-->H+/2e- stoichiometry in NADH-quinone reductase reactions catalyzed by bovine heart submitochondrial particles

Essential regions in the membrane domain of bacterial complex I (NDH-1): the machinery for proton translocation

The proton-translocating NADH-quinone oxidoreductase (complex I/NDH-1) is the first and largest enzyme of the respiratory chain which has a central role in cellular energy production and is implicated in many human neurodegenerative diseases and aging. It is believed that the peripheral domain of complex I/NDH-1 transfers the electron from NADH to Quinone (Q) and the redox energy couples the proton translocation in the membrane domain. To investigate the mechanism of the proton translocation, in a series of works we have systematically studied all membrane subunits in the Escherichia coli NDH-1 by site-directed mutagenesis. In this mini-review, we have summarized our strategy and results of the mutagenesis by depicting residues essential for proton translocation, along with those for subunit connection. It is suggested that clues to understanding the driving forces of proton translocation lie in the similarities and differences of the membrane subunits, highlighting the communication of essential charged residues among the subunits. A possible proton translocation mechanism with all membrane subunits operating in unison is described.

The mechanism of coupling between electron transfer and proton translocation in respiratory complex I

NADH-ubiquinone oxidoreductase (complex I) is the first and largest enzyme in the respiratory chain of mitochondria and many bacteria. It couples the transfer of two electrons between NADH and ubiquinone to the translocation of four protons across the membrane. Complex I is an L-shaped assembly formed by the hydrophilic (peripheral) arm, containing all the redox centres performing electron transfer and the membrane arm, containing proton-translocating machinery. Mitochondrial complex I consists of 44 subunits of about 1 MDa in total, whilst the prokaryotic enzyme is simpler and generally consists of 14 conserved "core" subunits. Recently we have determined the first atomic structure of the entire complex I, using the enzyme from Thermus thermophilus (536 kDa, 16 subunits, 9 Fe-S clusters, 64 TM helices). Structure suggests a unique coupling mechanism, with redox energy of electron transfer driving proton translocation via long-range (up to ~200 Å) conformational changes. It resembles a steam engine, with coupling elements (akin to coupling rods) linking parts of this molecular machine.

A long road towards the structure of respiratory complex I, a giant molecular proton pump

Complex I (NADH:ubiquinone oxidoreductase) is central to cellular energy production, being the first and largest enzyme of the respiratory chain in mitochondria. It couples electron transfer from NADH to ubiquinone with proton translocation across the inner mitochondrial membrane and is involved in a wide range of human neurodegenerative disorders. Mammalian complex I is composed of 44 different subunits, whereas the 'minimal' bacterial version contains 14 highly conserved 'core' subunits. The L-shaped assembly consists of hydrophilic and membrane domains. We have determined all known atomic structures of complex I, starting from the hydrophilic domain of Thermus thermophilus enzyme (eight subunits, nine Fe-S clusters), followed by the membrane domains of the Escherichia coli (six subunits, 55 transmembrane helices) and T. thermophilus (seven subunits, 64 transmembrane helices) enzymes, and finally culminating in a recent crystal structure of the entire intact complex I from T. thermophilus (536 kDa, 16 subunits, nine Fe-S clusters, 64 transmembrane helices). The structure suggests an unusual and unique coupling mechanism via long-range conformational changes. Determination of the structure of the entire complex was possible only through this step-by-step approach, building on from smaller subcomplexes towards the entire assembly. Large membrane proteins are notoriously difficult to crystallize, and so various non-standard and sometimes counterintuitive approaches were employed in order to achieve crystal diffraction to high resolution and solve the structures. These steps, as well as the implications from the final structure, are discussed in the present review.

A single amino acid residue controls ROS production in the respiratory Complex I from Escherichia coli

Reactive oxygen species (ROS) production by respiratory Complex I from Escherichia coli was studied in bacterial membrane fragments and in the isolated and purified enzyme, either solubilized or incorporated in proteoliposomes. We found that the replacement of a single amino acid residue in close proximity to the nicotinamide adenine dinucleotide (NADH)-binding catalytic site (E95 in the NuoF subunit) dramatically increases the reactivity of Complex I towards dioxygen (O2 ). In the E95Q variant short-chain ubiquinones exhibit strong artificial one-electron reduction at the catalytic site, also leading to a stronger increase in ROS production. Two mechanisms can contribute to the observed kinetic effects: (a) a change in the reactivity of flavin mononucleotide (FMN) towards dioxygen at the catalytic site, and (b) a change in the population of the ROS-generating state. We propose the existence of two (closed and open) states of the NAD(+) -bound enzyme as one feature of the substrate-binding site of Complex I. The analysis of the kinetic model of ROS production allowed us to propose that the population of Complex I with reduced FMN is always low in the wild-type enzyme even at low ambient redox potentials, minimizing the rate of reaction with O2 in contrast to E95Q variant.

NDUFAF7 methylates arginine 85 in the NDUFS2 subunit of human complex I

Complex I (NADH ubiquinone oxidoreductase) in mammalian mitochondria is an L-shaped assembly of 44 subunits. One arm is embedded in the inner membrane with the other protruding ∼100 Å into the matrix of the organelle. The extrinsic arm contains binding sites for NADH and the primary electron acceptor FMN, and it provides a scaffold for seven iron-sulfur clusters that form an electron pathway linking FMN to the terminal electron acceptor, ubiquinone, which is bound in the region of the junction between the arms. The membrane arm contains four antiporter-like domains, probably energetically coupled to the quinone site and involved in pumping protons from the matrix into the intermembrane space contributing to the proton motive force. Complex I is put together from preassembled subcomplexes. Their compositions have been characterized partially, and at least 12 extrinsic assembly factor proteins are required for the assembly of the complex. One such factor, NDUFAF7, is predicted to belong to the family of S-adenosylmethionine-dependent methyltransferases characterized by the presence in their structures of a seven-β-strand protein fold. In the present study, the presence of NDUFAF7 in the mitochondrial matrix has been confirmed, and it has been demonstrated that it is a protein methylase that symmetrically dimethylates the ω-N^G,N^G′ atoms of residue Arg-85 in the NDUFS2 subunit of complex I. This methylation step occurs early in the assembly of complex I and probably stabilizes a 400-kDa subcomplex that forms the initial nucleus of the peripheral arm and its juncture with the membrane arm.

