Peculiar spectroscopic and kinetic properties of Cys-47 in human placental glutathione transferase: Evidence for an atypical thiolate ion pair near the active site

Abstract

Cys-47, the most reactive cysteine in the homodimeric glutathione transferase (EC 2.5.1.18) from human placenta (class Pi), displays peculiar acid base and spectroecopic properties. The thiolate form of this residue is characterized by a sharp UV absorption spectrum centered at 229 nm with an ε = 7,500 M-1 cm-1. The dependence of the apparent extinction coefficient on pH indicates that the sulfhydryl group of Cys-47 has a pKa value of 4.2. Moreover the dependence of the reactivity of Cys-47 toward bromopyruvate and iodoacetamide with pH resembles that found for the functional sulfhydryls of thiol proteases, which have very low pKa values and exist mainly as a mercaptide-imidazole ion pair. T..