Expression and characterization of the flavoprotein subcomplex composed of 50-kDa (NQO1) and 25-kDa (NQO2) subunits of the proton-translocating NADH-quinone oxidoreductase of Paracoccus denitrificans

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.

Revisiting the Burden Borne by Fumarase: Enzymatic Hydration of an Olefin

Fumarate hydratase (FH) is a remarkable catalyst that decreases the free energy of the catalyzed reaction by 30 kcal mol^-1, much larger than most exceptional enzymes with extraordinary catalytic rates. Two classes of FH are observed in nature: class-I and class-II, which have different folds, yet catalyze the same reversible hydration/dehydration reaction of the dicarboxylic acids fumarate/malate, with equal efficiencies. Using class-I FH from the hyperthermophilic archaeon Methanocaldococcus jannaschii (Mj) as a model along with comparative analysis with the only other available class-I FH structure from Leishmania major (Lm), we provide insights into the molecular mechanism of catalysis in this class of enzymes. The structure of MjFH apo-protein has been determined, revealing that large intersubunit rearrangements occur across apo- and holo-protein forms, with a largely preorganized active site for substrate binding. Site-directed mutagenesis of active site residues, kinetic analysis, and computational studies, including density functional theory (DFT) and natural population analysis, together show that residues interacting with the carboxylate group of the substrate play a pivotal role in catalysis. Our study establishes that an electrostatic network at the active site of class-I FH polarizes the substrate fumarate through interactions with its carboxylate groups, thereby permitting an easier addition of a water molecule across the olefinic bond. We propose a mechanism of catalysis in FH that occurs through transition-state stabilization involving the distortion of the electronic structure of the substrate olefinic bond mediated by the charge polarization of the bound substrate at the enzyme active site.

Structure: Function Studies of the Cytosolic, Mo- and NAD+-Dependent Formate Dehydrogenase from Cupriavidus necator

Here, we report recent progress our laboratories have made in understanding the maturation and reaction mechanism of the cytosolic and NAD+-dependent formate dehydrogenase from Cupriavidus necator. Our recent work has established that the enzyme is fully capable of catalyzing the reverse of the physiological reaction, namely, the reduction of CO2 to formate using NADH as a source of reducing equivalents. The steady-state kinetic parameters in the forward and reverse directions are consistent with the expected Haldane relationship. The addition of an NADH-regenerating system consisting of glucose and glucose dehydrogenase increases the yield of formate approximately 10-fold. This work points to possible ways of optimizing the reverse of the enzyme’s physiological reaction with commercial potential as an effective means of CO2 remediation. New insight into the maturation of the enzyme comes from the recently reported structure of the FdhD sulfurase. In E. coli, FdhD transfers a catalytically essential sulfur to the maturing molybdenum cofactor prior to insertion into the apoenzyme of formate dehydrogenase FdhF, which has high sequence similarity to the molybdenum-containing domain of the C. necator FdsA. The FdhD structure suggests that the molybdenum cofactor may first be transferred from the sulfurase to the C-terminal cap domain of apo formate dehydrogenase, rather than being transferred directly to the body of the apoenzyme. Closing of the cap domain over the body of the enzymes delivers the Mo-cofactor into the active site, completing the maturation of formate dehydrogenase. The structural and kinetic characterization of the NADH reduction of the FdsBG subcomplex of the enzyme provides further insights in reversing of the formate dehydrogenase reaction. Most notably, we observe the transient formation of a neutral semiquinone FMNH·, a species that has not been observed previously with holoenzyme. After initial reduction of the FMN of FdsB by NADH to the hydroquinone (with a kred of 680 s−1 and Kd of 190 µM), one electron is rapidly transferred to the Fe2S2 cluster of FdsG, leaving FMNH·. The Fe4S4 cluster of FdsB does not become reduced in the process. These results provide insight into the function not only of the C. necator formate dehydrogenase but also of other members of the NADH dehydrogenase superfamily of enzymes to which it belongs.

Crystallographic and kinetic analyses of the FdsBG subcomplex of the cytosolic formate dehydrogenase FdsABG from Cupriavidus necator

Formate oxidation to carbon dioxide is a key reaction in one-carbon compound metabolism, and its reverse reaction represents the first step in carbon assimilation in the acetogenic and methanogenic branches of many anaerobic organisms. The molybdenum-containing dehydrogenase FdsABG is a soluble NAD⁺-dependent formate dehydrogenase and a member of the NADH dehydrogenase superfamily. Here, we present the first structure of the FdsBG subcomplex of the cytosolic FdsABG formate dehydrogenase from the hydrogen-oxidizing bacterium Cupriavidus necator H16 both with and without bound NADH. The structures revealed that the two iron-sulfur clusters, Fe₄S₄ in FdsB and Fe₂S₂ in FdsG, are closer to the FMN than they are in other NADH dehydrogenases. Rapid kinetic studies and EPR measurements of rapid freeze-quenched samples of the NADH reduction of FdsBG identified a neutral flavin semiquinone, FMNH^•, not previously observed to participate in NADH-mediated reduction of the FdsABG holoenzyme. We found that this semiquinone forms through the transfer of one electron from the fully reduced FMNH^-, initially formed via NADH-mediated reduction, to the Fe₂S₂ cluster. This Fe₂S₂ cluster is not part of the on-path chain of iron-sulfur clusters connecting the FMN of FdsB with the active-site molybdenum center of FdsA. According to the NADH-bound structure, the nicotinamide ring stacks onto the re-face of the FMN. However, NADH binding significantly reduced the electron density for the isoalloxazine ring of FMN and induced a conformational change in residues of the FMN-binding pocket that display peptide-bond flipping upon NAD⁺ binding in proper NADH dehydrogenases.

Heterologous production, isolation, characterization and crystallization of a soluble fragment of the NADH:ubiquinone oxidoreductase (complex I) from Aquifex aeolicus

The proton-pumping NADH:ubiquinone oxidoreductase (complex I) is the first enzyme complex of the respiratory chains in many bacteria and most eukaryotes. It is the least understood of all, due to its enormous size and unique energy conversion mechanism. The bacterial complex is in general made up of 14 different subunits named NuoA-N. Subunits NuoE, -F, and -G comprise the electron input part of the complex. We have cloned these genes from the hyperthermophilic bacterium Aquifex aeolicus and expressed them heterologously in Escherichia coli. A soluble subcomplex made up of NuoE and NuoF and containing the NADH binding site, the primary electron acceptor flavin mononucleotide (FMN), the binuclear iron-sulfur cluster N1a, and the tetranuclear iron-sulfur cluster N3 was isolated by chromatographic methods. The proteins were identified by N-terminal sequencing and mass spectrometry; the cofactors were characterized by UV/vis and EPR spectroscopy. Subunit NuoG was not produced in this strain. The preparation was thermostable and exhibited maximum NADH/ferricyanide oxidoreductase activity at 85 degrees C. Analytical size-exclusion chromatography and dynamic light scattering revealed the homogeneity of the preparation. First attempts to crystallize the preparation led to crystals diffracting more than 2 A.

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.

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.

