. Now I will talk about why the Krebs cycle is needed, where it goes and what are its features. I hope it will be clear and accessible.

First, let's understand what metabolism is. This is the basis without which the understanding of the Krebs Cycle is impossible.

Metabolism

One of the most important properties of living things (remember) is the metabolism with the environment. Indeed, only a living being can absorb something from the environment, and then release something into it.

In biochemistry, metabolism is called "metabolism". Metabolism, the exchange of energy with the environment is metabolism.

When we, say, ate a chicken sandwich, we got proteins (chicken) and carbohydrates (bread). During digestion, proteins break down into amino acids and carbohydrates into monosaccharides. What I have just described is called catabolism, that is, the breakdown of complex substances into simpler ones. The first part of metabolism is catabolism.

One more example. The tissues in our body are constantly being renewed. When the old tissue dies, its fragments are pulled apart by macrophages, and they are replaced by new tissue. New tissue is created in the process of protein synthesis from amino acids. Protein synthesis occurs in ribosomes. The creation of a new protein (complex substance) from amino acids (simple substance) is anabolism.

So anabolism is the opposite of catabolism. Catabolism is the destruction of substances, anabolism is the creation of substances. By the way, so as not to confuse them, remember the association: “Anabolics. Blood and sweat". This is a Hollywood movie (rather boring in my opinion) about athletes using anabolics for muscle growth. Anabolics - growth, synthesis. Catabolism is the reverse process.

Intersection point of decay and synthesis.

The Krebs cycle as a stage of catabolism.

How are metabolism and the Krebs cycle related? The fact is that it is the Krebs cycle that is one of the most important points at which the paths of anabolism and catabolism converge. This is where its significance lies.

Let's break it down in diagrams. Catabolism can be roughly thought of as the breakdown of proteins, fats, and carbohydrates in our digestive system. So, we ate food from proteins, fats, and carbohydrates, what's next?

  • Fats - into glycerin and fatty acids (there may be other components, I decided to take the simplest example);
  • Proteins - into amino acids;
  • Polysaccharide molecules of carbohydrates are divided into single monosaccharides.

Further, in the cytoplasm of the cell, the transformation of these simple substances into pyruvic acid(she is pyruvate). From the cytoplasm, pyruvic acid enters the mitochondria, where it turns into acetyl coenzyme A. Please remember these two substances, pyruvate and acetyl CoA, they are very important.

Let's now see how the stage that we have just painted happens:

An important detail: amino acids can turn into acetyl CoA immediately, bypassing the stage of pyruvic acid. Fatty acids are immediately converted to acetyl CoA. Let's take this into account and edit our schema to get it right:

The transformation of simple substances into pyruvate occurs in the cytoplasm of cells. After that, pyruvate enters the mitochondria, where it is successfully converted to acetyl CoA.

Why is pyruvate converted to acetyl CoA? Precisely in order to start our Krebs cycle. Thus, we can make one more inscription in the scheme, and we get the correct sequence:

As a result of the reactions of the Krebs cycle, substances important for life are formed, the main of which are:

  • NADH(NicotineAmideAdenineDiNucleotide + hydrogen cation) and FADH 2(Flavin Adenine DiNucleotide + hydrogen molecule). I specifically highlighted the constituent parts of the terms in capital letters to make it easier to read, normally they are written in one word. NADH and FADH 2 are released during the Krebs cycle in order to then take part in the transfer of electrons to the respiratory chain of the cell. In other words, these two substances play a crucial role in cellular respiration.
  • ATP i.e. adenosine triphosphate. This substance has two bonds, the breaking of which gives a large amount of energy. Many vital reactions are supplied with this energy;

Water and carbon dioxide are also released. Let's reflect this in our diagram:

By the way, the entire Krebs cycle takes place in the mitochondria. It is where the preparatory stage takes place, that is, the conversion of pyruvate into acetyl CoA. Not for nothing, by the way, mitochondria are called the "energy station of the cell."

