Except for glycine, all amino acids have an asymmetric carbon and exhibit optical activity. The absolute configuration of amino acids (l- or D-isomers) is defined with reference to glyceraldehyde. Except for proline, all protein amino acids have both a primary amino group and a carboxyl group linked to the a-carbon atom (hence a-amino acids). In p-amino acids (e.g., taurine and p-alanine), an amino group links to the p-carbon atom. Posttranslationally modified amino acids (e.g., 4-hydroxyproline, 5-hydroxylysine, 3-methylhistidine, and dimethylarginines) occur in some proteins. The biochemical properties of amino acids vary because of their different side chains. The amino and acid groups of all amino acids are completely ionized (zwitterionic form) at physiological pH.
Amino acids are stable in aqueous solution at physiological temperature, except for glutamine, which is slowly cyclized to pyroglutamate (<2%/day at 1 mM), and cysteine, which undergoes rapid oxidation to cystine. Acid hydrolysis of protein results in almost complete destruction of tryptophan, the oxidation of cysteine to cystine, and some degradation of methionine, serine, threonine, and tyrosine. Alkaline hydrolysis is used for tryptophan determination because of its relative stability. Both acid and alkaline hydrolysis are accompanied by deamination of glutamine and asparagine.
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