Exploring the inhibitor binding pocket of respiratory complex I

Asp563 of the horizontal helix of subunit NuoL is involved in proton translocation by the respiratory complex I

The NADH:ubiquinone oxidoreductase couples the electron transfer from NADH to ubiquinone with the translocation of protons across the membrane. It contains a 110Å long helix running parallel to the membrane part of the complex. Deletion of the helix resulted in a reduced H(+)/e(-) stoichiometry indicating its direct involvement in proton translocation. Here, we show that the mutation of the conserved amino acid D563(L), which is part of the horizontal helix of the Escherichia coli complex I, leads to a reduced H(+)/e(-) stoichiometry. It is discussed that this residue is involved in transferring protons to the membranous proton translocation site.

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.

During the stationary growth phase, Yarrowia lipolytica prevents the overproduction of reactive oxygen species by activating an uncoupled mitochondrial respiratory pathway

In the branched mitochondrial respiratory chain from Yarrowia lipolytica there are two alternative oxido-reductases that do not pump protons, namely an external type II NADH dehydrogenase (NDH2e) and the alternative oxidase (AOX). Direct electron transfer between these proteins is not coupled to ATP synthesis and should be avoided in most physiological conditions. However, under low energy-requiring conditions an uncoupled high rate of oxygen consumption would be beneficial, as it would prevent overproduction of reactive oxygen species (ROS). In mitochondria from high energy-requiring, logarithmic-growth phase cells, most NDH2e was associated to cytochrome c oxidase and electrons from NADH were channeled to the cytochromic pathway. In contrast, in the low energy requiring, late stationary-growth phase, complex IV concentration decreased, the cells overexpressed NDH2e and thus a large fraction of this enzyme was found in a non-associated form. Also, the NDH2e-AOX uncoupled pathway was activated and the state IV external NADH-dependent production of ROS decreased. Association/dissociation of NDH2e to/from complex IV is proposed to be the switch that channels electrons from external NADH to the coupled cytochrome pathway or allows them to reach an uncoupled, alternative, ΔΨ-independent pathway.

LHON/MELAS overlap mutation in ND1 subunit of mitochondrial complex I affects ubiquinone binding as revealed by modeling in Escherichia coli NDH-1

Defects in complex I due to mutations in mitochondrial DNA are associated with clinical features ranging from single organ manifestation like Leber hereditary optic neuropathy (LHON) to multiorgan disorders like mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) syndrome. Specific mutations cause overlap syndromes combining several phenotypes, but the mechanisms of their biochemical effects are largely unknown. The m.3376G>A transition leading to p.E24K substitution in ND1 with LHON/MELAS phenotype was modeled here in a homologous position (NuoH-E36K) in the Escherichia coli enzyme and it almost totally abolished complex I activity. The more conservative mutation NuoH-E36Q resulted in higher apparent K(m) for ubiquinone and diminished inhibitor sensitivity. A NuoH homolog of the m.3865A>G transition, which has been found concomitantly in the overlap syndrome patient with the m.3376G>A, had only a minor effect. Consequences of a primary LHON-mutation m.3460G>A affecting the same extramembrane loop as the m.3376G>A substitution were also studied in the E. coli model and were found to be mild. The results indicate that the overlap syndrome-associated m.3376G>A transition in MTND1 is the pathogenic mutation and m.3865A>G transition has minor, if any, effect on presentation of the disease. The kinetic effects of the NuoH-E36Q mutation suggest its proximity to the putative ubiquinone binding domain in 49kD/PSST subunits. In all, m.3376G>A perturbs ubiquinone binding, a phenomenon found in LHON, and decreases the activity of fully assembled complex I as in MELAS.

New developments on the functions of coenzyme Q in mitochondria

The notion of a mobile pool of coenzyme Q (CoQ) in the lipid bilayer has changed with the discovery of respiratory supramolecular units, in particular the supercomplex comprising complexes I and III; in this model, the electron transfer is thought to be mediated by tunneling or microdiffusion, with a clear kinetic advantage on the transfer based on random collisions. The CoQ pool, however, has a fundamental function in establishing a dissociation equilibrium with bound quinone, besides being required for electron transfer from other dehydrogenases to complex III. The mechanism of CoQ reduction by complex I is analyzed regarding recent developments on the crystallographic structure of the enzyme, also in relation to the capacity of complex I to generate superoxide. Although the mechanism of the Q-cycle is well established for complex III, involvement of CoQ in proton translocation by complex I is still debated. Some additional roles of CoQ are also examined, such as the antioxidant effect of its reduced form and the capacity to bind the permeability transition pore and the mitochondrial uncoupling proteins. Finally, a working hypothesis is advanced on the establishment of a vicious circle of oxidative stress and supercomplex disorganization in pathological states, as in neurodegeneration and cancer.

