Carboxypeptidase A: A Protein and an Enzyme

Abstract

Publisher Summary This chapter discusses the relationship of the three-dimensional structures of bovine carboxypeptidase A (CPA), and of its complexes with substrates and inhibitors, to the functional behavior of this enzyme. In particular, it discusses the basis for substrate specificity, modes of binding, and possible mechanisms of hydrolytic cleavage of substrates for this enzyme. CPA is a zinc-containing proteolytic enzyme, which catalyzes the hydrolysis of carboxy-terminal peptide bonds in protein and peptide substrates. Removal of Zn2+, either by lowering the pH below 5.5 or by the use of a variety of chelating agents at neutral pH, yields an inactive enzyme, apocarboxypeptidase A. Peptidase activity is known for Co2+, Ni2+, Mn2+ and Fe2+ in place of Zn2+, but substitution of Cu2+ for Zn2+ yields an enzyme that is inactive toward all substrates. Esters are also cleaved by CPA and the substitution of Hg2+, Cd2+, or Pb2+ retains esterase activity, although these heavy metal derivatives are not peptidases in solution. However, the crystals of the mercury derivative have shown some peptidase activity.