Energy transducing roles of antiporter-like subunits in Escherichia coli NDH-1 with main focus on subunit NuoN (ND2)

The proton-translocating NADH-quinone oxidoreductase (complex I/NDH-1) contains a peripheral and a membrane domain. Three antiporter-like subunits in the membrane domain, NuoL, NuoM, and NuoN (ND5, ND4 and ND2, respectively), are structurally similar. We analyzed the role of NuoN in Escherichia coli NDH-1. The lysine residue at position 395 in NuoN (NLys(395)) is conserved in NuoL (LLys(399)) but is replaced by glutamic acid (MGlu(407)) in NuoM. Our mutation study on NLys(395) suggests that this residue participates in the proton translocation. Furthermore, we found that MGlu(407) is also essential and most likely interacts with conserved LArg(175). Glutamic acids, NGlu(133), MGlu(144), and LGlu(144), are corresponding residues. Unlike mutants of MGlu(144) and LGlu(144), mutation of NGlu(133) scarcely affected the energy-transducing activities. However, a double mutant of NGlu(133) and nearby KGlu(72) showed significant inhibition of these activities. This suggests that NGlu(133) bears a functional role similar to LGlu(144) and MGlu(144) but its mutation can be partially compensated by the nearby carboxyl residue. Conserved prolines located at loops of discontinuous transmembrane helices of NuoL, NuoM, and NuoN were shown to play a similar role in the energy-transducing activity. It seems likely that NuoL, NuoM, and NuoN pump protons by a similar mechanism. Our data also revealed that NLys(158) is one of the key interaction points with helix HL in NuoL. A truncation study indicated that the C-terminal amphipathic segments of NTM14 interacts with the Mβ sheet located on the opposite side of helix HL. Taken together, the mechanism of H(+) translocation in NDH-1 is discussed.

Post-translational modifications near the quinone binding site of mammalian complex I

Complex I (NADH:ubiquinone oxidoreductase) in mammalian mitochondria is an L-shaped assembly of 44 protein subunits with one arm buried in the inner membrane of the mitochondrion and the orthogonal arm protruding about 100 Å into the matrix. The protruding arm contains the binding sites for NADH, the primary acceptor of electrons flavin mononucleotide (FMN), and a chain of seven iron-sulfur clusters that carries the electrons one at a time from FMN to a coenzyme Q molecule bound in the vicinity of the junction between the two arms. In the structure of the closely related bacterial enzyme from Thermus thermophilus, the quinone is thought to bind in a tunnel that spans the interface between the two arms, with the quinone head group close to the terminal iron-sulfur cluster, N2. The tail of the bound quinone is thought to extend from the tunnel into the lipid bilayer. In the mammalian enzyme, it is likely that this tunnel involves three of the subunits of the complex, ND1, PSST, and the 49-kDa subunit. An arginine residue in the 49-kDa subunit is symmetrically dimethylated on the ω-N^G and ω-N^G′ nitrogen atoms of the guanidino group and is likely to be close to cluster N2 and to influence its properties. Another arginine residue in the PSST subunit is hydroxylated and probably lies near to the quinone. Both modifications are conserved in mammalian enzymes, and the former is additionally conserved in Pichia pastoris and Paracoccus denitrificans, suggesting that they are functionally significant.

The optic nerve: a "mito-window" on mitochondrial neurodegeneration

Retinal ganglion cells (RGCs) project their long axons, composing the optic nerve, to the brain, transmitting the visual information gathered by the retina, ultimately leading to formed vision in the visual cortex. The RGC cellular system, representing the anterior part of the visual pathway, is vulnerable to mitochondrial dysfunction and optic atrophy is a very frequent feature of mitochondrial and neurodegenerative diseases. The start of the molecular era of mitochondrial medicine, the year 1988, was marked by the identification of a maternally inherited form of optic atrophy, Leber's hereditary optic neuropathy, as the first disease due to mitochondrial DNA point mutations. The field of mitochondrial medicine has expanded enormously over the last two decades and many neurodegenerative diseases are now known to have a primary mitochondrial etiology or mitochondrial dysfunction plays a relevant role in their pathogenic mechanism. Recent technical advancements in neuro-ophthalmology, such as optical coherence tomography, prompted a still ongoing systematic re-investigation of retinal and optic nerve involvement in neurodegenerative disorders. In addition to inherited optic neuropathies, such as Leber's hereditary optic neuropathy and dominant optic atrophy, and in addition to the syndromic mitochondrial encephalomyopathies or mitochondrial neurodegenerative disorders such as some spinocerebellar ataxias or familial spastic paraparesis and other disorders, we draw attention to the involvement of the optic nerve in classic age-related neurodegenerative disorders such as Parkinson and Alzheimer disease. We here provide an overview of optic nerve pathology in these different clinical settings, and we review the possible mechanisms involved in the pathogenesis of optic atrophy. This may be a model of general value for the field of neurodegeneration. This article is part of a Special Issue entitled 'Mitochondrial function and dysfunction in neurodegeneration'.

Mitochondrial complex I

Complex I (NADH:ubiquinone oxidoreductase) is crucial for respiration in many aerobic organisms. In mitochondria, it oxidizes NADH from the tricarboxylic acid cycle and β-oxidation, reduces ubiquinone, and transports protons across the inner membrane, contributing to the proton-motive force. It is also a major contributor to cellular production of reactive oxygen species. The redox reaction of complex I is catalyzed in the hydrophilic domain; it comprises NADH oxidation by a flavin mononucleotide, intramolecular electron transfer along a chain of iron-sulfur clusters, and ubiquinone reduction. Redox-coupled proton translocation in the membrane domain requires long-range energy transfer through the protein complex, and the molecular mechanisms that couple the redox and proton-transfer half-reactions are currently unknown. This review evaluates extant data on the mechanisms of energy transduction and superoxide production by complex I, discusses contemporary mechanistic models, and explores how mechanistic studies may contribute to understanding the roles of complex I dysfunctions in human diseases.

Structural characterization of HP1264 reveals a novel fold for the flavin mononucleotide binding protein

Complex I (NADH-quinone oxidoreductase) is an enzyme that catalyzes the initial electron transfer from nicotinamide adenine dinucleotide (NADH) to flavin mononucleotide (FMN) bound at the tip of the hydrophilic domain of complex I. The electron flow into complex I is coupled to the generation of a proton gradient across the membrane that is essential for the synthesis of ATP. However, Helicobacter pylori has an unusual complex I that lacks typical NQO1 and NQO2 subunits, both of which are generally included in the NADH dehydrogenase domain of complex I. Here, we determined the solution structure of HP1264, one of the unusual subunits of complex I from H. pylori, which is located in place of NQO2, by three-dimensional nuclear magnetic resonance (NMR) spectroscopy and revealed that HP1264 can bind to FMN through UV-visible, fluorescence, and NMR titration experiments. This result suggests that FMN-bound HP1264 could be involved in the initial electron transfer step of complex I. In addition, HP1264 is structurally most similar to Escherichia coli TusA, which belongs to the SirA-like superfamily having an IF3-like fold in the SCOP database, implying that HP1264 adopts a novel fold for FMN binding. On the basis of the NMR titration data, we propose the candidate residues Ile32, Met34, Leu58, Trp68, and Val71 of HP1264 for the interaction with FMN. Notably, these residues are not conserved in the FMN binding site of any other flavoproteins with known structure. This study of the relationship between the structure and FMN binding property of HP1264 will contribute to improving our understanding of flavoprotein structure and the electron transfer mechanism of complex I.