Engineering the respiratory complex I to energy-converting NADPH:ubiquinone oxidoreductase

The respiratory complex I couples the electron transfer from NADH to ubiquinone with a translocation of protons across the membrane. Its nucleotide-binding site is made up of a unique Rossmann fold to accommodate the binding of the substrate NADH and of the primary electron acceptor flavin mononucleotide. Binding of NADH includes interactions of the hydroxyl groups of the adenosine ribose with a conserved glutamic acid residue. Structural analysis revealed that due to steric hindrance and electrostatic repulsion, this residue most likely prevents the binding of NADPH, which is a poor substrate of the complex. We produced several variants with mutations at this position exhibiting up to 200-fold enhanced catalytic efficiency with NADPH. The reaction of the variants with NAD(P)H is coupled with proton translocation in an inhibitor-sensitive manner. Thus, we have created an energy-converting NADPH:ubiquinone oxidoreductase, an activity so far not found in nature. Remarkably, the oxidation of NAD(P)H by the variants leads to an enhanced production of reactive oxygen species.

Isoflurane selectively inhibits distal mitochondrial complex I in Caenorhabditis elegans

BACKGROUND

Complex I of the electron transport chain (ETC) is a possible target of volatile anesthetics (VAs). Complex I enzymatic activities are inhibited by VAs, and dysfunction of complex I can lead to hypersensitivity to VAs in worms and in people. Mutant analysis in Caenorhabditis (C.) elegans suggests that VAs may specifically interfere with complex I function at the binding site for its substrate ubiquinone. We hypothesized that isoflurane inhibits electron transport by competing with ubiquinone for binding to complex I.

METHODS

Wildtype and mutant C. elegans were used to study the effects of isoflurane on isolated mitochondria. Enzymatic activities of the ETC were assayed and dose-response curves determined using established techniques. Two-dimensional native gels of mitochondrial proteins were performed after exposure of mitochondria to isoflurane.

RESULTS

Complex I is the most sensitive component of the ETC to isoflurane inhibition; however, the proximal portion of complex I (the flavoprotein) is relatively insensitive to isoflurane. Isoflurane and quinone do not compete for a common binding site on complex I. The absolute rate of complex I enzymatic activity in vitro does not predict immobilization of the animal by isoflurane. Isoflurane had no measurable effect on stability of mitochondrial supercomplexes. Reduction of ubiquinone by complex I displayed positive cooperative kinetics not disrupted by isoflurane.

CONCLUSIONS

Isoflurane directly inhibits complex I at a site distal to the flavoprotein subcomplex. However, we have excluded our original hypothesis that isoflurane and ubiquinone compete for a common hydrophobic binding site on complex I. In addition, immobilization of the nematode by isoflurane is not due to limiting absolute amounts of complex I electron transport as measured in isolated mitochondria.

Spin labeling of the Escherichia coli NADH ubiquinone oxidoreductase (complex I)

The proton-pumping NADH:ubiquinone oxidoreductase, the respiratory complex I, couples the transfer of electrons from NADH to ubiquinone with the translocation of protons across the membrane. Electron microscopy revealed the two-part structure of the complex with a peripheral arm involved in electron transfer and a membrane arm most likely involved in proton translocation. It was proposed that the quinone binding site is located at the joint of the two arms. Most likely, proton translocation in the membrane arm is enabled by the energy of the electron transfer reaction in the peripheral arm transmitted by conformational changes. For the detection of the conformational changes and the localization of the quinone binding site, we set up a combination of site-directed spin labeling and EPR spectroscopy. Cysteine residues were introduced to the surface of the Escherichia coli complex I. The spin label (1-oxyl-2,2,5,5-tetramethyl-Δ3-pyrroline-3-methyl)-methanethiosulfonate (MTSL) was exclusively bound to the engineered positions. Neither the mutation nor the labeling had an effect on the NADH:decyl-ubiquinone oxidoreductase activity. The characteristic signals of the spin label were detected by EPR spectroscopy, which did not change by reducing the preparation with NADH. A decyl-ubiquinone derivative with the spin label covalently attached to the alkyl chain was synthesized in order to localize the quinone binding site. The distance between a MTSL labeled complex I variant and the bound quinone was determined by continuous-wave (cw) EPR allowing an inference on the location of the quinone binding site. The distances between the labeled quinone and other complex I variants will be determined in future experiments to receive further geometry information by triangulation.

