An organic acid containing both a basic amino group (NH2) and an acidic carboxyl group (COOH). Amino acids are amphoteric and exist in aqueous solution as dipolar ions. The 25 amino acids that have been established as protein constituents are α-amino acids (i.e., the —NH2 group is attached to the carbon atom next to the —COOH group). Many other amino acids occur in the free state in plant or animal tissue. At least 22 amino acids with structures identical with those that exist today have been identified in pre-Cambrian sedimentary rock, indicating an age of at least 3 million years. Amino acids have been created in the laboratory by passing an electrical discharge through a mixture of ammonia, methane, and water vapor; it is believed that a similar reaction may have accounted for the original synthesis of amino acids on earth. Amino acids can be obtained by hydrolysis of a protein; or they can be synthesized in various ways, especially by fermentation of glucose. An essential amino acid is one that cannot be synthesized by the body and is necessary for survival, namely, isoleucine phenylalanine, leucine, lysine, methionine, threonine, tryptophan, and valine. Nonessential amino acids (alanine, glycine, and about a dozen others) can be synthesized by the body in adequate quantities. Arginine and histidine are essential during periods of intensive growth. All the essential and most of the nonessential amino acids have one or more asymmetric carbon atoms and are optically active. Amino acids are the building blocks of molecular biology. Various combinations of amino acids form the proteins, which are highly complex molecules present in all living things. Note: Use of amino acids as fortification additives to foods is restricted by the FDA to foods containing proteins.See Genetic Code; Deoxyribonucleic Acid; Chromatin; Protein; Life, Origin.