Crystal structure of the entire respiratory complex I

Complex I is the first and largest enzyme of the respiratory chain and has a central role in cellular energy production through the coupling of NADH:ubiquinone electron transfer to proton translocation. It is also implicated in many common human neurodegenerative diseases. Here, we report the first crystal structure of the entire, intact complex I (from Thermus thermophilus) at 3.3 Å resolution. The structure of the 536-kDa complex comprises 16 different subunits, with a total of 64 transmembrane helices and 9 iron-sulphur clusters. The core fold of subunit Nqo8 (ND1 in humans) is, unexpectedly, similar to a half-channel of the antiporter-like subunits. Small subunits nearby form a linked second half-channel, which completes the fourth proton-translocation pathway (present in addition to the channels in three antiporter-like subunits). The quinone-binding site is unusually long, narrow and enclosed. The quinone headgroup binds at the deep end of this chamber, near iron-sulphur cluster N2. Notably, the chamber is linked to the fourth channel by a 'funnel' of charged residues. The link continues over the entire membrane domain as a flexible central axis of charged and polar residues, and probably has a leading role in the propagation of conformational changes, aided by coupling elements. The structure suggests that a unique, out-of-the-membrane quinone-reaction chamber enables the redox energy to drive concerted long-range conformational changes in the four antiporter-like domains, resulting in translocation of four protons per cycle.

Pyridine nucleotide regulation of cardiac intermediary metabolism

The pyridine nucleotides NAD(+) and NADP(+) play a pivotal role in regulating intermediary metabolism in the heart. The intracellular NAD(+)/NADH ratio controls flux through various dehydrogenase enzymes involved in both anaerobic and aerobic metabolism and also regulates posttranslational protein modification. The intracellular NADP(+)/NADPH ratio controls flux through the pentose phosphate pathway (PPP) and the polyol pathway, while also regulating ion channel function and oxidative stress. Not only does the NAD(+)/NADH ratio regulate the rates of ATP production, it can also modify energy substrate preference. For instance, in many forms of heart disease a greater contribution from fatty acids for oxidative energy metabolism increases fatty acid β-oxidation-derived NADH, which can activate pyruvate dehydrogenase (PDH) kinase isoforms that inhibit PDH and subsequent glucose oxidation. As such, novel therapies that overcome fatty acid β-oxidation-induced inhibition of PDH improve cardiac efficiency and subsequent function during ischemia/reperfusion and in heart failure. Furthermore, recent studies have implicated a pivotal role for increased PPP-derived NADPH in mediating oxidative stress observed in heart failure. In this article, we review the multiple actions of NAD(+)/NADH and NADP(+)/NADPH in regulating intermediary metabolism in the heart. A better understanding of the roles of NAD(+)/NADH and NADP(+)/NADPH in cellular physiology and pathology could potentially be used to exploit pyridine nucleotide modification in the treatment of a number of different forms of heart disease.

Roles of subunit NuoK (ND4L) in the energy-transducing mechanism of Escherichia coli NDH-1 (NADH:quinone oxidoreductase)

The bacterial H(+)-translocating NADH:quinone oxidoreductase (NDH-1) catalyzes electron transfer from NADH to quinone coupled with proton pumping across the cytoplasmic membrane. The NuoK subunit (counterpart of the mitochondrial ND4L subunit) is one of the seven hydrophobic subunits in the membrane domain and bears three transmembrane segments (TM1-3). Two glutamic residues located in the adjacent transmembrane helices of NuoK are important for the energy coupled activity of NDH-1. In particular, mutation of the highly conserved carboxyl residue ((K)Glu-36 in TM2) to Ala led to a complete loss of the NDH-1 activities. Mutation of the second conserved carboxyl residue ((K)Glu-72 in TM3) moderately reduced the activities. To clarify the contribution of NuoK to the mechanism of proton translocation, we relocated these two conserved residues. When we shifted (K)Glu-36 along TM2 to positions 32, 38, 39, and 40, the mutants largely retained energy transducing NDH-1 activities. According to the recent structural information, these positions are located in the vicinity of (K)Glu-36, present in the same helix phase, in an immediately before and after helix turn. In an earlier study, a double mutation of two arginine residues located in a short cytoplasmic loop between TM1 and TM2 (loop-1) showed a drastic effect on energy transducing activities. Therefore, the importance of this cytosolic loop of NuoK ((K)Arg-25, (K)Arg-26, and (K)Asn-27) for the energy transducing activities was extensively studied. The probable roles of subunit NuoK in the energy transducing mechanism of NDH-1 are discussed.

A missing link between complex I and group 4 membrane-bound [NiFe] hydrogenases

Complex I of respiratory chains is an energy transducing enzyme present in most bacteria, mitochondria and chloroplasts. It catalyzes the oxidation of NADH and the reduction of quinones, coupled to cation translocation across the membrane. The complex has a modular structure composed of several proteins most of which are identified in other complexes. Close relations between complex I and group 4 membrane-bound [NiFe] hydrogenases and some subunits of multiple resistance to pH (Mrp) Na(+)/H(+) antiporters have been observed before and the suggestion that complex I arose from the association of a soluble nicotinamide adenine dinucleotide (NAD(+)) reducing hydrogenase with a Mrp-like antiporter has been put forward. In this article we performed a thorough taxonomic profile of prokaryotic group 4 membrane-bound [NiFe] hydrogenases, complexes I and complex I-like enzymes. In addition we have investigated the different gene clustering organizations of such complexes. Our data show the presence of complexes related to hydrogenases but which do not contain the binding site of the catalytic centre. These complexes, named before as Ehr (energy-converting hydrogenases related complexes) are a missing link between complex I and group 4 membrane-bound [NiFe] hydrogenases. Based on our observations we put forward a different perspective for the relation between complex I and related complexes. In addition we discuss the evolutionary, functional and mechanistic implications of this new perspective. This article is part of a Special Issue entitled: The evolutionary aspects of bioenergetic systems.

