The most studied of the zinc proteases is bovine pancreatic carboxypeptidase A, a digestive enzyme.

• It is called a carboxy peptidase because the key catalytic residue is the carboxyl of a glutamic acid

• It is an exopeptidase, meaning that it hydrolyzes C-terminal amino acids from polypeptides

• Like chymotrypsin, carboxypeptidase is specific for aromatic side chains

Bovine Carboxypeptidase A (1yme)	Catalytic Site

The zinc sits at the bottom of a pocket open to the surface of the enzyme.

• It is coordinated to Glu72, His69, and His196
  • A water molecule is probably the fourth ligand

• The active carboxy is Glu270

• Tyr248 is coordinated to the Zn in about 20% of the molecules; previously thought to be catalytic, it has been shown by site-directed mutation to be inactive

Carboxypeptidase B is very similar to A, except for an aspartate that makes it selective for residues with positive side chains (Lys, Arg).

Carboxypeptidase B (1zg8)	Catalytic Site

To emphasize the similarity of the carboxypeptidases A and B, here is a superposition of the human proenzymes:

Superposition of Human CP-A and CP-B (1aye, 1kwm)

The active site residues are picked out as CPK structures, and almost exactly superpose, rmsd 1.01 A, with most of the difference in the activation segment.

Another well-studied example is the bacterial enzyme thermolysin

• Thermolysin is an endopeptidase

• It is not homologous with carboxypeptidase A, and has a much shallower binding site

Thermolysin (1tlx)	Catalytic Site

• The Zn⁺⁺ is bound by His142, His146, and Glu166

• The catalytic residues are Glu143 and His231, the latter corresponding in position to the inactive tyrosine in carboxypeptidase