Dehydrogenases, structure and action

Oxidation of organic substances in living tissues of plants and animals occurs with the participation of enzymes: dehydrogenases – activators and carriers of hydrogen (electron) of the respiratory material and oxidases – activators of molecular oxygen.

Dehydrogenases (from de … and novolat. hudrogenium – hydrogen), enzymes that catalyze the elimination of hydrogen from organic substances. D.’s coenzymes are usually dinucleotides:

– nicotinamide adenine dinucleotide (NAD),

– nicotinamide adenine dinucleotide phosphate (NADP) or flavin adenine dinucleotide (FAD)

– flavin mononucleotide (FMN), which accept the hydrogen of the oxidized substance. D. carry out the first stage of biological oxidation.

They play an important role in the Krebs cycle, in glycolysis and in the pentose phosphate cycle. Some D., not associated with coenzymes, catalyze the oxidation reactions of substances directly with oxygen. Most of the D. in the composition of the active center contains metals – zinc, manganese.

Dehydrogenases act throughout the entire process of respiration and, according to the nature of their action, are divided into aerobic and anaerobic. The former transfer hydrogen directly to molecular oxygen, while the latter transfer it to some hydrogen acceptor. Dehydrogenases are two-component enzymes – proteids with pyridine or flavin (alloxazine) coenzymes.

Pyridine dehydrogenases belong to the group of anaerobic dehydrogenases, which are substrate hydrogen acceptors. The specificity of their action lies in the ability to hydrogenate and dehydrogenate pyridine nuclei.

Rice. 1 – Pyridine

Flavoprotein dehydrogenases include enzymes containing the phosphate ester of riboflavin.

Rice. 2 – Riboflavin


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8. This is the process of oxygen consumption by cells of body tissues, which is involved in biological oxidation. This type of oxidation is called aerobic oxidation. If the final acceptor in the hydrogen transfer chain is not oxygen, but other substances (for example, pyruvic acid), then this type of oxidation is called anaerobic.

9. Thus. biological oxidation is the dehydrogenation of a substrate with the help of intermediate hydrogen carriers and its final acceptor. Biological oxidation of nutrients occurs in mitochondria. Enzymes involved in the citric acid cycle, the respiratory chain, oxidative phosphorylation, the breakdown of fatty acids and a number of amino acids were found in them.

10. The respiratory chain (enzymes of tissue respiration) are the carriers of protons and electrons from the oxidized substrate to oxygen. An oxidizing agent is a compound capable of accepting electrons. This ability is quantified by the redox potential relative to a standard hydrogen electrode, which has a pH of 7.0. The lower the potential of the compound, the stronger its reducing properties and vice versa.

11. Features of tissue respiration

12. The process of tissue respiration cannot be considered identical to the processes of biological oxidation (enzymatic processes of oxidation of various substrates occurring in animal, plant and microbial cells), since a significant part of such oxidative transformations in the body occurs under anaerobic conditions, i.e. without the participation of molecular oxygen, in contrast to tissue respiration.

Most of the energy in aerobic cells is generated by tissue respiration, and the amount of energy generated depends on its intensity. The intensity of D. t. is determined by the rate of oxygen uptake per unit mass of tissue; Normally, it is due to the tissue’s need for energy. Its intensity is highest in the retina, kidneys, liver; it is significant in the intestinal mucosa, thyroid, testes, cerebral cortex, pituitary, spleen, bone marrow, lungs, placenta, thymus, pancreas, diaphragm, heart, skeletal muscle at rest. In the skin, cornea and lens of the eye, the intensity of tissue respiration is low. Thyroid hormones, fatty acids and other biologically active substances can activate tissue respiration.

The intensity of such breathing is determined by polarographic or manometric method in the Warburg apparatus. In the latter case, the so-called respiratory coefficient is used to characterize – the ratio of the volume of carbon dioxide released to the volume of oxygen absorbed by a certain amount of the tissue under study over a certain period of time.

13. T. o. any compound can only donate electrons to a compound with a higher redox potential. In the respiratory chain, each subsequent link has a higher potential than the previous one.

14. Respiratory chain

15. The respiratory chain consists of: NAD – dependent dehydrogenase; FAD-dependent dehydrogenase; Ubiquinone (CoQ); Cytochromes b, c, a+a3.

16. NAD-dependent dehydrogenases. They contain NAD and NADP as coenzymes. The pyridine ring of nicotinamide is capable of attaching electrons and hydrogen protons.

