Lancaster C R, Kröger A
Max-Planck-Institut für Biophysik, Abteilung Molekulare Membranbiologie, Frankfurt am Main, Germany.
Biochim Biophys Acta. 2000 Aug 15;1459(2-3):422-31. doi: 10.1016/s0005-2728(00)00180-8.
Membrane-bound succinate dehydrogenases (succinate:quinone reductases, SQR) and fumarate reductases (quinol:fumarate reductases, QFR) couple the oxidation of succinate to fumarate to the reduction of quinone to quinol and also catalyse the reverse reaction. SQR (respiratory complex II) is involved in aerobic metabolism as part of the citric acid cycle and of the aerobic respiratory chain. QFR is involved in anaerobic respiration with fumarate as the terminal electron acceptor, and is part of an electron transport chain catalysing the oxidation of various donor substrates by fumarate. QFR and SQR complexes are collectively referred to as succinate:quinone oxidoreductases (EC 1.3.5.1), have very similar compositions and are predicted to share similar structures. The complexes consist of two hydrophilic and one or two hydrophobic, membrane-integrated subunits. The larger hydrophilic subunit A carries covalently bound flavin adenine dinucleotide and subunit B contains three iron-sulphur centres. QFR of Wolinella succinogenes and SQR of Bacillus subtilis contain only one hydrophobic subunit (C) with two haem b groups. In contrast, SQR and QFR of Escherichia coli contain two hydrophobic subunits (C and D) which bind either one (SQR) or no haem b group (QFR). The structure of W. succinogenes QFR has been determined at 2.2 A resolution by X-ray crystallography (C.R.D. Lancaster, A. Kröger, M. Auer, H. Michel, Nature 402 (1999) 377-385). Based on this structure of the three protein subunits and the arrangement of the six prosthetic groups, a pathway of electron transfer from the quinol-oxidising dihaem cytochrome b to the site of fumarate reduction and a mechanism of fumarate reduction was proposed. The W. succinogenes QFR structure is different from that of the haem-less QFR of E. coli, described at 3.3 A resolution (T.M. Iverson, C. Luna-Chavez, G. Cecchini, D.C. Rees, Science 284 (1999) 1961-1966), mainly with respect to the structure of the membrane-embedded subunits and the relative orientations of soluble and membrane-embedded subunits. Also, similarities and differences between QFR transmembrane helix IV and transmembrane helix F of bacteriorhodopsin and their implications are discussed.
膜结合琥珀酸脱氢酶（琥珀酸：醌还原酶，SQR）和延胡索酸还原酶（醌醇：延胡索酸还原酶，QFR）将琥珀酸氧化为延胡索酸与醌还原为醌醇偶联起来，并且也催化逆反应。SQR（呼吸复合体II）作为柠檬酸循环和有氧呼吸链的一部分参与有氧代谢。QFR以延胡索酸作为末端电子受体参与无氧呼吸，并且是一个电子传递链的一部分，该电子传递链催化各种供体底物被延胡索酸氧化。QFR和SQR复合体统称为琥珀酸：醌氧化还原酶（EC 1.3.5.1），具有非常相似的组成，并且预计具有相似的结构。这些复合体由两个亲水性亚基和一个或两个疏水性的膜整合亚基组成。较大的亲水性亚基A共价结合黄素腺嘌呤二核苷酸，亚基B含有三个铁硫中心。产琥珀酸沃林氏菌的QFR和枯草芽孢杆菌的SQR仅含有一个带有两个血红素b基团的疏水性亚基（C）。相比之下，大肠杆菌的SQR和QFR含有两个疏水性亚基（C和D），它们结合一个血红素b基团（SQR）或不结合血红素b基团（QFR）。产琥珀酸沃林氏菌QFR的结构已通过X射线晶体学在2.2 Å分辨率下确定（C.R.D. Lancaster，A. Kröger，M. Auer，H. Michel，《自然》402 (1999) 377 - 385）。基于这三个蛋白质亚基的结构以及六个辅基的排列，提出了从醌醇氧化双血红素细胞色素b到延胡索酸还原位点的电子传递途径以及延胡索酸还原机制。产琥珀酸沃林氏菌QFR的结构与大肠杆菌无血红素QFR的结构不同，后者在3.3 Å分辨率下被描述（T.M. Iverson，C. Luna-Chavez，G. Cecchini，D.C. Rees，《科学》284 (1999) 1961 - 1966），主要在膜嵌入亚基的结构以及可溶性和膜嵌入亚基的相对取向上。此外，还讨论了细菌视紫红质的QFR跨膜螺旋IV和跨膜螺旋F之间的异同及其意义。
