Glycine molecule. Molecular model of the simplest amino acid glycine (C2H5NO2). Amino acids are the monomers or building-blocks of the larger protein molecules. Some smaller molecules, for example peptides and oligopeptides, such as the hormone insulin, are also made from amino acids. There are 20 common amino acids, which all possess a similar core structure of a carboxyl (COOH) group at one end, and an amino (NH2) group at the other. Each is distinguished by having a different side group structure. Carbon atoms are black, hydrogen are white, nitrogen is blue and oxygen is red.