Localization of the C-terminus of rat glutathione S-transferase A1-1: crystal structure of mutants W21F and W21F/F220Y

Twelve C-terminal residues of human glutathione S-transferase A1-1 form a helix in the presence of glutathione-conjugate, or substrate alone, and partly cover the active site. According to X-ray structures, the helix is disordered in the absence of glutathione, but it is not known if it is helical and delocalized, or in a random-coil conformation. Mutation to a tyrosine of residue 220 within this helix was previously shown to affect the pK(a) of Tyr-9 at the active site, in the apo form of the enzyme, and it was proposed that an on-face hydrogen bond between Tyr-220 and Tyr-9 provided a means for affecting this pK(a). In the current study, X-ray structures of the W21F and of the C-terminal mutation, W21F/F220Y, with glutathione sulfonate bound, show that the C-terminal helix is disordered (or delocalized) in the W21F crystal but is visible and ordered in a novel location, a crystal packing crevice, in one of three monomers in the W21F/F220Y crystal, and the proposed hydrogen bond is not formed. Fluorescence spectroscopy studies using an engineered F222W mutant show that the C-terminus remains delocalized in the absence of glutathione or when only the glutathione binding site is occupied, but is ordered and localized in the presence of substrate or conjugate, consistent with these and previous crystallographic studies. Proteins 2001;42:192-200.