Biochemistry

At the first stage, monomers are converted in cells. They decompose into simpler compounds, which may be the same for different monomers. For example, during the oxidation of carbohydrates, fats and a number of amino acids, albeit in different ways, the same compound is formed - acetyl-CoA, which is called the “ universal metabolite ” . At this stage, about 1/3 of all energy released during the decay of substances is released. The second stage is the complete oxidation of acetyl-CoA, resulting in the formation of carbon monoxide and the release of hydrogen. (This process has been studied in detail by X.A.Krebs.) This process proceeds equally in all cells. The Krebs cycle is a closed system of reactions that begins with the interaction of acetyl-CoA and oxaloacetic acid with the formation of tricarboxylic citric acid, which, after a series of stages, is again converted to oxalacetic acid. Among other compounds of the Krebs cycle, isolytic, a-ketoglutaric, succinic, and malic acids arc of particular importance, since they undergo oxidation. This process is catalyzed by enzymes whose coenzymes are vitamins or their derivatives (dehydrogenases, etc.). In the third stage, the reduced form of dehydrogenase is oxidized in the respiratory chain to form water. A small part of the released hydrogen is used to synthesize new compounds, and most of it is oxidized by oxygen to form water. It is during this process that the energy contained in the electrons is released. Moreover, the formation of water in the body does not occur immediately, but through a series of reactions, for this reason the electron energy is not released simultaneously, but gradually. In addition to oxidative phosphorylation, there is another way in the body for the synthesis of ATP. In the course of transformations, certain substances accumulate energy in their chemical bonds that they can “ transfer ” for the synthesis of ATP. The latter can be formed by direct transfer of a “ macroergic ” phosphoric acid residue, for example, from diphosphoglyceric acid, which is formed during the breakdown of carbohydrates without oxygen. Such phosphorylation is called substrate. Its significance is less in comparison with oxidative phosphorylation. The macroergic residue of phosphoric acid can be transferred to other compounds, which also become energy sources that can be converted into other forms. For example, glycogen synthesis is carried out with the participation of guanosine triphosphate (GTP), where guanine is contained instead of adenine. 113