Anabolism and catabolism are processes carried out in our body. Some of them are building processes (anabolic), while others are degradation or destruction processes (catabolic). Probably many of you will say that anabolic processes are more important and you need to reduce catabolic processes to a minimum.

It is true, however, that the processes of structure and degradation in the body depend on each other, the cell cannot exist if it only absorbs substances without synthesizing new ones and vice versa. Anabolic and catabolic processes build a single biochemical and energetic essence of metabolism.

Probably many people still do not know that during exercise we stimulate the catabolic processes in our body, which destroy our muscle tissue. This may seem strange to some, but if we think about it, we will see the logic. We cannot have building processes if we do not oppose them with the opposite and they are caused precisely by muscle training.

In short, we break down muscles so they can build and get bigger and stronger. It's good to know how catabolism affects anabolic processes and vice versa, because the better we know their dependence, the better the results we have in the gym!

Anabolic processes

As we have already said, anabolic processes are initiated by catabolic processes. During exercise and normal daily activities, our body is energized and in a catabolic phase. The body's normal response to catabolic processes is building processes.

Anabolic processes are produced in the body with energy from swallowed food, a sufficient amount of rest and hormones: growth hormone, insulin-like growth factor, insulin, testosterone, estradiol. This process can be divided into three stages: synthesis of intermediates, synthesis of monomer units, and synthesis of polymers and monomers. Simply put, movement from simple to complex using the available energy of the body.

Catabolic processes

Catabolism (degradation) is the process of releasing energy during the degradation of substances. They are determined by the calorific value, referred to as kcal / g (kcal / g substance). Under the influence of catabolic processes, the main nutrients (proteins, fats and carbohydrates) are degraded to final products: water, CO2, ammonia, urea, uric acid, etc., which are subsequently discharged from the body through the excretory system.

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Catabolic processes occur during physical activity and they are actually the culprit in building stronger muscles and clearing subcutaneous fat.

Balancing processes

Many people try to stop catabolic processes or overdo it (in training) in the idea of ​​obtaining maximum results. This is not a good approach since the processes depend on each other. To maximize results, you need to balance the processes of anabolism and catabolism.

We must tense our muscles and degrade many substances in order to release energy, but we must also allow our bodies to rest long enough and receive the necessary food to succeed in repairing tissues and building new and stronger ones. When a person exercises too often and does not sleep enough or eats enough, the body has no choice but to stay longer in the catabolic phase and therefore results diminish or even transition from progress to regression!

How to balance

The most important thing is to monitor progress so that we know if we are on the right track (well balanced). Here are some things to keep in mind to minimize waste.

• Don't overdo it. Often embarrassed by acquaintances, friends, or magazines, we change our program to the point that our body doesn't stop exercising 7 days a week. Most people will think that when they exercise every day, they will have better results. This can only be true if you allow your body to rest and recover, which is difficult to do if you are heavy and for prolonged periods, sleep a little or not eat enough.
• Do not try to suppress catabolic processes... Because, as we've said several times, they are just as important to you as the anabolic ones.
• If you decrease your workout or intensity, your body will have very poor catabolism and there will be no need to create muscle building processes. If you train regularly and seriously, then do not overdo it, allow your body to use the maximum anabolic window. Try to go to bed before 11pm to get up early at 7am. Our body is designed so that when the sun goes down it goes into an anabolic phase, and when the sun rises in the morning, anabolism goes into catabolism, which lasts throughout the day.
• Avoid Estrogenic Foods (foodstuffs containing the hormone estrogen): soybeans, foods treated with pesticides, beef from butchers (estrogen is added to the diet of cows, and it accumulates in the cells of the body, respectively, and in meat, which is found in butcher shops). The Food and Drug Administration says they do not allow added hormones to be used in pork or poultry (chicken, eggs, turkey), so these foods should be estrogen-free. The type of food with estrogen will tilt the body towards catabolic processes, lowering the amount of anabolic hormones (such as testosterone) towards estrogen.
• Relax... Physical and mental stress has a great impact on body processes. Try to get rid of these situations, or if you cannot avoid them, try to relax.
• Avoid cigarettes, alcohol and, of course, all kinds of drugs. Not only will they slow down your progress, but as we all know, they are detrimental to your health.
• Eat fiber-rich foods(whole grains

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Metabolism and energy - it is a set of physical, chemical and physiological processes of transformation of substances and energy in the human body and the exchange of substances and energy between the body and environment .

