asparagine (əspârˈəjēn) [key], organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l -stereoisomer participates in the biosynthesis of mammalian proteins. Its structure is identical to that of the amino acid aspartic acid, except that the latter compound's acidic side-chain carboxyl group has been coupled with ammonia, yielding an amide. Like glutamine, asparagine is important in the metabolism of toxic ammonia in the body. The relatively unreactive, neutral amide group in the side chain of asparagine confers no special properties upon this amino acid once it is included within a protein by two peptide bonds. Asparagine is not essential to the human diet, since it can be synthesized from aspartic acid. The first amino acid to be isolated from its natural source, asparagine was purified from asparagus juice in 1806; proof of the occurrence of this amino acid in proteins was finally obtained in 1932.