Dissociation and unfolding of homodimeric glutathione S-transferase Y7F mutant from Schistosoma japonicum (SjGST-Y7F) were investigated at equilibrium using urea as denaturant. The conserved residue Tyr7 plays a central role in the catalytic mechanism and the mutation Tyr-Phe yields an inactive enzyme that is able to bind the substrate GSH with a higher binding constant than the wild type enzyme. Mutant SjGST-Y7F is a dimer at pH 6 or higher and a stable monomer at pH 5 that binds GSH (K value of 1.2 x 10(5) +/- 6.4 x 10(3) M-1 at pH 6.5 and 6.3 x 10(4) +/- 1.25 x 10(3) M-1 at pH 5). The stability of the SjGST-Y7F mutant was studied by urea induced unfolding techniques (DeltaG(W) = 13.86 +/-0.63 kcal mol(-1) at pH 6.5 and DeltaG(W) = 11.22 +/- 0.25 kcal mol(-1) at pH 5) and the monomeric form characterized by means of size exclusion chromatography, fluorescence, and electrophoretic techniques. (C) 2003 Elsevier Inc. All rights reserved.