Identification of six subunits constituting Na+-translocating NADH-quinone reductase from the marine Vibrio alginolyticus

Living in mangroves: a syntrophic scenario unveiling a resourceful microbiome

BACKGROUND

Mangroves are complex and dynamic coastal ecosystems under frequent fluctuations in physicochemical conditions related to the tidal regime. The frequent variation in organic matter concentration, nutrients, and oxygen availability, among other factors, drives the microbial community composition, favoring syntrophic populations harboring a rich and diverse, stress-driven metabolism. Mangroves are known for their carbon sequestration capability, and their complex and integrated metabolic activity is essential to global biogeochemical cycling. Here, we present a metabolic reconstruction based on the genomic functional capability and flux profile between sympatric MAGs co-assembled from a tropical restored mangrove.

RESULTS

Eleven MAGs were assigned to six Bacteria phyla, all distantly related to the available reference genomes. The metabolic reconstruction showed several potential coupling points and shortcuts between complementary routes and predicted syntrophic interactions. Two metabolic scenarios were drawn: a heterotrophic scenario with plenty of carbon sources and an autotrophic scenario with limited carbon sources or under inhibitory conditions. The sulfur cycle was dominant over methane and the major pathways identified were acetate oxidation coupled to sulfate reduction, heterotrophic acetogenesis coupled to carbohydrate catabolism, ethanol production and carbon fixation. Interestingly, several gene sets and metabolic routes similar to those described for wastewater and organic effluent treatment processes were identified.

CONCLUSION

The mangrove microbial community metabolic reconstruction reflected the flexibility required to survive in fluctuating environments as the microhabitats created by the tidal regime in mangrove sediments. The metabolic components related to wastewater and organic effluent treatment processes identified strongly suggest that mangrove microbial communities could represent a resourceful microbial model for biotechnological applications that occur naturally in the environment.

The transferred translocases: An old wine in a new bottle

The role of translocases was underappreciated and was not included as a separate class in the enzyme commission until August 2018. The recent research interests in proteomics of orphan enzymes, ionomics, and metallomics along with high-throughput sequencing technologies generated overwhelming data and revamped this enzyme into a separate class. This offers a great opportunity to understand the role of new or orphan enzymes in general and specifically translocases. The enzymes belonging to translocases regulate/permeate the transfer of ions or molecules across the membranes. These enzyme entries were previously associated with other enzyme classes, which are now transferred to a new enzyme class 7 (EC 7). The entries that are reclassified are important to extend the enzyme list, and it is the need of the hour. Accordingly, there is an upgradation of entries of this class of enzymes in several databases. This review is a concise compilation of translocases with reference to the number of entries currently available in the databases. This review also focuses on function as well as dysfunction of translocases during normal and disordered states, respectively.

Improving electron trans-inner membrane movements in microbial electrocatalysts

A novel nondestructive strategy of improving electron trans-inner membrane movements in bioelectrocatalysts is realized by overexpressing NADH dehydrogenase II in the inner membrane. A microbial fuel cell loaded with these improved bioelectrocatalysts shows significantly enhanced performance based on promoting the utilization of intracellular primary electron donors in bioelectrocatalysts.

NqrM (DUF539) Protein Is Required for Maturation of Bacterial Na+-Translocating NADH:Quinone Oxidoreductase

Na(+)-translocating NADH:quinone oxidoreductase (Na(+)-NQR) catalyzes electron transfer from NADH to ubiquinone in the bacterial respiratory chain, coupled with Na(+) translocation across the membrane. Na(+)-NQR maturation involves covalent attachment of flavin mononucleotide (FMN) residues, catalyzed by flavin transferase encoded by the nqr-associated apbE gene. Analysis of complete bacterial genomes has revealed another putative gene (duf539, here renamed nqrM) that usually follows the apbE gene and is present only in Na(+)-NQR-containing bacteria. Expression of the Vibrio harveyi nqr operon alone or with the associated apbE gene in Escherichia coli, which lacks its own Na(+)-NQR, resulted in an enzyme incapable of Na(+)-dependent NADH or reduced nicotinamide hypoxanthine dinucleotide (dNADH) oxidation. However, fully functional Na(+)-NQR was restored when these genes were coexpressed with the V. harveyi nqrM gene. Furthermore, nqrM lesions in Klebsiella pneumoniae and V. harveyi prevented production of functional Na(+)-NQR, which could be recovered by an nqrM-containing plasmid. The Na(+)-NQR complex isolated from the nqrM-deficient strain of V. harveyi lacks several subunits, indicating that nqrM is necessary for Na(+)-NQR assembly. The protein product of the nqrM gene, NqrM, contains a single putative transmembrane α-helix and four conserved Cys residues. Mutating one of these residues (Cys33 in V. harveyi NqrM) to Ser completely prevented Na(+)-NQR maturation, whereas mutating any other Cys residue only decreased the yield of the mature protein. These findings identify NqrM as the second specific maturation factor of Na(+)-NQR in proteobacteria, which is presumably involved in the delivery of Fe to form the (Cys)4[Fe] center between subunits NqrD and NqrE.

IMPORTANCE

Na(+)-translocating NADH:quinone oxidoreductase complex (Na(+)-NQR) is a unique primary Na(+) pump believed to enhance the vitality of many bacteria, including important pathogens such as Vibrio cholerae, Vibrio parahaemolyticus, Haemophilus influenzae, Neisseria gonorrhoeae, Pasteurella multocida, Porphyromonas gingivalis, Enterobacter aerogenes, and Yersinia pestis. Production of Na(+)-NQR in bacteria requires Na(+)-NQR-specific maturation factors. We earlier identified one such factor (ApbE) that covalently attaches flavin residues to Na(+)-NQR. Here we identify the other protein factor, designated NqrM, and show that NqrM and ApbE suffice to produce functional Na(+)-NQR from the Vibrio harveyi nqr operon. NqrM may be involved in Fe delivery to a unique Cys4[Fe] center during Na(+)-NQR assembly. Besides highlighting Na(+)-NQR biogenesis, these findings suggest a novel drug target to combat Na(+)-NQR-containing bacteria.

