Evaluation of the role of two conserved active-site residues in Beta class glutathione S-transferases

Abstract

Glutathione S-transferases (GSTs) normally use hydroxy-group-containing residues in the N-terminal domain of the enzyme for stabilizing the activated form of the co-substrate, glutathione. However, previous mutagenesis studies have shown that this is not true for Beta class GSTs and thus the origin of the stabilization remains a mystery. The recently determined crystal structure of Proteus mirabilis GST B1-1 (PmGST B1-1) suggested that the stabilizing role might be fulfilled in Beta class GSTs by one or more residues in the C-terminal domain of the enzyme. To test this hypothesis we mutated His106 and Lys107 of PmGST B1-1 to investigate their possible role in the enzyme's catalytic activity. His106 was mutated to Ala, Asn and Phe, and Lys107 to Ala and Arg. The effects of the replacement on the activity, thermal stability and antibiotic-binding capacity of the enzyme were examined. The results are consistent with the involvement of His106 and Lys107 in interacting with glutathione at the active site but these residues do not contribute significantly to catalysis, folding or antibiotic binding.