Energy-converting respiratory Complex I: on the way to the molecular mechanism of the proton pump

In respiring organisms the major energy transduction flux employs the transmembrane electrochemical proton gradient as a physical link between exergonic redox reactions and endergonic ADP phosphorylation. Establishing the gradient involves electrogenic, transmembrane H(+) translocation by the membrane-embedded respiratory complexes. Among others, Complex I (NADH:ubiquinone oxidoreductase) is the most structurally complex and functionally enigmatic respiratory enzyme; its molecular mechanism is as yet unknown. Here we highlight recent progress and discuss the catalytic events during Complex I turnover in relation to their role in energy conversion and to the enzyme structure.

Uncoupling of substrate-level phosphorylation in Escherichia coli during glucose-limited growth

The respiratory chain of Escherichia coli contains three different cytochrome oxidases. Whereas the cytochrome bo oxidase and the cytochrome bd-I oxidase are well characterized and have been shown to contribute to proton translocation, physiological data suggested a nonelectrogenic functioning of the cytochrome bd-II oxidase. Recently, however, this view was challenged by an in vitro biochemical analysis that showed that the activity of cytochrome bd-II oxidase does contribute to proton translocation with an H(+)/e(-) stoichiometry of 1. Here, we propose that this apparent discrepancy is due to the activities of two alternative catabolic pathways: the pyruvate oxidase pathway for acetate production and a pathway with methylglyoxal as an intermediate for the production of lactate. The ATP yields of these pathways are lower than those of the pathways that have so far always been assumed to catalyze the main catabolic flux under energy-limited growth conditions (i.e., pyruvate dehydrogenase and lactate dehydrogenase). Inclusion of these alternative pathways in the flux analysis of growing E. coli strains for the calculation of the catabolic ATP synthesis rate indicates an electrogenic function of the cytochrome bd-II oxidase, compatible with an H(+)/e(-) ratio of 1. This analysis shows for the first time the extent of bypassing of substrate-level phosphorylation in E. coli under energy-limited growth conditions.

The role of proton and sodium ions in energy transduction by respiratory complex I

Respiratory complex I plays a central role in energy transduction. It catalyzes the oxidation of NADH and the reduction of quinone, coupled to cation translocation across the membrane, thereby establishing an electrochemical potential. For more than half a century, data on complex I has been gathered, including recently determined crystal structures, yet complex I is the least understood complex of the respiratory chain. The mechanisms of quinone reduction, charge translocation and their coupling are still unknown. The H(+) is accepted to be the coupling ion of the system; however, Na(+) has also been suggested to perform such a role. In this article, we address the relation of those two ions with complex I and refer ion pump and Na(+)/H(+) antiporter as possible transport mechanisms of the system. We put forward a hypothesis to explain some apparently contradictory data on the nature of the coupling ion, and we revisit the role of H(+) and Na(+) cycles in the overall bioenergetics of the cell.

Study of ion translocation by respiratory complex I. A new insight using (23)Na NMR spectroscopy

The research on complex I has gained recently a new enthusiasm, especially after the resolution of the crystallographic structures of bacterial and mitochondrial complexes. Most attention is now dedicated to the investigation of the energy coupling mechanism(s). The proton has been identified as the coupling ion, although in the case of some bacterial complexes I Na(+) has been proposed to have that role. We have addressed the relation of some complexes I with Na(+) and developed an innovative methodology using (23)Na NMR spectroscopy. This allowed the investigation of Na(+) transport taking the advantage of directly monitoring changes in Na(+) concentration. Methodological aspects concerning the use of (23)Na NMR spectroscopy to measure accurately sodium transport in bacterial membrane vesicles are discussed here. External-vesicle Na(+) concentrations were determined by two different methods: 1) by integration of the resonance frequency peak and 2) using calibration curves of resonance frequency shift dependence on Na(+) concentration. Although the calibration curves are a suitable way to determine Na(+) concentration changes under conditions of fast exchange, it was shown not to be applicable to the bacterial membrane vesicle systems. In this case, the integration of the resonance frequency peak is the most appropriate analysis for the quantification of external-vesicle Na(+) concentration. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

Stoichiometry of proton translocation by respiratory complex I and its mechanistic implications

Complex I (NADH-ubiquinone oxidoreductase) in the respiratory chain of mitochondria and several bacteria functions as a redox-driven proton pump that contributes to the generation of the protonmotive force across the inner mitochondrial or bacterial membrane and thus to the aerobic synthesis of ATP. The stoichiometry of proton translocation is thought to be 4 H(+) per NADH oxidized (2 e(-)). Here we show that a H(+)/2 e(-) ratio of 3 appears more likely on the basis of the recently determined H(+)/ATP ratio of the mitochondrial F(1)F(o)-ATP synthase of animal mitochondria and of a set of carefully determined ATP/2 e(-) ratios for different segments of the mitochondrial respiratory chain. This lower H(+)/2 e(-) ratio of 3 is independently supported by thermodynamic analyses of experiments with both mitochondria and submitochondrial particles. A reduced H(+)/2 e(-) stoichiometry of 3 has important mechanistic implications for this proton pump. In a rough mechanistic model, we suggest a concerted proton translocation mechanism in the three homologous and tightly packed antiporter-like subunits L, M, and N of the proton-translocating membrane domain of complex I.

Molecular mechanisms of superoxide production by the mitochondrial respiratory chain

The mitochondrial respiratory chain is a major source of reactive oxygen species (ROS) in eukaryotic cells. Mitochondrial ROS production associated with a dysfunction of respiratory chain complexes has been implicated in a number of degenerative diseases and biological aging. Recent findings suggest that mitochondrial ROS can be integral components of cellular signal transduction as well. Within the respiratory chain, complexes I (NADH:ubiquinone oxidoreductase) and III (ubiquinol:cytochrome c oxidoreductase; cytochrome bc (1) complex) are generally considered as the main producers of superoxide anions that are released into the mitochondrial matrix and the intermembrane space, respectively. The primary function of both respiratory chain complexes is to employ energy supplied by redox reactions to drive the vectorial transfer of protons into the mitochondrial intermembrane space. This process involves a set of distinct electron carriers designed to minimize the unwanted leak of electrons from reduced cofactors onto molecular oxygen and hence ROS generation under normal circumstances. Nevertheless, it seems plausible that superoxide is derived from intermediates of the normal catalytic cycles of complexes I and III. Therefore, a detailed understanding of the molecular mechanisms driving these enzymes is required to understand mitochondrial ROS production during oxidative stress and redox signalling. This review summarizes recent findings on the chemistry and control of the reactions within respiratory complexes I and III that result in increased superoxide generation. Regulatory contributions of other components of the respiratory chain, especially complex II (succinate:ubiquinone oxidoreductase) and the redox state of the ubiquinone pool (Q-pool) will be briefly discussed.

