We are addressing the puzzling metal ion specificity of Fe- and Mn-containing superoxide dismutases (SODs) [see C.K.Vance, A.-F. Miller. J. Am. Chem. Soc. 120(3) (1998) 461-467]. Here, we test the significance to activity and active site integrity of the Gln side chain at the center of the active site hydrogen bond network. We have generated a mutant of MnSOD with the active site Gln in the location characteristic of Fe-specific SODs. The active site is similar to that of MnSOD when Mn2+, Fe3+ or Fe2+ are bound, based on EPR and NMR spectroscopy. However, the mutant's Fe-supported activity is at least 7% that of FeSOD, in contrast to Fe(Mn)SOD, which has 0% of FeSOD's activity. Thus, moving the active site Gln converts Mn-specific SOD into a cambialistic SOD and the Gln proves to be important but not the sole determinant of metal-ion specificity. Indeed, subtle differences in the spectra of Mn2+, Fe3+ and 1H in the presence of Fe2+ distinguish the G77Q, Q146A mut-(Mn)SOD from WT (Mn)SOD, and may prove to be correlated with metal ion activity. We have directly observed the side chain of the active site Gln in Fe2+ SOD and Fe2+ (Mn)SOD by 15N NMR. The very different chemical shifts indicate that the active site Gln interacts differently with Fe2+ in the two proteins. Since a shorter distance from Gln to Fe and stronger interaction with Fe correlate with a lower Em in Fe(Mn)SOD, Gln has the effect of destabilizing additional electron density on the metal ion. It may do this by stabilizing OH- coordinated to the metal ion.