Fachbereich Biologie , Philipps-Universität , 35032 Marburg , Germany.
Max-Planck-Institut für Terrestrische Mikrobiologie , 35043 Marburg , Germany.
Chem Rev. 2018 Apr 11;118(7):3862-3886. doi: 10.1021/acs.chemrev.7b00707. Epub 2018 Mar 21.
There are two types of electron bifurcation (EB), either quinone- or flavin-based (QBEB/FBEB), that involve reduction of a quinone or flavin by a two-electron transfer and two reoxidations by a high- and low-potential one-electron acceptor with a reactive semiquinone intermediate. In QBEB, the reduced low-potential acceptor (cytochrome b) is exclusively used to generate ΔμH. In FBEB, the "energy-rich" low-potential reduced ferredoxin or flavodoxin has dual function. It can give rise to ΔμH/Na via a ferredoxin:NAD reductase (Rnf) or ferredoxin:proton reductase (Ech) or conducts difficult reductions such as CO to CO. The QBEB membrane complexes are similar in structure and function and occur in all domains of life. In contrast, FBEB complexes are soluble and occur only in strictly anaerobic bacteria and archaea (FixABCX being an exception). The FBEB complexes constitute a group consisting of four unrelated families that contain (1) electron-transferring flavoproteins (EtfAB), (2) NAD(P)H dehydrogenase (NuoF homologues), (3) heterodisulfide reductase (HdrABC) or HdrABC homologues, and (4) NADH-dependent ferredoxin:NADP reductase (NfnAB). The crystal structures and electron transport of EtfAB-butyryl-CoA dehydrogenase and NfnAB are compared with those of complex III of the respiratory chain (cytochrome bc), whereby unexpected common features have become apparent.
有两种类型的电子分支（EB），醌或黄素依赖型（QBEB/FBEB），涉及通过双电子转移还原醌或黄素，并通过高和低电位单电子受体的两次再氧化，具有反应性半醌中间体。在 QBEB 中，还原的低电位受体（细胞色素 b）专门用于产生 ΔμH。在 FBEB 中，“能量丰富”的低电位还原型铁氧还蛋白或黄素蛋白具有双重功能。它可以通过铁氧还蛋白：NAD 还原酶（Rnf）或铁氧还蛋白：质子还原酶（Ech）产生 ΔμH/Na，或者进行困难的还原，例如 CO 到 CO。QBEB 膜复合物在结构和功能上相似，存在于所有生命领域。相比之下，FBEB 复合物是可溶性的，仅存在于严格厌氧细菌和古细菌中（FixABCX 是一个例外）。FBEB 复合物构成了一个由四个不相关家族组成的群体，包含（1）电子转移黄素蛋白（EtfAB），（2）NAD(P)H 脱氢酶（NuoF 同源物），（3）异二硫键还原酶（HdrABC）或 HdrABC 同源物，以及（4）NADH 依赖性铁氧还蛋白：NADP 还原酶（NfnAB）。EtfAB-丁酰辅酶 A 脱氢酶和 NfnAB 的晶体结构和电子传递与呼吸链（细胞色素 bc）的 III 复合物进行了比较，其中出现了一些意想不到的共同特征。