Novel insights into the role of Neurospora crassa NDUFAF2, an evolutionarily conserved mitochondrial complex I assembly factor

Complex I deficiency is commonly associated with mitochondrial oxidative phosphorylation diseases. Mutations in nuclear genes encoding structural subunits or assembly factors of complex I have been increasingly identified as the cause of the diseases. One such factor, NDUFAF2, is a paralog of the NDUFA12 structural subunit of the enzyme, but the mechanism by which it exerts its function remains unknown. Herein, we demonstrate that the Neurospora crassa NDUFAF2 homologue, the 13.4 L protein, is a late assembly factor that associates with complex I assembly intermediates containing the membrane arm and the connecting part but lacking the N module of the enzyme. Furthermore, we provide evidence that dissociation of the assembly factor is dependent on the incorporation of the putative regulatory module composed of the subunits of 13.4 (NDUFA12), 18.4 (NDUFS6), and 21 (NDUFS4) kDa. Our results demonstrate that the 13.4 L protein is a complex I assembly factor functionally conserved from fungi to mammals.

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.

Electron transfer in subunit NuoI (TYKY) of Escherichia coli NADH:quinone oxidoreductase (NDH-1)

Bacterial proton-translocating NADH:quinone oxidoreductase (NDH-1) consists of a peripheral and a membrane domain. The peripheral domain catalyzes the electron transfer from NADH to quinone through a chain of seven iron-sulfur (Fe/S) clusters. Subunit NuoI in the peripheral domain contains two [4Fe-4S] clusters (N6a and N6b) and plays a role in bridging the electron transfer from cluster N5 to the terminal cluster N2. We constructed mutants for eight individual Cys-coordinating Fe/S clusters. With the exception of C63S, all mutants had damaged architecture of NDH-1, suggesting that Cys-coordinating Fe/S clusters help maintain the NDH-1 structure. Studies of three mutants (C63S-coordinating N6a, P110A located near N6a, and P71A in the vicinity of N6b) were carried out using EPR measurement. These three mutations did not affect the EPR signals from [2Fe-2S] clusters and retained electron transfer activities. Signals at g(z) = 2.09 disappeared in C63S and P110A but not in P71A. Considering our data together with the available information, g(z,x) = 2.09, 1.88 signals are assigned to cluster N6a. It is of interest that, in terms of g(z,x) values, cluster N6a is similar to cluster N4. In addition, we investigated the residues (Ile-94 and Ile-100) that are predicted to serve as electron wires between N6a and N6b and between N6b and N2, respectively. Replacement of Ile-100 and Ile-94 with Ala/Gly did not affect the electron transfer activity significantly. It is concluded that conserved Ile-100 and Ile-94 are not essential for the electron transfer.

Structural contribution of C-terminal segments of NuoL (ND5) and NuoM (ND4) subunits of complex I from Escherichia coli

The proton-translocating NADH-quinone oxidoreductase (complex I/NDH-1) is a multisubunit enzymatic complex. It has a characteristic L-shaped form with two domains, a hydrophilic peripheral domain and a hydrophobic membrane domain. The membrane domain contains three antiporter-like subunits (NuoL, NuoM, and NuoN, Escherichia coli naming) that are considered to be involved in the proton translocation. Deletion of either NuoL or NuoM resulted in an incomplete assembly of NDH-1 and a total loss of the NADH-quinone oxidoreductase activity. We have truncated the C terminus segments of NuoM and NuoL by introducing STOP codons at different locations using site-directed mutagenesis of chromosomal DNA. Our results suggest an important structural role for the C-terminal segments of both subunits. The data further advocate that the elimination of the last transmembrane helix (TM14) of NuoM and the TM16 (at least C-terminal seven residues) or together with the HL helix and the TM15 of the NuoL subunit lead to reduced stability of the membrane arm and therefore of the whole NDH-1 complex. A region of NuoL critical for stability of NDH-1 architecture has been discussed.

The evolution of respiratory chain complex I from a smaller last common ancestor consisting of 11 protein subunits

The NADH:quinone oxidoreductase (complex I) has evolved from a combination of smaller functional building blocks. Chloroplasts and cyanobacteria contain a complex I-like enzyme having only 11 subunits. This enzyme lacks the N-module which harbors the NADH binding site and the flavin and iron–sulfur cluster prosthetic groups. A complex I-homologous enzyme found in some archaea contains an F₄₂₀ dehydrogenase subunit denoted as FpoF rather than the N-module. In the present study, all currently available whole genome sequences were used to survey the occurrence of the different types of complex I in the different kingdoms of life. Notably, the 11-subunit version of complex I was found to be widely distributed, both in the archaeal and in the eubacterial kingdoms, whereas the 14-subunit classical complex I was found only in certain eubacterial phyla. The FpoF-containing complex I was present in Euryarchaeota but not in Crenarchaeota, which contained the 11-subunit complex I. The 11-subunit enzymes showed a primary sequence variability as great or greater than the full-size 14-subunit complex I, but differed distinctly from the membrane-bound hydrogenases. We conclude that this type of compact 11-subunit complex I is ancestral to all present-day complex I enzymes. No designated partner protein, acting as an electron delivery device, could be found for the compact version of complex I. We propose that the primordial complex I, and many of the present-day 11-subunit versions of it, operate without a designated partner protein but are capable of interaction with several different electron donor or acceptor proteins.

Human ind1, an iron-sulfur cluster assembly factor for respiratory complex I

Respiratory complex I (NADH:ubiquinone oxidoreductase) is a large mitochondrial inner membrane enzyme consisting of 45 subunits and 8 iron-sulfur (Fe/S) clusters. While complex I dysfunction is the most common reason for mitochondrial diseases, the assembly of complex I and its Fe/S cofactors remains elusive. Here, we identify the human mitochondrial P-loop NTPase, designated huInd1, that is critically required for the assembly of complex I. huInd1 can bind an Fe/S cluster via a conserved CXXC motif in a labile fashion. Knockdown of huInd1 in HeLa cells by RNA interference technology led to strong decreases in complex I protein and activity levels, remodeling of respiratory supercomplexes, and alteration of mitochondrial morphology. In addition, huInd1 depletion resulted in massive decreases in several subunits (NDUFS1, NDUFV1, NDUFS3, and NDUFA13) of the peripheral arm of complex I, with the concomitant appearance of a 450-kDa subcomplex representing part of the membrane arm. By a novel radiolabeling technique, the amount of iron associated with complex I was also shown to reflect the dependence of this enzyme on huInd1 for assembly. Together, these data identify huInd1 as a new assembly factor for human respiratory complex I with a possible role in the delivery of one or more Fe/S clusters to complex I subunits.

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.

Engineering Escherichia coli for fermentative dihydrogen production: potential role of NADH-ferredoxin oxidoreductase from the hydrogenosome of anaerobic protozoa

Trichomonas vaginalis generates reduced ferredoxin within a unique subcellular organelle, hydrogenosome that is used as a reductant for H2 production. Pyruvate ferredoxin oxidoreductase and NADH dehydrogenase (NADH-DH) are the two enzymes catalyzing the production of reduced ferredoxin. The genes encoding the two subunits of NADH-DH were cloned and expressed in Escherichia coli. Kinetic properties of the recombinant heterodimer were similar to that of the native enzyme from the hydrogenosome. The recombinant holoenzyme contained 2.15 non-heme iron and 1.95 acid-labile sulfur atoms per heterodimer. The EPR spectrum of the dithionite-reduced protein revealed a [2Fe-2S] cluster with a rhombic symmetry of gxyz = 1.917, 1.951, and 2.009 corresponding to cluster N1a of the respiratory complex I. Based on the Fe content, absorption spectrum, and the EPR spectrum of the purified small subunit, the [2Fe-2S] cluster was located in the small subunit of the holoenzyme. This recombinant NADH-DH oxidized NADH and reduced low redox potential electron carriers, such as viologen dyes as well as Clostridium ferredoxin that can couple to hydrogenase for H2 production from NADH. These results show that this unique hydrogenosome NADH dehydrogenase with a critical role in H2 evolution in the hydrogenosome can be produced with near-native properties in E. coli for metabolic engineering of the bacterium towards developing a dark fermentation process for conversion of biomass-derived sugars to H2 as an energy source.

