Carboxypeptidase A is a protease enzyme that catalyses the hydrolysis of the C-terminal amino acid of a protein or polypeptide. The active site of the enzyme includes a zinc(2+) Lewis acid catalyst (purple). The pseudotetrahedral geometry of the metal ion comes from two histidine residues, one glutamic acid residue and a water molecule. The polypeptide substrate replaces the water molecule. The side chains are shown in stick format with the hydrogen atoms omitted for clarity. Atoms are represented as spheres and are colour-coded: carbon (black), hydrogen (white), nitrogen (blue) and oxygen (red).