Class pi glutathione S-transferase: unfolding and conformational stability in the absence and presence of G-site ligands

Erhardt, Julija
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The glutathione S-transferases (GST) are a supergene family of h0111o-or heterodimeric Phase II detoxification enzymes which catalyse the S-conjugation between glutathione and an electrophilic substrate. The active site can be divided into two adjacent functional regions; a highly specific Gssite for binding the physiological substrate glutathione and a nonspecific If-site for binding nonpolar electrophilic substrates. Unfolding of porcine class Pi isoenzyme (pGSTPl~l) was monitored under equilibrium conditions using different physicochemical parameters. The coincidence of unfolding curves obtained with functional and structural probes, the absence of thermodynamically stable intermediates such as a folded monomer, and the dependence of pGSTPl··l stability upon protein concentration, indicate a cooperative and concerted two-state unfolding transition between native dimeric pGSTPl-l and unfolded monomeric enzyme. Equilibrium and kinetic unfolding experiments employing tryptophan fluorescence and enzyme activity measurements were preformed to study the effect of ligand binding to the G-site on the unfolding and stability of the porcine class pi glutathione S-transferase against urea. The presence of glutathione caused a shift in the equilibrium-unfolding curves towards lower urea concentrations and enhanced the first-order rate constant for unfolding suggesting a destabilisation of the pGSTPl-l structure against urea. The presence of either glutathione sulphonate or S-hexylglutathione, however, produced the opposite effect in that their binding to the G-site appeared to exert a stabilising effect against urea. The binding of these glutathione analogues also reduced significantly the degree of cooperativity of unfolding indicating a possible change in the protein's unfolding pathway.
A thesis submitted to the Faculty of Science, University of the Witwatersrand, in fulfilment of the requirements for the degree of Doctor of Philosophy, Johannesburg, 1996
Erhardt, Julija (1996) Class pi glutathione S-transferase: unfolding and conformational stability in the absence and presence of G-site ligands, University of the Witwatersrand, Johannesburg,<>