Electrostatic interactions between FeS clusters in NADH:ubiquinone oxidoreductase (Complex I) from Escherichia coli

The redox properties of the cofactors of NADH:ubiquinone oxidoreductase (complex I) from Escherichia coli were studied by following the changes in electron paramagnetic resonance (EPR) and optical spectra upon electrochemical redox titration of the purified protein. At neutral pH, the FMN cofactor had a midpoint redox potential ( E m) approximately -350 mV ( n = 2). Binuclear FeS clusters were well-characterized: N1a was titrated with a single ( n = 1) transition, and E m = -235 mV. In contrast, the titration of N1b can only be fitted with the sum of at least two one-electron Nernstian curves with E m values of -245 and -320 mV. The tetranuclear clusters can also be separated into two groups, either having a single, n = 1, or more complex redox titration curves. The titration curves of the EPR bands attributed to the tetranuclear clusters N2 ( g = 2.045 and g = 1.895) and N6b ( g = 2.089 and g = 1.877) can be presented by the sum of at least two components, each with E m (app) approximately -200/-300 mV and -235/-315 mV, respectively. The titration of the signals at g = 1.956-1.947 (N3 or N7, E m = -315 mV), g = 2.022, and g = 1.932 (Nx, -365 mV) and the low temperature signal at g = 1.929 (N4 or N5, -330 mV) followed Nernstian n = 1 curves. The observed redox titration curves are discussed in terms of intrinsic electrostatic interactions between FeS centers in complex I. A model showing shifts of E m due to the electrostatic interaction between the centers is presented.

The Na+-translocating NADH:quinone oxidoreductase (Na+-NQR) from Vibrio cholerae enhances insertion of FeS in overproduced NqrF subunit

The Na+-translocating NADH:quinone oxidoreductase (Na+-NQR) from Vibrio cholerae is a membrane-bound, respiratory Na+ pump. Its NqrF subunit contains one FAD and a [2Fe-2S] cluster and catalyzes the initial oxidation of NADH. A soluble variant of NqrF lacking its hydrophobic, N-terminal helix (NqrF') was produced in V. cholerae wild type and nqr deletion strain. Under identical conditions of growth and induction, the yield of NqrF' increased by 30% in the presence of the Na+-NQR. FAD-containing NqrF' species with or without the FeS cluster were observed, indicating that assembly of the FeS center, but not insertion of the flavin cofactor, was limited during overproduction in V. cholerae. A comparison of these distinct NqrF' species with regard to specific NADH dehydrogenase activity, pH dependence of activity and thermal inactivation showed that NqrF' lacking the [2Fe-2S] cluster was less stable, partially unfolded, and therefore prone to proteolytic degradation in V. cholerae. We conclude that the overall yield of NqrF' critically depends on the amount of fully assembled, FeS-containing NqrF' in the V. cholerae host cells. The Na+-NQR is proposed to increase the stability of NqrF' by stimulating the maturation of FeS centers.

Reevaluating the relationship between EPR spectra and enzyme structure for the iron sulfur clusters in NADH:quinone oxidoreductase

NADH:quinone oxidoreductase (complex I) plays a pivotal role in cellular energy production. It employs a series of redox cofactors to couple electron transfer to the generation of a proton-motive force across the inner mitochondrial or bacterial cytoplasmic membrane. Complex I contains a noncovalently bound flavin mononucleotide at the active site for NADH oxidation and eight or nine iron-sulfur clusters to transfer electrons between the flavin and a quinone-binding site. Understanding the mechanism of complex I requires the properties of these clusters to be defined, both individually and as an ensemble. Most functional information on the clusters has been gained from EPR spectroscopy, but some clusters are not observed by EPR and attributing the observed signals to the structurally defined clusters is difficult. The current consensus picture relies on correlating the spectra from overexpressed subunits (containing one to four clusters) with those from intact complexes I. Here, we analyze spectra from the overexpressed NuoG subunit from Escherichia coli complex I and compare them with spectra from the intact enzyme. Consequently, we propose that EPR signals N4 and N5 have been misassigned: signal N4 is from NuoI (not NuoG) and signal N5 is from the conserved cysteine-ligated [4Fe-4S] cluster in NuoG (not from the cluster with a histidine ligand). The consequences of reassigning the EPR signals and their associated functional information on the free energy profile for electron transfer through complex I are discussed.

Characterization of a subcomplex of mitochondrial NADH:ubiquinone oxidoreductase (complex I) lacking the flavoprotein part of the N-module

Mitochondrial NADH:ubiquinone oxidoreductase is the largest and most complicated proton pump of the respiratory chain. Here we report the preparation and characterization of a subcomplex of complex I selectively lacking the flavoprotein part of the N-module. Removing the 51-kDa and the 24-kDa subunit resulted in loss of catalytic activity. The redox centers of the subcomplex could be reduced neither by NADH nor NADPH demonstrating that physiological electron input into complex I occurred exclusively via the N-module and that the NADPH binding site in the 39-kDa subunit and further potential nucleotide binding sites are isolated from the electron transfer pathway within the enzyme. Taking advantage of the selective removal of two of the eight iron-sulfur clusters of complex I and providing additional evidence by redox titration and site-directed mutagenesis, we could for the first time unambiguously assign cluster N1 of fungal complex I to mammalian cluster N1b.

Reconstitution and Characterization of Aminopyrrolnitrin Oxygenase, a Rieske N-Oxygenase That Catalyzes Unusual Arylamine Oxidation

Rieske oxygenases catalyze a wide variety of important oxidation reactions. Here we report the characterization of a novel Rieske N-oxygenase, aminopyrrolnitrin oxygenase (PrnD) that catalyzes the unusual oxidation of an arylamine to an arylnitro group. PrnD from Pseudomonas fluorescens Pf5 was functionally expressed in Escherichia coli, and the activity of the purified PrnD was reconstituted, which required in vitro assembly of the Rieske iron-sulfur cluster into the protein and the presence of NADPH, FMN, and an E. coli flavin reductase SsuE. Biochemical and bioinformatics studies indicated that the reconstituted PrnD contains a Rieske iron-sulfur cluster and a mononuclear iron center that are formed by residues Cys(69), Cys(88), His(71), His(91), Asp(323), His(186), and His(191), respectively. The enzyme showed a limited range of substrate specificity and catalyzed the conversion of aminopyrrolnitrin into pyrrolnitrin with K(m) = 191 microM and k(cat) = 6.8 min(-1). Isotope labeling experiments with (18)O(2) and H(2)(18)O suggested that the oxygen atoms in the pyrrolnitrin product are derived exclusively from molecular oxygen. In addition, it was found that the oxygenation of the arylamine substrates catalyzed by PrnD occurs at the enzyme active site and does not involve free radical chain reactions. By analogy to known examples of arylamine oxidation, a catalytic mechanism for the bioconversion of amino pyrrolnitrin into pyrrolnitrin was proposed. Our results should facilitate further mechanistic and crystallographic studies of this arylamine oxygenase and may provide a new enzymatic route for the synthesis of aromatic nitro compounds from their corresponding aromatic amines.

