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

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 spectroscopic properties. The thiolate form of this residue is characterized by a sharp UV absorption spectrum centered at 229 nm with an epsilon = 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. The apparent pKa value for Cys-47, calculated by this kinetic approach, is in good agreement with that determined spectroscopically. X-ray crystallographic data indicate that the protonated amino group of Lys-54, 4.9 A from the sulfur atom, is probably involved in the deprotonation of Cys-47. Calculation of the electrostatic potential on the sulfur atom of Cys-47 gives a theoretical pKa value of 3.5 for the sulfhydryl group. The simulated neutralization of Lys-54 shifts the pKa value of Cys-47 to a normal value of 9.5. These findings suggest that at physiological pH values, Cys-47 exists as the thiolate ion stabilized by an ion pair formation with the protonated amino group of Lys-54, and this probably accounts for its high reactivity.