The catalytic active site of Mn-specific superoxide dismutase (MnSOD) is organized around a redox-active Mn ion. The most highly conserved difference between MnSODs and the homologous FeSODs is the origin of a Gln in the second coordination sphere. In MnSODs it derives from the C-terminal domain whereas in FeSODs it derives from the N-terminal domain, yet its side chain occupies almost superimposable positions in the active sites of these two types of SODs. Mutation of this Gln69 to Glu in Escherichia coli FeSOD increased the Fe reduction midpoint potential by >0.6 V without disrupting the structure or Fe binding [ Yikilmaz, E., Rodgers, D. W., and Miller, A.-F. ( 2006 ) Biochemistry 45 ( 4 ), 1151 - 1161 ]. We now describe the analogous Q146E mutant of MnSOD, explaining its low Mn content in terms increased stability of the apo-Mn protein. In 0.8 M guanidinium HCl, Q146E-apoMnSOD displays an apparent melting midpoint temperature (T) 35 °C higher that of wild-type (WT) apoMnSOD, whereas the T of WT-holoMnSOD is only 20 °C higher than that of WT-apoMnSOD. In contrast, the T attributed to Q146E-holoMnSOD is 40 °C lower than that of Q146E-apoMnSOD. Thus, our data refute the notion that the WT residues optimize the structural stability of the protein and instead are consistent with conservation on the basis of enzyme function and therefore ability to bind metal ion. We propose that the WT-MnSOD protein conserves a destabilizing amino acid at position 146 as part of a strategy to favor metal ion binding.