Organization of iron-sulfur clusters in respiratory complex I

Complex I of respiratory chains plays a central role in bioenergetics and is implicated in many human neurodegenerative diseases. An understanding of its mechanism requires a knowledge of the organization of redox centers. The arrangement of iron-sulfur clusters in the hydrophilic domain of complex I from Thermus thermophilus has been determined with the use of x-ray crystallography. One binuclear and six tetranuclear clusters are arranged, maximally 14 angstroms apart, in an 84-angstrom-long electron transfer chain. The binuclear cluster N1a and the tetranuclear cluster N7 are not in this pathway. Cluster N1a may play a role in the prevention of oxidative damage. The structure provides a framework for the interpretation of the large amounts of data accumulated on complex I.

Amino acid residues associated with cluster N3 in the NuoF subunit of the proton-translocating NADH-quinone oxidoreductase fromEscherichia coli

The NuoF subunit, which harbors NADH-binding site, of Escherichia coli NADH-quinone oxidoreductase (NDH-1) contains five conserved cysteine residues, four of which are predicted to ligate cluster N3. To determine this coordination, we overexpressed and purified the NuoF subunit and NuoF+E subcomplex in E. coli. We detected two distinct EPR spectra, arising from a [4Fe-4S] cluster (g(x,y,z)=1.90, 1.95, and 2.05) in NuoF, and a [2Fe-2S] cluster (g(x,y,z)=1.92, 1.95, and 2.01) in NuoE subunit. These clusters were assigned to clusters N3 and N1a, respectively. Based on the site-directed mutagenesis experiments, we identified that cluster N3 is ligated to the 351Cx2Cx2Cx40C398 motif.

Characterization of the Iron-Sulfur Cluster N7 (N1c) in the Subunit NuoG of the Proton-translocating NADH-quinone Oxidoreductase from Escherichia coli

The proton-pumping NADH-quinone oxidoreductase from Escherichia coli houses nine iron-sulfur clusters, eight of which are found in its mitochondrial counterpart, complex I. The extra putative iron-sulfur cluster binding site with a CXXCXXXCX(27)C motif in the NuoG subunit has been assigned to ligate a [2Fe-2S] (N1c). However, we have shown previously that the Thermus thermophilus N1c fragment containing this motif ligates a [4Fe-4S] (Nakamaru-Ogiso, E., Yano, T., Ohnishi, T., and Yagi, T. (2002) J. Biol. Chem. 277, 1680-1688). In the current study, we individually inactivated four sets of the iron-sulfur binding motifs in the E. coli NuoG subunit by replacing all four ligands with Ala. Each mutant subunit, designated Delta N1b, Delta N1c, Delta N4, and Delta N5, was expressed as maltose-binding protein fusion proteins. After in vitro reconstitution, all mutant subunits were characterized by EPR. Although EPR signals from cluster N1b were not detected in any preparations, we detected two [4Fe-4S] EPR signals with g values of g(x,y,z) = 1.89, 1.94, and 2.06, and g(x,y,z) = 1.91, 1.94, and 2.05 at 6-20 K in wild type, Delta N1b, and Delta N5. The former signal was assigned to cluster N4, and the latter signal was assigned to cluster N1c because of their disappearance in Delta N4 and Delta N1c. Confirming that a [4Fe-4S] cluster ligates to the N1c motif, we propose to replace its misleading [2Fe-2S] name, N1c, with "cluster N7." In addition, because these mutations differently affected the assembly of peripheral subunits by in trans complementation analysis with the nuoG knock-out strain, the implicated structural importance of the iron-sulfur binding domains is discussed.

Non-mitochondrial complex I proteins in a hydrogenosomal oxidoreductase complex

Trichomonas vaginalis is a unicellular microaerophilic eukaryote that lacks mitochondria yet contains an alternative organelle, the hydrogenosome, involved in pyruvate metabolism. Pathways between the two organelles differ substantially: in hydrogenosomes, pyruvate oxidation is catalysed by pyruvate:ferredoxin oxidoreductase (PFOR), with electrons donated to an [Fe]-hydrogenase which produces hydrogen. ATP is generated exclusively by substrate-level phosphorylation in hydrogenosomes, as opposed to oxidative phosphorylation in mitochondria. PFOR and hydrogenase are found in eubacteria and amitochondriate eukaryotes, but not in typical mitochondria. Analyses of mitochondrial genomes indicate that mitochondria have a single endosymbiotic origin from an alpha-proteobacterial-type progenitor. The absence of a genome in trichomonad hydrogenosomes precludes such comparisons, leaving the endosymbiotic history of this organelle unclear. Although phylogenetic reconstructions of a few proteins indicate that trichomonad hydrogenosomes share a common origin with mitochondria, others do not. Here we describe a novel NADH dehydrogenase module of respiratory complex I that is coupled to the central hydrogenosomal fermentative pathway to form a hydrogenosomal oxidoreductase complex that seems to function independently of quinones. Phylogenetic analyses of hydrogenosomal complex I-like proteins Ndh51 and Ndh24 reveal that neither has a common origin with mitochondrial homologues. These studies argue against a vertical origin of trichomonad hydrogenosomes from the proto-mitochondrial endosymbiont.

Functional roles of four conserved charged residues in the membrane domain subunit NuoA of the proton-translocating NADH-quinone oxidoreductase from Escherichia coli

The H(+)(Na(+))-translocating NADH-quinone (Q) oxidoreductase (NDH-1) of Escherichia coli is composed of 13 different subunits (NuoA-N). Subunit NuoA (ND3, Nqo7) is one of the seven membrane domain subunits that are considered to be involved in H(+)(Na(+)) translocation. We demonstrated that in the Paracoccus denitrificans NDH-1 subunit, Nqo7 (ND3) directly interacts with peripheral subunits Nqo6 (PSST) and Nqo4 (49 kDa) by using cross-linkers (Di Bernardo, S., and Yagi, T. (2001) FEBS Lett. 508, 385-388 and Kao, M.-C., Matsuno-Yagi, A., and Yagi, T. (2004) Biochemistry 43, 3750-3755). To investigate the structural and functional roles of conserved charged amino acid residues, a nuoA knock-out mutant and site-specific mutants K46A, E51A, D79N, D79A, E81Q, E81A, and D79N/E81Q were constructed by utilizing chromosomal DNA manipulation. In terms of immunochemical and NADH dehydrogenase activity-staining analyses, all site-specific mutants are similar to the wild type, suggesting that those NuoA site-specific mutations do not significantly affect the assembly of peripheral subunits in situ. In addition, site-specific mutants showed similar deamino-NADH-K(3)Fe(CN)(6) reductase activity to the wild type. The K46A mutation scarcely inhibited deamino-NADH-Q reductase activity. In contrast, E51A, D79A, D79N, E81A, and E81Q mutation partially suppressed deamino-NADH-Q reductase activity to 30, 90, 40, 40, and 50%, respectively. The double mutant D79N/E81Q almost completely lost the energy-transducing NDH-1 activities but did not display any loss of deamino-NADH-K(3)Fe(CN)(6) reductase activity. The possible functional roles of residues Asp-79 and Glu-81 were discussed.

