A ternary mechanism for NADH oxidation by positively charged electron acceptors, catalyzed at the flavin site in respiratory complex I

Structures of 3-acetylpyridine adenine dinucleotide and ADP-ribose bound to the electron input module of respiratory complex I

Energy-converting NADH:ubiquinone oxidoreductase, respiratory complex I, is a major enzyme of energy metabolism that couples NADH oxidation and ubiquinone reduction with proton translocation. The NADH oxidation site features different enzymatic activities with various nucleotides. While the kinetics of these reactions are well described, only binding of NAD+ and NADH have been structurally characterized. Here, we report the structures of the electron input module of Aquifex aeolicus complex I with bound ADP-ribose and 3-acetylpyridine adenine dinucleotides at resolutions better than 2.0 Å. ADP-ribose acts as inhibitor by blocking the "ADP-handle" motif essential for nucleotide binding. The pyridine group of APADH is minimally offset from flavin, which could contribute to its poorer suitability as substrate. A comparison with other nucleotide co-structures surprisingly shows that the adenine ribose and the pyrophosphate moiety contribute most to nucleotide binding, thus all adenine dinucleotides share core binding modes to the unique Rossmann-fold in complex I.

Proton-translocating NADH:ubiquinone oxidoreductase of Paracoccus denitrificans plasma membranes catalyzes FMN-independent reverse electron transfer to hexaammineruthenium (III)

NADH-OH, the specific inhibitor of NADH-binding site of the mammalian complex I, is shown to completely block FMN-dependent reactions of P. denitrificans enzyme in plasma membrane vesicles: NADH oxidation (in a competitive manner with K_i of 1 nM) as well as reduction of pyridine nucleotides, ferricyanide and oxygen in the reverse electron transfer. In contrast to these activities, the reverse electron transfer to hexaammineruthenium (III) catalyzed by plasma membrane vesicles is insensitive to NADH-OH. To explain these results, we hypothesize the existence of a non-FMN redox group of P. denitrificans complex I that is capable of reducing hexaammineruthenium (III), which is corroborated by the complex kinetics of NADH: hexaammineruthenium (III)-reductase activity, catalyzed by this enzyme. A new assay procedure for measuring succinate-driven reverse electron transfer catalyzed by P. denitrificans complex I to hexaammineruthenium (III) is proposed.

The ins and outs of the flavin mononucleotide cofactor of respiratory complex I

The flavin mononucleotide (FMN) cofactor of respiratory complex I occupies a key position in the electron transport chain. Here, the electrons coming from NADH start the sequence of oxidoreduction reactions, which drives the generation of the proton-motive force necessary for ATP synthesis. The overall architecture and the general catalytic proprieties of the FMN site are mostly well established. However, several aspects regarding the complex I flavin cofactor are still unknown. For example, the flavin binding to the N-module, the NADH-oxidizing portion of complex I, lacks a molecular description. The dissociation of FMN from the enzyme is beginning to emerge as an important regulatory mechanism of complex I activity and ROS production. Finally, how mitochondria import and metabolize FMN is still uncertain. This review summarizes the current knowledge on complex I flavin cofactor and discusses the open questions for future research.

Quantitative proteomic analysis of Xanthoceras sorbifolium Bunge seedlings in response to drought and heat stress