The continuous exchange of substances and energy between the body and the environment is one of the most essential signs of life.

To maintain vital processes, metabolism and energy provides plastic and energetic the needs of the body. This is achieved by extracting energy from entering the body nutrients and transforming it into forms macroergic(ATP and other molecules) and restored(NADPH - nicotine amide - adenine dinucleotide phosphate) compounds. Their energy is used for the synthesis of proteins, nucleic acids, lipids, as well as components of cell membranes and cell organelles, for performing mechanical, chemical, osmotic and electrical work, and for transporting ions. In the course of metabolism, plastic substances are delivered to the body, which are necessary for biosynthesis, construction and renewal of biological structures.

Anabolism and Catabolism

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In metabolism (metabolism) and energies are emitted by two interrelated, but oppositely directed processes:

1. Anabolism, which is based on assimilation processes,

2. Catabolism, which is based on dissimilation processes.

Anabolism is a set of processes of biosynthesis of organic substances, cell components and other structures of organs and tissues. Anabolism provides growth, development, renewal of biological structures, as well as continuous resynthesis of macroergs and the accumulation of energy substrates.

Catabolism - is a set of processes of splitting complex molecules, components of cells, organs and tissues to simple substances, using some of them as precursors of biosynthesis, and to the final decay products with the formation of high-energy and reduced compounds. The interconnection of the main functional elements of metabolism is shown in Fig. 10.1.

The diagram shows that the relationship between the processes of catabolism and anabolism is based on the unity of biochemical transformations that provide energy for all vital processes and the constant renewal of body tissues... The driving force of life is catabolism. The conjugation of anabolic and catabolic processes can be carried out by various substances, but the main role is played by ATP, NADPH. Unlike other mediators of metabolic transformations, ATP is cyclically re-phosphorylated, and NADP-H is restored.

The provision of energy for vital processes is carried out at the expense of anaerobic and aerobic catabolism of proteins, fats and carbohydrates entering the body with food.

During anaerobic digestion, glucose(glycolysis) or its reserve substrate glycogen (glycogenolysis), the conversion of 1 mole of glucose into 2 moles of lactate leads to the formation of 2 moles of ATP. The energy generated during anaerobic metabolism is not enough to carry out the vital processes of animal organisms. Due to anaerobic glycolysis, only limited short-term energy requirements of the cell can be satisfied. It is known, for example, that the mature erythrocyte of mammals fully satisfies its energy needs through glycolysis.

In the body of animals and humans in the process of aerobic metabolism almost all organic substances, including products of anaerobic metabolism, completely decompose to CO 2 and H 2 O. The total number of ATP molecules formed during the complete oxidation of 1 mole of glucose to CO 2 and H 2 O is 25.5 moles. When a fat molecule is completely oxidized, more moles of ATP are formed than when a carbohydrate molecule is oxidized. So with the complete oxidation of 1 mole of palmitic acid, 91.8 moles of ATP are formed. The number of moles of ATP formed during the complete oxidation of amino acids and carbohydrates is approximately the same. ATP plays the role of an internal “energy currency” in the body, a carrier and accumulator of chemical energy.

The main source of recovery energy for the reaction of biosynthesis of fatty acids, cholesterol, amino acids, steroid hormones, precursors of the synthesis of nucleotides and nucleic acids is NADPH. The formation of this substance is carried out in the cytoplasm of the cell during the phosphogluconate pathway of glucose catabolism. With this splitting of 1 mole of glucose, 12 moles of NADPH are formed.

The processes of anabolism and catabolism are in the body in a state of dynamic equilibrium or the prevalence of one of them... The predominance of anabolic processes over catabolic processes leads to growth, the accumulation of tissue mass, and the predominance of catabolic processes leads to a partial destruction of tissue structures, the release of energy. The state of an equilibrium or non-equilibrium ratio of anabolism and catabolism depends on age (predominance of anabolism in childhood, balance in adults, predominance of catabolism in old age), the state of health, performed by the body of physical or psycho-emotional stress.

