Department of Physics, Stockholm University, 106 91 Stockholm, Sweden.
J Biol Inorg Chem. 2012 Mar;17(3):363-73. doi: 10.1007/s00775-011-0858-8. Epub 2011 Nov 15.
Two-electron chemistry with an iron dimer, a manganese dimer, and a manganese-iron dimer as a catalyst has been modeled using B3LYP* hybrid density functional theory. The recently discovered MnFe proteins form (at least) two functionally distinct groups, performing radical generation (class Ic ribonucleotide reductase subunit II) and substrate oxidations (subunit II-like ligand-binding oxidases, R2lox), respectively. Proteins from the latter group appear to be functionally similar to the diiron carboxylate proteins that perform two-electron oxidations of substrates, such as methane monooxygenase. To qualitatively determine the potential role of a MnFe center in R2lox, methane hydroxylation with the MnFe heterodimer and with the FeFe and MnMn homodimers is studied. The redox potential of the active state of the Mn(IV)Fe(IV) heterodimer is about 7 kcal mol(-1) lower than that of the active state of the Fe(IV)Fe(IV) homodimer, leading to a high barrier for the rate-limiting hydrogen abstraction with the MnFe site. If the entropy loss is not included, the barriers are lower, and the MnFe heterodimer can therefore have a role in R2lox as an oxidase for larger substrates exergonically bound to the protein. A MnMn center has a high barrier both with and without entropy loss. The higher stability of Fe(IV) in the presence of Mn(IV) in the other site compared with a second Fe(IV) suggests an explanation for the presence of the MnFe site in R2lox: to provide a metal center that is capable of two-electron chemistry, and which is more stable and less sensitive to external reductants than an Fe(IV)Fe(IV) site.
使用 B3LYP*杂化密度泛函理论对铁二聚体、锰二聚体和锰铁二聚体作为催化剂的双电子化学进行了建模。最近发现的 MnFe 蛋白形成（至少）两个功能不同的组，分别进行自由基生成（Ic 类核核苷酸还原酶亚基 II）和底物氧化（亚基 II 样配体结合氧化酶，R2lox）。后一组蛋白质的功能似乎与进行底物双电子氧化的二铁羧酸酯蛋白相似，例如甲烷单加氧酶。为了定性确定 MnFe 中心在 R2lox 中的潜在作用，研究了 MnFe 杂二聚体和 FeFe 和 MnMn 同二聚体对甲烷的羟化作用。Mn(IV)Fe(IV)杂二聚体活性态的氧化还原电位比 Fe(IV)Fe(IV)同二聚体活性态低约 7 kcal mol(-1)，导致 MnFe 位的氢提取限速反应具有高势垒。如果不包括熵损失，势垒会降低，因此 MnFe 杂二聚体可以在 R2lox 中作为更大底物的氧化酶发挥作用，这些底物与蛋白质结合的自由能较低。MnMn 中心无论是否存在熵损失，势垒都很高。与第二个 Fe(IV)相比，另一个位点存在 Mn(IV)时 Fe(IV)的稳定性更高，这为 R2lox 中存在 MnFe 位点提供了一个解释：提供一个能够进行双电子化学的金属中心，其稳定性比 Fe(IV)Fe(IV)位点更高，对外部还原剂的敏感性更低。
