Essential and Nonessential Amino Acids
Ten of the amino acids normally present in animal pro-teins can be synthesized in the cells, whereas the other 10 either cannot be synthesized or are synthesized in quantities too small to supply the body’s needs. This second group of amino acids that cannot be synthesized is called the essential amino acids. Use of the word “essential” does not mean that the other 10 “nonessen-tial” amino acids are not required for the formation of proteins, but only that the others are not essential in thediet because they can be synthesized in the body.
Synthesis of the nonessential amino acids depends mainly on the formation of appropriate a-keto acids, 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, glutamicacid, and aspartic acid.
Transamination is promoted by several enzymes, among which are the aminotransferases, which are deriv-atives of pyridoxine, one of the B vitamins (B6). Without this vitamin, the amino acids are synthesized only poorly, and protein formation cannot proceed normally.