Submitochondrial fragments of brain mitochondria: general characteristics and catalytic properties of NADH:ubiquinone oxidoreductase (complex I)

A number of genetic or drug-induced pathophysiological disorders, particularly neurodegenerative diseases, have been reported to correlate with catalytic impairments of NADH:ubiquinone oxidoreductase (mitochondrial complex I). The vast majority of the data on catalytic properties of this energy-transducing enzyme have been accumulated from studies on bovine heart complex I preparations of different degrees of resolution, whereas almost nothing is known about the functional activities of the enzyme in neuronal tissues. Here a procedure for preparation of coupled inside-out submitochondrial particles from brain is described and their NADH oxidase activity is characterized. The basic characteristics of brain complex I, particularly the parameters of A/D-transition are found to be essentially the same as those previously reported for heart enzyme. The results show that coupled submitochondrial particles prepared from either heart or brain can equally be used as a model system for in vitro studies aimed to delineate neurodegenerative-associated defects of complex I.

Probing the mechanistic role of the long α-helix in subunit L of respiratory Complex I from Escherichia coli by site-directed mutagenesis

The C-terminus of the NuoL subunit of Complex I includes a long amphipathic α-helix positioned parallel to the membrane, which has been considered to function as a piston in the proton pumping machinery. Here, we have introduced three types of mutations into the nuoL gene to test the piston-like function. First, NuoL was truncated at its C- and N-termini, which resulted in low production of a fragile Complex I with negligible activity. Second, we mutated three partially conserved residues of the amphipathic α-helix: Asp and Lys residues and a Pro were substituted for acidic, basic or neutral residues. All these variants exhibited almost a wild-type phenotype. Third, several substitutions and insertions were made to reduce rigidity of the amphipathic α-helix, and/or to change its geometry. Most insertions/substitutions resulted in a normal growth phenotype, albeit often with reduced stability of Complex I. In contrast, insertion of six to seven amino acids at a site of the long α-helix between NuoL and M resulted in substantial loss of proton pumping efficiency. The implications of these results for the proton pumping mechanism of Complex I are discussed.

Mammalian NADH:ubiquinone oxidoreductase (Complex I) and nicotinamide nucleotide transhydrogenase (Nnt) together regulate the mitochondrial production of H₂O₂--implications for their role in disease, especially cancer

Mammalian NADH:ubiquinone oxidoreductase (Complex I) in the mitochondrial inner membrane catalyzes the oxidation of NADH in the matrix. Excess NADH reduces nine of the ten prosthetic groups of the enzyme in bovine-heart submitochondrial particles with a rate of at least 3,300 s⁻¹. This results in an overall NADH→O₂ rate of ca. 150 s⁻¹. It has long been known that the bovine enzyme also has a specific reaction site for NADPH. At neutral pH excess NADPH reduces only three to four of the prosthetic groups in Complex I with a rate of 40 s⁻¹ at 22 °C. The reducing equivalents remain essentially locked in the enzyme because the overall NADPH→O₂ rate (1.4 s⁻¹) is negligible. The physiological significance of the reaction with NADPH is still unclear. A number of recent developments has revived our thinking about this enigma. We hypothesize that Complex I and the Δp-driven nicotinamide nucleotide transhydrogenase (Nnt) co-operate in an energy-dependent attenuation of the hydrogen-peroxide generation by Complex I. This co-operation is thought to be mediated by the NADPH/NADP⁺ ratio in the vicinity of the NADPH site of Complex I. It is proposed that the specific H₂O₂ production by Complex I, and the attenuation of it, is of importance for apoptosis, autophagy and the survival mechanism of a number of cancers. Verification of this hypothesis may contribute to a better understanding of the regulation of these processes.

Respiratory complex I: 'steam engine' of the cell?

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 α-helix, akin to the coupling rod of a steam engine.

Structure of the membrane domain of respiratory complex I

Complex I is the first and largest enzyme of the respiratory chain, coupling electron transfer between NADH and ubiquinone to the translocation of four protons across the membrane. It has a central role in cellular energy production and has been implicated in many human neurodegenerative diseases. The L-shaped enzyme consists of hydrophilic and membrane domains. Previously, we determined the structure of the hydrophilic domain. Here we report the crystal structure of the Esherichia coli complex I membrane domain at 3.0 Å resolution. It includes six subunits, NuoL, NuoM, NuoN, NuoA, NuoJ and NuoK, with 55 transmembrane helices. The fold of the homologous antiporter-like subunits L, M and N is novel, with two inverted structural repeats of five transmembrane helices arranged, unusually, face-to-back. Each repeat includes a discontinuous transmembrane helix and forms half of a channel across the membrane. A network of conserved polar residues connects the two half-channels, completing the proton translocation pathway. Unexpectedly, lysines rather than carboxylate residues act as the main elements of the proton pump in these subunits. The fourth probable proton-translocation channel is at the interface of subunits N, K, J and A. The structure indicates that proton translocation in complex I, uniquely, involves coordinated conformational changes in six symmetrical structural elements.

Decoupling of the catalytic and transport activities of complex I from Rhodothermus marinus by sodium/proton antiporter inhibitor

The energy transduction by complex I from Rhodothermusmarinus was addressed by studying the influence of 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) on the activities of this enzyme. EIPA is an inhibitor of both Na(+)/H(+) antiporter and complex I NADH:quinone oxidoreductase activity. We performed studies of NADH:quinone oxidoreductase and H(+) and Na(+) translocation activities of complex I from R. marinus at different concentrations of EIPA, using inside-out membrane vesicles. We observed that the oxidoreductase activity and both H(+) and Na(+) transports are inhibited by EIPA. Most interestingly, the catalytic and the two transport activities showed different inhibition profiles. The transports are inhibited at concentrations of EIPA at which the catalytic activity is not affected. In this way the catalytic and transport activities were decoupled. Moreover, the inhibition of the catalytic activity was not influenced by the presence of Na(+), whereas the transport of H(+) showed different inhibition behaviors in the presence and absence of Na(+). Taken together our observations indicate that complex I from R. marinus performs energy transduction by two different processes: proton pumping and Na(+)/H(+) antiporting. The decoupling of the catalytic and transport activities suggests the involvement of an indirect coupling mechanism, possibly through conformational changes.

