Available online at www.sciencedirect.com Respiratory complex I: ‘steam engine’ of the cell? Rouslan G Efremov 1 and Leonid A Sazanov Complex I is the first enzyme of the respiratory chain and plays a central role in cellular energy production. It has been implicated in many human neurodegenerative diseases, as well as in ageing. One of the biggest membrane protein complexes, it is an L-shaped assembly consisting of hydrophilic and membrane domains. Previously, we have determined structures of the hydrophilic domain in several redox states. Last year was marked by fascinating breakthroughs in the understanding of the complete structure. We described the architecture of the membrane domain and of the entire bacterial complex I. X-ray analysis of the larger mitochondrial enzyme has also been published. The core subunits of the bacterial and mitochondrial enzymes have remarkably similar structures. The proposed mechanism of coupling between electron transfer and proton translocation involves long-range conformational changes, coordinated in part by a long a-helix, akin to the coupling rod of a steam engine. Address Medical Research Council Mitochondrial Biology Unit, Wellcome Trust/ MRC Building, Hills Road, Cambridge CB2 0XY, UK Corresponding author: Sazanov, Leonid A (sazanov@mrc-mbu.cam.ac.uk) 1 Present address: Max-Planck-Institute for Molecular Physiology, Otto-Hahn Str. 11, Dortmund 44227, Germany. Current Opinion in Structural Biology 2011, 21:532–540 This review comes from a themed issue on Membranes Edited by Gebhard Schertler and Robert Stroud Available online 8th August 2011 0959-440X/$ – see front matter # 2011 Elsevier Ltd. All rights reserved. DOI 10.1016/j.sbi.2011.07.002 Introduction Complex I (NADH:ubiquinone oxidoreductase) is the first enzyme of the respiratory chain in mitochondria and many bacteria. It catalyses the transfer of two electrons from NADH to quinone, coupled to the translocation of four protons (current consensus value [1–3]) across the membrane. In doing so, it provides about 40% of the proton flux during proton-motive force (pmf) generation for the synthesis of ATP [4–9]. Mutations in complex I subunits, including most common pathological mtDNA mutations, have been associated with human neurode- generative diseases [8,10]. Complex I is a major source of reactive oxygen species (ROS) in mitochondria, which can damage mtDNA and lead to sporadic Parkinson’s disease [11] and possibly aging [12]. Mitochondrial com- plex I consists of 45 subunits (980 kDa combined mass) [13]. The simpler prokaryotic enzyme normally consists of 14 ‘core’ subunits (seven hydrophilic and seven hydro- phobic, 550 kDa combined mass), all conserved from bacteria to humans [4,5,8]. The mitochondrial and bac- terial enzymes contain the same redox components (eight to nine iron–sulphur (Fe–S) clusters and flavin mononucleotide (FMN)) and have a similar L-shaped structure [5,14  ]. The hydrophobic arm is embedded in the membrane and the hydrophilic peripheral arm pro- trudes into the mitochondrial matrix or the bacterial cytoplasm [5,8]. High sequence conservation of core subunits indicates that the mechanism is likely to be the same throughout all species, and so the bacterial enzyme represents a ‘minimal’ model of human mito- chondrial complex I. The hydrophilic domain contains all the known redox cofactors of complex I, involved in electron transfer from NADH to quinone. Understanding how this process is coupled to the translocation of protons across the mem- brane remains the major question in complex I research [4– 6,8]. Two possible mechanisms of coupling have been proposed: ‘direct’ (redox-driven) and ‘indirect’ (confor- mation-driven) [5,6,8,15]. The membrane-spanning part of the enzyme contains the proton translocation machinery but lacks any covalently bound prosthetic groups. This is akin to F-ATPase (which operates by conformational coupling [16]) and is in contrast to cytochrome c oxidase (direct coupling involving heme cofactors [17]). The three largest hydrophobic subunits of complex I, NuoL, M and N (Escherichia coli nomenclature; subunit names differ be- tween species), are homologous to each other and to Na + / H + antiporter complex (Mrp) subunits [18,19]. They are likely to participate in proton translocation, but reside at a large distance from the hydrophilic domain [20]. A range of cross-linking [21–23] and proteolysis [24] studies suggested conformational changes upon reduction of com- plex I. All these facts indicate that the coupling mechanism involves long-range conformational changes. Complex I has for many years resisted attempts to deter- mine its structure and is considered as one of the most difficult membrane protein targets. The paucity of struc- tural data until 2006 has hindered progress in understand- ing its mechanism. Though the complete atomic structure of this large molecular machine is still unknown, the last two years were marked by major breakthroughs in crystallographic studies of the enzyme, which will be the main subject of current review. Current Opinion in Structural Biology 2011, 21:532–540 www.sciencedirect.com