13.4.1. Krebs cycle reactions belong to the third stage of nutrient catabolism and occur in the mitochondria of the cell. These reactions belong to the general pathway of catabolism and are characteristic of the breakdown of all classes of nutrients (proteins, lipids and carbohydrates).

The main function of the cycle is the oxidation of the acetyl residue with the formation of four molecules of reduced coenzymes (three NADH molecules and one FADH2 molecule), as well as the formation of a GTP molecule by substrate phosphorylation. The carbon atoms of the acetyl residue are released in the form of two CO2 molecules.

13.4.2. The Krebs cycle includes 8 sequential stages, with particular attention to the reaction of substrate dehydrogenation:

Figure 13.6. Krebs cycle reactions, including the formation of α-ketoglutarate

a) condensation of acetyl-CoA with oxaloacetate, as a result of which citrate is formed (Fig. 13.6, reaction 1); therefore, the Krebs cycle is also called citrate cycle... In this reaction, the methyl carbon of the acetyl group reacts with the keto group of oxaloacetate; at the same time, the thioether bond is cleaved. In the reaction, CoA-SH is released, which can take part in the oxidative decarboxylation of the next pyruvate molecule. The reaction is catalyzed citrate synthase, it is a regulatory enzyme, it is inhibited by high concentrations of NADH, succinyl-CoA, citrate.

b) the conversion of citrate to isocitrate through the intermediate formation of cis-aconitate. The citrate formed in the first reaction of the cycle contains a tertiary hydroxyl group and is not capable of being oxidized under cell conditions. By the action of an enzyme aconitase there is a splitting off of a water molecule (dehydration), and then its addition (hydration), but in a different way (Fig. 13.6, reactions 2-3). As a result of these transformations, the hydroxyl group moves to a position favorable for its subsequent oxidation.

v) dehydrogenation of isocitrate followed by the release of the CO2 molecule (decarboxylation) and the formation of α-ketoglutarate (Fig. 13.6, reaction 4). This is the first redox reaction in the Krebs cycle to form NADH. Isocitrate dehydrogenase, catalyzing the reaction, is a regulatory enzyme that is activated by ADP. Excess NADH inhibits the enzyme.

Figure 13.7. Krebs cycle reactions starting with α-ketoglutarate.

G) oxidative decarboxylation of α-ketoglutarate, catalyzed by a multienzyme complex (Fig. 13.7, reaction 5), accompanied by the release of CO2 and the formation of a second NADH molecule. This reaction is analogous to the pyruvate dehydrogenase reaction. The inhibitor is the reaction product, succinyl-CoA.

e) substrate phosphorylation at the level of succinyl-CoA, during which the energy released during the hydrolysis of the thioether bond is stored in the form of a GTP molecule. Unlike oxidative phosphorylation, this process proceeds without the formation of an electrochemical potential of the mitochondrial membrane (Fig. 13.7, reaction 6).

e) dehydrogenation of succinate with the formation of fumarate and the FADH2 molecule (Fig. 13.7, reaction 7). The enzyme succinate dehydrogenase is tightly bound to the inner mitochondrial membrane.

g) hydration of fumarate, as a result of which a readily oxidizable hydroxyl group appears in the reaction product molecule (Fig. 13.7, reaction 8).

h) malate dehydrogenation leading to the formation of oxaloacetate and a third NADH molecule (Figure 13.7, reaction 9). The oxaloacetate formed in the reaction can be used again in the condensation reaction with the next acetyl-CoA molecule (Fig. 13.6, reaction 1). Therefore, this process is cyclical.

13.4.3. Thus, as a result of the described reactions, the acetyl residue undergoes complete oxidation CH3 -CO-... The number of acetyl-CoA molecules converted in mitochondria per unit time depends on the concentration of oxaloacetate. The main ways to increase the concentration of oxaloacetate in mitochondria (the corresponding reactions will be discussed later):

13.4.4. Some metabolites of the Krebs cycle can be used to synthesis building blocks for building complex molecules. Thus, oxaloacetate can be converted into the amino acid aspartate, and α-ketoglutarate into the amino acid glutamate. Succinyl-CoA takes part in the synthesis of heme - the prosthetic group of hemoglobin. Thus, the reactions of the Krebs cycle can participate both in the processes of catabolism and anabolism, that is, the Krebs cycle performs amphibolic function(see 13.1).

