Effects of Cys10 mutation to Ala in glutathione transferase from Escherichia coli

The residue Cys10 in E. coli glutathione transferase (GST) is apparently equivalent in primary structure to the catalytic Ser residue of the class theta GST and is located near the thiol group of GSH bound to the enzyme. The mutation of Cys10 to Ala, however, increased the specific activity toward 1-chloro-2,4-dinitrobenzene at pH 6.5. This mutation increased both the kcat and Km values for GSH and affected the pH-activity profile the enzyme. The side chain of Cys10 is thought to be important for construction of the GSH-binding site and partly for lowering the pKa of the GSH thiol, but not to be essential for the catalytic activity.