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Glycolysis and respiration. Krebs cycle. Krebs cycle - the central metabolic pathway Accelerated Krebs course

13.05.2024

Hello! Summer is coming, which means that all second-year medical students will take biochemistry. A difficult subject, indeed. To help a little those who are repeating 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, these are all synonyms.

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

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

Metabolism

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

In biochemistry, metabolism is usually called “metabolism”. Metabolism, the exchange of energy with the environment is metabolism.

When we, say, ate a chicken sandwich, we received proteins (chicken) and carbohydrates (bread). During the digestion process, proteins are broken down into amino acids, and carbohydrates into monosaccharides. What I have described now 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 renewed. When old tissue dies, its fragments are taken away by macrophages, and they are replaced by new tissue. New tissue is created through the process of protein synthesis from amino acids. Protein synthesis occurs in ribosomes. Creating 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 (quite boring, in my opinion) about athletes using anabolic steroids to grow muscles. Anabolics - growth, synthesis. Catabolism is the reverse process.

The 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 the Krebs cycle is one of the most important points at which the paths of anabolism and catabolism converge. This is precisely its meaning.

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

  • Fats - into glycerol 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, these simple substances will be converted into pyruvic acid(aka pyruvate). From the cytoplasm, pyruvic acid enters the mitochondrion, where it is converted 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 now described occurs:

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

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

Why is pyruvate converted to acetyl CoA? Precisely to start our Krebs cycle. Thus, we can make one more inscription in the diagram, and we will 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(Nicotine Amide Adenine DiNucleotide + 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 as one word. NADH and FADH 2 are released during the Krebs cycle to then take part in the transfer of electrons into the cell's respiratory chain. In other words, these two substances play a critical role in cellular respiration.
  • ATP, that is, adenosine triphosphate. This substance has two bonds, the rupture of which provides 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 occurs in mitochondria. This is where the preparatory stage takes place, that is, the conversion of pyruvate into acetyl CoA. It’s not for nothing that mitochondria are called the “energy station of the cell.”

The Krebs cycle as the beginning of synthesis

The Krebs cycle is amazing because 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 catabolic processes. 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 reaction products of the Krebs cycle can be partially used 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 cascading. 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 subsequent 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 precisely as the point of intersection of decay and synthesis.

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

The most important

  • The Krebs cycle is a cross-point of metabolic pathways. It ends catabolism (breakdown), it begins anabolism (synthesis);
  • The reaction products of the Krebs Cycle are partly used to launch the next reaction of the cycle, and partly are sent to create new complex substances;
  • The Krebs cycle produces the coenzymes NADH and FADH 2, which transport electrons for cellular respiration, as well as energy in the form of ATP;
  • The Krebs cycle occurs in the mitochondria of cells.

TRICARBOXYLIC ACIDS CYCLE (KREBS CYCLE)

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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


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

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


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

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

With the complete oxidation of a glucose molecule under aerobic conditions to C0 2 and H 2 0, the formation of energy in the form of ATP is:

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

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

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

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

In the early stages of the tricarboxylic acid cycle, its acids can leave the cycle to participate in the synthesis of other cell compounds without disrupting the functioning of the cycle itself. Various factors are involved in the regulation of tricarboxylic acid cycle activity. Among them, primarily the supply of acetyl-CoA molecules, the activity of the pyruvate dehydrogenase complex, the activity of the components of the respiratory chain and associated oxidative phosphorylation, as well as the level of oxaloacetic acid should be mentioned.

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

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

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

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

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




In eukaryotes, all reactions of the Krebs cycle occur inside mitochondria, and the enzymes that catalyze them, except one, are in a free state in the mitochondrial matrix. In prokaryotes, the reactions of the cycle occur in the cytoplasm. During the operation of the Krebs cycle, various metabolic products are oxidized, in particular toxic under-oxidized products of alcohol breakdown, therefore stimulation of the Krebs cycle can be considered as a measure of biochemical detoxification.



