Reactivity of small thiolate anions and cysteine-25 in papain toward methyl methanethiosulfonate

Publisher Website
Google Scholar
Abstract
The dependence on thiol pK of the second-order rate constant (kS) for reaction of thiolate anions with MMTS was shown to follow the Bronsted equation log kS = log G + .beta.pK with log G = 1.44 and 3.54 and .beta. = 0.635 and 0.309 for aryl and alkyl thiols, respectively. The reactivity toward MMTS of the protonated thiol group was found to be negligible in comparison to that of the thiolate anion. For 2-mercaptoethanol the reactivity toward MMTS of the protonated form of the thiol group was shown to be at least 5 .times. 109 smaller than that of the thiolate anion. The pH dependence of the second-order rate constant for reaction of the thiolate group of Cys-25 at the active site of papain was determined and shown to be consistent with the previously determined low pK for Cys-35 and its electrostatic interaction with His-159. The small dependence of the reactivity of Cys-25 on thiol pK (.beta. .apprx. 0.09) suggested that the charge-charge interactions that act through space to perturb the pK of the nucleophile at the acte site of papain and perhaps other enzymes may serve to increase the fraction of nucleophile present in the reactive basic form without introducing the decrease in nucleophilic reactivity seen in model systems where pK''s are lowered primarily by charge-dipole interactions.

This publication has 16 references indexed in Scilit: