A mutagenic analysis of the role of Trp-115 in the human Theta class glutathione transferase GSTT2-2

Previous homology modelling studies suggested that the helix 4 residue Trp-115 may be involved in the novel sulphatase activity of the human Theta class glutathione transferase GSTT2-2 through the formation of ring stacking interactions with the naphthalene ring of the substrate I-menaphthyl sulphate. Unexpectedly the mutation of Trp-115 to Leu or Ala resulted in increased specific activities with 1-menaphthyl sulphate and cumene hydroperoxide, suggesting that the bulky side chain of Trp-115 does not allow optimal activity with these substrates. Mutation of Trp-115 appears to have indirect effects on catalysis by modifying H-site stability, substrate specificity and active site architecture, thereby influencing the exchange of substrates and products in the active site. Thus, although Trp-115 does not function exactly as predicted, its mutation in this study illustrates the fine balance between activity and stability.