Identification of the coupling step in Na(+)-translocating NADH:quinone oxidoreductase from real-time kinetics of electron transfer

Bacterial Na(+)-translocating NADH:quinone oxidoreductase (Na(+)-NQR) uses a unique set of prosthetic redox groups-two covalently bound FMN residues, a [2Fe-2S] cluster, FAD, riboflavin and a Cys4[Fe] center-to catalyze electron transfer from NADH to ubiquinone in a reaction coupled with Na(+) translocation across the membrane. Here we used an ultra-fast microfluidic stopped-flow instrument to determine rate constants and the difference spectra for the six consecutive reaction steps of Vibrio harveyi Na(+)-NQR reduction by NADH. The instrument, with a dead time of 0.25 ms and optical path length of 1 cm allowed collection of visible spectra in 50-μs intervals. By comparing the spectra of reaction steps with the spectra of known redox transitions of individual enzyme cofactors, we were able to identify the chemical nature of most intermediates and the sequence of electron transfer events. A previously unknown spectral transition was detected and assigned to the Cys4[Fe] center reduction. Electron transfer from the [2Fe-2S] cluster to the Cys4[Fe] center and all subsequent steps were markedly accelerated when Na(+) concentration was increased from 20 μM to 25 mM, suggesting coupling of the former step with tight Na(+) binding to or occlusion by the enzyme. An alternating access mechanism was proposed to explain electron transfer between subunits NqrF and NqrC. According to the proposed mechanism, the Cys4[Fe] center is alternatively exposed to either side of the membrane, allowing the [2Fe-2S] cluster of NqrF and the FMN residue of NqrC to alternatively approach the Cys4[Fe] center from different sides of the membrane.

Draft Genome Sequence of the Moderately Thermophilic Bacterium Schleiferia thermophila Strain Yellowstone (Bacteroidetes)

The draft genome sequence of the moderately thermophilic bacterium Schleiferia thermophila strain Yellowstone (Bacteroidetes), isolated from Octopus Spring (Yellowstone National Park, WY, USA) was sequenced and comprises 2,617,694 bp in 35 contigs. The draft genome is predicted to encode 2,457 protein coding genes and 37 tRNA encoding genes and two rRNA operons.

The sodium pumping NADH:quinone oxidoreductase (Na⁺-NQR), a unique redox-driven ion pump

The Na(+)-translocating NADH:quinone oxidoreductase (Na(+)-NQR) is a unique Na(+) pumping respiratory complex found only in prokaryotes, that plays a key role in the metabolism of marine and pathogenic bacteria, including Vibrio cholerae and other human pathogens. Na(+)-NQR is the main entrance for reducing equivalents into the respiratory chain of these bacteria, catalyzing the oxidation of NADH and the reduction of quinone, the free energy of this redox reaction drives the selective translocation of Na(+) across the cell membrane, which energizes key cellular processes. In this review we summarize the unique properties of Na(+)-NQR in terms of its redox cofactor composition, electron transfer reactions and a possible mechanism of coupling and pumping.

Localization-controlled specificity of FAD:threonine flavin transferases in Klebsiella pneumoniae and its implications for the mechanism of Na(+)-translocating NADH:quinone oxidoreductase

The Klebsiella pneumoniae genome contains genes for two putative flavin transferase enzymes (ApbE1 and ApbE2) that add FMN to protein Thr residues. ApbE1, but not ApbE2, has a periplasm-addressing signal sequence. The genome also contains genes for three target proteins with the Dxx(s/t)gAT flavinylation motif: two subunits of Na(+)-translocating NADH:quinone oxidoreductase (Na(+)-NQR), and a 99.5kDa protein, KPK_2907, with a previously unknown function. We show here that KPK_2907 is an active cytoplasmically-localized fumarate reductase. K. pneumoniae cells with an inactivated kpk_2907 gene lack cytoplasmic fumarate reductase activity, while retaining this activity in the membrane fraction. Complementation of the mutant strain with a kpk_2907-containing plasmid resulted in a complete recovery of cytoplasmic fumarate reductase activity. KPK_2907 produced in Escherichia coli cells contains 1mol/mol each of covalently bound FMN, noncovalently bound FMN and noncovalently bound FAD. Lesion in the ApbE1 gene in K. pneumoniae resulted in inactive Na(+)-NQR, but cytoplasmic fumarate reductase activity remained unchanged. On the contrary, lesion in the ApbE2 gene abolished the fumarate reductase but not the Na(+)-NQR activity. Both activities could be restored by transformation of the ApbE1- or ApbE2-deficient K. pneumoniae strains with plasmids containing the Vibrio cholerae apbE gene with or without the periplasm-directing signal sequence, respectively. Our data thus indicate that ApbE1 and ApbE2 bind FMN to Na(+)-NQR and fumarate reductase, respectively, and that, contrary to the presently accepted view, the FMN residues are on the periplasmic side of Na(+)-NQR. A new, "electron loop" mechanism is proposed for Na(+)-NQR, involving an electroneutral Na(+)/electron symport. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference.

Alternative pyrimidine biosynthesis protein ApbE is a flavin transferase catalyzing covalent attachment of FMN to a threonine residue in bacterial flavoproteins

Na(+)-translocating NADH:quinone oxidoreductase (Na(+)-NQR) contains two flavin residues as redox-active prosthetic groups attached by a phosphoester bond to threonine residues in subunits NqrB and NqrC. We demonstrate here that flavinylation of truncated Vibrio harveyi NqrC at Thr-229 in Escherichia coli cells requires the presence of a co-expressed Vibrio apbE gene. The apbE genes cluster with genes for Na(+)-NQR and other FMN-binding flavoproteins in bacterial genomes and encode proteins with previously unknown function. Experiments with isolated NqrC and ApbE proteins confirmed that ApbE is the only protein factor required for NqrC flavinylation and also indicated that the reaction is Mg(2+)-dependent and proceeds with FAD but not FMN. Inactivation of the apbE gene in Klebsiella pneumoniae, wherein the nqr operon and apbE are well separated in the chromosome, resulted in a complete loss of the quinone reductase activity of Na(+)-NQR, consistent with its dependence on covalently bound flavin. Our data thus identify ApbE as a novel modifying enzyme, flavin transferase.

Sodium-dependent movement of covalently bound FMN residue(s) in Na(+)-translocating NADH:quinone oxidoreductase

Na(+)-translocating NADH:quinone oxidoreductase (Na(+)-NQR) is a component of respiratory electron-transport chain of various bacteria generating redox-driven transmembrane electrochemical Na(+) potential. We found that the change in Na(+) concentration in the reaction medium has no effect on the thermodynamic properties of prosthetic groups of Na(+)-NQR from Vibrio harveyi, as was revealed by the anaerobic equilibrium redox titration of the enzyme's EPR spectra. On the other hand, the change in Na(+) concentration strongly alters the EPR spectral properties of the radical pair formed by the two anionic semiquinones of FMN residues bound to the NqrB and NqrC subunits (FMN(NqrB) and FMN(NqrC)). Using data obtained by pulse X- and Q-band EPR as well as by pulse ENDOR and ELDOR spectroscopy, the interspin distance between FMN(NqrB) and FMN(NqrC) was found to be 15.3 Å in the absence and 20.4 Å in the presence of Na(+), respectively. Thus, the distance between the covalently bound FMN residues can vary by about 5 Å upon changes in Na(+) concentration. Using these results, we propose a scheme of the sodium potential generation by Na(+)-NQR based on the redox- and sodium-dependent conformational changes in the enzyme.