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: II. Comparison of the proposed working hypothesis with literature data

The first purification of bovine NADH:ubiquinone oxidoreductase (Complex I) was reported nearly half a century ago (Hatefi et al. J Biol Chem 237:1676-1680, 1962). The pathway of electron-transfer through the enzyme is still under debate. A major obstacle is the assignment of EPR signals to the individual iron-sulfur clusters in the subunits. The preceding paper described a working model based on the kinetics with NADPH. This model is at variance with current views in the field. The present paper provides a critical overview on the possible causes for the discrepancies. It is concluded that the stability of all purified preparations described thus far, including Hatefi's Complex I, is compromised due to removal of the enzyme from the protective membrane environment. In addition, most preparations described during the last two decades are purified by methods involving synthetic detergents and column chromatography. This results in delipidation, loss of endogenous quinones and loss of reactions with (artificial) quinones in a rotenone-sensitive way. The Fe:FMN ratio's indicate that FMN-a is absent, but that all Fe-S clusters may be present. In contrast to the situation in bovine SMP and Hatefi's Complex I, three of the six expected [4Fe-4S] clusters are not detected in EPR spectra. Qualitatively, the overall EPR lineshape of the remaining three cubane signals may seem similar to that of Hatefi's Complex I, but quantitatively it is not. It is further proposed that point mutations in any of the TYKY, PSST, 49-kDa or 30-kDa subunits, considered to make up the delicate structural heart of Complex I, may have unpredictable effects on any of the other subunits of this quartet. The fact that most point mutations led to inactive enzymes makes a correct interpretation of such mutations even more ambiguous. In none of the Complex-I-containing membrane preparations from non-bovine origin, the pH dependencies of the NAD(P)H-->O(2) reactions and the pH-dependent reduction kinetics of the Fe-S clusters with NADPH have been determined. This excludes a proper discussion on the absence or presence of FMN-a in native Complex I from other organisms.

Mammalian mitochondrial complex I: biogenesis, regulation, and reactive oxygen species generation

Virtually every mammalian cell contains mitochondria. These double-membrane organelles continuously change shape and position and contain the complete metabolic machinery for the oxidative conversion of pyruvate, fatty acids, and amino acids into ATP. Mitochondria are crucially involved in cellular Ca2+ and redox homeostasis and apoptosis induction. Maintenance of mitochondrial function and integrity requires an inside-negative potential difference across the mitochondrial inner membrane. This potential is sustained by the electron-transport chain (ETC). NADH:ubiquinone oxidoreductase or complex I (CI), the first and largest protein complex of the ETC, couples the oxidation of NADH to the reduction of ubiquinone. During this process, electrons can escape from CI and react with ambient oxygen to produce superoxide and derived reactive oxygen species (ROS). Depending on the balance between their production and removal by antioxidant systems, ROS may function as signaling molecules or induce damage to a variety of biomolecules or both. The latter ultimately leads to a loss of mitochondrial and cellular function and integrity. In this review, we discuss (a) the role of CI in mitochondrial functioning; (b) the composition, structure, and biogenesis of CI; (c) regulation of CI function; (d) the role of CI in ROS generation; and (e) adaptive responses to CI deficiency.

Glutaredoxin 2 prevents H(2)O(2)-induced cell apoptosis by protecting complex I activity in the mitochondria