Yellowhorn (Xanthoceras sorbifolium Bunge) is a woody oil species that is widely distributed in northwestern China. To investigate the molecular mechanisms underlying the drought and heat tolerance response of yellowhorn seedlings, changes in protein abundance were analyzed via comparative proteomics. Drought and heat treatment of seedlings was applied in growth chamber, and the leaves were harvested after 7 days of treatment. The total protein was extracted, and comparative proteomic analysis was performed via isobaric tag for relative and absolute quantitation (iTRAQ). The abundance of most of the proteins associated with oxidative phosphorylation, NADH dehydrogenase and superoxide dismutase (SOD) was reduced. The differential proteins associated with photosynthesis enzymes indicated that stress had different effects on photosystem I (PSI) and photosystem II (PSII). After comprehensively analyzing the results, we speculated that drought and heat stress could hinder the synthesis of riboflavin, reducing NADH dehydrogenase content, which might further have an impact on energy utilization. Yellowhorn seedlings relied on Fe-Mn SOD enzymes rather than Cu/Zn SOD enzymes to remove reactive oxygen species (ROS). In addition, heat-shock proteins (HSPs) had significant increase and played a key role in stress response, which could be divided into two categories according to their transcription and translation efficiency. Over all, the results can provide a basis for understanding the molecular mechanism underlying resistance to drought and heat stress in yellowhorn and for subsequent research of posttranslational modification-related omics of key proteins.

A mechanism to prevent production of reactive oxygen species by Escherichia coli respiratory complex I

Respiratory complex I plays a central role in cellular energy metabolism coupling NADH oxidation to proton translocation. In humans its dysfunction is associated with degenerative diseases. Here we report the structure of the electron input part of Aquifex aeolicus complex I at up to 1.8 Å resolution with bound substrates in the reduced and oxidized states. The redox states differ by the flip of a peptide bond close to the NADH binding site. The orientation of this peptide bond is determined by the reduction state of the nearby [Fe-S] cluster N1a. Fixation of the peptide bond by site-directed mutagenesis led to an inactivation of electron transfer and a decreased reactive oxygen species (ROS) production. We suggest the redox-gated peptide flip to represent a previously unrecognized molecular switch synchronizing NADH oxidation in response to the redox state of the complex as part of an intramolecular feed-back mechanism to prevent ROS production.

Respiratory complex I plays a central role in cellular energy metabolism coupling NADH oxidation to proton translocation. Here, the authors report the structure of the electron input part of Aquifex aeolicus complex I at up to 1.8 Å resolution with bound substrates in the reduced and oxidized states.

Selective mitochondrial superoxide generation in vivo is cardioprotective through hormesis

Reactive oxygen species (ROS) have an equivocal role in myocardial ischaemia reperfusion injury. Within the cardiomyocyte, mitochondria are both a major source and target of ROS. We evaluate the effects of a selective, dose-dependent increase in mitochondrial ROS levels on cardiac physiology using the mitochondria-targeted redox cycler MitoParaquat (MitoPQ). Low levels of ROS decrease the susceptibility of neonatal rat ventricular myocytes (NRVMs) to anoxia/reoxygenation injury and also cause profound protection in an in vivo mouse model of ischaemia/reperfusion. However higher doses of MitoPQ resulted in a progressive alteration of intracellular [Ca²⁺] homeostasis and mitochondrial function in vitro, leading to dysfunction and death at high doses. Our data show that a primary increase in mitochondrial ROS can alter cellular function, and support a hormetic model in which low levels of ROS are cardioprotective while higher levels of ROS are cardiotoxic.

FMN site-independent energy-linked reverse electron transfer in mitochondrial respiratory complex I

A simple assay procedure for measuring ATP-dependent reverse electron transfer from ubiquinol to hexaammineruthenium (III) (HAR) catalyzed by mitochondrial respiratory complex I is introduced. The specific activity of the enzyme in this reaction and its sensitivity to the standard inhibitors and uncoupling are the same as with other well-known electron acceptors, NAD⁺ and ferricyanide. In contrast to the reactions with these acceptors, the energy-dependent HAR reduction is not inhibited by NADH-OH, the specific inhibitor of NADH-binding site. These results suggest that a catalytically competent electron connection exists between HAR and a redox component of complex I that is different from flavin mononucleotide bound at the substrate-binding site.

