Synthesis of alanine from pyruvic acid by transamination.
which are the precursors of the respective amino acids. For instance, pyruvic acid, which is formed in large quantities during the glycolytic breakdown of glucose, is the keto acid precursor of the amino acid alanine. Then, by the process of transamination, an amino radical is transferred to the a-keto acid, and the keto oxygen is transferred to the donor of the amino radical. This reaction is shown in Figure 69-3. Note in this figure that the amino radical is transferred to the pyruvic acid from another chemical that is closely allied to the amino acids, glutamine. Glutamine is present in the tissues in large quantities, and one of its principal functions is to serve as an amino radical storehouse. In addition, amino radicals can be transferred from asparagine, glutamic acid, and aspartic acid.
Transamination is promoted by several enzymes, among which are the aminotransferases, which are derivatives of pyridoxine, one of the B vitamins (B6). Without this vitamin, the amino acids are synthesized only poorly, and protein formation cannot proceed normally.
Once the cells are filled to their limits with stored protein, any additional amino acids in the body fluids are degraded and used for energy or are stored mainly as fat or secondarily as glycogen. This degradation occurs almost entirely in the liver, and it begins with deamina-tion, which is explained in the following section.
Deamination. Deamination means removal of the amino groups from the amino acids. This occurs mainly by transamination, which means transfer of the amino group to some acceptor substance, which is the reverse of the transamination explained earlier in relation to the synthesis of amino acids.
The greatest amount of deamination occurs by the following transamination schema:
Note from this schema that the amino group from the amino acid is transferred to a-ketoglutaric acid, which then becomes glutamic acid. The glutamic acid can then transfer the amino group to still other substances or release it in the form of ammonia (NH3). In the process of losing the amino group, the glutamic acid once again becomes a-ketoglutaric acid, so that the cycle can be repeated again and again. To initiate this process, the excess amino acids in the cells, especially in the liver, induce the activation of large quantities of aminotrans-ferases, the enzymes responsible for initiating most deamination.
Urea Formation by the Liver. The ammonia released during deamination of amino acids is removed from the blood almost entirely by conversion into urea; two molecules of ammonia and one molecule of carbon dioxide combine in accordance with the following net reaction:
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