Enzymes catalyze reactions by doing one or more of the following: bringing reacting substances into close proximity, binding substrates in precise orientations, polarizing substrates, destabilizing reagents or stabilizing transition states. Enzymes often bring reactants together in the presence of catalytic functional groups within the active site. Substrates are usually bound to enzymes exclusively by noncovalent interactions but sometimes other forms of binding may be involved. For example, the pancreatic enzyme carboxypeptidase A binds its substrate in the active site by intermolecular forces, a coordinate covalent bond, and a salt bridge. With this enzyme, other side chains serve respectively as the nucleophile for hydrolytic acyl exchange and proton donation.