Insights into the mechanism of electron transfer and sodium translocation of the Na(+)-pumping NADH:quinone oxidoreductase

Na(+)-NQR is a unique energy-transducing complex, widely distributed among marine and pathogenic bacteria. It converts the energy from the oxidation of NADH and the reduction of quinone into an electrochemical Na(+)-gradient that can provide energy for the cell. Na(+)-NQR is not homologous to any other respiratory protein but is closely related to the RNF complex. In this review we propose that sodium pumping in Na(+)-NQR is coupled to the redox reactions by a novel mechanism, which operates at multiple sites, is indirect and mediated by conformational changes of the protein. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

Cys377 residue in NqrF subunit confers Ag(+) sensitivity of Na+-translocating NADH:quinone oxidoreductase from Vibrio harveyi

The Na(+)-translocating NADH:quinone oxidoreductase (Na(+)-NQR) is a component of the respiratory chain of various bacteria that generates a redox-driven transmembrane electrochemical Na(+) potential. The Na(+)-NQR activity is known to be specifically inhibited by low concentrations of silver ions. Replacement of the conserved Cys377 residue with alanine in the NqrF subunit of Na(+)-NQR from Vibrio harveyi resulted in resistance of the enzyme to Ag(+) and to other heavy metal ions. Analysis of the catalytic activity also showed that the rate of electron input into the mutant Na(+)-NQR decreased by about 14-fold in comparison to the wild type enzyme, whereas all other properties of (NqrF)C377A Na(+)-NQR including its stability remained unaffected.

Localization of ubiquinone-8 in the Na+-pumping NADH:quinone oxidoreductase from Vibrio cholerae

Na(+) is the second major coupling ion at membranes after protons, and many pathogenic bacteria use the sodium-motive force to their advantage. A prominent example is Vibrio cholerae, which relies on the Na(+)-pumping NADH:quinone oxidoreductase (Na(+)-NQR) as the first complex in its respiratory chain. The Na(+)-NQR is a multisubunit, membrane-embedded NADH dehydrogenase that oxidizes NADH and reduces quinone to quinol. Existing models describing redox-driven Na(+) translocation by the Na(+)-NQR are based on the assumption that the pump contains four flavins and one FeS cluster. Here we show that the large, peripheral NqrA subunit of the Na(+)-NQR binds one molecule of ubiquinone-8. Investigations of the dynamic interaction of NqrA with quinones by surface plasmon resonance and saturation transfer difference NMR reveal a high affinity, which is determined by the methoxy groups at the C-2 and C-3 positions of the quinone headgroup. Using photoactivatable quinone derivatives, it is demonstrated that ubiquinone-8 bound to NqrA occupies a functional site. A novel scheme of electron transfer in Na(+)-NQR is proposed that is initiated by NADH oxidation on subunit NqrF and leads to quinol formation on subunit NqrA.

Localization and function of the membrane-bound riboflavin in the Na+-translocating NADH:quinone oxidoreductase (Na+-NQR) from Vibrio cholerae

The sodium ion-translocating NADH:quinone oxidoreductase (Na(+)-NQR) from the human pathogen Vibrio cholerae is a respiratory membrane protein complex that couples the oxidation of NADH to the transport of Na(+) across the bacterial membrane. The Na(+)-NQR comprises the six subunits NqrABCDEF, but the stoichiometry and arrangement of these subunits are unknown. Redox-active cofactors are FAD and a 2Fe-2S cluster on NqrF, covalently attached FMNs on NqrB and NqrC, and riboflavin and ubiquinone-8 with unknown localization in the complex. By analyzing the cofactor content and NADH oxidation activity of subcomplexes of the Na(+)-NQR lacking individual subunits, the riboflavin cofactor was unequivocally assigned to the membrane-bound NqrB subunit. Quantitative analysis of the N-terminal amino acids of the holo-complex revealed that NqrB is present in a single copy in the holo-complex. It is concluded that the hydrophobic NqrB harbors one riboflavin in addition to its covalently attached FMN. The catalytic role of two flavins in subunit NqrB during the reduction of ubiquinone to ubiquinol by the Na(+)-NQR is discussed.

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

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

Redox properties of the prosthetic groups of Na(+)-translocating nadh:quinone oxidoreductase. 1. Electron paramagnetic resonance study of the enzyme

Redox properties of all EPR-detectable prosthetic groups of Na(+)-translocating NADH:quinone oxidoreductase (Na(+)-NQR) from Vibrio harveyi were studied at pH 7.5 using cryo-EPR spectroelectrochemistry. Titration shows five redox transitions. One with E(m) = -275 mV belongs to the reduction of the [2Fe-2S] cluster, and the four others reflect redox transitions of flavin cofactors. Two transitions (E(m)(1) = -190 mV and E(m)(2) = -275 mV) originate from the formation of FMN anion radical, covalently bound to the NqrC subunit, and its subsequent reduction. The remaining two transitions arise from the two other flavin cofactors. A high potential (E(m) = -10 mV) transition corresponds to the reduction of riboflavin neutral radical, which is stable at rather high redox potentials. An E(m) = -130 mV transition reflects the formation of FMN anion radical from a flavin covalently bound to the NqrB subunit, which stays as a radical down to very low potentials. Taking into account the EPR-silent, two-electron transition of noncovalently bound FAD located in the NqrF subunit, there are four flavins in Na(+)-NQR all together. Defined by dipole-dipole magnetic interaction measurements, the interspin distance between the [2Fe-2S](+) cluster and the NqrB subunit-bound FMN anion radical is found to be 22.5 +/- 1.5 A, which means that for the functional electron transfer between these two centers another cofactor, most likely FMN bound to the NqrC subunit, should be located.

Redox properties of the prosthetic groups of Na(+)-translocating NADH:quinone oxidoreductase. 2. Study of the enzyme by optical spectroscopy

Redox titration of the electronic spectra of the prosthetic groups of the Na(+)-translocating NADH:quinone oxidoreductase (Na(+)-NQR) from Vibrio harveyi at different pH values showed five redox transitions corresponding to the four flavin cofactors of the enzyme and one additional transition reflecting oxidoreduction of the [2Fe-2S] cluster. The pH dependence of the measured midpoint redox potentials showed that the two-electron reduction of the FAD located in the NqrF subunit was coupled with the uptake of only one H(+). The one-electron reduction of neutral semiquinone of riboflavin and the formation of anion flavosemiquinone from the oxidized FMN bound to the NqrB subunit were not coupled to any proton uptake. The two sequential one-electron reductions of the FMN residue bound to the NqrC subunit showed pH-independent formation of anion radical in the first step and the formation of fully reduced flavin coupled to the uptake of one H(+) in the second step. All four flavins stayed in the anionic form in the fully reduced enzyme. None of the six redox transitions in Na(+)-NQR showed dependence of its midpoint redox potential on the concentration of sodium ions. A model of the sequence of electron transfer steps in the enzyme is suggested.

