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

Fe and Mn are both entrained to the same chemical reaction in apparently su perimposable superoxide dismutase (SOD) proteins. However, neither Fe-subst ituted 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 redu ction potential (E-m) for the other metal ion that is either too low (Fe) o r too high (Mn) [Vance and Miller (1998) J. Am. Chem. Soc. 120, 461-467]. W e have demonstrated that this is true for Fe-substituted MnSOD from Escheri chia coli and that this metal ion-protein combination retains the ability t o 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. Specifi cally, we have measured the E-m of E. coli MnSOD to be 290 mV vs NHE. We ha ve 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 th e different tuning stems from different hydrogen bonding between the protei ns and a molecule of solvent that is coordinated to the metal ion in both c ases. Because a proton is taken up by SOD upon reduction, the protein can e xert very strong control over the E-m, by modulating the degree to which co ordinated solvent is protonated, in both oxidation states. Thus, coordinate d solvent molecules may have widespread significance as "adapters" by which proteins can control the reactivity of bound metal ions.