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It's called dissimilation. It is a collection of organic compounds in which a certain amount of energy is released.

Dissimilation takes place in two or three stages, depending on the type of living organisms. So, in aerobes it consists of preparatory, oxygen-free and oxygen stages. In anaerobes (organisms that are able to function in an anoxic environment), dissimilation does not require the last step.

The final stage of energy metabolism in aerobes ends with complete oxidation. In this case, the breakdown of glucose molecules occurs with the formation of energy, which partially goes to the formation of ATP.

It is worth noting that ATP synthesis occurs in the process of phosphorylation, when inorganic phosphate is added to ADP. At the same time, it is synthesized in mitochondria with the participation of ATP synthase.

What reaction occurs during the formation of this energy compound?

Adenosine diphosphate and phosphate combine to form ATP and the formation of which takes about 30.6 kJ / mol. Adenosine triphosphate, since a significant amount of it is released during the hydrolysis of precisely the high-energy bonds of ATP.

The molecular machine that is responsible for the synthesis of ATP is a specific synthase. It consists of two parts. One of them is located in the membrane and is a channel through which protons enter the mitochondria. This releases energy, which is captured by another structural part of ATP called F1. It contains a stator and a rotor. The stator in the membrane is fixed and consists of a delta region, as well as alpha and beta subunits, which are responsible for the chemical synthesis of ATP. The rotor contains gamma as well as epsilon subunits. This part spins using the energy of protons. This synthase ensures the synthesis of ATP if the protons from the outer membrane are directed towards the middle of the mitochondria.

It should be noted that the cell is characterized by spatial order. The products of chemical interactions of substances are distributed asymmetrically (positively charged ions go in one direction, and negatively charged particles go in the other direction), creating an electrochemical potential on the membrane. It consists of a chemical and an electrical component. It should be said that it is this potential on the surface of mitochondria that becomes the universal form of energy storage.

This pattern was discovered by the English scientist P. Mitchell. He suggested that substances after oxidation do not look like molecules, but positively and negatively charged ions, which are located on opposite sides of the mitochondrial membrane. This assumption made it possible to elucidate the nature of the formation of macroergic bonds between phosphates during the synthesis of adenosine triphosphate, and also to formulate the chemiosmotic hypothesis of this reaction.

light phase

The transformation of substances and energy in the process of dissimilation includes the following steps:

I stage- preparatory: complex organic substances under the action of digestive enzymes break down into simple ones, while only thermal energy is released.
Proteins ® amino acids

Fats ® glycerol and fatty acids

Starch ® glucose

II stage- glycolysis (oxygen-free): carried out in the hyaloplasm, not associated with membranes; it involves enzymes; glucose is broken down:

Stage III- oxygen: carried out in mitochondria, associated with the mitochondrial matrix and the inner membrane, enzymes participate in it, pyruvic acid undergoes cleavage

CO 2 (carbon dioxide) is released from mitochondria into the environment. The hydrogen atom is included in a chain of reactions, the end result of which is the synthesis of ATP. These reactions go in the following order:

1. The hydrogen atom H, with the help of carrier enzymes, enters the inner membrane of the mitochondria, which forms cristae, where it is oxidized:

2. Proton H + (hydrogen cation) is carried by carriers to the outer surface of the membrane of the cristae. For protons, this membrane, as well as the outer membrane of the mitochondria, is impermeable, so they accumulate in the intermembrane space, forming a proton reservoir.

3. Hydrogen electrons are transferred to the inner surface of the cristae membrane and immediately attached to oxygen with the help of the oxidase enzyme, forming a negatively charged active oxygen (anion):

4. Cations and anions on both sides of the membrane create an oppositely charged electric field, and when the potential difference reaches 200 mV, the proton channel begins to operate. It occurs in the enzyme molecules of ATP synthetase, which are embedded in the inner membrane that forms the cristae.

