amino acid (AA) /əmē″nō/ , an organic chemical compound composed of one or more basic amino groups and one or more acidic carboxyl groups. A total of 20 of the more than 100 amino acids that occur in nature are the building blocks of proteins. The eight essential amino acids are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Arginine and histidine are essential in infants. Cysteine and tyrosine are semiessential because they may be synthesized from methionine and phenylalanine, respectively. The main nonessential amino acids are alanine, asparagine, aspartic acid, glutamine, glutamic acid, glycine, proline, and serine. From their structures, the amino acids can be classified as basic (arginine, histidine, lysine), acidic (aspartic acid, glumatic acid), or neutral (the remainder); each group is transported across cell membranes by different carrier methods. Individual amino acids represent the monomeric units that can be connected via peptide linkages (amide bonds) to produce polymeric structures called proteins according to the scheme below.