Reduction of the off-pathway iron-sulphur cluster N1a of Escherichia coli respiratory complex I restrains NAD+ dissociation

Respiratory complex I couples the electron transfer from NADH to ubiquinone with the translocation of protons across the membrane. The reaction starts with NADH oxidation by a flavin cofactor followed by transferring the electrons through a chain of seven iron-sulphur clusters to quinone. An eighth cluster called N1a is located proximally to flavin, but on the opposite side of the chain of clusters. N1a is strictly conserved although not involved in the direct electron transfer to quinone. Here, we show that the NADH:ferricyanide oxidoreductase activity of E. coli complex I is strongly diminished when the reaction is initiated by an addition of ferricyanide instead of NADH. This effect is significantly less pronounced in a variant containing N1a with a 100 mV more negative redox potential. Detailed kinetic analysis revealed that the reduced activity is due to a lower dissociation constant of bound NAD⁺. Thus, reduction of N1a induces local structural rearrangements of the protein that stabilise binding of NAD⁺. The variant features a considerably enhanced production of reactive oxygen species indicating that bound NAD⁺ represses this process.

Purification of Ovine Respiratory Complex I Results in a Highly Active and Stable Preparation

NADH-ubiquinone oxidoreductase (complex I) is the largest (∼1 MDa) and the least characterized complex of the mitochondrial electron transport chain. Because of the ease of sample availability, previous work has focused almost exclusively on bovine complex I. However, only medium resolution structural analyses of this complex have been reported. Working with other mammalian complex I homologues is a potential approach for overcoming these limitations. Due to the inherent difficulty of expressing large membrane protein complexes, screening of complex I homologues is limited to large mammals reared for human consumption. The high sequence identity among these available sources may preclude the benefits of screening. Here, we report the characterization of complex I purified from Ovis aries (ovine) heart mitochondria. All 44 unique subunits of the intact complex were identified by mass spectrometry. We identified differences in the subunit composition of subcomplexes of ovine complex I as compared with bovine, suggesting differential stability of inter-subunit interactions within the complex. Furthermore, the 42-kDa subunit, which is easily lost from the bovine enzyme, remains tightly bound to ovine complex I. Additionally, we developed a novel purification protocol for highly active and stable mitochondrial complex I using the branched-chain detergent lauryl maltose neopentyl glycol. Our data demonstrate that, although closely related, significant differences exist between the biochemical properties of complex I prepared from ovine and bovine mitochondria and that ovine complex I represents a suitable alternative target for further structural studies.

Reversible FMN dissociation from Escherichia coli respiratory complex I

Respiratory complex I transfers electrons from NADH to quinone, utilizing the reaction energy to translocate protons across the membrane. It is a key enzyme of the respiratory chain of many prokaryotic and most eukaryotic organisms. The reversible NADH oxidation reaction is facilitated in complex I by non-covalently bound flavin mononucleotide (FMN). Here we report that the catalytic activity of E. coli complex I with artificial electron acceptors potassium ferricyanide (FeCy) and hexaamineruthenium (HAR) is significantly inhibited in the enzyme pre-reduced by NADH. Further, we demonstrate that the inhibition is caused by reversible dissociation of FMN. The binding constant (K_d) for FMN increases from the femto- or picomolar range in oxidized complex I to the nanomolar range in the NADH reduced enzyme, with an FMN dissociation time constant of ~5s. The oxidation state of complex I, rather than that of FMN, proved critical to the dissociation. Such dissociation is not observed with the T. thermophilus enzyme and our analysis suggests that the difference may be due to the unusually high redox potential of Fe-S cluster N1a in E. coli. It is possible that the enzyme attenuates ROS production in vivo by releasing FMN under highly reducing conditions.

