Human Glutathione Transferase T2-2 Discloses Some Evolutionary Strategies for Optimization of Substrate Binding to the Active Site of Glutathione Transferases

Rapid kinetic, spectroscopic, and potentiometric studies have been performed on human Theta class glutathione transferase T2-2 to dissect the mechanism of interaction of this enzyme with its natural substrate GSH. Theta class glutathione transferases are considered to be older than Alpha, Pi, and Mu classes in the evolutionary pathway. As in the more recently evolved GSTs, the activation of GSH in the human Theta enzyme proceeds by a forced deprotonation of the sulfhydryl group (pK_a = 6.1). The thiol proton is released quantitatively in solution, but above pH 6.5, a protein residue acts as an internal base. Unlike Alpha, Mu, and Pi class isoenzymes, the GSH-binding mechanism occurs via a simple bimolecular reaction with k _on and k_off values at least hundred times lower (k _on = (2.7 ± 0.8) × 10⁴ M^-1 s ^-1, k_off = 36 ± 9 s^-1 at 37 °C). Replacement of Arg-107 by alanine, using site-directed mutagenesis, remarkably increases the pK_a value of the bound GSH and modifies the substrate binding modality. Y107A mutant enzyme displays a mechanism and rate constants for GSH binding approaching those of Alpha, Mu, and Pi isoenzymes. Comparison of available crystallographic data for all these GSTs reveals an unexpected evolutionary trend in terms of flexibility, which provides a basis for understanding our experimental results.