The Na+-translocating NADH : ubiquinone oxidoreductase of Azotobacter vinelandii negatively regulates alginate synthesis

Azotobacter vinelandii is a nitrogen-fixing soil bacterium that produces the exopolysaccharide alginate. In this report we describe the isolation and characterization of A. vinelandii strain GG4, which carries an nqrE : : Tn5 mutation resulting in alginate overproduction. The nqrE gene encodes a subunit of the Na+-translocating NADH : ubiquinone oxidoreductase (Na+-NQR). As expected, Na+-NQR activity was abolished in mutant GG4. When this strain was complemented with the nqrEF genes this activity was restored and alginate production was reduced to wild-type levels. Na+-NQR may be the main sodium pump of A. vinelandii under the conditions tested (∼2 mM Na+) since no Na+/H+-antiporter activity was detected. Collectively our results indicate that in A. vinelandii the lack of Na+-NQR activity caused the absence of a transmembrane Na+ gradient and an increase in alginate production.

Catalytic properties of Na+-translocating NADH:quinone oxidoreductases from Vibrio harveyi, Klebsiella pneumoniae, and Azotobacter vinelandii

The catalytic properties of sodium-translocating NADH:quinone oxidoreductases (Na+-NQRs) from the marine bacterium Vibrio harveyi, the enterobacterium Klebsiella pneumoniae, and the soil microorganism Azotobacter vinelandii have been comparatively analyzed. It is shown that these enzymes drastically differ in their affinity to sodium ions. The enzymes also possess different sensitivity to inhibitors. Na+-NQR from A. vinelandii is not sensitive to low 2-n-heptyl-4-hydroxyquinoline N-oxide (HQNO) concentrations, while Na+-NQR from K. pneumoniae is fully resistant to either Ag+ or N-ethylmaleimide. All the Na+-NQR-type enzymes are sensitive to diphenyliodonium, which is shown to modify the noncovalently bound FAD of the enzyme.

Redox-Dependent Sodium Binding by the Na+-Translocating NADH:Quinone Oxidoreductase from Vibrio harveyi

Relaxation characteristics of the 23Na nuclei magnetization were used to determine the sodium-binding properties of the Na+-translocating NADH:quinone oxidoreductase from Vibrio harveyi (NQR). The dissociation constant of Na+ for the oxidized enzyme was found to be 24 mM and for the reduced enzyme about 30 microM. Such large (3 orders in magnitude) redox dependence of the NQR affinity to sodium ions shows that the molecular machinery was designed to use the drop in redox energy for creating an electrochemical sodium gradient. Redox titration of NQR monitored by changes in line width of the 23Na NMR signal at 2 mM Na+ showed that the enzyme affinity to sodium ions follows the Nernst law for a one-electron carrier with Em about -300 mV (vs SHE). The data indicate that energy conservation by NQR involves a mechanism modulating ion affinity by the redox state of an enzyme redox cofactor.

Regulation of expression of Na+ -translocating NADH:quinone oxidoreductase genes in Vibrio harveyi and Klebsiella pneumoniae

The expression of genes encoding sodium-translocating NADH:quinone oxidoreductase (Na(+)-NQR) was studied in the marine bacterium Vibrio harveyi and in the enterobacterium Klebsiella pneumoniae. It has been shown that such parameters as NaCl concentration, pH value, and presence of an uncoupler in the growth media do not influence significantly the level of nqr expression. However, nqr expression depends on the growth substrates used by these bacteria. Na(+)-NQR is highly repressed in V. harveyi during anaerobic growth, and nqr expression is modulated by electron acceptors and values of their redox potentials. The latter effect was shown to be independent of the ArcAB regulatory system.

The three families of respiratory NADH dehydrogenases

Most reducing equivalents extracted from foodstuffs during oxidative metabolism are fed into the respiratory chains of aerobic bacteria and mitochondria by NADH:quinone oxidoreductases. Three families of enzymes can perform this task and differ remarkably in their complexity and role in energy conversion. Alternative or NDH-2-type NADH dehydrogenases are simple one subunit flavoenzymes that completely dissipate the redox energy of the NADH/quinone couple. Sodium-pumping NADH dehydrogenases (Nqr) that are only found in procaryotes contain several flavins and are integral membrane protein complexes composed of six different subunits. Proton-pumping NADH dehydrogenases (NDH-1 or complex I) are highly complicated membrane protein complexes, composed of up to 45 different subunits, that are found in bacteria and mitochondria. This review gives an overview of the origin, structural and functional properties and physiological significance of these three types of NADH dehydrogenase.

Oxygen, cyanide and energy generation in the cystic fibrosis pathogen Pseudomonas aeruginosa

Pseudomonas aeruginosa is a gram-negative, rod-shaped bacterium that belongs to the gamma-proteobacteria. This clinically challenging, opportunistic pathogen occupies a wide range of niches from an almost ubiquitous environmental presence to causing infections in a wide range of animals and plants. P. aeruginosa is the single most important pathogen of the cystic fibrosis (CF) lung. It causes serious chronic infections following its colonisation of the dehydrated mucus of the CF lung, leading to it being the most important cause of morbidity and mortality in CF sufferers. The recent finding that steep O2 gradients exist across the mucus of the CF-lung indicates that P. aeruginosa will have to show metabolic adaptability to modify its energy metabolism as it moves from a high O2 to low O2 and on to anaerobic environments within the CF lung. Therefore, the starting point of this review is that an understanding of the diverse modes of energy metabolism available to P. aeruginosa and their regulation is important to understanding both its fundamental physiology and the factors significant in its pathogenicity. The main aim of this review is to appraise the current state of knowledge of the energy generating pathways of P. aeruginosa. We first look at the organisation of the aerobic respiratory chains of P. aeruginosa, focusing on the multiple primary dehydrogenases and terminal oxidases that make up the highly branched pathways. Next, we will discuss the denitrification pathways used during anaerobic respiration as well as considering the ability of P. aeruginosa to carry out aerobic denitrification. Attention is then directed to the limited fermentative capacity of P. aeruginosa with discussion of the arginine deiminase pathway and the role of pyruvate fermentation. In the final part of the review, we consider other aspects of the biology of P. aeruginosa that are linked to energy metabolism or affected by oxygen availability. These include cyanide synthesis, which is oxygen-regulated and can affect the operation of aerobic respiratory pathways, and alginate production leading to a mucoid phenotype, which is regulated by oxygen and energy availability, as well as having a role in the protection of P. aeruginosa against reactive oxygen species. Finally, we consider a possible link between cyanide synthesis and the mucoid switch that operates in P. aeruginosa during chronic CF lung infection.