Energy conversion, redox catalysis and generation of reactive oxygen species by respiratory complex I

Complex I (NADH:ubiquinone oxidoreductase) is critical for respiration in mammalian mitochondria. It oxidizes NADH produced by the Krebs' tricarboxylic acid cycle and β-oxidation of fatty acids, reduces ubiquinone, and transports protons to contribute to the proton-motive force across the inner membrane. Complex I is also a significant contributor to cellular oxidative stress. In complex I, NADH oxidation by a flavin mononucleotide, followed by intramolecular electron transfer along a chain of iron–sulfur clusters, delivers electrons and energy to bound ubiquinone. Either at cluster N2 (the terminal cluster in the chain) or upon the binding/reduction/dissociation of ubiquinone/ubiquinol, energy from the redox process is captured to initiate long-range energy transfer through the complex and drive proton translocation. This review focuses on current knowledge of how the redox reaction and proton transfer are coupled, with particular emphasis on the formation and role of semiquinone intermediates in both energy transduction and reactive oxygen species production. This article is part of a Special Issue entitled Respiratory complex I, edited by Volker Zickermann and Ulrich Brandt.

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Current knowledge of the redox reactions catalyzed by complex I is reviewed.

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Possible quinone reduction pathways are presented.

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The presence and number of semiquinone intermediates are deliberated.

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The involvement of cluster N2/semiquinones in coupled proton transfer is discussed.

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Evidence for reactive oxygen species production by semiquinones is examined.

Resveratrol Directly Binds to Mitochondrial Complex I and Increases Oxidative Stress in Brain Mitochondria of Aged Mice

Resveratrol is often described as a promising therapeutic molecule for numerous diseases, especially in metabolic and neurodegenerative disorders. While the mechanism of action is still debated, an increasing literature reports that resveratrol regulates the mitochondrial respiratory chain function. In a recent study we have identified mitochondrial complex I as a direct target of this molecule. Nevertheless, the mechanisms and consequences of such an interaction still require further investigation. In this study, we identified in silico by docking study a binding site for resveratrol at the nucleotide pocket of complex I. In vitro, using solubilized complex I, we demonstrated a competition between NAD+ and resveratrol. At low doses (<5μM), resveratrol stimulated complex I activity, whereas at high dose (50 μM) it rather decreased it. In vivo, in brain mitochondria from resveratrol treated young mice, we showed that complex I activity was increased, whereas the respiration rate was not improved. Moreover, in old mice with low antioxidant defenses, we demonstrated that complex I activation by resveratrol led to oxidative stress. These results bring new insights into the mechanism of action of resveratrol on mitochondria and highlight the importance of the balance between pro- and antioxidant effects of resveratrol depending on its dose and age. These parameters should be taken into account when clinical trials using resveratrol or analogues have to be designed.

Selective superoxide generation within mitochondria by the targeted redox cycler MitoParaquat

Superoxide is the proximal reactive oxygen species (ROS) produced by the mitochondrial respiratory chain and plays a major role in pathological oxidative stress and redox signaling. While there are tools to detect or decrease mitochondrial superoxide, none can rapidly and specifically increase superoxide production within the mitochondrial matrix. This lack impedes progress, making it challenging to assess accurately the roles of mitochondrial superoxide in cells and in vivo. To address this unmet need, we synthesized and characterized a mitochondria-targeted redox cycler, MitoParaquat (MitoPQ) that comprises a triphenylphosphonium lipophilic cation conjugated to the redox cycler paraquat. MitoPQ accumulates selectively in the mitochondrial matrix driven by the membrane potential. Within the matrix, MitoPQ produces superoxide by redox cycling at the flavin site of complex I, selectively increasing superoxide production within mitochondria. MitoPQ increased mitochondrial superoxide in isolated mitochondria and cells in culture ~a thousand-fold more effectively than untargeted paraquat. MitoPQ was also more toxic than paraquat in the isolated perfused heart and in Drosophila in vivo. MitoPQ enables the selective generation of superoxide within mitochondria and is a useful tool to investigate the many roles of mitochondrial superoxide in pathology and redox signaling in cells and in vivo.

Oxidation of NADH and ROS production by respiratory complex I

Kinetic characteristics of the proton-pumping NADH:quinone reductases (respiratory complexes I) are reviewed. Unsolved problems of the redox-linked proton translocation activities are outlined. The parameters of complex I-mediated superoxide/hydrogen peroxide generation are summarized, and the physiological significance of mitochondrial ROS production is discussed. This article is part of a Special Issue entitled Respiratory complex I, edited by Volker Zickermann and Ulrich Brandt.

