Energy of muscular activity
As already mentioned, both phases of muscle activity - contraction and relaxation - proceed with the mandatory use of energy, which is released during the hydrolysis of ATP.
However, ATP stores in muscle cells are insignificant (ATP concentration in muscles is about 5 mmol/l at rest), and they are sufficient for muscle work for 1-2 seconds. Therefore, to ensure a longer muscular activity in the muscles, there must be replenishment of ATP stores. The formation of ATP in muscle cells directly during physical work is called the resynthesis of ATP and comes with energy consumption.
Thus, in the functioning of muscles in them simultaneously proceed two processes: hydrolysis of ATP, giving the necessary energy for contraction and relaxation, and the resynthesis of ATP, replenishing the loss of this substance. If only the chemical energy of ATP is used to provide muscle contraction and relaxation, then the chemical energy of a wide variety of compounds, carbohydrates, fats, amino acids and creatine phosphate, is suitable for the resynthesis of ATP.
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Structure and biological role of ATP
Adenosine triphosphate (ATP) is a nucleotide. The molecule ATP (adenosine triphosphate) consists of the nitrogen base of adenine, five-carbon ribose sugar and three residues of phosphoric acid, connected by a macroergic bond. During its hydrolysis, a large amount of energy is released. ATP is the main macroerger of the cell, the energy accumulator in the form of energy of high-energy chemical bonds.
Under physiological conditions, that is, under the conditions that exist in a living cell, the cleavage of a mole of ATP (506 g) is accompanied by the release of 12 kcal, or 50 kJ of energy.
ATP generation pathways
Aerobic oxidation (tissue respiration)
Synonyms: oxidative phosphorylation, respiratory phosphorylation, aerobic phosphorylation.
This path takes place in the mitochondria.
The cycle of tricarboxylic acids was first discovered by the English biochemist G. Krebs (Figure 4).
The first reaction is catalyzed by the citrate synthase enzyme, while the acetyl-CoA acetyl group condenses with oxaloacetate, resulting in the formation of citric acid. Apparently, in this reaction, the citric-CoA bound to the enzyme is formed as an intermediate product. Then the latter spontaneously and irreversibly hydrolyses with the formation of citrate and HS-CoA.
As a result of the second reaction, the resulting citric acid undergoes dehydration with the formation of cis-aconitic acid, which, attaching the water molecule, transforms to isonic acid (isocitrate). Catalyzes these reversible hydration-dehydration reactions of the enzyme aconitate hydratase (aconitase). As a result, H and OH are displaced in the citrate molecule.
Fig. 4. A cycle of tricarboxylic acids (Krebs cycle)
The third reaction, apparently, limits the speed of the Krebs cycle. The isolimonic acid is dehydrogenated in the presence of NAD-dependent isocitrate dehydrogenase. During the isocitrate dehydrogenase reaction, the isocitric acid is simultaneously decarboxylated. NAD-dependent isocitrate dehydrogenase is an allosteric enzyme, which requires ADP as a specific activator. In addition, the enzyme needs the ions or to manifest its activity.
During the fourth reaction, oxidative decarboxylation of α-ketoglutaric acid occurs to form a high-energy succinyl-CoA compound. By mechanism this reaction is similar to the reaction of oxidative decarboxylation of pyruvate to acetyl-CoA; The α-ketoglutarate dehydrogenase complex resembles in its structure the pyruvate dehydrogenase complex. In both cases, 5 coenzymes take part in the reaction: TPP, amide of lipoic acid, HS-CoA, FAD and NAD +.
The fifth reaction is catalyzed by the enzyme succinyl-CoA synthetase. During this reaction, succinyl-CoA, with the participation of GTP and inorganic phosphate, is converted to succinic acid (succinate). At the same time, the high-energy phosphate bond of GTP is formed due to the highlyergic thioether bond of succinyl-CoA.
As a result of the sixth reaction, the succinate dehydrogenates into fumaric acid. Oxidation of succinate is catalyzed by succinate dehydrogenase,
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in the molecule of which the coenzyme FAD is linked with the protein firmly (covalently). In turn, succinate dehydrogenase is strongly bound to the inner mitochondrial membrane.
The seventh reaction is carried out under the influence of the enzyme fumarate hydratase (fumarase). The resulting fumaric acid is hydrated, the reaction product is malic acid (malate).
Finally, during the eighth reaction of the tricarboxylic acid cycle under the influence of mitochondrial NAD-dependent malate dehydrogenase, oxidation of L-malate to oxaloacetate occurs.
One molecule of ATP can be formed in one cycle of the cycle during the oxidation of one molecule of acetyl-CoA in the Krebs cycle and the system of oxidative phosphorylation.
Synonyms: substrate phosphorylation, anaerobic synthesis of ATP. It is in the cytoplasm, the split hydrogen is attached to some other substance. Depending on the substrate, there are two ways of anaerobic resynthesis of ATP: creatine phosphate (creatine kinase, alaktate) and glycolytic (glycolysis, lactate). In the nervous case, the substrate is creatine phosphate, in the second - glucose.
These paths take place without oxygen.
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