A model of the proton translocation mechanism of complex I

Despite decades of speculation, the proton pumping mechanism of complex I (NADH-ubiquinone oxidoreductase) is unknown and continues to be controversial. Recent descriptions of the architecture of the hydrophobic region of complex I have resolved one vital issue: this region appears to have multiple proton transporters that are mechanically interlinked. Thus, transduction of conformational changes to drive the transmembrane transporters linked by a "connecting rod" during the reduction of ubiquinone (Q) can account for two or three of the four protons pumped per NADH oxidized. The remaining proton(s) must be pumped by direct coupling at the Q-binding site. Here, we present a mixed model based on a crucial constraint: the strong dependence on the pH gradient across the membrane (ΔpH) of superoxide generation at the Q-binding site of complex I. This model combines direct and indirect coupling mechanisms to account for the pumping of the four protons. It explains the observed properties of the semiquinone in the Q-binding site, the rapid superoxide production from this site during reverse electron transport, its low superoxide production during forward electron transport except in the presence of inhibitory Q-analogs and high protonmotive force, and the strong dependence of both modes of superoxide production on ΔpH.

Sodium influence on energy transduction by complexes I from Escherichia coli and Paracoccus denitrificans

The nature of the ions that are translocated by Escherichia coli and Paracoccus denitrificans complexes I was investigated. We observed that E. coli complex I was capable of proton translocation in the same direction to the established deltapsi, showing that in the tested conditions, the coupling ion is the H(+). Furthermore, Na(+) transport to the opposite direction was also observed, and, although Na(+) was not necessary for the catalytic or proton transport activities, its presence increased the latter. We also observed H(+) translocation by P. denitrificans complex I, but in this case, H(+) transport was not influenced by Na(+) and also Na(+) transport was not observed. We concluded that E. coli complex I has two energy coupling sites (one Na(+) independent and the other Na(+) dependent), as previously observed for Rhodothermus marinus complex I, whereas the coupling mechanism of P. denitrificans enzyme is completely Na(+) independent. This work thus shows that complex I energy transduction by proton pumping and Na(+)/H(+) antiporting is not exclusive of the R. marinus enzyme. Nevertheless, the Na(+)/H(+) antiport activity seems not to be a general property of complex I, which may be correlated with the metabolic characteristics of the organisms.

A cytochrome c fusion protein domain for convenient detection, quantification, and enhanced production of membrane proteins in Escherichia coli--expression and characterization of cytochrome-tagged Complex I subunits

Overproduction of membrane proteins can be a cumbersome task, particularly if high yields are desirable. NADH:quinone oxidoreductase (Complex I) contains several very large membrane-spanning protein subunits that hitherto have been impossible to express individually in any appreciable amounts in Escherichia coli. The polypeptides contain no prosthetic groups and are poorly antigenic, making optimization of protein production a challenging task. In this work, the C-terminal ends of the Complex I subunits NuoH, NuoL, NuoM, and NuoN from E. coli Complex I and the bona fide antiporters MrpA and MrpD were genetically fused to the cytochrome c domain of Bacillus subtilis cytochrome c(550). Compared with other available fusion-protein tagging systems, the cytochrome c has several advantages. The heme is covalently bound, renders the proteins visible by optical spectroscopy, and can be used to monitor, quantify, and determine the orientation of the polypeptides in a plethora of experiments. For the antiporter-like subunits NuoL, NuoM, and NuoN and the real antiporters MrpA and MrpD, unprecedented amounts of holo-cytochrome fusion proteins could be obtained in E. coli. The NuoHcyt polypeptide was also efficiently produced, but heme insertion was less effective in this construct. The cytochrome c(550) domain in all the fusion proteins exhibited normal spectra and redox properties, with an E(m) of about +170 mV. The MrpA and MrpD antiporters remained functional after being fused to the cytochrome c-tag. Finally, a his-tag could be added to the cytochrome domain, without any perturbations to the cytochrome properties, allowing efficient purification of the overexpressed fusion proteins.

Truncation of subunit ND2 disrupts the threefold symmetry of the antiporter-like subunits in complex I from higher metazoans

Three of the conserved, membrane-bound subunits in NADH:ubiquinone oxidoreductase (complex I) are related to one another, and to Mrp sodium-proton antiporters. Recent structural analysis of two prokaryotic complexes I revealed that the three subunits each contain fourteen transmembrane helices that overlay in structural alignments: the translocation of three protons may be coordinated by a lateral helix connecting them together (Efremov, R.G., Baradaran, R. and Sazanov, L.A. (2010). The architecture of respiratory complex I. Nature 465, 441-447). Here, we show that in higher metazoans the threefold symmetry is broken by the loss of three helices from subunit ND2; possible implications for the mechanism of proton translocation are discussed.

A new hypothesis on the simultaneous direct and indirect proton pump mechanisms in NADH-quinone oxidoreductase (complex I)

Recently, Sazanov's group reported the X-ray structure of whole complex I [Nature, 465, 441 (2010)], which presented a strong clue for a "piston-like" structure as a key element in an "indirect" proton pump. We have studied the NuoL subunit which has a high sequence similarity to Na(+)/H(+) antiporters, as do the NuoM and N subunits. We constructed 27 site-directed NuoL mutants. Our data suggest that the H(+)/e(-) stoichiometry seems to have decreased from (4H(+)/2e(-)) in the wild-type to approximately (3H(+)/2e(-)) in NuoL mutants. We propose a revised hypothesis that each of the "direct" and the "indirect" proton pumps transports 2H(+) per 2e(-).

The membrane subunit NuoL(ND5) is involved in the indirect proton pumping mechanism of Escherichia coli complex I

Complex I pumps protons across the membrane by using downhill redox energy. Here, to investigate the proton pumping mechanism by complex I, we focused on the largest transmembrane subunit NuoL (Escherichia coli ND5 homolog). NuoL/ND5 is believed to have H(+) translocation site(s), because of a high sequence similarity to multi-subunit Na(+)/H(+) antiporters. We mutated thirteen highly conserved residues between NuoL/ND5 and MrpA of Na(+)/H(+) antiporters in the chromosomal nuoL gene. The dNADH oxidase activities in mutant membranes were mostly at the control level or modestly reduced, except mutants of Glu-144, Lys-229, and Lys-399. In contrast, the peripheral dNADH-K(3)Fe(CN)(6) reductase activities basically remained unchanged in all the NuoL mutants, suggesting that the peripheral arm of complex I was not affected by point mutations in NuoL. The proton pumping efficiency (the ratio of H(+)/e(-)), however, was decreased in most NuoL mutants by 30-50%, while the IC(50) values for asimicin (a potent complex I inhibitor) remained unchanged. This suggests that the H(+)/e(-) stoichiometry has changed from 4H(+)/2e(-) to 3H(+) or 2H(+)/2e(-) without affecting the direct coupling site. Furthermore, 50 μm of 5-(N-ethyl-N-isopropyl)-amiloride (EIPA), a specific inhibitor for Na(+)/H(+) antiporters, caused a 38 ± 5% decrease in the initial H(+) pump activity in the wild type, while no change was observed in D178N, D303A, and D400A mutants where the H(+) pumping efficiency had already been significantly decreased. The electron transfer activities were basically unaffected by EIPA in both control and mutants. Taken together, our data strongly indicate that the NuoL subunit is involved in the indirect coupling mechanism.