Characterization of clinically identified mutations in NDUFV1, the flavin-binding subunit of respiratory complex I, using a yeast model system

Dysfunctions in mitochondrial complex I (NADH:ubiquinone oxidoreductase) are both genetically and clinically highly diverse and a major cause of human mitochondrial diseases. The genetic determinants of individual clinical cases are increasingly being described, but how these genetic defects affect complex I on the molecular and cellular level, and have different clinical consequences in different individuals, is little understood. Furthermore, without molecular-level information innocent genetic variants may be misassigned as pathogenic. Here, we have used a yeast model system (Yarrowia lipolytica) to study the molecular consequences of 16 single amino acid substitutions, classified as pathogenic, in the NDUFV1 subunit of complex I. NDUFV1 binds the flavin cofactor that oxidizes NADH and is the site of complex I-mediated reactive oxygen species production. Seven mutations caused loss of complex I expression, suggesting they are detrimental but precluding further study. In two variants complex I was fully assembled but did not contain any flavin, and four mutations led to functionally compromised enzymes. Our study provides a molecular rationale for assignment of all these variants as pathogenic. However, three variants provided complex I that was functionally equivalent to the wild-type enzyme, challenging their assignment as pathogenic. By combining structural, bioinformatic and functional data, a simple scoring system for the initial evaluation of future NDUFV1 variants is proposed. Overall, our results broaden understanding of how mutations in this centrally important core subunit of complex I affect its function and provide a basis for understanding the role of NDUFV1 mutations in mitochondrial dysfunction.

Assembly defects induce oxidative stress in inherited mitochondrial complex I deficiency

Complex I (CI) deficiency is the most common respiratory chain defect representing more than 30% of mitochondrial diseases. CI is an L-shaped multi-subunit complex with a peripheral arm protruding into the mitochondrial matrix and a membrane arm. CI sequentially assembled into main assembly intermediates: the P (pumping), Q (Quinone) and N (NADH dehydrogenase) modules. In this study, we analyzed 11 fibroblast cell lines derived from patients with inherited CI deficiency resulting from mutations in the nuclear or mitochondrial DNA and impacting these different modules. In patient cells carrying a mutation located in the matrix arm of CI, blue native-polyacrylamide gel electrophoresis (BN-PAGE) revealed a significant reduction of fully assembled CI enzyme and an accumulation of intermediates of the N module. In these cell lines with an assembly defect, NADH dehydrogenase activity was partly functional, even though CI was not fully assembled. We further demonstrated that this functional N module was responsible for ROS production through the reduced flavin mononucleotide. Due to the assembly defect, the FMN site was not re-oxidized leading to a significant oxidative stress in cell lines with an assembly defect. These findings not only highlight the relationship between CI assembly and oxidative stress, but also show the suitability of BN-PAGE analysis in evaluating the consequences of CI dysfunction. Moreover, these data suggest that the use of antioxidants may be particularly relevant for patients displaying a CI assembly defect.

Effects of metformin and other biguanides on oxidative phosphorylation in mitochondria

The biguanide metformin is widely prescribed for Type II diabetes and has anti-neoplastic activity in laboratory models. Despite evidence that inhibition of mitochondrial respiratory complex I by metformin is the primary cause of its cell-lineage-specific actions and therapeutic effects, the molecular interaction(s) between metformin and complex I remain uncharacterized. In the present paper, we describe the effects of five pharmacologically relevant biguanides on oxidative phosphorylation in mammalian mitochondria. We report that biguanides inhibit complex I by inhibiting ubiquinone reduction (but not competitively) and, independently, stimulate reactive oxygen species production by the complex I flavin. Biguanides also inhibit mitochondrial ATP synthase, and two of them inhibit only ATP hydrolysis, not synthesis. Thus we identify biguanides as a new class of complex I and ATP synthase inhibitor. By comparing biguanide effects on isolated complex I and cultured cells, we distinguish three anti-diabetic and potentially anti-neoplastic biguanides (metformin, buformin and phenformin) from two anti-malarial biguanides (cycloguanil and proguanil): the former are accumulated into mammalian mitochondria and affect oxidative phosphorylation, whereas the latter are excluded so act only on the parasite. Our mechanistic and pharmacokinetic insights are relevant to understanding and developing the role of biguanides in new and existing therapeutic applications, including cancer, diabetes and malaria.