5. Through the proton channel, H + protons rush into the mitochondria, creating a high level of energy, most of which goes to the synthesis of ATP from ADP and F ( ), and the H + protons themselves interact with active oxygen, forming water and molecular O 2:

Thus, O 2 entering the mitochondria during the respiration of the organism is necessary for the addition of H + protons. In its absence, the entire process in mitochondria stops, since the electron transport chain ceases to function. General reaction of stage III:

As a result of the breakdown of one glucose molecule, 38 ATP molecules are formed: at stage II - 2 ATP and at stage III - 36 ATP. The resulting ATP molecules go beyond the mitochondria and participate in all cell processes where energy is needed. Splitting, ATP gives off energy (one phosphate bond contains 46 kJ) and returns to the mitochondria in the form of ADP and F (phosphate).

The work of respiratory enzymes is regulated by an effect called respiratory control.

- this is the direct effect of the electrochemical gradient on the speed of movement of electrons along the respiratory chain (i.e., on the amount of respiration). In turn, the magnitude of the gradient directly depends on ratios of ATP / ADP, the quantitative sum of which in the cell is practically constant ([ATP] + [ADP] = const). Catabolism reactions are aimed at maintaining a constantly high level of ATP and low ADP.

An increase in the proton gradient occurs with a decrease in the amount of ADP and the accumulation of ATP ( dormant state), i.e. when ATP synthase is deprived of its substrate and H + ions do not penetrate into the mitochondrial matrix. In this case, the inhibitory effect of the gradient is enhanced and the movement of electrons along the chain slows down. Enzyme complexes remain in a reduced state. The consequence is a decrease in the oxidation of NADH and FADH 2 on complexes I and II, inhibition of TCA enzymes with the participation of NADH and slowing catabolism in a cage.

Dependence of the Electrochemical Gradient on the Electron Velocity

A decrease in the proton gradient occurs when ATP reserves are exhausted and ADP is in excess, i.e. during cell operation. In this case ATP synthase is actively working and H + ions pass through the F o channel into the matrix. In this case, the proton gradient naturally decreases, the flow of electrons along the chain increases, and as a result, the pumping out of H + ions into the intermembrane space and again their rapid “falling through” through ATP synthase into mitochondria with ATP synthesis increases. Enzyme complexes I and II enhance the oxidation of NADH and FADH 2 (as electron sources) and the inhibitory effect of NADH is removed on the citric acid cycle and the pyruvate dehydrogenase complex. As a result - catabolism reactions are activated carbohydrates and fats.

The mechanism of ATP synthesis during glycolysis is relatively simple and can be easily reproduced in a test tube. However, it has never been possible to simulate the respiratory synthesis of ATP in the laboratory. In 1961, the English biochemist Peter Mitchell suggested that enzymes - neighbors in the respiratory chain - observe not only a strict sequence of reactions, but also a clear order in the space of the cell. The respiratory chain, without changing its order, is fixed in the inner shell (membrane) of the mitochondria and “sews” it several times like stitches. Attempts to reproduce the respiratory synthesis of ATP failed because the role of the membrane was underestimated by researchers. But the reaction also involves enzymes concentrated in mushroom-shaped growths on the inside of the membrane. If these growths are removed, then ATP will not be synthesized.

Oxidative phosphorylation, the synthesis of ATP from adenosine diphosphate and inorganic phosphate, carried out in living cells, due to the energy released during the oxidation of org. substances during cellular respiration. In general, oxidative phosphorylation and its place in metabolism can be represented by the scheme:

AN2 - organic substances oxidized into respiratory chains (the so-called substrates of oxidation, or respiration), ADP-adenosine diphosphate, P-inorganic phosphate.

Since ATP is necessary for the implementation of many processes that require energy (biosynthesis, mechanical work, transport of substances, etc.), oxidative phosphorylation plays a crucial role in the life of aerobic organisms. The formation of ATP in the cell also occurs due to other processes, for example, in the course of glycolysis and various types of fermentation. proceeding without the participation of oxygen. Their contribution to ATP synthesis under conditions of aerobic respiration is an insignificant part of the contribution of oxidative phosphorylation (about 5%).

In animals, plants, and fungi, oxidative phosphorylation occurs in specialized subcellular structures—mitochondria (Fig. 1); in bacteria, the enzyme systems that carry out this process are located in the cell membrane.

