proline (prōˈlēn) [key], organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l -stereoisomer appears in mammalian protein. It is not essential to the human diet, since it can be synthesized in the body from glutamic acid. The amino group through which it can link to other amino acids (see peptide, protein) is part of a circlelike array of atoms—unique to proline. This is significant because when the amino acid is incorporated into protein, its peculiar structure leads to sharp bends, or kinks, in the peptide chain, thus figuring prominently in the determination of the protein's shape. Proline and its derivate hydroxyproline, make up some 21% of the amino-acid residues found in collagen, the fibrous protein of connective tissue. Its chemical synthesis was accomplished in 1900; in 1901 proline was isolated from casein, the milk protein, and its structure was shown to be the same as that of the synthetic compound.