amino acid

A molecule containing both carboxylic acid and amino groups and having the general formula RCHNH₂CO OH. The nature of the R group varies widely, from a hydrogen atom in glycine to aromatic and heterocyclic ring structures in such amino acids as tyrosine and tryptophan. Amino acids can be classified as nonpolar, polar uncharged, acidic, or basic, according to the nature of the R group (see tables). Amino acids are the basic structural units of proteins with some 20 amino acids commonly occurring in proteins. In addition to these there are a few unusual amino acids that occur only in a few proteins and over 200 nonprotein amino acids that have been isolated from various plant sources. In many cases the function of these nonprotein amino acids is unclear but some are intermediates in the synthesis of common amino acids while others may have protective or storage functions. Because they contain both acidic and basic groups, amino acids will react with both acids and bases. Consequently the charge on an amino acid varies with its pH. Each amino acid has a specific pH, known as its isoelectric point, at which the net charge on the molecule is zero. All naturally occurring amino acids, except glycine, are optically active due to the asymmetry of the α carbon atom. Most amino acids in nature are in the L form, although some D-amino acids are found in bacterial cell walls.