Abstract
The 2.1-Angstrom resolution crystal structure of native uncomplexed iron superoxide dismutase (EC 1.15.1.1) from Pseudomonas ovalis was solved and refined to a final R factor of 24%. The dimeric structure contains one catalytic iron center per monomer with an asymmetric trigonal-bipyramidal coordination of protein ligands to the metal. Each monomer contains two domains, with the trigonal ligands (histidines 74 and 160; aspartate 156) contributed by the large domain and stabilized by an extended hydrogen-bonded network, including residues from opposing monomers. The molecule contains a glutamine residue in the active site which is conserved between all iron enzymes sequenced to data but which is conserved among all manganese SODs at a separate position in the sequence. This residue shows the same structural interactions in both cases, implying that iron and manganese SODs are second-site revertants of one another.