Glutathione
S-transferases are a family of multifunctional enzymes involved in the metabolismof drugs and xenobiotics. Two tyrosine residues, Tyr 7 and Tyr 111, in the active site of the enzyme playan important role in the binding and catalysis of substrate ligands. The crystal structures of
Schistosomajaponicum glutathione
S-transferase tyrosine 7 to phenylalanine mutant [SjGST(Y7F)] in complex withthe substrate glutathione (GSH) and the competitive inhibitor
S-octylglutathione (
S-octyl-GSH) have beenobtained. These new structural data combined with fluorescence spectroscopy and thermodynamic data,obtained by means of isothermal titration calorimetry, allow for detailed characterization of the ligand-binding process. The binding of
S-octyl-GSH to SjGST(Y7F) is enthalpically and entropically driven attemperatures below 30
C. The stoichiometry of the binding is one molecule of
S-octyl-GSH per mutantdimer, whereas shorter alkyl derivatives bind with a stoichiometry of two molecules per mutant dimer.The SjGST(Y7F)·GSH structure showed no major structural differences compared to the wild-type enzyme.In contrast, the structure of SjGST(Y7F)·
S-octyl-GSH showed asymmetric binding of
S-octyl-GSH. Thislack of symmetry is reflected in the lower symmetry space group of the SjGST(Y7F)·
S-octyl-GSH crystals(
P6
3) compared to that of the SjGST(Y7F)·GSH crystals (
P6
322). Moreover, the binding of
S-octyl-GSHto the A subunit is accompanied by conformational changes that may be responsible for the lack of bindingto the B subunit.