Identification and clarification of the role of key active site residues in bacterial glutathione S-transferase zeta/maleylpyruvate isomerase

The maleylpyruvate isomerase NagL from Ralstonia sp. strain U2, which has been structurally characterized previously, catalyzes the isomerization of maleylpyruvate to fumarylpyruvate. It belongs to the class zeta glutathione S-transferases (GSTZs), part of the cytosolic GST family (cGSTs). In this study, site-directed mutagenesis was conducted to probe the functions of 13 putative active site residues. Steady-state kinetic information for mutants in the reduced glutathione (GSH) binding site, suggested that (a) Gln64 and Asp102 interact directly with the glutamyl moiety of glutathione, (b) Gln49 and Gln64 are involved in a potential electron-sharing network that influences the ionization of the GSH thiol. The information also suggests that (c) His38, Asn108 and Arg109 interact with the GSH glycine moiety, (d) His104 has a role in the ionization of the GSH sulfur and the stabilization of the maleyl terminal carboxyl group in the reaction intermediate and (e) Arg110 influences the electron distribution in the active site and therefore the ionization of the GSH thiolate. Kinetic data for mutants altered in the substrate-binding site imply that (a) Arg8 and Arg176 are critical for maleylpyruvate orientation and enolization, and (b) Arg109 (exclusive to NagL) participates in kcat regulation. Surprisingly, the T11A mutant had a decreased GSH Km value, whereas little impact on maleylpyruvate kinetics was observed, suggesting that this residue plays an important role in GSH binding. An evolutionary trend in this residue appears to have developed not only in prokaryotic and eukaryotic GSTZs, but also among the wider class of cGSTs.

► Application of site-directed mutagenesis to probe the active site residues of glutathione-dependent maleylpyruvate isomerase.
► Two conserved residues, Arg8 and Arg176, in zeta class glutathione S-transferases are critical for maleylpyruvate orientation and enolization.
► Arg109, found exclusively in NagL, participates in kcat regulation.
► The T11A mutant exhibited a significantly decreased Km value for glutathione with little impact on maleylpyruvate kinetics.
► The Thr11 residue appears to have significance in the evolution of glutathione S-transferase classes.