Mitochondria are surrounded by a protein-phospholipid membrane. Inside the mitochondria (in the so-called matrix) there is a series of metabolic processes of decomposition of nutrients that supply the substrates for the oxidation of AH2 for oxidative phosphorylation of Naib. important of these processes is the tricarboxylic acid cycle and the so-called. -oxidation of fatty acids (oxidative cleavage of a fatty acid to form acetyl coenzyme A and an acid containing 2 C atoms less than the original; the newly formed fatty acid can also undergo -oxidation). The intermediates of these processes undergo dehydrogenation (oxidation) with the participation of dehydrogenase enzymes; the electrons are then transferred to the mitochondrial respiratory chain, an ensemble of redox enzymes embedded in the inner mitochondrial membrane. The respiratory chain carries out a multistage exergonic transfer of electrons (accompanied by a decrease in free energy) from substrates to oxygen, and the released energy is used by the ATP synthetase enzyme located in the same membrane to phosphorylate ADP to ATP. In an intact (intact) mitochondrial membrane, electron transfer in the respiratory chain and phosphorylation are closely coupled. So, for example, the shutdown of phosphorylation after the exhaustion of ADP or inorganic phosphate is accompanied by inhibition of respiration (the effect of respiratory control). A large number of mitochondrial membrane-damaging effects disrupt the coupling between oxidation and phosphorylation, allowing electron transfer to proceed even in the absence of ATP synthesis (the uncoupling effect).

The mechanism of oxidative phosphorylation can be represented by the scheme: Electron transfer (respiration) A ~ B ATP A ~ B is a high-energy intermediate. It was assumed that A ~ B is a chemical compound with a macroergic bond, for example, a phosphorylated enzyme of the respiratory chain (chemical conjugation hypothesis), or a strained conformation of some protein involved in oxidative phosphorylation (conformational conjugation hypothesis). However, these hypotheses have not received experimental confirmation. The chemiosmotic concept of conjugation, proposed in 1961 by P. Mitchell, enjoys the greatest recognition (he was awarded the Nobel Prize in 1979 for the development of this concept). According to this theory, the free energy of electron transport in the respiratory chain is spent on the transfer of H+ ions from mitochondria through the mitochondrial membrane to its outer side (Fig. 2, process 1). As a result, an electric difference occurs on the membrane. potentials and chemical difference. activities of H+ ions (inside the mitochondria pH is higher than outside). In sum, these components give the transmembrane difference in the electrochemical potentials of hydrogen ions between the mitochondrial matrix and the outer aqueous phase separated by a membrane:

where R is the universal gas constant, T is the absolute temperature, F is the Faraday number. The value is usually about 0.25 V, with the main part (0.15-0.20 V) being the electrical component. The energy released when protons move inside the mitochondria along the electric field towards their lower concentration (Fig. 2, process 2) is used by ATP synthetase for ATP synthesis. Thus, the scheme of oxidative phosphorylation, according to this concept, can be represented as follows:

Electron transport (respiration) ATP

The conjugation of oxidation and phosphorylation via makes it possible to explain why oxidative phosphorylation, in contrast to glycolytic (“substrate”) phosphorylation occurring in solution, is possible only in closed membrane structures, and also why all influences that reduce the electrical resistance and increase the proton conductivity of the membrane suppress (uncoupling) oxidative phosphorylation Energy, in addition to ATP synthesis, can be directly used by the cell for other purposes - transport of metabolites, movement (in bacteria), reduction of nicotinamide coenzymes, etc.

There are several sections in the respiratory chain that are characterized by a significant drop in the redox potential and are associated with energy storage (generation). There are usually three such sites, called points or conjugation points: NADH: ubiquinone reductase unit (0.35-0.4 V), ubiquinol: cytochrome-c-reductase unit (~ ~ 0.25 V) and cytochrome-c- oxidase complex (~0.6 V) - conjugation points 1, 2 and 3 resp. (Fig. 3). Each of the points of conjugation of the respiratory chain can be isolated from the membrane in the form of an individual enzyme complex with redox activity. Such a complex, embedded in the phospholipid membrane, is able to function as a proton pump.

