Any of a class of organic compounds in which a carbon atom has bonds to an amino group ( 2015; NH 2 ), a carboxyl group ( 2015; COOH), a hydrogen atom ( 2015; H), and an organic side group (called 2015; R).
They are therefore both carboxylic acids and amines . The physical and chemical properties unique to each result from the properties of the R group, particularly its tendency to interact with water and its charge (if any). Amino acids joined linearly by peptide bonds (see covalent bond ) in a particular order make up peptides and proteins . Of over 100 natural amino acids, each with a different R group, only 20 make up the proteins of all living organisms. Humans can synthesize 10 of them (by interconversions) from each other or from other molecules of intermediary metabolism , but the other 10 (essential amino acids: arginine , histidine , isoleucine , leucine , lysine , methionine , phenylalanine , threonine , tryptophan , and valine ) must be consumed in the diet.