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.
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Research Article| January 01 1993
Fluxionate Lewis acidity of the Zn2+ ion in carboxypeptidase A
W L Mock;
D J Freeman;
Biochem J (1993) 289 (1): 185–193.
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W L Mock, D J Freeman, M Aksamawati; Fluxionate Lewis acidity of the Zn2+ ion in carboxypeptidase A. Biochem J 1 January 1993; 289 (1): 185–193. doi: https://doi.org/10.1042/bj2890185
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