In a living organism, various processes take place constantly to ensure its vital activity. One of these is metabolism (metabolism), which converts the food consumed into energy. It is about metabolism that will be discussed in this article. We will consider the essence of metabolic processes, clarify their stages and answer the question - what are catabolism and anabolism.

The essence of metabolism and its role for living organisms

In order for living organisms to grow, repair and reproduce, nature has endowed them with the extremely important ability to convert calories from outside food into valuable and vital energy. The combination of these biochemical processes is called metabolism or metabolism.

The metabolism of each individual can be expressed by a quantitative coefficient that determines the rate at which the body converts food into usable energy. It has been established that a number of factors affect the activity of metabolic processes:

• Floor. In men, the metabolic rate is 10-20% higher than in women.
• Age. In general, metabolism (anabolism, catabolism) proceeds faster in the first 25-30 years of life, after which there is a decrease in the metabolic rate by 3% every decade.
• Being overweight leads to the rapid absorption of nutrients, which are stored as fat in muscle tissue and liver.
• Physical activity accelerates metabolism in the next progression - by 20% during the first 2-3 hours, and on the next day - by 5%.

Metabolic processes in living organisms inevitably take place in two opposite forms: dissimilation (catabolism) and assimilation (anabolism). We'll take a closer look at these processes below.

More on what catabolism is

Catabolism is a process during which complex substances, in the form of a collection of cells, tissues, organs and other things, are broken down into simpler ones. The processes of catabolism are necessarily accompanied by the formation and enrichment of energy cells in the form of ATP, which can subsequently be spent on synthesis and other vital processes, for example, movement.

The following hormones affect catabolism, or rather the rate of breakdown of complex substances:

• cortisol - produced in the body due to stressful situations;
• adrenaline, the concentration of which can be increased during fasting, as well as when a person is experiencing strong emotions;
• glucagon is a fat-burning hormone that is actively produced when the amount of carbohydrate intake from food is limited (in dietetics, this is possible due to diets with a predominant intake of protein foods).

Stages of catabolism

Dissimilation of complex compounds goes through several successive stages, including:

1. The breakdown of organic molecules of fats, proteins and complex carbohydrates into monomers (for example, fatty acid, amino acids, monosaccharides). The process takes place outside the cell - in the digestive tract.
2. The penetration of monomers through the circulation of blood and lymph into tissue cells, where the cleavage process continues until the formation of smaller structures, such as the acetyl group of coenzymes.
3. Oxidation under the influence of oxygen during the respiration of low molecular weight compounds, resulting in the formation carbon dioxide and water, and also accumulates cellular energy ATP.

How anabolism works

After we have considered what catabolism is, it will be appropriate to give the concept of its opposite form - anabolism. So, if catabolism is the process of breaking down polymers, then anabolism is nothing more than the synthesis of simple substances into complex compounds, from which new cells and tissues of the body are built.

Anabolism promotes the growth, development and regeneration of all types of body tissues.

Assimilation metabolism also takes place in three stages:

• Initially, low molecular weight compounds form precursor molecules.
• In the next stage, these molecules are converted into active forms, while the cellular energy accumulated during catabolism is expended.
• The final stage includes the process of formation of the body's building material - polymers, in the form of proteins, carbohydrates and fats.

The relationship between catabolism and anabolism

Catabolism and anabolism are closely related processes. The first ensures the decomposition of organic compounds to simple substances and the accumulation of energy, which is needed for assimilation metabolism. The second supplies the necessary enzymes to the catabolic processes.

These two forms of exchange continuously occur in living organisms and can be in two types of interaction:

• in a state of equilibrium;
• in the predominance of one species over another.

The maintenance or imbalance of metabolic processes depends on age and on the psycho-emotional state of the body. So, for example, in children, especially in the first year of life, there is a predominance of anabolism over catabolism, and in older people, on the contrary.

Stress and physical activity also lead to a shift in the balance of metabolic processes towards dissimilation. After all, what is catabolism? In practical terms, this is weight loss and calorie burning.

