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固氮酶中单电子还原的活性位点 FeFe-辅因子含有一个氢化物与一个形式上氧化的金属离子核心结合。

The One-Electron Reduced Active-Site FeFe-Cofactor of Fe-Nitrogenase Contains a Hydride Bound to a Formally Oxidized Metal-Ion Core.

机构信息

Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States.

Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322, United States.

出版信息

Inorg Chem. 2022 Apr 11;61(14):5459-5464. doi: 10.1021/acs.inorgchem.2c00180. Epub 2022 Mar 31.

Abstract

The nitrogenase active-site cofactor must accumulate 4e/4H (E(4H) state) before N can bind and be reduced. Earlier studies demonstrated that this E(4H) state stores the reducing-equivalents as two hydrides, with the cofactor metal-ion core formally at its resting-state redox level. This led to the understanding that N binding is mechanistically coupled to reductive-elimination of the two hydrides that produce H. The state having acquired 2e/2H (E(2H)) correspondingly contains one hydride with a resting-state core redox level. How the cofactor accommodates addition of the first e/H (E(H) state) is unknown. The Fe-nitrogenase FeFe-cofactor was used to address this question because it is EPR-active in the E(H) state, unlike the FeMo-cofactor of Mo-nitrogenase, thus allowing characterization by EPR spectroscopy. The freeze-trapped E(H) state of Fe-nitrogenase shows an = 1/2 EPR spectrum with = [1.965, 1.928, 1.779]. This state is photoactive, and under 12 K cryogenic , 450 nm photolysis converts to a new and likewise photoactive = 1/2 state (denoted E(H)) with = [2.009, 1.950, 1.860], which results in a photostationary state, with E(H) relaxing to E(H) at temperatures above 145 K. An H/D kinetic isotope effect of 2.4 accompanies the 12 K E(H)/E(H)* photointerconversion. These observations indicate that the addition of the first e/H to the FeFe-cofactor of Fe-nitrogenase produces an Fe-bound hydride, not a sulfur-bound proton. As a result, the cluster metal-ion core is one-electron oxidized relative to the resting state. It is proposed that this behavior applies to all three nitrogenase isozymes.

摘要

固氮酶活性部位辅因子在 N 结合并被还原之前必须积累 4e/4H(E(4H) 态)。早期的研究表明,这种 E(4H) 态将还原当量存储为两个氢化物,辅因子金属离子核心在其静止状态的氧化还原水平处。这导致了这样的理解,即 N 结合与产生 H 的两个氢化物的还原消除机械偶联。相应地,获得 2e/2H(E(2H))的状态包含一个具有静止状态核心氧化还原水平的氢化物。辅因子如何适应第一个 e/H 的添加(E(H) 态)是未知的。使用 Fe-固氮酶 FeFe-辅因子来解决这个问题,因为它在 E(H) 态中具有 EPR 活性,而不同于 Mo-固氮酶的 FeMo-辅因子,因此允许通过 EPR 光谱学进行表征。Fe-固氮酶的冷冻捕获 E(H) 态显示出 g = 1/2 的 EPR 谱,g = [1.965,1.928,1.779]。该状态是光活性的,并且在 12 K 低温下,450nm 光解转化为新的同样光活性的 g = 1/2 状态(表示为 E(H)*),g = [2.009,1.950,1.860],这导致光稳定状态,E(H)*在 145 K 以上的温度下弛豫至 E(H)。12 K 下 E(H)/E(H)*光互变伴随 H/D 动力学同位素效应为 2.4。这些观察结果表明,向 Fe-固氮酶的 FeFe-辅因子添加第一个 e/H 会产生一个与 Fe 结合的氢化物,而不是与 S 结合的质子。结果,簇金属离子核心相对于静止状态被单电子氧化。据推测,这种行为适用于所有三种固氮酶同工酶。

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