Thermodynamic Properties of the Redox Centers of Na+-Translocating NADH:Quinone Oxidoreductase

Redox titration of all optically detectable prosthetic groups of Na(+)-translocating NADH:quinone oxidoreductase (Na(+)-NQR) at pH 7.5 showed that the functionally active enzyme possesses only three titratable flavin cofactors, one noncovalently bound FAD and two covalently bound FMN residues. All three flavins undergo different redox transitions during the function of the enzyme. The noncovalently bound FAD works as a "classical" two-electron carrier with a midpoint potential (E(m)) of -200 mV. Each of the FMN residues is capable of only one-electron reduction: one from neutral flavosemiquinone to fully reduced flavin (E(m) = 20 mV) and the other from oxidized flavin to flavosemiquinone anion (E(m) = -150 mV). The lacking second half of the redox transitions for the FMNs cannot be reached under our experimental conditions and is most likely not employed in the catalytic cycle. Besides the flavins, a [2Fe-2S] cluster was shown to function in the enzyme as a one-electron carrier with an E(m) of -270 mV. The midpoint potentials of all the redox transitions determined in the enzyme were found to be independent of Na(+) concentration. Even the components that exhibit very strong retardation in the rate of their reduction by NADH at low sodium concentrations experienced no change in the E(m) values when the concentration of the coupling ion was changed 1000 times. On the basis of these data, plausible mechanisms for the translocation of transmembrane sodium ions by Na(+)-NQR are discussed.

Marine enzymes

Marine enzyme biotechnology can offer novel biocatalysts with properties like high salt tolerance, hyperthermostability, barophilicity, cold adaptivity, and ease in large-scale cultivation. This review deals with the research and development work done on the occurrence, molecular biology, and bioprocessing of marine enzymes during the last decade. Exotic locations have been accessed for the search of novel enzymes. Scientists have isolated proteases and carbohydrases from deep sea hydrothermal vents. Cold active metabolic enzymes from psychrophilic marine microorganisms have received considerable research attention. Marine symbiont microorganisms growing in association with animals and plants were shown to produce enzymes of commercial interest. Microorganisms isolated from sediment and seawater have been the most widely studied, proteases, carbohydrases, and peroxidases being noteworthy. Enzymes from marine animals and plants were primarily studied for their metabolic roles, though proteases and peroxidases have found industrial applications. Novel techniques in molecular biology applied to assess the diversity of chitinases, nitrate, nitrite, ammonia-metabolizing, and pollutant-degrading enzymes are discussed. Genes encoding chitinases, proteases, and carbohydrases from microbial and animal sources have been cloned and characterized. Research on the bioprocessing of marine-derived enzymes, however, has been scanty, focusing mainly on the application of solid-state fermentation to the production of enzymes from microbial sources.

Na(+)-Translocating NADH:quinone oxidoreductase: progress achieved and prospects of investigations

Structural and catalytic properties of bacterial Na+-translocating NADH:quinone oxidoreductases are briefly described. Special attention is given to studies on kinetics of the enzyme interaction with NADH and the role of sodium ions in this process. Based on the existing data, possible model mechanisms of sodium transfer by Na+-translocating NADH:quinone oxidoreductase are proposed.

Mutagenesis study of the 2Fe-2S center and the FAD binding site of the Na(+)-translocating NADH:ubiquinone oxidoreductase from Vibrio cholerae

Many marine and pathogenic bacteria have a unique sodium-translocating NADH:ubiquinone oxidoreductase (Na(+)-NQR), which generates an electrochemical Na(+) gradient during aerobic respiration. Na(+)-NQR consists of six subunits (NqrA-F) and contains five known redox cofactors: two covalently bound FMNs, one noncovalently bound FAD, one riboflavin, and one 2Fe-2S center. A stable neutral flavin-semiquinone radical is observed in the air-oxidized enzyme, while the NADH- or dithionite-reduced enzyme exhibits a stable anionic flavin-semiquinone radical. The NqrF subunit has been implicated in binding of both the 2Fe-2S cluster and the FAD. Four conserved cysteines (C70, C76, C79, and C111) in NqrF match the canonical 2Fe-2S motif, and three conserved residues (R210, Y212, S246) have been predicted to be part of a flavin binding domain. In this work, these two motifs have been altered by site-directed mutagenesis of individual residues and are confirmed to be essential for binding, respectively, the 2Fe-2S cluster and FAD. EPR spectra of the FAD-deficient mutants in the oxidized and reduced forms exhibit neutral and anionic flavo-semiquinone radical signals, respectively, demonstrating that the FAD in NqrF is not the source of either radical signal. In both the FAD and 2Fe-2S center mutants the line widths of the neutral and anionic flavo-semiquinone EPR signals are unchanged from the wild-type enzyme, indicating that neither of these centers is nearby or coupled to the radicals. Measurements of steady-state turnover using NADH, Q-1, and the artificial electron acceptor ferricyanide strongly support an electron transport pathway model in which the noncovalently bound FAD in the NqrF subunit is the initial electron acceptor and electrons then flow to the 2Fe-2S center.

Enhanced photosynthesis and redox energy production contribute to salinity tolerance in Dunaliella as revealed by homology-based proteomics

Salinity is a major limiting factor for the proliferation of plants and inhibits central metabolic activities such as photosynthesis. The halotolerant green alga Dunaliella can adapt to hypersaline environments and is considered a model photosynthetic organism for salinity tolerance. To clarify the molecular basis for salinity tolerance, a proteomic approach has been applied for identification of salt-induced proteins in Dunaliella. Seventy-six salt-induced proteins were selected from two-dimensional gel separations of different subcellular fractions and analyzed by mass spectrometry (MS). Application of nanoelectrospray mass spectrometry, combined with sequence-similarity database-searching algorithms, MS BLAST and MultiTag, enabled identification of 80% of the salt-induced proteins. Salinity stress up-regulated key enzymes in the Calvin cycle, starch mobilization, and redox energy production; regulatory factors in protein biosynthesis and degradation; and a homolog of a bacterial Na(+)-redox transporters. The results indicate that Dunaliella responds to high salinity by enhancement of photosynthetic CO(2) assimilation and by diversion of carbon and energy resources for synthesis of glycerol, the osmotic element in Dunaliella. The ability of Dunaliella to enhance photosynthetic activity at high salinity is remarkable because, in most plants and cyanobacteria, salt stress inhibits photosynthesis. The results demonstrated the power of MS BLAST searches for the identification of proteins in organisms whose genomes are not known and paved the way for dissecting molecular mechanisms of salinity tolerance in algae and higher plants.

