Electronic and molecular structures of the active-site H-cluster in [FeFe]-hydrogenase determined by site-selective X-ray spectroscopy and quantum chemical calculations
Electronic and molecular structures of the active-site H-cluster in [FeFe]-hydrogenase determined by site-selective X-ray spectroscopy and quantum chemical calculations
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DOI：
10.1039/c3sc52703d
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发表时间：
2014-03-01
期刊：
影响因子：
8.4
通讯作者：
Haumann, Michael
中科院分区：
文献类型：
--
作者：
Lambertz, Camilla;Chernev, Petko;Haumann, Michael
来自绿色藻类的[FeFe]-氢化酶（HydA 1）是生物高效产氢的最小酶。它的活性中心六铁中心（H-簇）由一个立方烷[4Fe 4S]（H）、一个半胱氨酸连接到一个二铁中心[2Fe]（H）组成。我们利用铁K β X射线荧光发射的自旋极化进行位点选择性X射线吸收实验的光谱歧视的两个子复合物。对于还原的HydA 1蛋白中的H簇，分别获得了[4Fe 4S]（H）和（2Fe）（H）的XANES和EXAFS光谱、K β发射线（3 p-> 1 s跃迁）、核-价（前边缘）吸收（1 s-> 3d）和价-核（K β（2，5））发射（3d -> 1 s）光谱。解析了[2Fe]（H）和[4Fe 4S]（H）中的铁-配体键长和金属间距离，以及高自旋含铁立方烷的精细结构。密度泛函理论计算再现了X射线光谱特征并指定了分子轨道构型，强调了非顺磁状态下[2Fe]（H）中近端（Fe-p）和远端（Fe-d）低自旋铁的不对称d能级简并。这产生了一个特定的模型结构的H-簇的一个桥接的一氧化碳配体和顶端开放的配位位点在Fe-d [2Fe]（H）。小的HOMO-LUMO间隙（类似于0.3eV）使得能够在类似的电势下氧化和还原活性位点以通过HydAl进行可逆的H-2转换，LUMO在[4Fe 4S]（H）上的分布支持其作为电子转移中继的作用，并且制备携带HOMO的Fe-d用于瞬时氢化物结合。这些功能和从体相的Fe-d的可访问性可以解释区域特定的氧化还原转变以及H-2-形成和O-2-抑制在H-簇。我们提供了一个概念和实验框架，在非均匀材料的催化机制的选址研究。
The [FeFe]-hydrogenase (HydA1) from green algae is the minimal enzyme for efficient biological hydrogen (H-2) production. Its active-site six-iron center (H-cluster) consists of a cubane, [4Fe4S](H), cysteine-linked to a diiron site, [2Fe](H). We utilized the spin-polarization of the iron K beta X-ray fluorescence emission to perform site-selective X-ray absorption experiments for spectral discrimination of the two sub-complexes. For the H-cluster in reduced HydA1 protein, XANES and EXAFS spectra, K beta emission lines (3p -> 1s transitions), and core-to-valence (pre-edge) absorption (1s -> 3d) and valence-to-core (K beta(2,5)) emission (3d -> 1s) spectra were obtained, individually for [4Fe4S](H) and (2Fe)(H). Iron-ligand bond lengths and intermetal distances in [2Fe](H) and [4Fe4S](H) were resolved, as well as fine structure in the high-spin iron containing cubane. Density functional theory calculations reproduced the X-ray spectral features and assigned the molecular orbital configurations, emphasizing the asymmetric d-level degeneracy of the proximal (Fe-p) and distal (Fe-d) low-spin irons in [2Fe](H) in the non-paramagnetic state. This yielded a specific model structure of the H-cluster with a bridging carbon monoxide ligand and an apical open coordination site at Fe-d in [2Fe](H). The small HOMO-LUMO gap (similar to 0.3 eV) enables oxidation and reduction of the active site at similar potentials for reversible H-2 turnover by HydA1, the LUMO spread over [4Fe4S](H) supports its role as an electron transfer relay, and Fe-d carrying the HOMO is prepared for transient hydride binding. These features and the accessibility of Fe-d from the bulk phase can account for regio-specific redox transitions as well as H-2-formation and O-2-inhibition at the H-cluster. We provide a conceptual and experimental framework for site-selective studies on catalytic mechanisms in inhomogeneous materials.