Cryoelectron microscopy structure and mechanism of the membrane-associated electron-bifurcating flavoprotein Fix/EtfABCX
Cryoelectron microscopy structure and mechanism of the membrane-associated electron-bifurcating flavoprotein Fix/EtfABCX
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DOI：
10.1073/pnas.2016978118
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发表时间：
2021-01-12
影响因子：
11.1
通讯作者：
Adams, Michael W. W.
中科院分区：
文献类型：
--
作者：
Feng, Xiang;Schut, Gerrit J.;Adams, Michael W. W.
电子转移黄蛋白-甲基萘醌氧化还原酶ABCX (EtfABCX)，也因其在固氮生物中的作用而被称为FixABCX，是催化电子分岔的电子转移黄蛋白家族的成员。EtfABCX通过将NADH （E度’-320 mV）作为电子供体，与甲萘醌（E度’-80 mV）的自激还原（E度’-450 mV）结合，实现了铁氧还蛋白（E度’-450 mV）的自激还原。本文报道了一种来自嗜热细菌的膜相关黄素基电子分岔（FBEB）复合物EtfABCX的2.9 A结构。EtfABCX与两个膜相关etfc在二聚体界面形成超二聚体，其中包含两个结合的甲基萘醌。该结构表明，与之前的预测相反，EtfAB分叉的低电位电子最有可能直接转移到铁氧还蛋白上，而高电位电子通过EtfX中的两个[4Fe-4S]簇还原醌。令人惊讶的是，EtfX与哺乳动物[4Fe-4S]含簇的ETF泛醌氧化还原酶（ETF- qo）具有显著的结构相似性，这表明分岔系统和非分岔系统之间存在意想不到的进化联系。基于对密切相关的EtfABCX的结构和光谱研究，我们提出了该膜相关FBEB体系的催化循环和伴随的结构变化的详细机制。
The electron-transferring flavoprotein-menaquinone oxidoreductase ABCX (EtfABCX), also known as FixABCX for its role in nitrogen-fixing organisms, is a member of a family of electron-transferring flavoproteins that catalyze electron bifurcation. EtfABCX enables endergonic reduction of ferredoxin (E degrees' similar to-450 mV) using NADH (E degrees' -320 mV) as the electron donor by coupling this reaction to the exergonic reduction of menaquinone (E degrees' -80 mV). Here we report the 2.9 A structure of EtfABCX, a membrane-associated flavin-based electron bifurcation (FBEB) complex, from a thermophilic bacterium. EtfABCX forms a superdimer with two membrane-associated EtfCs at the dimer interface that contain two bound menaquinones. The structure reveals that, in contrast to previous predictions, the low-potential electrons bifurcated from EtfAB are most likely directly transferred to ferredoxin, while high-potential electrons reduce the quinone via two [4Fe-4S] clusters in EtfX. Surprisingly, EtfX shares remarkable structural similarity with mammalian [4Fe-4S] cluster-containing ETF ubiquinone oxidoreductase (ETF-QO), suggesting an unexpected evolutionary link between bifurcating and nonbifurcating systems. Based on this structure and spectroscopic studies of a closely related EtfABCX, we propose a detailed mechanism of the catalytic cycle and the accompanying structural changes in this membrane-associated FBEB system.