The structurally homologous mononuclear iron and man-ganese superoxide dismutases (FeSOD and MnSOD, respectively) containa highly conservedglutamine residue in the active site which projects toward the active-site metal centre and participates in an extensive hydrogen bonding network. The position of this residue is different for each SOD isoenzyme (Q69 in FeSOD and Q146 in MnSOD of Escherichia coli).