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首页> 外文期刊>Dalton transactions: An international journal of inorganic chemistry >How feasible is the reversible S-dissociation mechanism for the activation of FeMo-co, the catalytic site of nitrogenase?
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How feasible is the reversible S-dissociation mechanism for the activation of FeMo-co, the catalytic site of nitrogenase?

机译：可行的是可逆的S-解离机制，用于激活股骨CO，氢酶的催化位点？

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The active site of the enzyme nitrogenase (N-2 NH3) is a Fe7MoS9C cluster that contains three doubly-bridging -S atoms around a central belt. A vanadium nitrogenase variant has a slightly different cluster, containing two -S atoms. Recent crystal structures have revealed substitution of one -S (S2B, bridging Fe2 and Fe6), by CO in Mo-nitrogenase and an uncertain light atom in V-nitrogenase. These systems retained catalytic activity, and were able to recover the lost -S atom. Electron density attributed to the dissociated S is displaced by 7 angstrom in the crystal structure of the non-standard V-protein. The hypothesis arising from these observations is that the chemical mechanism of nitrogenase involves reversible dissociation of S2B, leaving Fe2 and Fe6 seriously under-coordinated and reactive in trapping N-2 and binding reaction intermediates. Accumulated experimental evidence points to the Fe2-S2B-Fe6 domain as the centre of catalytic hydrogenation of N-2. Using DFT simulations of a large model (488 atoms) containing all relevant surrounding protein residues, I have investigated the chemical steps that could allow dissociation of S2B. The participation of H atoms is crucial, as is involvement of the nearby side chain of His195 that can function as proton donor to S2B and hydrogen-bonding supporter of displaced S2B. A significant result is that after ingress and binding of N-2 at Fe2 the breaking of the Fe2-S2B bond can be strongly exergonic with negligible kinetic barrier. Subsequent extension of the Fe6-S2B bond and dissociation as H2S (or SH-) is endergonic by 20-25 kcal mol(-1), partly because the separating H2S is restricted by surrounding amino-acids. I present a number of reaction sequences and energy landscapes, and derive thirteen chemical principles relevant to the postulated S-dissociation mechanism. A key conclusion is that unhooking of S2BH or S2BH(2) from Fe2 is favourable, likely, and propitious for subsequent H transfer to bound N-2 or reactio
机译：酶氮酶的活性位点（N-2 NH 3）是Fe7MOS9C簇，其围绕中心带含有三个双桥接原子。钒硝酸酯变体具有略微不同的簇，含有两个原子原子。最近的晶体结构揭示了在Mo-unitogenase中的CO和V-氮酶中的不确定光原子取代单（S2B，桥接Fe2和Fe6）。这些系统保留了催化活性，并且能够回收丢失的原子。归因于离解的S的电子密度在非标准V-蛋白的晶体结构中置换7埃。由这些观察结果产生的假设是氮酶的化学机制涉及S2B的可逆解离，在捕获N-2和结合反应中间体时严重地欠下和反应的Fe2和Fe6。累积的实验证据指向Fe2-S2B-Fe6结构域作为N-2的催化氢化中心。使用含有所有相关周围蛋白质残留物的大型模型（＆ 488原子）的DFT模拟，我研究了可以允许解离S2B的化学步骤。 H原子的参与至关重要，与他195的附近侧链的参与一样，可以用作质子供体至S2B和移位的S2B的氢键配子。显着的结果是，在Fe2处的N-2的进入和结合后，Fe2-S2B键的破碎可以强烈地具有可忽略的动力学屏障。作为H 2 S（或SH-）的Fe 6-S2B键和解离的后续延伸，通过20-25kcal（-1），部分地是因为分离的H 2被周围氨基酸限制。我呈现了许多反应序列和能量景观，并导出与假设的S-解离机制相关的十三个化学原理。关键的结论是从Fe2中脱离S2BH或S2BH（2）是有利的，可能和有利于随后的H转移到结合的N-2或反应

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