The Krebs cycle as the beginning of synthesis

The Krebs cycle is amazing in that it not only provides us with valuable ATP (energy) and coenzymes for cellular respiration. If you look at the previous diagram, you will understand that the Krebs cycle is a continuation of the processes of catabolism. But at the same time, it is also the first step of anabolism. How is this possible? How can the same cycle both destroy and create?

It turns out that individual products of the reactions of the Krebs cycle can be partially sent for the synthesis of new complex substances, depending on the needs of the body. For example, gluconeogenesis is the synthesis of glucose from simple substances that are not carbohydrates.

  • The reactions of the Krebs cycle are cascaded. They occur one after another, and each previous reaction triggers the next one;
  • The reaction products of the Krebs cycle are partly used to start the next reaction, and partly to the synthesis of new complex substances.

Let's try to reflect this on the diagram so that the Krebs cycle is designated exactly as the point of intersection of decay and synthesis.

With blue arrows, I marked the paths of anabolism, that is, the creation of new substances. As you can see, the Krebs cycle is indeed the point of intersection of many processes of both destruction and creation.

The most important

  • The Krebs cycle is the crossroads of metabolic pathways. They end catabolism (decay), they begin anabolism (synthesis);
  • The reaction products of the Krebs Cycle are partly used to start the next reaction of the cycle, and partly sent to create new complex substances;
  • The Krebs cycle produces the coenzymes NADH and FADH 2, which carry electrons for cellular respiration, as well as energy in the form of ATP;
  • The Krebs cycle occurs in the mitochondria of cells.

tricarboxylic acid cycle

CIRCOXIC ACID CYCLE - the citric acid cycle or the Krebs cycle - a path of oxidative transformations of di- and tricarboxylic acids, which are formed as intermediate products during the breakdown and synthesis of proteins, fats and carbohydrates, is widely represented in the organisms of animals, plants and microbes. 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 major 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 CO2 molecules,

2) three molecules of reduced nicotinamide adenine dinucleotide (NADH) and one reduced flavin adenine dinucleotide (FADH2) 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 favorable (DG0 "= -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, oxalosuccinic (oxalosuccinate) and dicarboxylic acids - 2-ketoglutaric (KG), succinic, fumaric, malic (malate) and oxaloacetic acids. The substrates of the Krebs cycle should 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 that enters the structure of citric acid that is oxidized and undergoes oxidation; carbon atoms are oxidized to CO2, hydrogen atoms are partially accepted by the coenzymes of dehydrogenases, partially in the protonated form they pass into solution, that is, into the environment.

As the starting compound for the formation of acetyl-CoA, pyruvic acid (pyruvate) is usually indicated, which is formed during glycolysis and occupies one of the central places in the crossing metabolic pathways. Under the influence of an enzyme of complex structure - pyruvate dehydrogenase (EC1.2.4.1 - PDGas), pyruvate is oxidized to form CO2 (first decarboxylation), acetyl-CoA and NAD is reduced (see diagram). However, pyruvate oxidation is far from the only way to form acetyl-CoA, which is also a characteristic product of fatty acid oxidation (the enzyme thiolase or fatty acid synthetase) and other decomposition reactions of carbohydrates and amino acids. All enzymes involved in the reactions of the Krebs cycle are localized in mitochondria, and most of them are soluble, and succinate dehydrogenase (EC1.3.99.1) is strongly associated with membrane structures.

