Amino acids. Molecular graphics of three different amino acids. Clockwise from top left: methionine (C5H11SNO2), lysine (C6H14N2.O2) and glutamic acid (C5H9NO4). The spheres represent different atoms: oxygen (O, red), carbon (C, grey), hydrogen (H, white), nitrogen (N, blue) and sulphur (S, yellow). These are three of the 20 standard amino acids, organic molecules that are the building blocks of the millions of proteins that control the biochemistry of life. All amino acids have an adjacent carboxyl (CO2H, pair of red O atoms) and amino (NH2, blue N atom) chemical group attached to a variable group which determines the shape, properties and function of the amino acid.