Communication between the Two Active Sites of Glutathione S-Transferase A1-1, Probed Using Wild-Type−Mutant Heterodimers

Abstract

To study the communication between the two active sites of dimeric glutathione S-transferase A1-1, we used heterodimers containing one wild-type (WT) active site and one active site with a single mutation at either Tyr9, Arg15, or Arg131. Tyr9 and Arg15 are part of the active site of the same subunit, while Arg131 contributes to the active site of the opposite subunit. The V(max) values of Tyr9 and Arg15 mutant enzymes were less than 2% that of WT, indicating their importance in catalysis. In contrast, V(max) values of Arg131 mutant enzymes were about 50-90% of that of WT enzyme while K(m)(GSH) values were approximately 3-8 times that of WT, suggesting that Arg131 plays a role in glutathione binding. The mutant enzyme (with a His(6) tag) and the WT enzyme (without a His(6) tag) were used to construct heterodimers (WT-Y9F, WT-Y9T, WT-R15Q, WT-R131M, WT-R131Q, and WT-R131E) by incubation of a mixture of wild-type and mutant enzyme at pH 7.5 in buffer containing 1,6-hexanediol, followed by dialysis against buffer lacking the organic solvent. The resultant heterodimers were separated from the wild-type and mutant homodimers using chromatography on nickel-nitrilotriacetic acid agarose. The V(max) values of all heterodimers were lower than expected for independent active sites. Our experiments demonstrate that mutation of an amino acid residue in one active site affects the activity in the other active site. Modeling studies show that key amino acid residues and water molecules connect the two active sites. This connectivity is responsible for the cross-talk between the active sites.