Any of a class of organic compounds in which a carbon atom has bonds to an amino group (―NH 2 ), a carboxyl group (―COOH), a hydrogen atom (―H), and an organic side group (called ―R).
They are therefore both 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 {{link=metabolism">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.