title: A topologically conserved aliphatic residue in alpha-helix 6 stabilizes the hydrophobic core in domain II of glutathione transferases and is a structural determinant for the unfolding pathway.
abstract: A topologically conserved residue in alpha-helix 6 of domain II of human glutathione transferase (hGST) A1-1 was mutated to investigate its contribution to protein stability and the unfolding pathway. The replacement of Leu-164 with alanine (L164A) did not impact on the functional and gross structural properties of native hGST A1-1. The wild-type protein unfolds via a three-state pathway in which only folded dimer and unfolded monomer were highly populated at equilibrium; a native-like dimeric intermediate with partially dissociated domains I and II was detected using stopped-flow fluorescence studies [Wallace, Sluis-Cremer and Dirr (1998) Biochemistry 37, 5320-5328]. In the present study, urea-induced equilibrium unfolding of L164A hGST A1-1 indicated a destabilization of the native state and suggested the presence of a stable dimeric intermediate. The unfolding kinetic pathway for L164A hGST A1-1, like that for the wild type, is biphasic, with a fast and a slow unfolding event; the cavity-forming mutation has a substantially greater effect on the rate of unfolding of the fast event. The equilibrium and kinetic unfolding data for L164A hGST A1-1 suggest that a rapid pre-equilibrium is established between the native dimer and a dimeric intermediate before complete domain and subunit dissociation and unfolding. It is proposed that the topologically conserved bulky residue in alpha-helix 6 plays a role in specifying and stabilizing the core of domain II and the interface of domains I and II.
authors.affiliation: Protein Structure-Function Research Programme, Department of Biochemistry, University of the Witwatersrand, Johannesburg 2050, South Africa.
authors.forename: L A
authors.initials: LA
authors.lastname: Wallace
authors.forename: G L
authors.initials: GL
authors.lastname: Blatch
authors.forename: H W
authors.initials: HW
authors.lastname: Dirr
mesh_terms: D000595:Amino Acid Sequence; D017124:Conserved Sequence; D003600:Cytosol / Q000201:enzymology; D004795:Enzyme Stability; D005982:Glutathione Transferase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D017510:Protein Folding; D013329:Structure-Activity Relationship; D014508:Urea
chemical_list: D014508:Urea; D005982:Glutathione Transferase