An adequate amount of all twenty amino acids is required to build proteins for growth and to replace the proteins that are turned over. However, only eight of these (nine in children) cannot be produced by the body and must be obtained in the diet. These are the essential amino acids (table 5.2). The remaining amino acids are "nonessential" only in the sense that the body can produce them if provided with a sufficient amount of carbohydrates and the essential amino acids.
Pyruvic acid and the Krebs cycle acids are collectively termed keto acids because they have a ketone group; these should not be confused with the ketone bodies (derived from acetyl CoA) discussed in the previous section. Keto acids can be converted to amino acids by the addition of an amine (NH2) group. This amine group is usually obtained by "cannibalizing" another amino acid; in this process, a new amino acid is formed as the one that was cannibalized is converted to a new keto acid. This type of reaction, in which the amine group is transferred from one amino acid to form another, is called transamination (fig. 5.14).
Each transamination reaction is catalyzed by a specific enzyme (a transaminase) that requires vitamin B6 (pyridoxine) as a coenzyme. The amine group from glutamic acid, for example, may be transferred to either pyruvic acid or oxaloacetic acid. The former reaction is catalyzed by the enzyme alanine transaminase (ALT); the latter reaction is catalyzed by aspartate transaminase
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