The formation of citric acid, with the synthesis of which the cycle itself begins, with the help of citrate synthase (EC4.1.3.7 - the condensing enzyme in the scheme), 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 [CH3CO~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 conversion of isocitric acid under the influence of the corresponding dehydrogenase (isocitrate dehydrogenase EC1.1.1.41) is, apparently, oxalosuccinic acid, the decarboxylation of which (the second CO2 molecule) leads to CH. This stage is also highly regulated. According to a number of characteristics (high molecular weight, complex multicomponent structure, stepwise reactions, partially the same coenzymes, etc.), CH dehydrogenase (EC1.2.4.2) resembles PDGas. The reaction products are CO2 (third decarboxylation), H+ and succinyl-CoA. At this stage, succinyl-CoA synthetase, otherwise called succinatethiokinase (EC6.2.1.4), is activated, catalyzing the reversible reaction of formation of free succinate: Succinyl-CoA + Phneorg + GDP = Succinate + KoA + GTP. During this reaction, the so-called substrate phosphorylation is carried out, i.e. the formation of energy-rich guanosine triphosphate (GTP) due to guanosine diphosphate (GDP) and mineral phosphate (Pneorg) using the energy of succinyl-CoA. After the formation of succinate, succinate dehydrogenase (EC1.3.99.1), a flavoprotein that leads to fumaric acid, enters into action. FAD is connected to the protein part of the enzyme and is the metabolically active form of riboflavin (vitamin B2). This enzyme is also characterized by absolute stereospecificity of 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 needs the NAD + coenzyme, is also stereospecific) leads to the completion of the Krebs cycle, that is, to the formation of oxaloacetic acid. After that, 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 a more complex succinate dehydrogenase complex (complex II) of the respiratory chain, supplying reducing equivalents (NAD-H2) formed during the reaction to the respiratory chain.

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

tricarboxylic acid cycle

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 a more efficient cycle.

The energy efficiency of the considered processes 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 respiratory chain enzymes also in mitochondria and is associated with phosphorylation, i.e. the formation of ATP due to the esterification (formation of organophosphorus esters) of mineral phosphate. Glycolysis, the enzymatic action of PDGase and the Krebs cycle - a total of 19 reactions - determine the complete oxidation of one glucose molecule to 6 CO2 molecules with the formation of 38 ATP molecules - this exchange "energy currency" of the cell. The process of oxidation of NADH and FADH2 by respiratory chain enzymes is energetically very efficient, occurs with the use of atmospheric oxygen, leads to the formation of water and serves as the main source of cell energy resources (more than 90%). However, Krebs cycle enzymes are not involved in its direct implementation. Each human cell has from 100 to 1000 mitochondria that provide energy for life.

The integrating function of the Krebs cycle in metabolism is based on the fact that carbohydrates, fats and amino acids from proteins can ultimately be converted into intermediates (intermediate compounds) 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, which is necessary for biosynthesis. Thus, the loop must perform two functions at the same time. In this case, the concentration of intermediates (especially OA) can decrease, which can lead to a dangerous decrease in energy production. To prevent, "safety valves" called anaplerotic reactions (from the Greek "to fill") are used. 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 is accumulated, which ensures the synthesis of citrate and other intermediates, which allows the Krebs cycle to function normally and, at the same time, ensure the excretion 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 up to KG (reactions 1, 2, 3) and the reduction branch, from OA to succinate (reactions 8®7®6). At the same time, much energy is not stored and the cycle supplies only intermediates for cellular syntheses.

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

Glutamate + OA = KG + aspartate

Another modification of the Krebs cycle (the so-called 4-aminobutyrate shunt) is the conversion of CG 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 association 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 action of ATP, predetermine a decrease in cycle activity at a high phosphoryl potential of the cell, i.e. at a high ratio of ATP/ADP concentrations. In most plants, bacteria, and many fungi, close coupling is overcome by the development of non-conjugated alternative oxidation pathways, which allow both respiratory and cycle activity to be maintained at a high level even at a high phosphoryl potential.

Igor Rapanovich

tricarboxylic acid cycle

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Literature

Strayer L. Biochemistry. Per. from English. M., Mir, 1985

Bohinski R. Modern views in biochemistry. Translated from English, M., Mir, 1987

Knorre D.G., Myzina S.D. Biological chemistry. M., Higher School, 2003

Kolman J., Rem K.-G. Visual biochemistry. M., Mir, 2004