Iron-sulfur cluster N2 of the Escherichia coli NADH:ubiquinone oxidoreductase (complex I) is located on subunit NuoB

The proton-pumping NADH:ubiquinone oxidoreductase, also called respiratory complex I, couples the transfer of electrons from NADH to ubiquinone with the translocation of protons across the membrane. One FMN and up to 9 iron-sulfur (Fe/S) clusters participate in the redox reaction. There is discussion that the EPR-detectable Fe/S cluster N2 is involved in proton pumping. However, the assignment of this cluster to a distinct subunit of the complex as well as the number of Fe/S clusters giving rise to the EPR signal are still under debate. Complex I from Escherichia coli consists of 13 polypeptides called NuoA to N. Either subunit NuoB or NuoI could harbor Fe/S cluster N2. Whereas NuoB contains a unique motif for the binding of one Fe/S cluster, NuoI contains a typical ferredoxin motif for the binding of two Fe/S clusters. Individual mutation of all four conserved cysteine residues in NuoB resulted in a loss of complex I activity and of the EPR signal of N2 in the cytoplasmic membrane as well as in the isolated complex. Individual mutations of all eight conserved cysteine residues of NuoI revealed a variable phenotype. Whereas cluster N2 was lost in most NuoI mutants, it was still present in the cytoplasmic membranes of the mutants NuoI C63A and NuoI C102A. N2 was also detected in the complex isolated from the mutant NuoI C102A. From this we conclude that the Fe/S cluster N2 is located on subunit NuoB.

The nuclear encoded subunits of complex I from bovine heart mitochondria

NADH:ubiquinone oxidoreductase (complex I) from bovine heart mitochondria is a complicated, multi-subunit, membrane-bound assembly. Recently, the subunit compositions of complex I and three of its subcomplexes have been reevaluated comprehensively. The subunits were fractionated by three independent methods, each based on a different property of the subunits. Forty-six different subunits, with a combined molecular mass of 980 kDa, were identified. The three subcomplexes, I alpha, I beta and I lambda, correlate with parts of the membrane extrinsic and membrane-bound domains of the complex. Therefore, the partitioning of subunits amongst these subcomplexes has provided information about their arrangement within the L-shaped structure. The sequences of 45 subunits of complex I have been determined. Seven of them are encoded by mitochondrial DNA, and 38 are products of the nuclear genome, imported into the mitochondrion from the cytoplasm. Post-translational modifications of many of the nuclear encoded subunits of complex I have been identified. The seven mitochondrially encoded subunits, and seven of the nuclear encoded subunits, are homologues of the 14 subunits found in prokaryotic complexes I. They are considered to be sufficient for energy transduction by complex I, and they are known as the core subunits. The core subunits bind a flavin mononucleotide (FMN) at the active site for NADH oxidation, up to eight iron-sulfur clusters, and one or more ubiquinone molecules. The locations of some of the cofactors can be inferred from the sequences of the core subunits. The remaining 31 subunits of bovine complex I are the supernumerary subunits, which may be important either for the stability of the complex, or for its assembly. Sequence relationships suggest that some of them carry out reactions unrelated to the NADH:ubiquinone oxidoreductase activity of the complex.

Proton pumping by NADH:ubiquinone oxidoreductase. A redox driven conformational change mechanism?

The modular evolutionary origin of NADH:ubiquinone oxidoreductase (complex I) provides useful insights into its functional organization. Iron-sulfur cluster N2 and the PSST and 49 kDa subunits were identified as key players in ubiquinone reduction and proton pumping. Structural studies indicate that this 'catalytic core' region of complex I is clearly separated from the membrane. Complex I from Escherichia coli and Klebsiella pneumoniae was shown to pump sodium ions rather than protons. These new insights into structure and function of complex I strongly suggest that proton or sodium pumping in complex I is achieved by conformational energy transfer rather than by a directly linked redox pump.

Characterization of cluster N5 as a fast-relaxing [4Fe-4S] cluster in the Nqo3 subunit of the proton-translocating NADH-ubiquinone oxidoreductase from Paracoccus denitrificans

The NADH-quinone oxidoreductase from Paracoccus denitrificans consists of 14 subunits (Nqo1-14) and contains one FMN and eight iron-sulfur clusters. The Nqo3 subunit possesses fully conserved 11 Cys and 1 His in its N-terminal region and is considered to harbor three iron-sulfur clusters; however, only one binuclear (N1b) and one tetranuclear (N4) were previously identified. In this study, the Nqo3 subunit containing 1x[2Fe-2S] and 2x[4Fe-4S] clusters was expressed in Escherichia coli. The second [4Fe-4S](1+) cluster is detected by EPR spectroscopy below 6 K, exhibiting very fast spin relaxation. The resolved EPR spectrum of this cluster is broad and nearly axial. The subunit exhibits an absorption-type EPR signal around g approximately 5 region below 6 K, most likely arising from an S = 3/2 ground state of the fast-relaxing [4Fe-4S](1+) species. The substitution of the conserved His(106) with Cys specifically affected the fast-relaxing [4Fe-4S](1+) cluster, suggesting that this cluster is coordinated by His(106). In the cholate-treated NDH-1-enriched P. denitrificans membranes, we observed EPR signals arising from a [4Fe-4S] cluster below 6 K, exhibiting properties similar to those of cluster N5 detected in other complex I/NDH-1 and of the fast-relaxing [4Fe-4S](1+) cluster in the expressed Nqo3 subunit. Hence, we propose that the His-coordinated [4Fe-4S] cluster corresponds to cluster N5.

Two EPR-detectable [4Fe-4S] clusters, N2a and N2b, are bound to the NuoI (TYKY) subunit of NADH:ubiquinone oxidoreductase (Complex I) from Rhodobacter capsulatus

NADH:ubiquinone oxidoreductases (Complex I) contain a subunit, TYKY in the bovine enzyme and NuoI in the enzyme from Rhodobacter capsulatus, which is assumed to bind two [4Fe-4S] clusters because it contains two sets of conserved cysteine motifs similar to those found in the 2[4Fe-4S] ferredoxins. It was recently shown that the TYKY subunit is not an ordinary 2[4Fe-4S] ferredoxin, but has a unique amino acid sequence, which is only found in NAD(P)H:quinone oxidoreductases and certain membrane-bound [NiFe]-hydrogenases expected to be involved in redox-linked proton translocation [FEBS Lett. 485 (2000) 1]. We have generated a set of R. capsulatus mutants in which five out of the eight conserved cysteine residues in NuoI were replaced by other amino acids. The resulting mutants fell into three categories with virtually no, intermediate or quite normal Complex I activities. EPR-spectroscopic analysis of the membranes of the C67S and C106S mutants, two mutants belonging to the second and third group, respectively, showed a specific 50% decrease of the EPR signal attributed to cluster N2. It is concluded that the NuoI (TYKY) subunit binds two clusters N2, called N2a and N2b, which exhibit very similar spectral features when analyzed by X-band EPR spectroscopy.