Anabolism and catabolism are the main metabolic processes.

Catabolism is the enzymatic breakdown of complex organic compounds that occurs inside the cell through oxidation reactions. Catabolism is accompanied by the release of energy and its storage in the high-energy phosphate bonds of ATP.

Anabolism is the synthesis of complex organic compounds - proteins, nucleic acids, polysaccharides - from simple precursors that enter the cell from the environment or are formed during catabolism. The synthesis processes are associated with the consumption of free energy, which is supplied by ATP (Fig. 31).

Rice. 31 Diagram of metabolic pathways in a bacterial cell

Depending on the biochemistry of the dissimilation (catabolism) process, respiration and fermentation are distinguished.

Breath Is a complex process of biological oxidation of various compounds), coupled with the formation of a large amount of energy accumulated in the form of high-energy bonds in the structure of ATP (adenosine triphosphate), UTP (uridine triphosphate), etc., and the formation of carbon dioxide and water. Distinguish between aerobic and anaerobic respiration.

Fermentation- incomplete decomposition of organic compounds with the formation of a small amount of energy and energy-rich products.

Anabolism involves fusion processes that use the energy generated by catabolism. In a living cell, the processes of catabolism and anabolism occur simultaneously and continuously. Many reactions and intermediates are common to them.

Living organisms are classified according to which source of energy or carbon they use. Carbon is the main element of living matter. In constructive metabolism, he plays a leading role.

Depending on the source of cellular carbon, all organisms, including prokaryotic organisms, are divided into autotrophs and heterotrophs.

Autotrophs use CO 2 as the only source of carbon, reducing it with hydrogen, which is split off from water or other substance. They synthesize organic substances from simple inorganic compounds in the process of photo- or chemosynthesis.

Heterotrophs get carbon from organic compounds.

Living organisms can use light or chemical energy. Organisms that live off the energy of light are called phototrophic. They synthesize organic substances by absorbing the electromagnetic radiation of the Sun (light). These include plants, blue-green algae, green and purple sulfur bacteria.

Organisms that receive energy from substrates, food sources (oxidation energy of inorganic substances) are called chemotrophs. TO chemoheterotrophs include most bacteria, as well as fungi and animals.

There is a small group chemoautotrophs... Such chemosynthetic microorganisms include nitrifying bacteria, which, by oxidizing ammonia to nitrous acid, release the energy necessary for synthesis. Chemosynthetics also include hydrogen bacteria, which receive energy during the oxidation of molecular hydrogen.

Carbohydrates as a source of energy

In most organisms, the breakdown of organic matter occurs in the presence of oxygen - aerobic metabolism. As a result of this exchange, energy-poor end products (CO 2 and H 2 O) remain, but a lot of energy is released. The process of aerobic metabolism is called respiration, and anaerobic metabolism is called fermentation.

Carbohydrates are the main energy material that cells use primarily for chemical energy. In addition, proteins and fats can also be used during respiration, and alcohols and organic acids during fermentation.

Organisms carry out the breakdown of carbohydrates in different ways, in which the most important intermediate product is pyruvic acid (pyruvate). Pyruvate is central to respiration and fermentation metabolism. There are three main mechanisms for the formation of PVCs.

1. Fructose diphosphate (glycolysis) or the Embden-Meyerhof-Parnassus pathway- a universal way.

The process begins with phosphorylation (Fig. 32). With the participation of the enzyme hexokinase and ATP, glucose is phosphorylated at the sixth carbon atom to form glucose-6-phosphate. It is the active form of glucose. It serves as the starting product for the breakdown of carbohydrates in any of three ways.

During glycolysis, glucose-6-phosphate is isomerized into fructose-6-phosphate, and then, under the action of 6-phosphofructokinase, it is phosphorylated at the first carbon atom. The resulting fructose-1,6-diphosphate under the action of the enzyme aldolase easily decomposes into two trioses: phosphoglyceric aldehyde and dihydroxyacetone phosphate. Further conversion of C 3 -carbohydrates is carried out due to the transfer of hydrogen and phosphorus residues through a number of organic acids with the participation of specific dehydrogenases. All reactions of this pathway, with the exception of three, involving hexokinase, 6-phosphofructokinase, and pyruvate kinase, are completely reversible. At the stage of formation of pyruvic acid, the anaerobic phase of the conversion of carbohydrates ends.