The origin of the sodium-dependent NADH oxidation by the respiratory chain ofKlebsiella pneumoniae

Properties of Klebsiella pneumoniae respiratory chain enzymes catalyzing NADH oxidation have been studied. Using constructed K. pneumoniae mutant strains, it was shown that three enzymes belonging to different families of NADH:quinone oxidoreductases operate in this bacterium. The NDH-2-type enzyme is not coupled with energy conservation, the NDH-1-type enzyme is a primary proton pump, and the NQR-type enzyme is homologous to the sodium-motive NADH dehydrogenase of Vibrio and is shown to be a primary Na(+) pump. It is concluded that the NQR-type enzyme, not the NDH-1-type enzyme, catalyzes sodium-dependent NADH oxidation in K. pneumoniae.

NADH oxidation by the Na+-translocating NADH:quinone oxidoreductase from Vibrio cholerae: functional role of the NqrF subunit

The Na(+)-translocating NADH:quinone oxidoreductase from Vibrio cholerae is a six subunit enzyme containing four flavins and a single motif for the binding of a Fe-S cluster on its NqrF subunit. This study reports the production of a soluble variant of NqrF (NqrF') and its individual flavin and Fe-S-carrying domains using V. cholerae or Escherichia coli as expression hosts. NqrF' and the flavin domain each contain 1 mol of FAD/mol of enzyme and exhibit high NADH oxidation activity (20,000 micromol min(-1) mg(-1)). EPR, visible absorption, and circular dichroism spectroscopy indicate that the Fe-S cluster in NqrF' and its Fe-S domain is related to 2Fe ferredoxins of the vertebrate-type. The addition of NADH to NqrF' results in the formation of a neutral flavosemiquinone and a partial reduction of the Fe-S cluster. The NqrF subunit harbors the active site of NADH oxidation and acts as a converter between the hydride donor NADH and subsequent one-electron reaction steps in the Na(+)-translocating NADH:quinone oxidoreductase complex. The observed electron transfer NADH --> FAD --> [2Fe-2S] in NqrF requires positioning of the FAD and the Fe-S cluster in close proximity in accordance with a structural model of the subunit.

Mutagenesis of subunit N of the Escherichia coli complex I. Identification of the initiation codon and the sensitivity of mutants to decylubiquinone

The last gene in the nuo operon of Escherichia coli, nuoN, encodes a membrane-bound subunit of Complex I (NADH:ubiquinone oxidoreductase). In this report, the gene for subunit N was disrupted by a 163 bp deletion in the chromosome, resulting in the loss of Complex I function, as measured by deamino-NADH oxidase activity. This activity could be recovered after transformation of the mutant strain by a plasmid that contains the previously identified nuoN gene and the upstream intergenic region between nuoM and nuoN. Mutagenesis of the first ATG downstream of nuoM led to a loss of function, indicating that this is the likely initiation codon for nuoN, and predicting a protein of 485 amino acids and 52 044 Da. Thirty site-specific mutations in nuoN at 19 different positions were constructed in a vector that expresses the full-length subunit N with both an octahistidine tag and an HA epitope tag at the carboxyl terminus. Highly conserved charged and aromatic residues were selected for mutagenesis, as well as a substitution that occurs as a secondary mutation in Leber's hereditary optic neuropathy (LHON). Membranes from the mutant strains were tested for production of subunit N by immunoblots and for NADH-linked activities. Mutants with substitutions at six different positions (K158, K217, H224, K247, Y300, and K395) had rates of deamino-NADH oxidase activity that were no more than 50% of that of the wild type and had reduced rates of proton translocation. These mutants also showed enhanced inhibition by decylubiquinone, indicating that subunit N interacts with quinones. The mutation associated with LHON, G391S, had little effect on these functions.

Kinetics of the spectral changes during reduction of the Na+-motive NADH:quinone oxidoreductase from Vibrio harveyi

Two radical signals with different line widths are seen in the Na+-translocating NADH:ubiquinone oxidoreductase (Na+-NQR) from Vibrio harveyi by EPR spectroscopy. The first radical is observed in the oxidized enzyme, and is assigned as a neutral flavosemiquinone. The second radical is observed in the reduced enzyme and is assigned to be the anionic form of flavosemiquinone. The time course of Na+-NQR reduction by NADH, as monitored by stopped-flow optical spectroscopy, shows three distinct phases, the spectra of which suggest that they correspond to the reduction of three different flavin species. The first phase is fast both in the presence and absence of sodium, and is assigned to reduction of FAD to FADH2 at the NADH dehydrogenating site. The rates of the other two phases are strongly dependent on sodium concentration, and these phases are attributed to reduction of two covalently bound FMN's. Combination of the optical and EPR data suggests that a neutral FMN flavosemiquinone preexists in the oxidized enzyme, and that it is reduced to the fully reduced flavin by NADH. The other FMN moiety is initially oxidized, and is reduced to the anionic flavosemiquinone. One-electron transitions of two discrete flavin species are thus assigned as sodium-dependent steps in the catalytic cycle of Na+-NQR.

Korormicin insensitivity in Vibrio alginolyticus is correlated with a single point mutation of Gly-140 in the NqrB subunit of the Na(+)-translocating NADH-quinone reductase

Na(+)-translocating NADH-quinone reductase (NQR) from the marine Vibrio alginolyticus is strongly inhibited by a new antibiotic korormicin. Korormicin specifically inhibits the Na(+)-dependent reaction of the NQR complex and acts as a purely non-competitive inhibitor for Q-1 with the inhibitor constant of 82 pM. Korormicin-resistant mutants were isolated from V. alginolyticus and the NQR complex was purified from a mutant KR2. Similar to 2-n-heptyl-4-hydroxyquinoline N-oxide (HQNO), korormicin acted as a purely noncompetitive inhibitor to the NQR complex from the mutant KR2, but the inhibitor constant increased to 8 microM, which is 10(5)-fold higher than that of the wild-type NQR complex. The inhibitor constant of HQNO, however, was only slightly affected by the acquisition of korormicin resistance. The spontaneous mutation was caused by a single mutation of G-422 to T-422 in the nucleotide sequence of the nqrB gene, which resulted in the conversion of Gly-140 to Val-140. Thus, Gly-140 seems to play an important role for the binding of korormicin to the NqrB subunit. The fact that korormicin is a purely noncompetitive inhibitor for Q-1 strongly supports the presence of one of Q-1 binding sites in the NqrB subunit, which also has a covalently bound FMN at Thr-235.

