Ck. Vance et Af. Miller, Novel insights into the basis for Escherichia coli superoxide dismutase's metal ion specificity from Mn-substituted FeSOD and its very high E-m, BIOCHEM, 40(43), 2001, pp. 13079-13087
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.