Fluxionate Lewis acidity of the Zn2+ ion in carboxypeptidase A.
Competitive inhibition constants Ki for a series of phenol-ring-substituted derivatives of alpha-(2-hydroxyphenyl)benzenepropanoic acid have been ascertained by observing their influence on the catalytic hydrolysis of a peptide substrate by the zinc enzyme carboxypeptidase A. The pH-dependence of Ki shows that binding is maximal between two pKa values: one is that of the phenol group of the inhibitor, and the other uniformly has a value of 6, the pKa of a Zn(2+)-bound water molecule on the enzyme in the absence of substrate or inhibitor. This is the dependence expected if phenolate binds to the Zn2+ displacing its bound H2O/HO-. A log-log plot of the dissociation constants for the productive forms of inhibitor plus enzyme versus the acid dissociation constants of the phenolic residues in the inhibitors yields a straight line with a slope of +0.76. This number indicates that the active-site metal ion has special capacity for dispersing negative charge, such as builds up on the oxygen atom of a carboxamide group undergoing nucleophilic addition.
Mock
W L
WL
Department of Chemistry, University of Illinois, Chicago 60607-7061.
Freeman
D J
DJ
Aksamawati
M
M
D001665:Binding Sites; D002268:Carboxypeptidases / Q000737:chemistry* / Q000378:metabolism; D043422:Carboxypeptidases A; D002384:Catalysis; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D015032:Zinc / Q000737:chemistry*
D002268:Carboxypeptidases; D043422:Carboxypeptidases A; D015032:Zinc