Hydrolysis of a thiopeptide by cadmium carboxypeptidase A.

Abstract Substitution of the active site zinc ion of carboxypeptidase A by cadmium yields an enzyme inactive towards ordinary peptide substrates. However, a substrate analog (BzGlyNHCH2CSPheOH) containing a thioamide linkage at the scissile position is cleaved to the thioacid. The kinetic parameters and their pH dependencies are k cat K m = 5.04 × 10 4 min −1 M −1 , decreasing with either acid or base (PKE1 = 5.64, pKE2 = 9.55), and kcat = 1.02 × 102 min−1, decreasing with acid (pKES = 6.61). The thiopeptide is less efficiently cleaved by native (zinc) carboxypeptidase A. This cadmium-sulfur synergism supports a mechanism wherein the substrate amide is activated by metal ion coordination to its (thio) carbonyl.

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