The maximum amount of energy received by the cell during the oxidation of one carbohydrate molecule by the glycolytic pathway is 2 · 10 5 J.

Fig. 32. Fructose diphosphate pathway of glucose breakdown

2. Pentose Phosphate (Warburg-Dickens-Horeker)path also typical for most organisms (mostly for plants, and plays an auxiliary role for microorganisms). Unlike glycolysis, the PP pathway does not form pyruvate.

Glucose-6-phosphate is converted to 6-phosphoglucolactone, which is decarboxylated (Fig. 33). In this case, ribulose-5-phosphate is formed, on which the oxidation process is completed. Subsequent reactions are considered as processes of conversion of pentose phosphates to hexose phosphates and vice versa, i.e. a cycle is formed. It is believed that the pentose phosphate pathway transforms into glycolysis at one of the stages.

When every six glucose molecules pass through the IF pathway, one molecule of glucose-6-phosphate is completely oxidized to CO 2 and 6 NADP + molecules are reduced to NADPH 2. As a mechanism for obtaining energy, this pathway is two times less efficient than glycolytic: for each glucose molecule, 1 ATP molecule is formed.

Rice. 33. Pentose phosphate pathway for the cleavage of glucose-6-phosphate

The main purpose of this pathway is to supply pentoses necessary for the synthesis of nucleic acids and to ensure the formation of most of the NADPH 2 necessary for the synthesis of fatty acids, steroids.

3. Entner-Dudorov pathway (ketodeoxyphosphogluconate or CDPG pathway) found only in bacteria. Glucose is phosphorylated by the ATP molecule with the participation of the enzyme hexokinase (Fig. 34).

Fig. 34. Entner-Dudorov pathway of glucose breakdown

The phosphorylation product, glucose-6-phosphate, is dehydrated to 6-phosphogluconate. Under the action of the enzyme phosphogluconate dehydrogenase, water is cleaved from it and 2-keto-3-deoxy-6-phosphogluconate (CDPG) is formed. The latter is cleaved by a specific aldolase into pyruvate and glyceraldehyde-3-phosphate. Glyceraldehyde is further acted upon by enzymes of the glycolytic pathway and is transformed into a second molecule of pyruvate. In addition, this pathway supplies the cell with 1 ATP molecule and 2 NAD · H 2 molecules.

Thus, the main intermediate product of the oxidative degradation of carbohydrates is pyruvic acid, which, with the participation of enzymes, is converted into various substances. The PVA formed by one of the pathways in the cell undergoes further oxidation. The released carbon and hydrogen are removed from the cell. Carbon is released in the form of CO 2, hydrogen is transferred to various acceptors. Moreover, either a hydrogen ion or an electron can be transferred, therefore the transfer of hydrogen is equivalent to the transfer of an electron. Depending on the final acceptor of hydrogen (electron), aerobic respiration, anaerobic respiration and fermentation are distinguished.

Breath

Breathing is a redox process that leads to the formation of ATP; the role of hydrogen donors (electrons) in it is played by organic or inorganic compounds; in most cases, inorganic compounds serve as hydrogen (electron) acceptors.

If the final electron acceptor is molecular oxygen, the respiratory process is called aerobic breathing... In some microorganisms, the final electron acceptor is compounds such as nitrates, sulfates, and carbonates. This process is called anaerobic respiration.

Aerobic breathing- the process of complete oxidation of substrates to CO 2 and H 2 O with the formation of a large amount of energy in the form of ATP.

Complete oxidation of pyruvic acid occurs under aerobic conditions in the tricarboxylic acid cycle (CTA or Krebs cycle) and the respiratory chain.

Aerobic breathing consists of two phases:

one). The pyruvate formed during glycolysis is oxidized to acetyl-CoA, and then to CO 2, and the released hydrogen atoms move to acceptors. This is how the CTK is carried out.

2). Hydrogen atoms split off by dehydrogenases are accepted by the coenzymes of anaerobic and aerobic dehydrogenases. Then they are carried along the respiratory chain, in some parts of which a significant amount of free energy is formed in the form of high-energy phosphates.