Usually, to characterize the effectiveness of oxidative phosphorylation, the H + / 2e or q / 2e values ​​\u200b\u200bare used, indicating how many protons (or electric charges) are transferred through the membrane during the transport of a pair of electrons through a given section of the respiratory chain, as well as the H + / ATP ratio, showing how many protons must be transferred from the outside to the inside of the mitochondria through ATP synthetase for the synthesis of 1 ATP molecule. The value q/2e is for junction points 1, 2, and 3, respectively. 3-4, 2 and 4. The value of H+/ATP during ATP synthesis inside mitochondria is 2; however, one more H+ can be spent on the removal of the synthesized ATP4- from the matrix to the cytoplasm by the carrier of adenine nucleotides in exchange for ADP-3. Therefore, the apparent value of H + / ATP outward is 3.

In the body, oxidative phosphorylation is suppressed by many toxic substances, which can be divided into three groups according to their site of action: 1) inhibitors of the respiratory chain, or the so-called respiratory poisons. 2) ATP synthetase inhibitors. The most common inhibitors of this class used in laboratory studies are the antibiotic oligomycin and the protein carboxyl group modifier dicyclohexylcarbodiimide. 3) The so-called uncouplers of oxidative phosphorylation. They do not suppress either electron transfer or ADP phosphorylation proper, but they have the ability to reduce the value on the membrane, due to which the energy conjugation between respiration and ATP synthesis is disrupted. The uncoupling effect is exhibited by a large number of compounds of the most diverse chemical structure. Classical uncouplers are substances that have weak acidic properties and are able to penetrate the membrane both in ionized (deprotonated) and neutral (protonated) forms. Such substances include, for example, 1-(2-dicyanomethylene)hydrazino-4-trifluoromethoxybenzene, or carbonyl cyanide-p-trifluoromethoxyphenylhydrazone, and 2,4-dinitrophenol (formulas I and II, respectively; protonated and deprotonated forms are shown) .

Moving through the membrane in an electric field in ionized form, the uncoupler reduces; returning back in the protonated state, the uncoupler lowers (Fig. 4). So arr., such a "shuttle" type of action of the uncoupler leads to a decrease

Ionophores (for example, gramicidin) that increase the electrical conductivity of the membrane as a result of the formation of ion channels or substances that destroy the membrane (for example, detergents) also have an uncoupling effect.

Oxidative phosphorylation was discovered by V. A. Engelgardt in 1930 while working with bird erythrocytes. In 1939, V. A. Belitser and E. T. Tsybakova showed that oxidative phosphorylation is associated with the transfer of electrons during respiration; GM Kalkar came to the same conclusion somewhat later.

ATP synthesis mechanism. The diffusion of protons back through the inner mitochondrial membrane is coupled with the synthesis of ATP by the ATPase complex, called the coupling factor F,. On electron microscopic images, these factors look like globular mushroom-shaped formations on the inner membrane of mitochondria, and their “heads” protrude into the matrix. F1 is a water-soluble protein composed of 9 subunits of five different types. The protein is an ATPase and is bound to the membrane through another F0 protein complex that ligates the membrane. F0 does not exhibit catalytic activity, but serves as a channel for the transport of H+ ions across the membrane to Fx.

The mechanism of ATP synthesis in the Fi ~ F0 complex has not been fully elucidated. There are a number of hypotheses in this regard.

One of the hypotheses explaining the formation of ATP through the so-called direct mechanism was proposed by Mitchell.

According to this scheme, at the first stage of phosphorylation, the phosphate ion and ADP bind to the r component of the enzyme complex (A). The protons travel through a channel in the F0 component and combine in the phosphate with one of the oxygen atoms, which is removed as a water molecule (B). The oxygen atom of ADP combines with the phosphorus atom, forming ATP, after which the ATP molecule is separated from the enzyme (B).

Various options are possible for the indirect mechanism. ADP and inorganic phosphate are attached to the active site of the enzyme without an influx of free energy. Ions H + , moving along the proton channel along the gradient of their electrochemical potential, bind in certain areas of Fb causing conformational. changes in the enzyme (P. Boyer), as a result of which ATP is synthesized from ADP and Pi. The release of protons into the matrix is ​​accompanied by the return of the ATP-synthetase complex to its original conformational state and the release of ATP.

When energized, F1 functions as an ATP synthetase. In the absence of conjugation between the electrochemical potential of H+ ions and ATP synthesis, the energy released as a result of the reverse transport of H+ ions in the matrix can be converted into heat. Sometimes this is beneficial, as increasing the temperature in the cells activates the work of enzymes.