Glutaredoxin 2 (Grx2) belongs to the oxidoreductase family and is an isozyme of glutaredoxin 1 (Grx1) present in the mitochondria, however its function is not well understood. The purpose of this study is to evaluate the potential anti-apoptotic function of Grx2 by examining its ability to protect complex I in the mitochondrial electron transport system using human lens epithelial cells as a model. We found that cells treated with 200muM hydrogen peroxide (H(2)O(2)) for 24h exhibited decreased viability and became apoptotic with corresponding Bax up-regulation, Bcl-2 down-regulation, caspase 3 activation and mitochondrial cytochrome c leakage. Grx2 over-expression (OE) could protect cells against H(2)O(2)-induced damage while Grx2 knockdown (KD) showed the opposite effect. Under the same conditions, H(2)O(2) treatment caused 50% inactivation of complex I activity in control cells (vector only), 75% in Grx2 KD cells but only 20% in Grx2 OE cells. Furthermore, the inactivated complex I in the H(2)O(2)-treated cells could be protected mostly by importing the purified nascent Grx2 protein, but not the Grx2 protein mutated at the active site with C70S, or C73S, or with C70S plus C73S. Immunoprecipitation study also revealed that Grx2 co-precipitated with complex I, but not complex II, in the mitochondrial lysate. Thus, the mechanism of Grx2 protection against H(2)O(2)-induced apoptosis is likely associated with its ability to preserve complex I.

Rotenone enhances the antifungal properties of staurosporine

We studied staurosporine-induced cell death in the filamentous fungus Neurospora crassa. The generation of reactive oxygen species during the process appears to be an important signaling event, since addition of the antioxidant glutathione prevents the effects of staurosporine on fungal growth. Selected mutants with mutations in respiratory chain complex I are extremely sensitive to the drug, stressing the involvement of complex I in programmed cell death. Following this finding, we determined that the complex I-specific inhibitor rotenone used in combination with staurosporine results in a synergistic and specific antifungal activity, likely through a concerted action on intracellular glutathione depletion. Paradoxically, the synergistic antifungal activity of rotenone and staurosporine is observed in N. crassa complex I mutants and in Saccharomyces cerevisiae, which lacks complex I. In addition, it is not observed when other complex I inhibitors are used instead of rotenone. These results indicate that the rotenone effect is independent of complex I inhibition. The combination of rotenone and staurosporine is effective against N. crassa as well as against the common pathogens Aspergillus fumigatus and Candida albicans, pointing to its usefulness as an antifungal agent.

Structure and organization of mitochondrial respiratory complexes: a new understanding of an old subject

The enzymatic complexes of the mitochondrial respiratory chain have been extensively investigated in their structural and functional properties. A clear distinction is possible today between three complexes in which the difference in redox potential allows proton translocation (complexes I, III, and IV) and those having the mere function to convey electrons to the respiratory chain. We also have a clearer understanding of the structure and function of most respiratory complexes, of their biogenesis and regulation, and of their capacity to generate reactive oxygen species. Past investigations led to the conclusion that the complexes are randomly dispersed and functionally connected by diffusion of smaller redox components, coenzyme Q and cytochrome c. More-recent investigations by native gel electrophoresis and single-particle image processing showed the existence of supramolecular associations. Flux-control analysis demonstrated that complexes I and III in mammals and I, III, and IV in plants kinetically behave as single units, suggesting the existence of substrate channeling. This review discusses conditions affecting the formation of supercomplexes that, besides kinetic advantage, have a role in the stability and assembly of the individual complexes and in preventing excess oxygen radical formation. Disruption of supercomplex organization may lead to functional derangements responsible for pathologic changes.

Redox-induced conformational changes within the Escherichia coli NADH ubiquinone oxidoreductase (complex I): an analysis by mutagenesis and FT-IR spectroscopy

The proton-pumping NADH:ubiquinone oxidoreductase couples the transfer of electrons from NADH to ubiquinone with the translocation of protons across the membrane. This process is suggested to be accompanied by conformational changes of the enzyme that may be monitored by redox-induced FT-IR difference spectroscopy. Signals observed in the amide I range are partially attributed to local rearrangements that occur as an electrostatic response to the redox reactions of the FeS clusters. In addition, conformational changes can be reported that depend on pH and at the same time can be perturbed by site-directed mutagenesis of residue E67 on subunit B (the bacterial homologue of the mitochondrial PSST subunit). This residue is located in the vicinity of the cluster N2. Re-evaluating these previous data we here discuss a mechanism, by which the redox reaction of N2 induces conformational changes possibly leading to proton translocation.

