Vance C K, Miller A F
Department of Chemistry, The Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
Biochemistry. 2001 Oct 30;40(43):13079-87. doi: 10.1021/bi0113317.
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 and 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.
铁（Fe）和锰（Mn）在看似可叠加的超氧化物歧化酶（SOD）蛋白中都参与相同的化学反应。然而，铁取代的锰超氧化物歧化酶（MnSOD）和锰取代的铁超氧化物歧化酶（FeSOD）均无活性。我们提出，这两种SOD蛋白必定对其各自的金属离子采用了非常不同的氧化还原调节方式，并且适合一种金属离子的调节会导致另一种金属离子的还原电位（E(m)）过低（Fe）或过高（Mn）[万斯和米勒（1998年）《美国化学会志》120，461 - 467]。我们已经证明，对于来自大肠杆菌的铁取代的MnSOD确实如此，并且这种金属离子 - 蛋白质组合保留了还原超氧化物但不能氧化超氧化物的能力。我们现在证明其推论也是正确的：锰取代的FeSOD [Mn(Fe)SOD]具有非常高的E(m)。具体而言，我们测得大肠杆菌MnSOD的E(m)相对于标准氢电极（NHE）为290 mV。我们制备了Mn(Fe)SOD，发现锰结合的环境与天然（Mn）SOD蛋白的环境相似。然而，相对于NHE，其E(m)大于960 mV，远高于MnSOD的290 mV的E(m)。我们提出，不同的调节源于两种情况下蛋白质与配位到金属离子的溶剂分子之间不同的氢键作用。由于SOD在还原时会吸收一个质子，蛋白质可以通过调节配位溶剂在两种氧化态下的质子化程度，对E(m)施加非常强的控制。因此，配位溶剂分子作为“适配器”可能具有广泛的意义，通过它蛋白质可以控制结合金属离子的反应性。