Inactivation of the Na+-translocating NADH:ubiquinone oxidoreductase from Vibrio alginolyticus by reactive oxygen species

The Na+-translocating NADH:quinone oxidoreductase (Na+-NQR) from Vibrio alginolyticus was inactivated by reactive oxygen species. Highest Na+-NQR activity was observed in anaerobically prepared membranes that exhibited 1:1 coupling of NADH oxidation and Q reduction activities (1.6 U x mg(-1)). Optical and EPR spectroscopy documented the presence of b-type cytochromes, a [2Fe-2S] cluster and an organic radical signal in anaerobically prepared membranes from V. alginolyticus. It is shown that the [2Fe-2S] cluster previously assigned to the Na+-NQR originates from the succinate dehydrogenase or the related enzyme fumarate reductase.

Residue aspartate-147 from the third transmembrane region of Na(+)/H(+) antiporter NhaB of Vibrio alginolyticus plays a role in its activity

NhaB is a bacterial Na(+)/H(+) antiporter with unique topology. The pH dependence of NhaB from Vibrio alginolyticus differs from that of the Escherichia coli NhaB homolog. Replacement of Asp-147 with Glu made high H(+) concentrations a requirement for the NhaB activity. Replacement of Asp-147 with neutral amino acids inactivated NhaB.

Sodium ion cycle in bacterial pathogens: evidence from cross-genome comparisons

Analysis of the bacterial genome sequences shows that many human and animal pathogens encode primary membrane Na+ pumps, Na+-transporting dicarboxylate decarboxylases or Na+ translocating NADH:ubiquinone oxidoreductase, and a number of Na+ -dependent permeases. This indicates that these bacteria can utilize Na+ as a coupling ion instead of or in addition to the H+ cycle. This capability to use a Na+ cycle might be an important virulence factor for such pathogens as Vibrio cholerae, Neisseria meningitidis, Salmonella enterica serovar Typhi, and Yersinia pestis. In Treponema pallidum, Chlamydia trachomatis, and Chlamydia pneumoniae, the Na+ gradient may well be the only energy source for secondary transport. A survey of preliminary genome sequences of Porphyromonas gingivalis, Actinobacillus actinomycetemcomitans, and Treponema denticola indicates that these oral pathogens also rely on the Na+ cycle for at least part of their energy metabolism. The possible roles of the Na+ cycling in the energy metabolism and pathogenicity of these organisms are reviewed. The recent discovery of an effective natural antibiotic, korormicin, targeted against the Na+ -translocating NADH:ubiquinone oxidoreductase, suggests a potential use of Na+ pumps as drug targets and/or vaccine candidates. The antimicrobial potential of other inhibitors of the Na+ cycle, such as monensin, Li+ and Ag+ ions, and amiloride derivatives, is discussed.

Sodium-dependent steps in the redox reactions of the Na+-motive NADH:quinone oxidoreductase from Vibrio harveyi

The Na+-translocating NADH:ubiquinone oxidoreductase (Na+-NQR) from Vibrio harveyi was purified and studied by EPR and visible spectroscopy. Two EPR signals in the NADH-reduced enzyme were detected: one, a radical signal, and the other a line around g = 1.94, which is typical for a [2Fe-2S] cluster. An E(m) of -267 mV was found for the Fe-S cluster (n = 1), independent of sodium concentration. The spin concentration of the radical in the enzyme was approximately the same under a variety of redox conditions. The time course of Na+-NQR reduction by NADH indicated the presence of at least two different flavin species. Reduction of the first species (most likely, a FAD near the NADH dehydrogenase site) was very rapid in both the presence and absence of sodium. Reduction of the second flavin species (presumably, covalently bound FMN) was slower and strongly dependent on sodium concentration, with an apparent activation constant for Na+ of approximately 3.4 mM. This is very similar to the Km for Na+ in the steady-state quinone reductase reaction catalyzed by this enzyme. These data led us to conclude that the sodium-dependent step within the Na+-NQR is located between the noncovalently bound FAD and the covalently bound FMN.

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.

Role of sodium bioenergetics in Vibrio cholerae

The ability of the bacterium to use sodium in bioenergetic processes appears to play a key role in both the environmental and pathogenic phases of Vibrio cholerae. Aquatic environments, including fresh, brackish, and coastal waters, are an important factor in the transmission of cholera and an autochthonous source. The organism is considered to be halophilic and has a strict requirement for Na(+) for growth. Furthermore, expression of motility and virulence factors of V. cholerae is intimately linked to sodium bioenergetics and to each other. Several lines of evidence indicated that the activity of the flagellum of V. cholerae might have an impact on virulence gene regulation. As the V. cholerae flagellum is sodium-driven and the Na(+)-NQR enzyme is known to create a sodium motive force across the bacterial membrane, it was recently suggested that the increased toxT expression observed in a nqr-negative strain is mediated by affecting flagella activity. It was suggested that the V. cholerae flagellum might respond to changes in membrane potential and the resulting changes in flagellar rotation might serve as a signal for virulence gene expression. However, we recently demonstrated that although the flagellum of V. cholerae is not required for the effects of ionophores on virulence gene expression, changes in the sodium chemical potential are sensed and thus alternative mechanisms, perhaps involving the TcpP/H proteins, for the detection of these conditions must exist. Analyzing the underlying mechanisms by which bacteria respond to changes in the environment, such as their ability to monitor the level of membrane potential, will probably reveal complex interplays between basic physiological processes and virulence factor expression in a variety of pathogenic species.

Na(+) translocation by bacterial NADH:quinone oxidoreductases: an extension to the complex-I family of primary redox pumps

The current knowledge on the Na(+)-translocating NADH:ubiquinone oxidoreductase of the Na(+)-NQR type from Vibrio alginolyticus, and on Na(+) transport by the electrogenic NADH:Q oxidoreductases from Escherichia coli and Klebsiella pneumoniae (complex I, or NDH-I) is summarized. A general mode of redox-linked Na(+) transport by NADH:Q oxidoreductases is proposed that is based on the electrostatic attraction of a positively charged Na(+) towards a negatively charged, enzyme-bound ubisemiquinone anion in a medium of low dielectricity. A structural model of the [2Fe-2S]- and FAD-carrying NqrF subunit of the Na(+)-NQR from V. alginolyticus based on ferredoxin and ferredoxin:NADP(+) oxidoreductase suggests that a direct participation of the Fe/S center in Na(+) transport is rather unlikely. A ubisemiquinone-dependent mechanism of Na(+) translocation is proposed that results in the transport of two Na(+) ions per two electrons transferred. Whereas this stoichiometry of the pump is in accordance with in vivo determinations of Na(+) transport by the respiratory chain of V. alginolyticus, higher (Na(+) or H(+)) transport stoichiometries are expected for complex I, suggesting the presence of a second coupling site.

