Amino acids, whose general formula is , where R is a variable radical, constitute an important class of organic compounds that contain both the amino group (-NH2) and the carboxyl one (-COOH). Twenty of these substances are the fundamental blocks which conform proteins. They are called alpha amino acids, to wit: alanine, arginine, aspargine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophane, tyrosine, and valine. The amino and carboxyl groups are glued to the same carbon atom, called the alpha carbon. Also joined to this atom, one finds the variable radical ("R" in the formula) where alpha amino acids differ from each other. In the case of the simplest of these substances, glycine, the radical consists solely of a hydrogen atom. Others have more complex radicals consisting of carbon and hydrogen, and also oxygen, nitrogen or sulphur.