Tricarboxylic acid cycle (Krebs cycle, CTA)

Pyruvate formed during glycolysis is decarboxylated to acetaldehyde with the participation of the pyruvate dehydrogenase multienzyme complex. Acetaldehyde, combining with the coenzyme of one of the oxidizing enzymes - coenzyme A (CoA-SH), forms "activated acetic acid" - acetyl-CoA - a high-energy compound.

Acetyl-CoA, under the action of citrate synthetase, reacts with oxaloacetic acid (oxaloacetate) to form citric acid (citrate C 6), which is the main link of CTA (Fig. 35). Citrate is converted to isocitrate after isomerization. This is followed by oxidative (elimination of H) decarboxylation (elimination of CO 2) of isocitrate, the product of which is 2-oxoglutarate (C 5). Under the influence of the enzyme complex ɑ-ketoglutarate dehydrogenase with the active group NAD, it turns into succinate, losing CO 2 and two hydrogen atoms. The succinate is then oxidized to fumarate (C 4), and the latter is hydrated (addition of H 2 O) to malate. In the final Krebs cycle of the reaction, malate is oxidized, which leads to the regeneration of oxaloacetate (C 4). Oxaloacetate reacts with acetyl-CoA and the cycle repeats again. Each of the 10 TCA reactions, except one, is easily reversible. Two carbon atoms enter the cycle in the form of acetyl-CoA and the same number of carbon atoms leave this cycle in the form of CO 2.

Rice. 35. Krebs cycle (according to V.L. Kretovich):

1, 6 - oxidative decarboxylation system; 2 - citrate synthetase, coenzyme A; 3, 4 - aconitate hydratase; 5 - isocitrate dehydrogenase; 7 - succinate dehydrogenase; 8 - fumarate hydratase; 9 - malate dehydrogenase; 10 - spontaneous transformation; 11 - pyruvate carboxylase

As a result of four redox reactions of the Krebs cycle, three pairs of electrons are transferred to NAD and one pair of electrons to FAD. The electron carriers NAD and FAD, reduced in this way, then undergo oxidation already in the electron transport chain. The cycle produces one ATP molecule, 2 CO 2 molecules and 8 hydrogen atoms.

The biological significance of the Krebs cycle is that it is a powerful supplier of energy and "building blocks" for biosynthetic processes. The Krebs cycle operates only under aerobic conditions, in anaerobic conditions it is open at the level of α-ketoglutarate dehydrogenase.

Respiratory chain

The last stage of catabolism is oxidative phosphorylation. During this process, most of the metabolic energy is released.

The NAD and FAD electron carriers reduced in the Krebs cycle undergo oxidation in the respiratory or electron transport chain. The carrier molecules are dehydrogenases, quinones and cytochromes.

Both enzyme systems are located in the plasma membrane in prokaryotes, and in the inner mitochondrial membrane in eukaryotes. Electrons from hydrogen atoms (NAD, FAD) pass through a complex carrier chain to molecular oxygen, reducing it, and water is formed.

Balance. Calculations of the energy balance showed that when glucose is cleaved by the glycolytic pathway and through the Krebs cycle, followed by oxidation in the respiratory chain to CO 2 and H 2 O, 38 ATP molecules are formed for each glucose molecule. Moreover, the maximum amount of ATP is formed in the respiratory chain - 34 molecules, 2 molecules - in the EMF pathway and 2 molecules - in the CTX (Fig. 36).

Incomplete oxidation of organic compounds

Respiration is usually associated with complete oxidation of the organic substrate, i.e. the end products of decomposition are CO 2 and H 2 O.

However, some bacteria and some fungi do not fully oxidize carbohydrates. The end products of incomplete oxidation are organic acids: acetic, citric, fumaric, gluconic, etc., which accumulate in the environment. This oxidative process is used by microorganisms for energy. However, the overall energy yield is significantly lower than with complete oxidation. Part of the energy of the initial substrate being oxidized is stored in the resulting organic acids.

Microorganisms that develop due to the energy of incomplete oxidation are used in the microbiological industry to obtain organic acids and amino acids.