title: Outer sphere mutations perturb metal reactivity in manganese superoxide dismutase.
abstract: Tyrosine 34 and glutamine 146 are highly conserved outer sphere residues in the mononuclear manganese active site of Escherichia coli manganese superoxide dismutase. Biochemical and spectroscopic characterization of site-directed mutants has allowed functional characterization of these residues in the wild-type (wt) enzyme. X-ray crystallographic analysis of three mutants (Y34F, Q146L, and Q146H) reveal subtle changes in the protein structures. The Y34A mutant, as well as the previously reported Y34F mutant, retained essentially the full superoxide dismutase activity of the wild-type enzyme, and the X-ray crystal structure of Y34F manganese superoxide dismutase shows that mutation of this strictly conserved residue has only minor effects on the positions of active site residues and the organized water in the substrate access funnel. Mutation of the outer sphere solvent pocket residue Q146 has more dramatic effects. The Q146E mutant is isolated as an apoprotein lacking dismutase activity. Q146L and Q146H mutants retain only 5-10% of the dismutase activity of the wild-type enzyme. The absorption and circular dichroism spectra of the Q146H mutant resemble corresponding data for the superoxide dismutase from a hyperthermophilic archaeon, Pyrobaculum aerophilum, which is active in both Mn and Fe forms. Interestingly, the iron-substituted Q146H protein also exhibits low dismutase activity, which increases at lower pH. Mutation of glutamine 146 disrupts the hydrogen-bonding network in the active site and has a greater effect on protein structure than does the Y34F mutant, with rearrangement of the tyrosine 34 and tryptophan 128 side chains.
authors.affiliation: Centre for Structural Biology, Institutes of Fundamental Sciences and Molecular BioSciences, Massey University, Palmerston North, New Zealand.
authors.forename: R A
authors.initials: RA
authors.lastname: Edwards
authors.forename: M M
authors.initials: MM
authors.lastname: Whittaker
authors.forename: J W
authors.initials: JW
authors.lastname: Whittaker
authors.forename: E N
authors.initials: EN
authors.lastname: Baker
authors.forename: G B
authors.initials: GB
authors.lastname: Jameson
mesh_terms: D001665:Binding Sites / Q000235:genetics; D002384:Catalysis; D002942:Circular Dichroism; D003198:Computer Simulation; D017124:Conserved Sequence; D004789:Enzyme Activation / Q000235:genetics; D004926:Escherichia coli / Q000201:enzymology; D005973:Glutamine / Q000235:genetics; D006639:Histidine / Q000235:genetics; D008345:Manganese / Q000737:chemistry*; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D010649:Phenylalanine / Q000235:genetics; D013056:Spectrophotometry, Ultraviolet; D013482:Superoxide Dismutase / Q000737:chemistry*; D014443:Tyrosine / Q000235:genetics
chemical_list: D005973:Glutamine; D014443:Tyrosine; D008345:Manganese; D010649:Phenylalanine; D006639:Histidine; D013482:Superoxide Dismutase