Carboxypeptidase E

This chapter examines activity, specificity and structural chemistry of carboxypeptidase E (CPE). CPE removes C-terminal basic residues from a variety of substrates, with no detectable activity towards nonbasic residues. All residues are tolerated in the PI position, although the Pro–Arg bond is cleaved several orders of magnitude more slowly than substrates with other residues in PI. Amongst N-terminally blocked small peptides, CPE shows lower activity with dipeptides than with tri- or tetrapeptides. The optimal pH for CPE activity is 5.0–5.5, and activity falls off dramatically with increasing pH. The sensitivity of CPE to pH over the range 5.5–7 may serve as a mechanism to keep CPE inactive in the Golgi and allow for the activation of CPE in mature secretory vesicles. As found with other metallocarboxypeptidases, CPE is stimulated by millimolar concentrations of Co²⁺ and inhibited by 1,10-phenanthroline. CPE is potently inhibited by some thiol-directed reagents. CPE is a single-chain protein of 52–56 kDa, with a pi around 5. It is likely that CPE contains lmol of zinc per mol of protein based on the homology between CPE and other metallocarboxypeptidases.