Characterization of the activity and folding of the glutathione transferase from Escherichia coli and the roles of residues Cys¹⁰ and His¹⁰⁶

GSTs (glutathione transferases) are an important class of enzymes involved in cellular detoxification. GSTs are found in all classes of organisms and are implicated in resistance towards drugs, pesticides, herbicides and antibiotics. The activity, structure and folding, particularly of eukaryotic GSTs, have therefore been widely studied. The crystal structure of EGST (GST from Escherichia coli) was reported around 10 years ago and it suggested Cys¹⁰ and His¹⁰⁶ as potential catalytic residues. However, the role of these residues in catalysis has not been further investigated, nor have the folding properties of the protein been described. In the present study we investigated the contributions of residues Cys¹⁰ and His¹⁰⁶ to the activity and stability of EGST. We found that EGST shows a complex equilibrium unfolding profile, involving a population of at least two partially folded intermediates, one of which is dimeric. Mutation of residues Cys¹⁰ and His¹⁰⁶ leads to stabilization of the protein and affects the apparent steady-state kinetic parameters for enzyme catalysis. The results suggest that the imidazole ring of His¹⁰⁶ plays an important role in the catalytic mechanism of the enzyme, whereas Cys¹⁰ is involved in binding of the substrate, glutathione. Engineering of the Cys¹⁰ site can be used to increase both the stability and GST activity of EGST. However, in addition to GST activity, we discovered that EGST also possesses thiol:disulfide oxidoreductase activity, for which the residue Cys¹⁰ plays an essential role. Further, tryptophan quenching experiments indicate that a mixed disulfide is formed between the free thiol group of Cys¹⁰ and the substrate, glutathione.