17. FAD and FMN-dependent dehydrogenases contain vitamin B2 phosphate ester (FAD) as a coenzyme.

18. Ubiquinone (CoQ) removes hydrogen from flavoproteins and is converted into hydroquinone.

19. Cytochromes are chromoprotein proteins capable of attaching electrons due to the presence of iron porphyrins as prosthetic groups in their composition. They accept an electron from a slightly stronger reducing agent and donate it to a stronger oxidizing agent. The iron atom is bonded to the nitrogen atom of the imidazole ring of the histidine amino acid on one side of the plane of the porphyrin ring, and on the other side to the sulfur atom of methionine. Therefore, the potential ability of the iron atom in cytochromes to bind oxygen is suppressed.

20. In cytochrome c, the porphyrin plane is covalently bound to the protein through two cysteine residues, while in cytochromes b and a, it is not covalently bound to the protein.

21. In cytochrome a + a3 (cytochrome oxidase), instead of protoporphyrin, porphyrin A is contained, which differs in a number of structural features. The fifth coordination position of iron is occupied by an amino group belonging to an amino sugar residue that is part of the protein itself.

22. In contrast to the heme of hemolgobin, the iron atom in cytochromes can reversibly change from the two to the trivalent state, which ensures the transport of electrons.

23. Needs of tissues for oxygen and its reserves

The needs of tissues for oxygen depend on the functional state of the cells that make up it. The rate of oxygen consumption is usually expressed in ml of oxygen per gram of weight per minute. At rest, oxygen is relatively intensively absorbed by the myocardium, the gray matter of the brain (in particular, the cerebral cortex), the liver and the cortical substance of the kidneys. At the same time, skeletal muscles, the spleen, and the white matter of the brain at rest consume little oxygen.
With an increase in the activity of any organ, its need for oxygen increases. During exercise, oxygen consumption by the myocardium can increase 3-4 times, and by working skeletal muscles – more than 20-50 times compared with rest. Oxygen consumption by the kidneys increases with an increase in the intensity of reabsorption of sodium ions.

The amount of oxygen that cells can use for oxidative processes depends on the convective transport of oxygen by the blood and the diffusion of oxygen from the capillaries into the tissues. Since the only supply of oxygen in most tissues is its physically dissolved fraction, a decrease in oxygen supply leads to oxygen starvation and to a slowdown in oxidative processes.
Muscle tissue is the only tissue that has oxygen reserves. The role of the oxygen depot is played by the pigment myoglobin, which is capable of reversibly binding oxygen. However, the content of myoglobin in human muscles is low, so the average content of myoglobin in the heart is 4 mg/g. Since 1 g of myoglobin can bind up to about 1.34 ml of oxygen, the oxygen stores in the heart are about 0.005 ml of oxygen per 1 g of tissue. This amount, under conditions of a complete cessation of oxygen delivery to the myocardium, may be enough to maintain oxidative processes only for about 3-4 seconds.
Myoglobin plays the role of a short-term oxygen depot. In the myocardium, oxygen bound to myoglobin provides oxidative processes in those areas whose blood supply is reduced for a short time or completely stops during systole.
In the initial period of intense muscular exercise, the increased oxygen demand of the skeletal muscles is partly met by the oxygen released by myoglobin. In the future, muscle blood flow increases, and the supply of oxygen to the muscles again becomes adequate. Replenishment of oxymyoglobin stores is an integral part of the oxygen debt, which must be covered by each muscle fiber after work is completed.

Oxygen starvation of tissues

In a number of pathological conditions, the supply of tissues with oxygen suffers. In these cases, the energy needs of cells can be met for a short time due to limited energy reserves in the form of ATP and creatine phosphate, as well as due to anaerobic glycolysis. However, these energy sources are insufficient and can be used for a short time, since under anaerobic conditions the demand for glucose sharply increases, the supply of which usually cannot satisfy this need, and secondly, a large amount of lactate is formed during glycolysis, which is slowly removed from the tissue for subsequent processing (for example, for cleavage in the liver, kidneys or myocardium, or for glycogen synthesis). With a significant lack of oxygen, the content of lactate in the blood constantly increases, which leads to non-respiratory acidosis. When the pH of the intracellular environment falls below the level of the optimal activity of enzyme systems, pronounced disturbances in cellular metabolism occur.
The main reasons leading to oxygen starvation (tissue hypoxia) are a decrease in oxygen tension in the arterial blood (arterial hypoxia), a decrease in the oxygen capacity of the blood (anemia) and a decrease in the blood supply to a particular organ (ischemia).

NADH dehydrogenase contains several Fe 2 S 2 and Fe 4 S 4 type centers. Iron atoms in such centers can accept and donate electrons in turn, turning into ferro- (Fe 2 +) and ferry- (Fe 3 +) states. From the iron-sulfur centers, electrons are transferred to coenzyme Q (ubiquinone) (Fig.).

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