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- W2085869920 abstract "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." @default.
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- W2085869920 date "1997-03-01" @default.
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- W2085869920 title "pH Profiles Indicative of Rate-Limiting Nucleophilic Displacement in Thioltransferase Catalysis" @default.
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- W2085869920 doi "https://doi.org/10.1021/bi962017t" @default.
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