Hydroxyproline
4-hydroxy-2-pyrrolidinecarboxylic acid, a heterocyclic amino acid (in terms of chemical structure, an imino acid).
Hydroxyproline was first isolated from gelatin hydrolyzates in 1902 by E. Fischer. Because of the presence of two asymmetric carbon atoms per molecule, it has four optically active forms (L-, D-, allo-L-, and allo-D-hydroxyprolines) and two racemic forms. Natural L-hydroxyproline is a specific constituent of proteins in connective tissue, collagen and elastin (up to 13 percent), as well as of certain vegetable proteins; other proteins contain negligible amounts of L-hydroxyproline or none at all. Allo-L-hydroxypro-line was discovered in free form in sandalwood and is a component of toxic peptides in the death-cap. The formation of L-hydroxyproline in living cells is effected by hydroxylation of the proline bonded to the protein components (the O atom of the hydroxyl group is incorporated into hydroxyproline upon fixation of molecular O2). One of the products formed during the conversion of L-hydroxyproline in the organism is glutamic acid.
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