Characterization of the iron-sulfur cluster coordinated by a cysteine cluster motif (CXXCXXXCX27C) in the Nqo3 subunit in the proton-translocating NADH-quinone oxidoreductase (NDH-1) of Thermus thermophilus HB-8

The proton-translocating NADH-quinone oxidoreductase (NDH-1) of Thermus thermophilus HB-8 is composed of 14 subunits (designated Nqo1-14). This NDH-1 houses nine putative iron-sulfur binding sites, eight of which are generally found in bacterial NDH-1 and its mitochondrial counterpart (complex I). The extra site contains a CXXCXXXCX(27)C motif and is located in the Nqo3 subunit. This motif was originally found in Escherichia coli NDH-1 and was assigned to a binuclear cluster (g(z, y, x) = 2.00, 1.95, 1.92) and named N1c. In this report, the Thermus Nqo3 fragment containing this motif was heterologously overexpressed, using a glutathione S-transferase fusion system. This fragment contained a small amount of iron-sulfur cluster, whose content was significantly increased by in vitro reconstitution. The UV-visible and EPR spectroscopic properties of this fragment indicate that the ligated iron-sulfur cluster is tetranuclear with nearly axial symmetry (g( parallel, perpendicular) = 2.045, approximately 1.94). Site-directed mutants show that all four cysteines participate in the ligation of a [4Fe-4S] cluster. Considering the fact that the same motif coordinates only tetranuclear clusters in other enzymes so far known, we propose that the CXXCXXXCX(27)C motif in the Nqo3 subunit most likely ligates the [4Fe-4S] cluster.

Direct interaction between a membrane domain subunit and a connector subunit in the H(+)-translocating NADH-quinone oxidoreductase

When Paracoccus denitrificans membranes were treated with a crosslinker, m-maleimidobenzoyl-N-hydroxysuccinimide ester (MBS), a cross-linked product of M(r) approximately 31 kDa was found which reacted with antibodies against the hydrophobic subunit Nqo7 and the connector subunit Nqo6. NaI treatment of the Paracoccus membranes before, but not after, the crosslinking step prevented the formation of the 31 kDa band. When Nqo7 and Nqo6 were coexpressed in Escherichia coli, both subunits were located in the membrane fraction. MBS treatment of the E. coli membranes generated the 31 kDa band as in the Paracoccus membranes. These results indicate that Nqo7 interacts with probable N2-binding Nqo6.

The origin of cluster N2 of the energy-transducing NADH-quinone oxidoreductase: comparisons of phylogenetically related enzymes

NADH-quinone (Q) oxidoreductase is a large and complex redox proton pump, which utilizes the free energy derived from oxidation of NADH with lipophilic electron/proton carrier Q to translocate protons across the membrane to generate an electrochemical proton gradient. Although its molecular mechanism is largely unknown, recent biochemical, biophysical, and molecular biological studies have revealed that particular subunits and cofactors play an essential role in the energy-coupling reaction. Based on these latest experimental data, we exhaustively analyzed the sequence information available from evolutionarily related enzymes such as [NiFe] hydrogenases. We found significant and conserved sequence differences in the PSST/Nqo6/NuoB, 49kDa/Nqo4/NuoD, and ND1/Nqo8/NuoH subunit homologs between complex I/NDH-1 and [NiFe] hydrogenases. The alterations, especially in the postulated ligand motif for cluster N2 in the PSST/Nqo6/NuoB subunits, appear to be evolutionarily important in determining the physiological function of complex I/NDH-1. These observations led us to propose a hypothetical evolutionary scheme: during the course of evolution, drastic changes have occurred in the putative cluster N2 binding site in the PSST/Nqo6/NuoB subunit and the progenitors of complex I/NDH-1 have concurrently become to utilize a lipophilic electron/proton carrier such as Q as its physiological substrate. This scheme provides new insights into the structure and function relationship of complex I/NDH-1 and may help us understand its energy-coupling mechanism.

NADH dehydrogenases: from basic science to biomedicine

This review article is concerned with two on-going research projects in our laboratory, both of which are related to the study of the NADH dehydrogenase enzyme complexes in the respiratory chain. The goal of the first project is to decipher the structure and mechanism of action of the proton-translocating NADH-quinone oxidoreductase (NDH-1) from two bacteria, Paracoccus denitrificans and Thermus thermophilus HB-8. These microorganisms are of particular interest because of the close resemblance of the former (P. denitrificans) to a mammalian mitochondria, and because of the thermostability of the enzymes of the latter (T. thermophilus). The NDH-1 enzyme complex of these and other bacteria is composed of 13 to 14 unlike subunits and has a relatively simple structure relative to the mitochondrial proton-translocating NADH-quinone oxidoreductase (complex I), which is composed of at least 42 different subunits. Therefore, the bacterial NDH-I is believed to be a useful model for studying the mitochondrial complex I, which is understood to have the most intricate structure of all the membrane-associated enzyme complexes. Recently, the study of the NADH dehydrogenase complex has taken on new urgency as a result of reports that complex I defects are involved in many human mitochondrial diseases. Thus the goal of the second project is to develop possible gene therapies for mitochondrial diseases caused by complex I defects. This project involves attempting to repair complex I defects in the mammalian system using Saccharomyces cerevisiae NDI1 genes, which code for the internal, rotenone-insensitive NADH-quinone oxidoreductase. In this review, we will discuss our progress and the data generated by these two projects to date. In addition, background information and the significance of various approaches employed to pursue these research objectives will be described.

Complex I: a chimaera of a redox and conformation-driven proton pump?

From phylogenetic sequence analysis, it can be concluded that the proton-pumping NADH:ubiquinone oxidoreductase (complex I) has evolved from preexisting modules for electron transfer and proton translocation. It is built up by a peripheral NADH dehydrogenase module, an amphipatic hydrogenase module, and a membrane-bound transporter module. These modules, or at least part of them, are also present in various other bacterial enzymes. It is assumed that they fulfill a similar function in complex I and related enzymes. Based on the function of the individual modules, it is possible to speculate about the mechanism of complex I. The hydrogenase module might work as a redox-driven proton pump, while the transporter module might act as a conformation-driven proton pump. This implies that complex I contains two energy-coupling sites. The NADH dehydrogenase module seems to be involved in electron transfer and not in proton translocation.

The multiple nicotinamide nucleotide-binding subunits of bovine heart mitochondrial NADH:ubiquinone oxidoreductase (complex I)

Direct photoaffinity labeling of purified bovine heart NADH:ubiquinone oxidoreductase (complex I) with 32P-labeled NAD(H), NADP(H) and ADP has shown that five polypeptides become labeled, with molecular masses of 51, 42, 39, 30, and 18-20 kDa. The 51 and the 30-kDa polypeptides were labeled with either [32P]NAD(H), [32P]NADP(H) or [beta-32P]ADP. The 42-kDa polypeptide was labeled with [32P]NAD(H) and to a small extent with [beta-32P]ADP. It was not labeled with [32P]NADP(H). The 39-kDa polypeptide was labeled with [32P]NADPH and to a small extent with [beta-32P]ADP. Our previous studies had shown that this subunit also binds NADP, but not NAD(H) [Yamaguchi, M., Belogrudov, G.I. & Hatefi, Y. (1998) J. Biol. Chem. 273, 8094-8098]. The 18-20-kDa polypeptide was labeled only with [32P]NADPH. Among these polypeptides, the 51-kDa subunit is known to contain FMN and a [4Fe-4S] cluster, and is the NAD(P)H-binding subunit of the primary dehydrogenase domain of complex I. The possible roles of the other nucleotide-binding subunits of complex I have been discussed.

