Iron and manganese superoxide dismutases

Abstract

Fe and Mn SDs are enigmatic metalloproteins for several reasons: They are found in strict anaerobes as well as aerobes. In some cases the Fe and Mn ions appear to be interchangeable while in others they are not. In some organisms the Mn-protein is strongly induced by O 2 while in others the Fe-protein is induced. The two distinct types of proteins are structurally related as judged from published amino acid sequences. The Fe-protein appears to have a non-covalently associated organic co-factor bound near the iron (see below). Recently, the three dimensional structures of FeSD from E. coli and Ps. ovalis have been elucidated in the laboratories of M. Ludwig and G. Petsko. The FeSD molecule is composed of a dimer of identical subunits related within the crystal by a dyad axis. The distance between the Fe atoms is 18 Å and each is close to the subunit interface. Access to the Fe appears to be from a region near the interface. Each monomer has two distinct structural domains, connected by a single strand and the Fe is bound at the interface of the two domains receiving two ligands from each. The Fe is surrounded by four ligands from the protein and probably a water molecule. The geometry, at 3 Å resolution has the appearance of a flattened pyramid. According to present interpretations of the electron density map the four protein ligands are His-26, residue 69, and residues 148 and 152. The spectral properties of the protein would appear to exclude tyrosine as a ligand to the Fe. A region of unconnected electron density is found in the region of the molecule separating the two domains and about 6 Å from the Fe. This is a dominant feature in FeSD from both sources and may represent an organic co-factor. To date we have not isolated and identified this material. We are involved in a program to correlate structural functional properties of the FeSD from E. coli. The presentation will be concerned with redox probes of the Fe, steady state and transient kinetic studies of superoxide dismutation, and relations between anion inhibition and binding to the Fe. If time allows a comparison will be made between the mechanism of FeSD action and the weak superoxide dismutase activity of Fe-EDTA.