Recent progress in the Na(+)-translocating NADH-quinone reductase from the marine Vibrio alginolyticus

The respiratory chain of Gram-negative marine and halophilic bacteria has a Na(+)-dependent NADH-quinone reductase that functions as a primary Na(+) pump. The Na(+)-translocating NADH-quinone reductase (NQR) from the marine Vibrio alginolyticus is composed of six structural genes (nqrA to nqrF). The NqrF subunit has non-covalently bound FAD. There are conflicting results on the existence of other flavin cofactors. Recent studies revealed that the NqrB and NqrC subunits have a covalently bound flavin, possibly FMN, which is attached to a specified threonine residue. A novel antibiotic, korormicin, was found to specifically inhibit the NQR complex. From the homology search of the nqr operon, it was found that the Na(+)-pumping NQR complex is widely distributed among Gram-negative pathogenic bacteria.

Plasma membrane electron transport coupled to Na(+) extrusion in the halotolerant alga Dunaliella

The halotolerant alga Dunaliella adapts to exceptionally high salinity and maintains low [Na(+)](in) at hypersaline solutions, suggesting that it possesses efficient mechanisms for regulating intracellular Na(+). In this work we examined the possibility that Na(+) export in Dunaliella is linked to a plasma membrane electron transport (redox) system. Na(+) extrusion was induced in Dunaliella cells by elevation of intracellular Na(+) with Na(+)-specific ionophores. Elevation of intracellular Na(+) was found to enhance the reduction of an extracellular electron acceptor ferricyanide (FeCN). The quinone analogs NQNO and dicumarol inhibited FeCN reduction and led to accumulation of Na(+) by inhibition of Na(+) extrusion. These inhibitors also diminished the plasma membrane potential in Dunaliella. Anaerobic conditions elevated, whereas FeCN partially decreased intracellular Na(+) content. Cellular NAD(P)H level decreased upon enhancement of plasma membrane electron transport. These results are consistent with the operation of an electrogenic NAD(P)H-driven redox system coupled to Na(+) extrusion in Dunaliella plasma membrane. We propose that redox-driven Na(+) extrusion and recycling in Dunaliella evolved as means of adaptation to hypersaline environments.

Fungal respiration: a fusion of standard and alternative components

In animals, electron transfer from NADH to molecular oxygen proceeds via large respiratory complexes in a linear respiratory chain. In contrast, most fungi utilise branched respiratory chains. These consist of alternative NADH dehydrogenases, which catalyse rotenone insensitive oxidation of matrix NADH or enable cytoplasmic NADH to be used directly. Many also contain an alternative oxidase that probably accepts electrons directly from ubiquinol. A few fungi lack Complex I. Although the alternative components are non-energy conserving, their organisation within the fungal electron transfer chain ensures that the transfer of electrons from NADH to molecular oxygen is generally coupled to proton translocation through at least one site. The alternative oxidase enables respiration to continue in the presence of inhibitors for ubiquinol:cytochrome c oxidoreductase and cytochrome c oxidase. This may be particularly important for fungal pathogens, since host defence mechanisms often involve nitric oxide, which, whilst being a potent inhibitor of cytochrome c oxidase, has no inhibitory effect on alternative oxidase. Alternative NADH dehydrogenases may avoid the active oxygen production associated with Complex I. The expression and activity regulation of alternative components responds to factors ranging from oxidative stress to the stage of fungal development.

Expression and mutagenesis of the NqrC subunit of the NQR respiratory Na(+) pump from Vibrio cholerae with covalently attached FMN

The Na(+)-translocating NADH:quinone oxidoreductase (Na(+)-NQR) is present in the membranes of a number of marine bacteria and pathogenic bacteria. Two of the six subunits of the Na(+)-NQR, NqrB and NqrC, have been previously shown to contain covalently bound flavin adenine mononucleotide (FMN). In the current work, the cloning of nqrC from Vibrio cholerae is reported. The gene has been expressed in V. cholerae and shown to contain one equivalent of covalently bound FMN. In contrast, no covalent flavin was detected when threonine-225 was replaced by leucine. The data show that the FMN attachment does not require assembly of the enzyme and are consistent with the unusual threonine attachment site.

FMN is covalently attached to a threonine residue in the NqrB and NqrC subunits of Na(+)-translocating NADH-quinone reductase from Vibrio alginolyticus

The Na(+)-translocating NADH-quinone reductase (NQR) from Vibrio alginolyticus is composed of six subunits (NqrA to NqrF). We previously demonstrated that both NqrB and NqrC subunits contain a flavin cofactor covalently attached to a threonine residue. Fluorescent peptide fragments derived from the NqrB and NqrC subunits were applied to a matrix-assisted laser desorption ionization time-of-flight mass spectrometer, and covalently attached flavin was identified as FMN in both subunits. From post-source decay fragmentation analysis, it was concluded that FMN is attached by a phosphate group to Thr-235 in the NqrB subunit and to Thr-223 in the NqrC subunit. The phosphoester binding of FMN to a threonine residue reported here is a new type of flavin attachment to a polypeptide.

Covalently bound flavin in the NqrB and NqrC subunits of Na(+)-translocating NADH-quinone reductase from Vibrio alginolyticus

Na(+)-translocating NADH-quinone reductase (NQR) from the marine bacterium Vibrio alginolyticus is composed of six subunits (NqrA to NqrF). On SDS-PAGE of the purified complex, NqrB and NqrC subunits were found to give yellow-green fluorescent bands under UV illumination. Both the NqrB and NqrC, electroeluted from the gel, had an absorption maximum at 448 nm, and the fluorescence excitation maxima at 365 and 448 nm and the emission maximum at 514 nm. The electroeluted NqrB and NqrC, respectively, were identified from their N-terminal amino acid sequences. These results clearly indicated that the NqrB and NqrC subunits have covalently bound flavins. The two subunits were digested by protease and then the fluorescent peptide fragments were separated by a reversed-phase high performance liquid chromatography. N-Terminal amino acid sequence analyses of the fluorescent peptides revealed that the flavin is linked to Thr-235 in the NqrB and Thr-223 in the NqrC subunits. This is the first example that the flavin is linked to a threonine residue. The amino acid sequence around the flavin-linked threonine was well conserved between NqrB and NqrC. Identification of the flavin group is in progress.