Origin of rate-acceleration in ester hydrolysis with metalloprotease mimics Original Research Article

Mimics of carboxypeptidase A, a prototypical metalloprotease, were synthesized by linking macrocyclicpolyamines to the primary side of β-cyclodextrin followed by complexing with Zn(II). These enzyme mimics exhibit saturation kinetics in hydrolysis of p-nitrophenyl acetate (PNPA) and enhance the rate of hydrolysis reaction by almost 300-fold. The effective molarities (EM) of the mimics range from 0.2 to 1.9 M. Origin of the rate acceleration was examined: the reactivity of Zn(II) complexes of [12]aneN3 [12]aneN4, and [14]aneN4 for hydrolyzing PNPA increases with increase in basicity of the zinc bound hydroxides [Zn(II)–OH], yielding a linear Brönsted plot. Free hydroxide fits well on this plot. A similar plot was obtained with the enzyme mimics. The Brönsted relationships indicate that the Zn(II)–OH in the catalytic systems hydrolyzes the ester by direct nucleophilic attack on the ester carbonyl of cyclodextrin-bound but not Zn(II)-coordinated PNPA.