glycine (glĪˈsēn) [key], organic compound, one of the 20 amino acids commonly found in animal proteins. Glycine is the only one of these amino acids that is not optically active, i.e., it does not have d - and l -stereoisomers. It is structurally the simplest of the α-amino acids, having merely a hydrogen atom for a side chain, and is thus very unreactive when incorporated into proteins. Nevertheless, in the free state glycine participates in several important reactions, including the biosynthesis of heme, an important constituent of hemoglobin, and the biosyntheses of serine (another amino acid), purines (constituents of genetic material), and glutathione (a coenzyme). Defects of glycine metabolism are very rare. The amino acid is not essential to the diet since it can be made from other substances in the body. Glycine was the first amino acid to be isolated from a protein, in this case gelatin, and has been found in the halo of a comet, Wild 2.