The reaction of NADPH with bovine mitochondrial NADH:ubiquinone oxidoreductase revisited: I. Proposed consequences for electron transfer in the enzyme

Bovine NADH:ubiquinone oxidoreductase (Complex I) is the first complex in the mitochondrial respiratory chain. It has long been assumed that it contained only one FMN group. However, as demonstrated in 2003, the intact enzyme contains two FMN groups. The second FMN was proposed to be located in a conserved flavodoxin fold predicted to be present in the PSST subunit. The long-known reaction of Complex I with NADPH differs in many aspects from that with NADH. It was proposed that the second flavin group was specifically involved in the reaction with NADPH. The X-ray structure of the hydrophilic domain of Complex I from Thermus thermophilus (Sazanov and Hinchliffe 2006, Science 311, 1430-1436) disclosed the positions of all redox groups of that enzyme and of the subunits holding them. The PSST subunit indeed contains the predicted flavodoxin fold although it did not contain FMN. Inspired by this structure, the present paper describes a re-evaluation of the enigmatic reactions of the bovine enzyme with NADPH. Published data, as well as new freeze-quench kinetic data presented here, are incompatible with the general opinion that NADPH and NADH react at the same site. Instead, it is proposed that these pyridine nucleotides react at opposite ends of the 90 A long chain of prosthetic groups in Complex I. Ubiquinone is proposed to react with the Fe-S clusters in the TYKY subunit deep inside the hydrophilic domain. A new model for electron transfer in Complex I is proposed. In the accompanying paper this model is compared with the one advocated in current literature.

Energy conservation by Rhodothermus marinus respiratory complex I

A sodium ion efflux, together with a proton influx and an inside-positive DeltaPsi, was observed during NADH-respiration by Rhodothermus marinus membrane vesicles. Proton translocation was monitored by fluorescence spectroscopy and sodium ion transport by (23)Na-NMR spectroscopy. Specific inhibitors of complex I (rotenone) and of the dioxygen reductase (KCN) inhibited the proton and the sodium ion transport, but the KCN effect was totally reverted by the addition of menaquinone analogues, indicating that both transports were catalyzed by complex I. We concluded that the coupling ion of the system is the proton and that neither the catalytic reaction nor the establishment of the delta-pH are dependent on sodium, but the presence of sodium increases proton transport. Moreover, studies of NADH oxidation at different sodium concentrations and of proton and sodium transport activities allowed us to propose a model for the mechanism of complex I in which the presence of two different energy coupling sites is suggested.

Sodium-translocating NADH:quinone oxidoreductase as a redox-driven ion pump

The Na+-translocating NADH:ubiquinone oxidoreductase (Na+-NQR) is a component of the respiratory chain of various bacteria. This enzyme is an analogous but not homologous counterpart of mitochondrial Complex I. Na+-NQR drives the same chemistry and also uses released energy to translocate ions across the membrane, but it pumps Na+ instead of H+. Most likely the mechanism of sodium pumping is quite different from that of proton pumping (for example, it could not accommodate the Grotthuss mechanism of ion movement); this is why the enzyme structure, subunits and prosthetic groups are completely special. This review summarizes modern knowledge on the structural and catalytic properties of bacterial Na+-translocating NADH:quinone oxidoreductases. The sequence of electron transfer through the enzyme cofactors and thermodynamic properties of those cofactors is discussed. The resolution of the intermediates of the catalytic cycle and localization of sodium-dependent steps are combined in a possible molecular mechanism of sodium transfer by the enzyme.

Features of subunit NuoM (ND4) in Escherichia coli NDH-1: TOPOLOGY AND IMPLICATION OF CONSERVED GLU144 FOR COUPLING SITE 1

The bacterial H(+)-pumping NADH-quinone oxidoreductase (NDH-1) is an L-shaped membrane-bound enzymatic complex. Escherichia coli NDH-1 is composed of 13 subunits (NuoA-N). NuoM (ND4) subunit is one of the hydrophobic subunits that constitute the membrane arm of NDH-1 and was predicted to bear 14 helices. We attempted to clarify the membrane topology of NuoM by the introduction of histidine tags into different positions by chromosomal site-directed mutagenesis. From the data, we propose a topology model containing 12 helices (helices I-IX and XII-XIV) located in transmembrane position and two (helices X and XI) present in the cytoplasm. We reported previously that residue Glu(144) of NuoM was located in the membrane (helix V) and was essential for the energy-coupling activities of NDH-1 (Torres-Bacete, J., Nakamaru-Ogiso, E., Matsuno-Yagi, A., and Yagi, T. (2007) J. Biol. Chem. 282, 36914-36922). Using mutant E144A, we studied the effect of shifting the glutamate residue to all sites within helix V and three sites each in helix IV and VI on the function of NDH-1. Twenty double site-directed mutants including the mutation E144A were constructed and characterized. None of the mutants showed alteration in the detectable levels of expressed NuoM or on the NDH-1 assembly. In addition, most of the double mutants did not restore the energy transducing NDH-1 activities. Only two mutants E144A/F140E and E144A/L147E, one helix turn downstream and upstream restored the energy transducing activities of NDH-1. Based on these results, a role of Glu(144) for proton translocation has been discussed.

Critical roles of subunit NuoH (ND1) in the assembly of peripheral subunits with the membrane domain of Escherichia coli NDH-1

The bacterial proton-translocating NADH:quinone oxidoreductase (NDH-1) consists of two domains, a peripheral arm and a membrane arm. NuoH is a counterpart of ND1, which is one of seven mitochondrially encoded hydrophobic subunits, and is considered to be involved in quinone/inhibitor binding. Sequence comparison in a wide range of species showed that NuoH is comprehensively conserved, particularly with charged residues in the cytoplasmic side loops. We have constructed 40 mutants of 27 conserved residues predicted to be in the cytoplasmic side loops of Escherichia coli NuoH by utilizing the chromosomal DNA manipulation technique and investigated roles of these residues. Mutants of Arg(37), Arg(46), Asp(63), Gly(134), Gly(145), Arg(148), Glu(220), and Glu(228) showed low deamino-NADH-K(3)Fe(CN)(6) reductase activity, undetectable NDH-1 in Blue Native gels, low contents of peripheral subunits (especially NuoB and NuoCD) bound to the membranes, and a significant loss of the membrane potential and proton-pumping function coupled to deamino-NADH oxidation. The results indicated that these conserved residues located in the cytoplasmic side loops are essential for the assembly of the peripheral subunits with the membrane arm. Implications for the involvement of NuoH (ND1) in maintaining the structure and function of NDH-1 are discussed.