Inhibition of Escherichia coli respiratory complex I by Zn(2+)

The energy-converting NADH:ubiquinone oxidoreductase, respiratory complex I, couples NADH oxidation and quinone reduction with the translocation of protons across the membrane. Complex I exhibits a unique L shape with a peripheral arm extending in the aqueous phase and a membrane arm embedded in the lipid bilayer. Both arms have a length of ∼180 Å. The electron transfer reaction is catalyzed by a series of cofactors in the peripheral arm, while the membrane arm catalyzes proton translocation. We used the inhibition of complex I by zinc to shed light on the coupling of the two processes, which is not yet understood. Enzyme kinetics revealed the presence of two high-affinity binding sites for Zn(2+) that are attributed to the proton translocation pathways in the membrane arm. Electrochemically induced Fourier transform infrared difference spectroscopy demonstrated that zinc binding involves at least two protonated acidic residues. Electron paramagnetic resonance spectroscopy showed that one of the cofactors is only partially reduced by NADH in the presence of Zn(2+). We conclude that blocking the proton channels in the membrane arm leads to a partial block of the electron transfer in the peripheral arm, indicating the long-range coupling between both processes.

Solid-state NMR of flavins and flavoproteins

Why apply solid-state NMR (SSNMR) to flavins and flavoproteins? NMR provides information on an atom-specific basis about chemical functionality, structure, proximity to other groups, and dynamics of the system. Thus, it has become indispensable to the study of chemicals, materials, catalysts, and biomolecules. It is no surprise then that NMR has a great deal to offer in the study of flavins and flavoenzymes. In general, their catalytic or electron-transfer activity resides essentially in the flavin, a molecule eminently accessible by NMR. However, the specific reactivity displayed depends on a host of subtle interactions whereby the protein biases and reshapes the flavin's propensities to activate it for one reaction while suppressing other aspects of this cofactor's prodigious repertoire (Massey et al., J Biol Chem 244:3999-4006, 1969; Müller, Z Naturforsch 27B:1023-1026, 1972; Joosten and van Berkel, Curr Opin Struct Biol 11:195-202, 2007). Thus, we are fascinated to learn about how the flavin cofactor of one enzyme is, and is not, like the flavin cofactor of another. In what follows, we describe how the capabilities of SSNMR can help and are beginning to bear fruit in this exciting endeavor.

Investigating the function of [2Fe-2S] cluster N1a, the off-pathway cluster in complex I, by manipulating its reduction potential

NADH:quinone oxidoreductase (complex I) couples NADH oxidation and quinone reduction to proton translocation across an energy-transducing membrane. All complexes I contain a flavin to oxidize NADH, seven iron–sulfur clusters to transfer electrons from the flavin to quinone and an eighth cluster (N1a) on the opposite side of the flavin. The role of cluster N1a is unknown, but Escherichia coli complex I has an unusually high-potential cluster N1a and its reduced flavin produces H₂O₂, not superoxide, suggesting that cluster N1a may affect reactive oxygen species production. In the present study, we combine protein film voltammetry with mutagenesis in overproduced N1a-binding subunits to identify two residues that switch N1a between its high- (E. coli, valine and asparagine) and low- (Bos taurus and Yarrowia lipolytica, proline and methionine) potential forms. The mutations were incorporated into E. coli complex I: cluster N1a could no longer be reduced by NADH, but H₂O₂ and superoxide production were unaffected. The reverse mutations (that increase the potential by ~0.16 V) were incorporated into Y. lipolytica complex I, but N1a was still not reduced by NADH. We conclude that cluster N1a does not affect reactive oxygen species production by the complex I flavin; it is probably required for enzyme assembly or stability.

