Mutations in human copper zinc superoxide dismutase (hSOD) that are associated with amyotrophic lateral sclerosis (ALS) have been proposed to destabilize the protein and thereby enhance toxic protein aggregation. In previous studies, denaturation of metallated (holo) hSODs was found to be irreversible, and complicated by the formation of intermolecular disulfide bonds. Here, ALS-associated mutations (E100G, G93A, G85R and A4V) are introduced into a pseudo wild-type background containing no free cysteine residues. The guanidinium chloride-induced denaturation of the holo proteins is generally found to be highly reversible (except for A4V, which tended to aggregate), enabling quantitative analysis of the effects of the mutations on protein stability. Denaturation and renaturation curves were monitored by tryptophan fluorescence, circular dichroism, enzyme activity, chemical cross-linking and analytical sedimentation, as a function of equilibration time and protein concentration. There is strong kinetic hysteresis, with curves requiring exceptionally long times (many days for pseudo wild-type) to reach equilibrium, and evidence for the formation of kinetic and equilibrium intermediate(s), which are more highly populated at lower protein concentrations. The effects of metal dissociation were included in the data fitting. The full protein concentration dependence is best described using a three-state model involving metallated native dimer, metallated monomeric intermediate and unfolded monomers with no bound metals; however, at high protein concentrations the unfolding approaches a two-state transition with metal binding to both the native dimers and unfolded monomers. We show that the E100G, G93A and G85R mutations decrease overall protein stability, largely by decreasing monomer stability with little effect on dimer dissociation. Comparison of the chemical denaturation data with ALS disease characteristics suggests that aggregation of some mutant hSOD may occur through increased population of partially folded states that are less stable than the monomeric intermediate and accessed from the destabilized holo protein. Study holds ProTherm entries: 18920, 18921, 18922, 18923, 18924, 18925, 18926, 18927, 18928, 18929, 18930, 18931, 18932, 18933, 18934, 18935, 18936, 18937, 18938 Extra Details: dG_H2O data are in units of kcal/mol.dimer; m in kcal/mol.dimer/M superoxide dismutase; amyotrophic lateral sclerosis; protein stability and folding; dimeric protein; guanidine hydrochloride denaturation
Submitter: Connie Wang
Submission Date: April 24, 2018, 8:51 p.m.
|Number of data points||38|
|Proteins||Superoxide dismutase [Cu-Zn] ; Superoxide dismutase [Cu-Zn]|
|Assays/Quantities/Protocols||Experimental Assay: m prot_conc:0.2 microM ; Experimental Assay: dG_H2O prot_conc:0.2 microM ; Experimental Assay: m prot_conc:10 microM ; Experimental Assay: dG_H2O prot_conc:10 microM ; Experimental Assay: m prot_conc:10 microM ; Experimental Assay: dG_H2O prot_conc:10 microM ; Experimental Assay: m prot_conc:10 microM ; Experimental Assay: dG_H2O prot_conc:10 microM ; Experimental Assay: m prot_conc:5 microM ; Experimental Assay: dG_H2O prot_conc:5 microM ; Experimental Assay: m prot_conc:5 microM ; Experimental Assay: dG_H2O prot_conc:5 microM|
|Libraries||Mutations for sequence ATKAVCVLKGDGPVQGIINFEQKESNGPVKVWGSIKGLTEGLHGFHVHEFGDNTAGCTSAGPHFNPLSRKHGGPKDEERHVGDLGNVTADKDGVADVSIEDSVISLSGDHCIIGRTLVVHEKADDLGKGGNEESTKTGNAGSRLACGVIGIAQ|