Carboxypeptidase A is a protease enzyme that catalyses the hydrolysis of the C-terminal amino acid of a protein or polypeptide. The enzyme is involved in the digestion of food as well as in post-translational modification of protein structure for many biochemical functions. The mechanism of hydrolysis involves a catalytic zinc(2+) ion (purple) ligated to two histidine and one glutamic acid residues.