Precise detection of pH inside large unilamellar vesicles using membrane-impermeable dendritic porphyrin-based nanoprobes

Accurate real-time measurements of proton concentration gradients are pivotal to mechanistic studies of proton translocation by membrane-bound enzymes. Here we report a detailed characterization of the pH-sensitive fluorescent nanoprobe Glu(3), which is well suited for pH measurements in microcompartmentalized biological systems. The probe is a polyglutamic porphyrin dendrimer in which multiple carboxylate termini ensure its high water solubility and prevent its diffusion across phospholipid membranes. The probe's pK is in the physiological pH range, and its protonation can be followed ratiometrically by absorbance or fluorescence in the ultraviolet-visible spectral region. The usefulness of the probe was enhanced by using a semiautomatic titration system coupled to a charge-coupled device (CCD) spectrometer, enabling fast and accurate titrations and full spectral coverage of the system at millisecond time resolution. The probe's pK was measured in bulk solutions as well as inside large unilamellar vesicles in the presence of physiologically relevant ions. Glu(3) was found to be completely membrane impermeable, and its distinct spectroscopic features permit pH measurements inside closed membrane vesicles, enabling quantitative mechanistic studies of membrane-spanning proteins. Performance of the probe was demonstrated by monitoring the rate of proton leakage through the phospholipid bilayer in large vesicles with and without the uncoupler gramicidin present. Overall, as a probe for biological proton translocation measurements, Glu(3) was found to be superior to the commercially available pH indicators.

Identification of the mitochondrial ND3 subunit as a structural component involved in the active/deactive enzyme transition of respiratory complex I

Mitochondrial complex I (NADH:ubiquinone oxidoreductase) undergoes reversible deactivation upon incubation at 30-37 degrees C. The active/deactive transition could play an important role in the regulation of complex I activity. It has been suggested recently that complex I may become modified by S-nitrosation under pathological conditions during hypoxia or when the nitric oxide:oxygen ratio increases. Apparently, a specific cysteine becomes accessible to chemical modification only in the deactive form of the enzyme. By selective fluorescence labeling and proteomic analysis, we have identified this residue as cysteine-39 of the mitochondrially encoded ND3 subunit of bovine heart mitochondria. Cysteine-39 is located in a loop connecting the first and second transmembrane helix of this highly hydrophobic subunit. We propose that this loop connects the ND3 subunit of the membrane arm with the PSST subunit of the peripheral arm of complex I, placing it in a region that is known to be critical for the catalytic mechanism of complex I. In fact, mutations in three positions of the loop were previously reported to cause Leigh syndrome with and without dystonia or progressive mitochondrial disease.

NADH/NAD+ interaction with NADH: ubiquinone oxidoreductase (complex I)

The quantitative data on the binding affinity of NADH, NAD(+), and their analogues for complex I as emerged from the steady-state kinetics data and from more direct studies under equilibrium conditions are summarized and discussed. The redox-dependency of the nucleotide binding and the reductant-induced change of FMN affinity to its tight non-covalent binding site indicate that binding (dissociation) of the substrate (product) may energetically contribute to the proton-translocating activity of complex I.

Functional role of coenzyme Q in the energy coupling of NADH-CoQ oxidoreductase (Complex I): stabilization of the semiquinone state with the application of inside-positive membrane potential to proteoliposomes

Coenzyme Q10 (which is also designated as CoQ10, ubiquinone-10, UQ10, CoQ, UQ or simply as Q) plays an important role in energy metabolism. For NADH-Q oxidoreductase (complex I), Ohnishi and Salerno proposed a hypothesis that the proton pump is operated by the redox-driven conformational change of a Q-binding protein, and that the bound form of semiquinone (SQ) serves as its gate [FEBS Letters 579 (2005) 45-55]. This was based on the following experimental results: (i) EPR signals of the fast-relaxing SQ anion (designated as QNf(.-)) are observable only in the presence of the proton electrochemical potential (DeltamuH+); (ii) iron-sulfur cluster N2 and QNf(.-) are directly spin-coupled; and (iii) their center-to-center distance was calculated as 12angstroms, but QNf(.-) is only 5angstroms deeper than N2 perpendicularly to the membrane. After the priming reduction of Q to QNf(.-), the proton pump operates only in the steps between the semiquinone anion (QNf(.-)) and fully reduced quinone (QH2). Thus, by cycling twice for one NADH molecule, the pump transports 4H+ per 2e(-). This hypothesis predicts the following phenomena: (a) Coupled with the piericidin A sensitive NADH-DBQ or Q1 reductase reaction, DeltamuH+ would be established; (b) DeltamuH+ would enhance the SQ EPR signals; and (c) the dissipation of DeltamuH+ with the addition of an uncoupler would increase the rate of NADH oxidation and decrease the SQ signals. We reconstituted bovine heart complex I, which was prepared at Yoshikawa's laboratory, into proteoliposomes. Using this system, we succeeded in demonstrating that all of these phenomena actually took place. We believe that these results strongly support our hypothesis.

Human mitochondrial complex I assembly: A dynamic and versatile process

One can but admire the intricate way in which biomolecular structures are formed and cooperate to allow proper cellular function. A prominent example of such intricacy is the assembly of the five inner membrane embedded enzymatic complexes of the mitochondrial oxidative phosphorylation (OXPHOS) system, which involves the stepwise combination of >80 subunits and prosthetic groups encoded by both the mitochondrial and nuclear genomes. This review will focus on the assembly of the most complicated OXPHOS structure: complex I (NADH:ubiquinone oxidoreductase, EC 1.6.5.3). Recent studies into complex I assembly in human cells have resulted in several models elucidating a thus far enigmatic process. In this review, special attention will be given to the overlap between the various assembly models proposed in different organisms. Complex I being a complicated structure, its assembly must be prone to some form of coordination. This is where chaperone proteins come into play, some of which may relate complex I assembly to processes such as apoptosis and even immunity.