The bulk of the chemical energy of carbon is released under aerobic conditions with the participation of oxygen. The Krebs cycle is also called the citric acid cycle, or cellular respiration. Many scientists took part in decoding individual reactions of this process: A. Saint-György, A. Leninger, X. Krebs, whose name is given to the cycle, S. E. Severin and others.
There is a close correlation between anaerobic and aerobic breakdown of carbohydrates. First of all, it is expressed in the presence of pyruvic acid, which completes the anaerobic breakdown of carbohydrates and begins cellular respiration (Krebs cycle). Both phases are catalyzed by the same enzymes. Chemical energy is released during phosphorylation, is reserved in the form of ATP macroergs. The same coenzymes (NAD, NADPH) and cations participate in chemical reactions. The differences are as follows: if the anaerobic breakdown of carbohydrates is predominantly localized in the hyaloplasm, then cellular respiration reactions occur mainly in the mitochondria.
Under certain conditions, antagonism is observed between both phases. So, in the presence of oxygen, the rate of glycolysis reaction decreases sharply (Pasteur effect). Glycolysis products can inhibit the aerobic metabolism of carbohydrates (Crabtree effect).
The Krebs cycle has a number of chemical reactions, as a result of which carbohydrate breakdown products are oxidized to carbon dioxide and water, and chemical energy is accumulated in macroergic compounds. During cellular respiration , a “carrier” is formed - oxaloacetic acid (alkali acid). Subsequently, condensation occurs with the “carrier” of the activated residue of acetic acid. There arises tricarboxylic acid - citric. In the course of chemical reactions, a “turn” of the residue of acetic acid in the cycle occurs. Eighteen adenosine triphosphate molecules are formed from each pyruvic acid molecule. At the end of the cycle, a “carrier” is released, which reacts with new molecules of the activated residue of acetic acid.
Krebs cycle: reactions
If the final product of anaerobic digestion of carbohydrates is lactic acid, then it is oxidized to pyruvic acid under the influence of lactate dehydrogenase. A part of the pyruvic acid molecules is used for the synthesis of the “carrier” of alkaline phosphatase under the influence of the pyruvate carboxylase enzyme and in the presence of Mg2 + ions. Some molecules of pyruvic acid are the source of the formation of “active acetate” - acetyl coenzyme A (acetyl-CoA). The reaction is carried out under the influence of pyruvate dehydrogenase. Acetyl-CoA contains a macroergic bond in which about 5-7% of energy is accumulated. The bulk of the chemical energy is generated as a result of the oxidation of "active acetate".
Under the influence of citrate synthetase, the Krebs cycle itself begins to function, which leads to the formation of citrate acid. This acid, under the influence of aconitate hydratase, is dehydrogenated and converted into cis-aconitic acid, which, after the addition of a water molecule, is converted to isolimonic. A dynamic equilibrium is established between the three tricarboxylic acids.
Isolimonic acid oxidizes to oxalo-succinic acid, which is decarboxylated and converted to alpha-ketoglutaric acid. The reaction is catalyzed by the enzyme isocitrate dehydrogenase. Alpha-ketoglutaric acid under the influence of the enzyme 2-oxo- (alpha-keto) -glutarate dehydrogenase is decarboxylated, resulting in the formation of succinyl-CoA containing a macroergic bond.
At the next stage, succinyl-CoA, under the action of the enzyme succinyl-CoA synthetase, transmits the macroergic bond of HDF (guanosine diphosphate acid). GTP (guanosine triphosphate acid), under the influence of the enzyme GTP-adenylate kinase, gives the macroergic bond of AMP (adenosine monophosphate acid). Krebs cycle: formulas - GTP + AMP - GDF + ADP.
Succinic acid under the influence of the enzyme succinate dehydrogenase (LDH) is oxidized to fumaric. Coenzyme LDH is flavin adenine dinucleotide. Fumarate, under the influence of the enzyme fumarate hydratase, turns into malic acid, which in turn oxidizes, forming alkaline phosphatase. If acetyl-CoA is present in the reacting system, the alkali nitrate is again included in the tricarboxylic acid cycle.
So, up to 38 ATP molecules are formed from one glucose molecule (two due to anaerobic glycolysis, six due to the oxidation of two NAD · H + H + molecules that were formed during glycolytic oxidation reduction, and 30 due to CTK). The coefficient of performance of the CTK is 0.5. The rest of the energy is dissipated in the form of heat. 16-33% of lactic acid is oxidized in the TCA, the rest of its mass goes to glycogen resynthesis.