Characterization of the putative 2x[4Fe-4S]-binding NQO9 subunit of the proton-translocating NADH-quinone oxidoreductase (NDH-1) of Paracoccus denitrificans. Expression, reconstitution, and EPR characterization

Molecular properties of the NQO9 subunit of Paracoccus denitrificans NDH-1, which is predicted to contain 2x[4Fe-4S] clusters, were investigated using recombinant expression techniques and EPR spectroscopy. The full-length form of NQO9 subunit co-expressed with thioredoxin in Escherichia coli at ambient temperature was found dominantly in the cytoplasmic membrane with low amplification. Genetic deletion of relatively hydrophobic and less conserved N-terminal stretches (30 or 40 amino acid residues long) of the NQO9 subunit resulted in the overexpression of the truncated soluble form of the subunit in a high yield in the cytoplasm. The purified soluble form of the NQO9 subunit contained only a small quantity of Fe and S(2-) (2.0-2.2 mol each per mol of subunit). However, the iron-sulfur content was considerably increased by in vitro reconstitution. The reconstituted NQO9 subunit contained 7.6-7.7 mol each of Fe and S(2-) per molecule and exhibited optical absorption spectra similar to those of 2x[4Fe-4S] ferredoxins. Two sets of relatively broad axial-type EPR signals with different temperature dependence and power saturation profile were detected in the dithionite-reduced preparations at a low temperature range (8-18 K). Due to a negative shift (<600 mV) of the apparent redox midpoint potential of the iron-sulfur clusters in the soluble form of the truncated NQO9 subunit, the following two possible cases could not be discriminated: (i) two sets of EPR signals arise from two distinct species of tetranuclear iron-sulfur clusters with two intrinsically different spectral parameters g(, perpendicular) = 2.05, approximately 1.93, and g(parallel, perpendicular) = 2.08, approximately 1.90, and respective slow (P((1)/(2)) = 8 milliwatts) and fast (P((1)/(2)) = 342 milliwatts) spin relaxation; (ii) two clusters exhibit similar intrinsic EPR spectra (g(parallel, perpendicular) = 2.05, approximately 1.93) with slow spin relaxation. When both clusters in the same subunit are concomitantly paramagnetic, their spin-spin interactions cause a shift of spectra to g(parallel, perpendicular) = 2.08, approximately 1.90, with enhanced spin relaxation. In either case, our EPR data provide the first experimental evidence for the presence of two [4Fe-4S] iron-sulfur clusters in the NQO9 subunit.

H(+)-translocating NADH-quinone oxidoreductase (NDH-1) of Paracoccus denitrificans. Studies on topology and stoichiometry of the peripheral subunits

The proton-translocating NADH-quinone oxidoreductase (NDH-1) of Paracoccus denitrificans is composed of at least 14 subunits (NQO1-14) and is located in the cytoplasmic membrane. In the present study, topological properties and stoichiometry of the 7 subunits (NQO1-6 and NQO9) of the P. denitrificans NDH-1 in the membranes were investigated using immunological techniques. Treatments with chaotropic reagents (urea, NaI, or NaBr) or with alkaline buffer (pH 10-12) resulted in partial or complete extraction of all the subunits from the membranes. Of interest is that when NaBr or urea were used, the NQO6 and NQO9 subunits remained in the membranes, whereas the other subunits were completely extracted, suggesting their direct association with the membrane part of the enzyme complex. Both deletion study and homologous expression study of the NQO9 subunit provided a clue that its hydrophobic N-terminal stretch plays an important role in such an association. In light of this observation and others, topological properties of the subunits in the NDH-1 enzyme complex are discussed. In addition, determination of stoichiometry of the peripheral subunits of the P. denitrificans NDH-1 was completed by radioimmunological methods. All the peripheral subunits are present as one molecule each in the enzyme complex. These results estimated the total number of cofactors in the P. denitrificans NDH-1; the enzyme complex contains one molecule of FMN and up to eight iron-sulfur clusters, 2x[2Fe-2S] and 6x[4Fe-4S], provided that the NQO6 subunit bears one [4Fe-4S] cluster.

The hyperthermophilic bacterium, Thermotoga maritima, contains an unusually complex iron-hydrogenase: amino acid sequence analyses versus biochemical characterization

The hyperthermophilic bacterium, Thermotoga maritima, grows up to 90 degrees C by fermenting carbohydrates and it disposes of excess reductant by H(2) production. The H(2)-evolving cytoplasmic hydrogenase of this organism was shown to consist of three different subunits of masses 73 (alpha), 68 (beta) and 19 (gamma) kDa and to contain iron as the only metal. The genes encoding the subunits were clustered in a single operon in the order hydC (gamma), hydB (beta), and hydA (alpha). Sequence analyses indicated that: (a) the enzyme is an Fe-S-cluster-containing flavoprotein which uses NADH as an electron donor; and (b) the catalytic Fe-S cluster resides within the alpha-subunit, which is equivalent to the single subunit that constitutes most mesophilic Fe-hydrogenases. The alpha- and beta-subunits of the purified enzyme were separated by chromatography in the presence of 4 M urea. As predicted, the H(2)-dependent methyl viologen reduction activity of the holoenzyme (45-70 U mg(-1)) was retained in the alpha-subunit (130-160 U mg(-1)) after subunit separation. However, the holoenzyme did not contain flavin and neither it nor the alpha-subunit used NAD(P)(H) or T. maritima ferredoxin as an electron carrier. The holoenzyme, but not the alpha-subunit, reduced anthraquinone-2,6-disulfonate (apparent K(m), 690 microM) with H(2). The EPR properties of the reduced holoenzyme, when compared with those of the separated and reduced subunits, indicate the presence of a catalytic 'H-cluster' and three [4Fe-4S] and one [2Fe-2S] cluster in the alpha-subunit, together with one [4Fe-4S] and two [2Fe-2S] clusters in the beta-subunit. Sequence analyses predict that the alpha-subunit should contain an additional [2Fe-2S] cluster, while the beta-subunit should contain one [2Fe-2S] and three [4Fe-4S] clusters. The latter cluster contents are consistent with the measured Fe contents of about 32, 20 and 14 Fe mol(-1) for the holoenzyme and the alpha- and beta-subunits, respectively. The T. maritima enzyme is the first 'complex' Fe-hydrogenase to be purified and characterized, although the reason for its complexity remains unclear.

Structure-function studies of iron-sulfur clusters and semiquinones in the NADH-Q oxidoreductase segment of the respiratory chain

Our recent experimental data on iron-sulfur clusters and semiquinones in the complex I segment of the respiratory chain is presented, focusing on the Paracoccus (P.) denitrificans and bovine heart studies. The iron-sulfur cluster N2 has attracted the attention of investigators in the research field of complex I, since the mid-point redox potential of this cluster is the highest among all clusters in complex I, and is pH dependent (60 mV/pH). It is known that this cluster is located either in the NQO6 (NuoB in E. coli/PSST in bovine heart nomenclature) or in the NQO9 (NuoI/TYKY) subunit in the amphipathic domain of complex I. Our preliminary data indicate that the cluster N2 is located in the NuoB rather than the long-advocated NuoI subunit, and may have a unique ligand structure. We previously reported spin-spin interactions between cluster N2 and two distinct species of semiquinone (designated SQNf and SQNs) associated with complex I. A parallel intensity change was observed between the SQNf (g = 2.00) signal and the N2 split g parallel signal, further supporting our proposed interaction between SQNf and N2 spins.

