Hidden Allostery in Human Glutathione Transferase P1-1 Unveiled by Unnatural Amino Acid Substitutions and Inhibition Studies

Conventional steady-state kinetic studies of the dimeric human glutathione transferase (GST) P1-1 do not reveal obvious deviations from Michaelis–Menten behavior. By contrast, engineering of the key residue Y50 of the lock-and-key motif in the subunit interface reveals allosteric properties of the enzyme. The low-activity mutant Y50C, characterized by 150-fold decreased kcat and 300-fold increased KMGSH values, displays an apparent Hill coefficient of 0.82 ± 0.22. Chemical alkylation of the sulfhydryl group of Y50C by unnatural n-butyl or n-pentyl substitutions enhances the catalytic efficiency kcat/KMGSH to near the wild-type value but still yields Hill coefficients of 0.61 ± 0.08 and 0.86 ± 0.1, respectively. Thus, allosteric kinetic behavior is not dependent on low activity of the enzyme. On the other hand, S-cyclobutylmethyl-substituted Y50C, which also displays high catalytic efficiency, has a Hill coefficient of 0.99 ± 0.11, showing that subtle differences in structure at the subunit interface influence the complex kinetic behavior. Furthermore, inhibition studies of native GST P1-1 using ethacrynic acid demonstrate that a ligand bound noncovalently to the wild-type enzyme also can elicit allosteric kinetic behavior. Thus, we conclude that the GST P1-1 structure has intrinsic allostery that becomes overt under some, but not all, ambient conditions.