Two residues that determine the potential of cluster N1a in respiratory complex I were identified, and their effects on its flavin-site reactions were determined. Reduction of cluster N1a by NADH does not affect reactive oxygen species production by the flavin.

Investigation of NADH binding, hydride transfer, and NAD(+) dissociation during NADH oxidation by mitochondrial complex I using modified nicotinamide nucleotides

NADH:ubiquinone oxidoreductase (complex I) is a complicated respiratory enzyme that conserves the energy from NADH oxidation, coupled to ubiquinone reduction, as a proton motive force across the mitochondrial inner membrane. During catalysis, NADH oxidation by a flavin mononucleotide is followed by electron transfer to a chain of iron–sulfur clusters. Alternatively, the flavin may be reoxidized by hydrophilic electron acceptors, by artificial electron acceptors in kinetic studies, or by oxygen and redox-cycling molecules to produce reactive oxygen species. Here, we study two steps in the mechanism of NADH oxidation by complex I. First, molecular fragments of NAD(H), tested as flavin-site inhibitors or substrates, reveal that the adenosine moiety is crucial for binding. Nicotinamide-containing fragments that lack the adenosine do not bind, and ADP-ribose binds more strongly than NAD⁺, suggesting that the nicotinamide is detrimental to binding. Second, the primary kinetic isotope effects from deuterated nicotinamide nucleotides confirm that hydride transfer is from the pro-S position and reveal that hydride transfer, along with NAD⁺ dissociation, is partially rate-limiting. Thus, the transition state energies are balanced so that no single step in NADH oxidation is completely rate-limiting. Only at very low NADH concentrations does weak NADH binding limit NADH:ubiquinone oxidoreduction, and at the high nucleotide concentrations of the mitochondrial matrix, weak nucleotide binding constants assist product dissociation. Using fast nucleotide reactions and a balance between the nucleotide binding constants and concentrations, complex I combines fast and energy-conserving NADH oxidation with minimal superoxide production from the nucleotide-free site.

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.

Partitioning of superoxide and hydrogen peroxide production by mitochondrial respiratory complex I

Membrane-bound respiratory complex I in inside-out submitochondrial particles (SMP) catalyzes both superoxide and hydrogen peroxide formation in NADH- and/or succinate-supported reactions. At optimal NADH concentration (50μM), the complex I-mediated process results in a formation of two superoxide anions and H(2)O(2) as the reaction products in approximately 0.7 ratio. Almost the same ratio is found for purified complex I (0.6) and for the aerobic succinate-supported reverse electron transfer reaction. Superoxide production is depressed at high, more physiologically relevant NADH concentrations, whereas hydrogen peroxide formation is insensitive to the elevated level of NADH. The rates of H(2)O(2) formation at variable NAD(+)/NADH ratios satisfactorily fit the Nernst equation for a single reactive two-electron donor component equilibrated with ambient midpoint redox potential of -347mV (0.13 NAD(+)/NADH ratio, pH 8.0). Half-maximal superoxide production rate proceeds at significantly higher NAD(+)/NADH ratio (0.33). Guanidine strongly stimulates NADH-supported hydrogen peroxide and superoxide production at any NADH concentration and activates NADH:ferricyanide and inhibits NADH:hexaammineruthenium (III) reductase activities while showing no effects on NADH oxidase of SMP. In the low range of NADH concentration, superoxide production rate shows a simple hyperbolic dependence on NADH with apparent K(m)(NADH) of 0.5μM, whereas sigmoidal dependence of hydrogen peroxide production is seen with half-maximal rate at 25μM NADH. We interpret the data as to suggest that at least two sites participate in complex I-mediated ROS generation: FMNH(-) that produces hydrogen peroxide, and an iron-sulfur center (likely N-2) that produces superoxide anion.