Iron-sulfur clusters/semiquinones in complex I

NADH-quinone 1 oxidoreductase (Complex I) isolated from bovine heart mitochondria was, until recently, the major source for the study of this most complicated energy transducing device in the mitochondrial respiratory chain. Complex I has been shown to contain 43 subunits and possesses a molecular mass of about 1 million. Recently, Complex I genes have been cloned and sequenced from several bacterial sources including Escherichia coli, Paracoccus denitrificans, Rhodobacter capsulatus and Thermus thermophilus HB-8. These enzymes are less complicated than the bovine enzyme, containing a core of 13 or 14 subunits homologous to the bovine heart Complex I. From this data, important clues concerning the subunit location of both the substrate binding site and intrinsic redox centers have been gleaned. Powerful molecular genetic approaches used in these bacterial systems can identify structure/function relationships concerning the redox components of Complex I. Site-directed mutants at the level of bacterial chromosomes and over-expression and purification of single subunits have allowed detailed analysis of the amino acid residues involved in ligand binding to several iron-sulfur clusters. Therefore, it has become possible to examine which subunits contain individual iron-sulfur clusters, their location within the enzyme and what their ligand residues are. The discovery of g=2.00 EPR signals arising from two distinct species of semiquinone (SQ) in the activated bovine heart submitochondrial particles (SMP) is another line of recent progress. The intensity of semiquinone signals is sensitive to DeltamicroH+ and is diminished by specific inhibitors of Complex I. To date, semiquinones similar to those reported for the bovine heart mitochondrial Complex I have not yet been discovered in the bacterial systems. This mini-review describes three aspects of the recent progress in the study of the redox components of Complex I: (A) the location of the substrate (NADH) binding site, flavin, and most of the iron-sulfur clusters, which have been identified in the hydrophilic electron entry domain of Complex I; (B) experimental evidence indicating that the cluster N2 is located in the amphipathic domain of Complex I, connecting the promontory and membrane parts. Very recent data is also presented suggesting that the cluster N2 may have a unique ligand structure with an atypical cluster-ligation sequence motif located in the NuoB (NQO6/PSST) subunit rather than in the long advocated NuoI (NQO9/TYKY) subunit. The latter subunit contains the most primordial sequence motif for two tetranuclear clusters; (C) the discovery of spin-spin interactions between cluster N2 and two distinct Complex I-associated species of semiquinone. Based on the splitting of the g1 signal of the cluster N2 and concomitant strong enhancement of the semiquinone spin relaxation, one semiquinone species was localized 8-11 A from the cluster N2 within the inner membrane on the matrix side (N-side). Spin relaxation of the other semiquinone species is much less enhanced, and thus it was proposed to have a longer distance from the cluster N2, perhaps located closer to the other side (P-side) surface of the membrane. A brief introduction of EPR technique was also described in Appendix A of this mini-review.

Complex I from the fungus Neurospora crassa

Respiratory chain complex I is a complicated enzyme of mitochondria, that couples electron transfer from NADH to ubiquinone to the proton translocation across the inner membrane of the organelle. The fungus Neurospora crassa has been used as one of the main model organisms to study this enzyme. Complex I is composed of multiple polypeptide subunits of dual genetic origin and contains several prosthetic groups involved in its activity. Most subunits have been cloned and those binding redox centres have been identified. Yet, the functional role of certain complex I proteins remains unknown. Insight into the possible origin and the mechanisms of complex I assembly has been gained. Several mutant strains of N. crassa, in which specific subunits of complex I were disrupted, have been isolated and characterised. This review concerns many aspects of the structure, function and biogenesis of complex I that are being elucidated.

Mitochondrial NADH-ubiquinone oxidoreductase (Complex I). Effect of substrates on the fragmentation of subunits by trypsin

It has been shown that treatment of bovine mitochondrial complex I (NADH-ubiquinone oxidoreductase) with NADH or NADPH, but not with NAD or NADP, increases the susceptibility of a number of subunits to tryptic degradation. This increased susceptibility involved subunits that contain electron carriers, such as FMN and iron-sulfur clusters, as well as subunits that lack electron carriers. Results shown elsewhere on changes in the cross-linking pattern of complex I subunits when the enzyme was pretreated with NADH or NADPH (Belogrudov, G., and Hatefi, Y. (1994) Biochemistry 33, 4571-4576) also indicated that complex I undergoes extensive conformation changes when reduced by substrate. Furthermore, we had previously shown that in submitochondrial particles the affinity of complex I for NAD increases by >/=20-fold in electron transfer from succinate to NAD when the particles are energized by ATP hydrolysis. Together, these results suggest that energy coupling in complex I may involve protein conformation changes as a key step. In addition, it has been shown here that treatment of complex I with trypsin in the presence of NADPH, but not NADH or NAD(P), produced from the 39-kDa subunit a 33-kDa degradation product that resisted further hydrolysis. Like the 39-kDa subunit, the 33-kDa product bound to a NADP-agarose affinity column, and could be eluted with a buffer containing NADPH. It is possible that together with the acyl carrier protein of complex I the NADP(H)-binding 39-kDa subunit is involved in intramitochondrial fatty acid synthesis.

Genetic analysis of the nuo locus, which encodes the proton-translocating NADH dehydrogenase in Escherichia coli

Complex I (EC 1.6.99.3) of the bacterium Escherichia coli is considered to be the minimal form of the type I NADH dehydrogenase, the first enzyme complex in the respiratory chain. Because of its small size and relative simplicity, the E. coli enzyme has become a model used to identify and characterize the mechanism(s) by which cells regulate the synthesis and assembly of this large respiratory complex. To begin dissecting the processes by which E. coli cells regulate the expression of nuo and the assembly of complex I, we undertook a genetic analysis of the nuo locus, which encodes the 14 Nuo subunits comprising E. coli complex I. Here we present the results of studies, performed on an isogenic collection of nuo mutants, that focus on the physiological, biochemical, and molecular consequences caused by the lack of or defects in several Nuo subunits. In particular, we present evidence that NuoG, a peripheral subunit, is essential for complex I function and that it plays a role in the regulation of nuo expression and/or the assembly of complex I.

Immuno-purification of a dimeric subcomplex of the respiratory NADH-CoQ reductase of Rhodobacter capsulatus equivalent to the FP fraction of the mitochondrial complex I

The Rhodobacter capsulatus genes encoding the NUOE and NUOF subunits, equivalent to the 24 kDa and 51 kDa subunits of the mammalian mitochondrial complex I, have been sequenced. According to the nucleotide sequence, the NUOE subunit is 389 amino acids long and has a molecular mass of 41.3 kDa. In comparison to the mitochondrial equivalent subunit, NUOE is extended at the C terminus by more than 150 amino acids. The NUOF subunit is 431 amino acids long and has a molecular mass of 47.1 kDa. A subcomplex containing both the NUOE and NUOF subunits was extracted by detergent treatment of R. capsulatus membranes and immuno-purified. This subcomplex is homologous to the mitochondrial FP fragment. Mass spectrometry after trypsin treatment of the NUOE subunit validates the atypical primary structure deduced from the sequence of the gene.

cDNA sequence and chromosomal localization of the NDUFS8 human gene coding for the 23 kDa subunit of the mitochondrial complex I

We have sequenced the cDNA for the 23 kDa subunit of the human mitochondrial respiratory complex I. The deduced protein consists of 210 amino acids (Mr = 23705 Da) with a 34 amino acid N terminus presumably acting as a presequence for mitochondrial import. The predicted mature protein (Mr = 20290 Da) is 92% identical to the bovine mitochondrial subunit and 72% to the Rhodobacter capsulatus NUOI counterpart. Two clusters of four cysteine residues are conserved among these proteins. The gene (NDUFS8) coding for the human subunit has been mapped to chromosome 11q13.