amino acid (əmēˈnō) [key], any one of a class of simple organic compounds containing carbon, hydrogen, oxygen, nitrogen, and in certain cases sulfur. These compounds are the building blocks of proteins. They are characterized by the presence of a carboxyl group (COOH) and an amino group (NH2) attached to the same carbon at the end of the compound. The 20 amino acids commonly found in animals are alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine. In addition to these 20, scientists have synthesized more than 70 artificial amino acids that are not found in animals, and more than 100 less common amino acids also occur in biological systems, particularly in plants. Every amino acid except glycine can occur as either of two optically active stereoisomers, d or l ; the more common isomer in nature is the l -form. When the carboxyl carbon atom of one amino acid covalently binds to the amino nitrogen atom of another amino acid with the release of a water molecule, a peptide bond is formed. Amino acids are released in the intestinal tract by the digestion of food proteins and are then carried in the bloodstream to the body cells, where they are used for growth, maintenance, and repair. Cellular catabolism breaks amino acids down into smaller fragments. Many of the amino acids necessary in metabolism can be synthesized in the human or animal body when needed; these are called nonessential. Others cannot be synthesized in sufficient quantities; these are termed essential and must be provided in the diet. Synthetic amino acids have been used by scientists as markers to track biological processes and as components of disease treatments; artificial amino acids can in some cases increase the effectivenesss of treatments by slowing the normal breakdown of the hormone or other biologic into which they are incorporated.