pH profiles indicative of rate-limiting nucleophilic displacement in thioltransferase catalysis

The apparent pKa for the active site thiol of human thioltransferase (TTase) is about 3.5, but the pH dependence of TTase-catalyzed rates of glutathione (GSH)-dependent reduction of disulfide substrates displays an inflection point near pH 8.5. The similarity of the pH-rate profile with the titration of the GSH thiol moiety suggested rate-limiting nucleophilic attack by the glutathionyl thiolate species to regenerate reduced TTase from the TTase-SSG intermediate. To test this hypothesis pH-rate profiles for TTase-catalyzed dethiolation of the glutathionyl mixed disulfide of bovine serum albumin ([35S]BSA-SSG) were measured according to release of radiolabeled GS-equivalents. Various thiol compounds, whose thiol pKa values range on both sides of the pKa of GSH (pKa = 8.7), were used as reducing substrates, e.g., trifluoroethanethiol (pKa = 7.5) and 3-mercaptopropionic acid (pKa = 10.3). The pH-rate profiles paralleled the titration of the respective thiol groups of the reducing substrates, consistent with the hypothesis. In addition, second-order rate constants (k) were determined for the nonenzymatic and TTase-catalyzed reactions of the various thiols with BSA-SSG. A simple linear free energy relationship (log k vs pKa) was displayed for the nonenzymatic reactions. In contrast, the relationship for the enzymatic reactions revealed GSH to be different from the other thiol substrates, i.e., GSH gave a second-order rate constant greater than expected for its thiol pKa. This result suggests a special interaction of GSH with the TTase enzyme in the transition state that enhances the nucleophilicity of GSH.