SubstratesProductsEnzymeType of reaction Comment 1 Oxaloacetate t + Acetyl-CoA + H 2 O Citrate + CoA-SH Citrate synth for Aldol condensation is the limiting stage, converts C 4 oxaloacetate into C 6 2 Citrate cis-aconitate + H 2 O aconitase 3 cis-aconiate + H 2 O isocitrate hydration isocitrate dehydrogenase decarboxylation Oxidation 4 Isocitrate + NAD + Oxalosuccinate + NADH + H + 5 Oxalosuccinate α-ketoglutarate + CO 2 decarboxylation irreversible stage, C 5 is formed


SubstratesProductsEnzyme Reaction type Comment 6 α-ketoglutarate + NAD + + CoA-SH succinyl-CoA + NADH + H + + CO 2 alpha-ketoglutarate dehydrogenase complex (3 enzymes) Oxidative decarboxylation produces NADH (equivalent to 2.5 ATP), regeneration of C 4 chains (released by CoA-SH) 7 succinyl-CoA + GDP + P i succinate + CoA-SH + GTP succinyl coenzyme A synthetase substrate phosphorylation ADP->ATP, 1 ATP (or 1 GTF) is formed 8 succinate + ubiquinone (Q ) fumarate + ubiquinol (QH 2) succinate hydrogenase Oxidation uses FAD as a prosthetic group (FAD->FADH 2 in the first stage of the reaction) in the enzyme, producing the equivalent of 1.5 ATP ATP, produces 1 ATP (or 1 GTF) 8 succinate + ubiquinone (Q) fumarate + ubiquinol (QH 2) succinate hydrogenase Oxidation uses FAD as a prosthetic group (FAD->FADH 2 in the first stage of the reaction) in the enzyme, produces the equivalent of 1.5 ATP ">


SubstratesProductsEnzyme Reaction type Comment 9 fumarate + H 2 O L-malate fumarase H 2 O- addition 10 L-malate + NAD + oxaloacetate + NADH + H + malate dehydrogenase oxidation produces NADH (equivalent to 2.5 ATP) General equation for one turn of the Krebs cycle: Acetyl-CoAAcetyl-CoA 2CO 2 + CoA + 8e CoAe



The Krebs cycle is regulated “by a negative feedback mechanism”; in the presence of a large number of substrates, the cycle actively operates, and when there is an excess of reaction products, it is inhibited. Regulation is also carried out with the help of hormones. These hormones are: insulin and adrenaline. Glucagon stimulates glucose synthesis and inhibits Krebs cycle reactions. As a rule, the work of the Krebs cycle is not interrupted due to anaplerotic reactions that replenish the cycle with substrates: Pyruvate + CO 2 + ATP = Oxalacetate (substrate of the Krebs Cycle) + ADP + Fn.


1. The integrative function of the cycle is the link between the reactions of anabolism and catabolism. 2. Catabolic function - the conversion of various substances into cycle substrates: Fatty acids, pyruvate, Leu, Phen Acetyl-CoA. Arg, His, Glu α-ketoglutarate. Fen, tyre fumarate. 3. Anabolic function: use of cycle substrates for the synthesis of organic substances: Oxalacetate, glucose, Asp, Asn. Succinyl-CoA heme synthesis. CO 2 carboxylation reactions.


1. The hydrogen-donating function of the Krebs cycle supplies protons to the respiratory chain of mitochondria in the form of three NADH.H + and one FADH 2. 2. The energy function of 3 NADH.H + gives 7.5 mol of ATP, 1 FADH 2 gives 1.5 mol of ATP on the respiratory chain. In addition, in the cycle, 1 GTP is synthesized by substrate phosphorylation, and then ATP is synthesized from it by transphosphorylation: GTP + ADP = ATP + GDP.


To make it easier to remember the acids involved in the Krebs cycle, there is a mnemonic: A Whole Pineapple And A Piece Of Soufflé Today Is Actually My Lunch, which corresponds to the series citrate, (cis-)aconitate, isocitrate, (alpha-)ketoglutarate, succinyl-CoA, succinate, fumarate, malate, oxaloacetate.


There is also the following mnemonic poem: The pike was acetyl limonil, And the horse was afraid of the daffodil, He was isolimoniously alpha-keto-glutared at him. Succinated by coenzyme, Amber fumarovo, Stored the apple for the winter, Turned into a pike again. (oxaloacetic acid, citric acid, cis-aconitic acid, isocitric acid, α-ketoglutaric acid, succinyl-CoA, succinic acid, fumaric acid, malic acid, oxaloacetic acid).

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

Krebs cycle - miracles that happen in mitochondria

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

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

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

  • fatty acids;
  • amino acids;
  • monosaccharides.

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

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

1. Formation of citric acid:

2. Formation of isocitric acid:

3. Dehydrogenation and direct decarboxylation of isocitric acid.

4. Oxidative decarboxylation of α-ketoglutaric acid

5. Substrate phosphorylation

6. Dehydrogenation of succinic acid with succinate dehydrogenase

7. Formation of malic acid by the enzyme fumarase

8. Formation of oxalacetate

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

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

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

Functions of the Krebs cycle:

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

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

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

In the 30s of the twentieth century, the German scientist Hans Krebs, together with his student, studied the circulation of urea. During World War II, Krebs moved to England where he came to the conclusion that certain acids catalyze processes in our body. For this discovery he was awarded the Nobel Prize.

As you know, the energy potential of the body depends on the glucose contained in our blood. Also, the cells of the human body contain mitochondria, which help in processing glucose to convert it into energy. After some transformations, glucose is converted into a substance called adenosine triphosphate (ATP), the main source of energy for cells. Its structure is such that it can be incorporated into a protein, and this compound will provide energy to all human organ systems. Glucose cannot directly become ATP, so complex mechanisms are used to obtain the desired result. This is the Krebs cycle.

In very simple terms, the Krebs cycle is a chain of chemical reactions occurring in every cell of our body, which is called a cycle because it continues continuously. The end result of this cycle of reactions is the production of adenosine triphosphate, a substance that represents the energy basis of the body's functioning. This cycle is otherwise called cellular respiration, since most of its stages occur with the participation of oxygen. In addition, the most important function of the Krebs cycle is distinguished - plastic (construction), since during the cycle elements important for life are produced: carbohydrates, amino acids, etc.

To implement all of the above, it is necessary to have more than a hundred different elements, including vitamins. If at least one of them is absent or deficient, the cycle will not be efficient enough, which will lead to metabolic disorders throughout the human body.

Stages of the Krebs cycle

  1. The first step is the splitting of glucose molecules into two molecules of pyruvic acid. Pyruvic acid performs an important metabolic function; liver function directly depends on its action. It has been proven that this compound is found in some fruits, berries and even honey; it is successfully used in cosmetology as a way to combat dead epithelial cells (gommage). Also, as a result of the reaction, lactate (lactic acid) can be formed, which is found in striated muscles, blood (more precisely, in red blood cells) and the human brain. An important element in the functioning of the heart and nervous system. A decarboxylation reaction occurs, that is, the cleavage of the carboxyl (acidic) group of amino acids, during which coenzyme A is formed - it performs the function of transporting carbon in various metabolic processes. When combined with a molecule of oxaloacetate (oxalic acid), citrate is obtained, which appears in buffer exchanges, i.e., “itself” carries useful substances in our body and helps them to be absorbed. At this stage, coenzyme A is completely released, plus we get a water molecule. This reaction is irreversible.
  2. The second stage is characterized by dehydrogenation (cleavage of water molecules) from the citrate, giving us cis-aconitate (aconitic acid), which helps in the formation of isocitrate. By the concentration of this substance, for example, you can determine the quality of fruit or fruit juice.
  3. Third stage. Here the carboxyl group is separated from isocitric acid, resulting in ketoglutaric acid. Alpha-ketoglutarate is involved in improving the absorption of amino acids from incoming food, improves metabolism and prevents stress. NADH is also formed - a substance necessary for the normal functioning of oxidative and metabolic processes in cells.
  4. At the next stage, when the carboxyl group is separated, succinyl-CoA is formed, which is an essential element in the formation of anabolic substances (proteins, etc.). The process of hydrolysis occurs (combination with a water molecule) and ATP energy is released.
  5. At subsequent stages the cycle will begin to close, i.e. The succinate will again lose a water molecule, which turns it into fumarate (a substance that promotes the transfer of hydrogen to coenzymes). Water joins the fumarate to form malate (malic acid), which oxidizes, which again leads to the appearance of oxaloacetate. Oxaloacetate, in turn, acts as a catalyst in the above processes; its concentrations in cell mitochondria are constant, but rather low.

Thus, we can highlight the most important functions of this cycle:

  • energy;
  • anabolic (synthesis of organic substances - amino acids, fatty proteins, etc.);
  • catabolic: the transformation of certain substances into catalysts - elements that contribute to energy production;
  • transport, mainly transporting hydrogen involved in cell respiration.