lysine (lĪˈsēn) [key], organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l -stereoisomer appears in mammalian protein. It is one of several essential amino acids needed in the diet; the human body cannot synthesize it from simpler metabolites. Young adults need about 23 mg of this amino acid per day per kilogram (10 mg per lb) of body weight. Lysine is found in particularly low concentrations in the proteins of cereals; wheat gluten, for example, is relatively poor in lysine. This deficiency in lysine is the reason for the failure of diets in some parts of the world that employ cereal protein as a sole source of essential amino acids to support growth in children and general well-being in adults. Attempts to develop lysine-rich corn have been partly successful. Once lysine is incorporated into protein, its basic side chain often provides a positive electrical charge to the protein, thereby aiding its solubility in water. Its side chain has also been implicated in the binding of several coenzymes (pyridoxal phosphate, lipoic acid, and biotin) to enzymes. It also plays an important role in the functioning of histones. The amino acid was first isolated from casein (milk protein) in 1889, and its structure was elucidated in 1902.