Novel insights into the basis for Escherichia coli superoxide dismutase's metal ion specificity from Mn-substituted FeSOD and its very high Em

Fe and Mn are both entrained to the same chemical reaction in apparently superimposable superoxide dismutase (SOD) proteins. However, neither Fe-substituted MnSOD nor Mn-substituted FeSOD is active. We have proposed that the two SOD proteins must apply very different redox tuning to their respective metal ions and that tuning appropriate for one metal ion results in a reduction potential (E(m)) for the other metal ion that is either too low (Fe) or too high (Mn) [Vance and Miller (1998) J. Am. Chem. Soc. 120, 461-467]. We have demonstrated that this is true for Fe-substituted MnSOD from Escherichia coli and that this metal ion-protein combination retains the ability to reduce but not oxidize superoxide. We now demonstrate that the corollary is also true: Mn-substituted FeSOD [Mn(Fe)SOD] has a very high E(m). Specifically, we have measured the E(m) of E. coli MnSOD to be 290 mV vs NHE. We have generated Mn(Fe)SOD and find that Mn is bound in an environment similar to that of the native (Mn)SOD protein. However, the E(m) is greater than 960 mV vs NHE a fid much higher than MnSOD's E(m) of 290 mV. We propose that the different tuning stems from different hydrogen bonding between the proteins and a molecule of solvent that is coordinated to the metal ion in both cases. Because a proton is taken up by SOD upon reduction, the protein can exert very strong control over the E(m), by modulating the degree to which coordinated solvent is protonated, in both oxidation states. Thus, coordinated solvent molecules may have widespread significance as adapters by which proteins can control the reactivity of bound metal ions.