Structure-Function Relationships in Iron and Manganese Superoxide Dismutases

Using the complete sequences for MnSOD from Thermus thermophilus and for FeSOD from E. coli, structural models for both oxidized enzymes have been refined, the Mn protein to an R of 0.186 for all data between 10.0 and 1.8 A, and the Fe protein to an R of 0.22 for data between 10.0 and 2.5 A. The results of the refinements support the presence of a solvent as a fifth ligand to Mn(III) and Fe(III) and a coordination geometry that is close to trigonal bipyramidal. The putative substrate-entry channel is comprised of residues from both subunits of the dimer; several basic residues that are conserved may facilitate approach of O2-, while other conserved residues maintain interchain packing interactions. Analysis of the azide complex of Fe(III) dismutase suggests that during turnover O2- binds to the metal at a sixth coordination site without displacing the solvent ligand. Because crystals reduced with dithionite show no evidence for displacement of the protein ligands, the redox-linked proton acceptor (C. Bull and J.A. Fee (1985), Journal of the American Chemistry Society 107, 3295-3304) is unlikely to be one of the histidines which bind the metal ion. Structural, kinetic, titration, and spectroscopic data can be accommodated in a mechanistic scheme which accounts for the differential titration behaviour of the Fe(III) and Fe(II) enzymes at neutral and high pH.