Exploring the binding site of acetogenin in the ND1 subunit of bovine mitochondrial complex I

125I-labeled (trifluoromethyl)phenyldiazirinyl acetogenin, [125I]TDA, a photoaffinity labeling probe of acetogenin, photo-cross-links to the ND1 subunit of bovine heart mitochondrial NADH-ubiquinone oxidoreductase (complex I) with high specificity [M. Murai, A. Ishihara, T. Nishioka, T. Yagi, and H. Miyoshi, (2007) The ND1 subunit constructs the inhibitor binding domain in bovine heart mitochondrial complex I, Biochemistry 46 6409-6416.]. To identify the binding site of [125I]TDA in the ND1 subunit, we carried out limited proteolysis of the subunit cross-linked by [125I]TDA using various proteases and carefully analyzed the fragmentation patterns. Our results revealed that the cross-linked residue is located within the region of the 4th to 5th transmembrane helices (Val144-Glu192) of the subunit. It is worth noting that an excess amount of short-chain ubiquinones such as ubiquinone-2 (Q2) and 2-azido-Q2 suppressed the cross-linking by [125I]TDA in a concentration-dependent way. Although the question of whether the binding sites for ubiquinone and different inhibitors in complex I are identical remains to be answered, the present study provided, for the first time, direct evidence that an inhibitor (acetogenin) and ubiquinone competitively bind to the enzyme. Considering the present results along with earlier photoaffinity labeling studies, we propose that not all inhibitors acting at the terminal electron transfer step of complex I necessarily bind to the ubiquinone binding site itself.

Architecture of complex I and its implications for electron transfer and proton pumping

Proton pumping NADH:ubiquinone oxidoreductase (complex I) is the largest and remains by far the least understood enzyme complex of the respiratory chain. It consists of a peripheral arm harbouring all known redox active prosthetic groups and a membrane arm with a yet unknown number of proton translocation sites. The ubiquinone reduction site close to iron-sulfur cluster N2 at the interface of the 49-kDa and PSST subunits has been mapped by extensive site directed mutagenesis. Independent lines of evidence identified electron transfer events during reduction of ubiquinone to be associated with the potential drop that generates the full driving force for proton translocation with a 4H(+)/2e(-) stoichiometry. Electron microscopic analysis of immuno-labelled native enzyme and of a subcomplex lacking the electron input module indicated a distance of 35-60 A of cluster N2 to the membrane surface. Resolution of the membrane arm into subcomplexes showed that even the distal part harbours subunits that are prime candidates to participate in proton translocation because they are homologous to sodium/proton antiporters and contain conserved charged residues in predicted transmembrane helices. The mechanism of redox linked proton translocation by complex I is largely unknown but has to include steps where energy is transmitted over extremely long distances. In this review we compile the available structural information on complex I and discuss implications for complex I function.

Chapter 24 Quantification, localization, and tissue specificities of mouse mitochondrial reactive oxygen species production

Mitochondria play a critical role in many different pathologic conditions. Increasing evidence has shown that mitochondrial reactive oxygen species (ROS) production may provide an etiologic link between mitochondria and pathologics. The widespread use of laboratory mice as models for a host of human diseases makes the quantification and localization of ROS production from mice an important endeavor. This chapter presents approaches to the quantification and localization of ROS from mouse brain, liver, and beta cell mitochondria. Techniques for the isolation of mitochondria and mitochondrial fractions and the subsequent quantification of ROS with Amplex Red or a FACS-based method on intact cells are described.

Challenges in elucidating structure and mechanism of proton pumping NADH:ubiquinone oxidoreductase (complex I)

Proton pumping NADH:ubiquinone oxidoreductase (complex I) is the most complicated and least understood enzyme of the respiratory chain. All redox prosthetic groups reside in the peripheral arm of the L-shaped structure. The NADH oxidation domain harbouring the FMN cofactor is connected via a chain of iron-sulfur clusters to the ubiquinone reduction site that is located in a large pocket formed by the PSST- and 49-kDa subunits of complex I. An access path for ubiquinone and different partially overlapping inhibitor binding regions were defined within this pocket by site directed mutagenesis. A combination of biochemical and single particle analysis studies suggests that the ubiquinone reduction site is located well above the membrane domain. Therefore, direct coupling mechanisms seem unlikely and the redox energy must be converted into a conformational change that drives proton pumping across the membrane arm. It is not known which of the subunits and how many are involved in proton translocation. Complex I is a major source of reactive oxygen species (ROS) that are predominantly formed by electron transfer from FMNH(2). Mitochondrial complex I can cycle between active and deactive forms that can be distinguished by the reactivity towards divalent cations and thiol-reactive agents. The physiological role of this phenomenon is yet unclear but it could contribute to the regulation of complex I activity in-vivo.