Identification of histidine residues in Wolinella succinogenes hydrogenase that are essential for menaquinone reduction by H2

Geobacter sulfurreducens inner membrane cytochrome CbcBA controls electron transfer and growth yield near the energetic limit of respiration

Geobacter sulfurreducens utilizes extracellular electron acceptors such as Mn(IV), Fe(III), syntrophic partners, and electrodes that vary from +0.4 to -0.3 V versus standard hydrogen electrode (SHE), representing a potential energy span that should require a highly branched electron transfer chain. Here we describe CbcBA, a bc-type cytochrome essential near the thermodynamic limit of respiration when acetate is the electron donor. Mutants-lacking cbcBA ceased Fe(III) reduction at -0.21 V versus SHE, could not transfer electrons to electrodes between -0.21 and -0.28 V, and could not reduce the final 10%-35% of Fe(III) minerals. As redox potential decreased during Fe(III) reduction, cbcBA was induced with the aid of the regulator BccR to become one of the most highly expressed genes in G. sulfurreducens. Growth yield (CFU/mM Fe(II)) was 112% of WT in ∆cbcBA, and deletion of cbcL (an unrelated bc-cytochrome essential near -0.15 V) in ΔcbcBA increased yield to 220%. Together with ImcH, which is required at high redox potentials, CbcBA represents a third cytoplasmic membrane oxidoreductase in G. sulfurreducens. This expanding list shows how metal-reducing bacteria may constantly sense redox potential to adjust growth efficiency in changing environments.

The Aerobic and Anaerobic Respiratory Chain of Escherichia coli and Salmonella enterica: Enzymes and Energetics

Escherichia coli contains a versatile respiratory chain that oxidizes 10 different electron donor substrates and transfers the electrons to terminal reductases or oxidases for the reduction of six different electron acceptors. Salmonella is able to use two more electron acceptors. The variation is further increased by the presence of isoenzymes for some substrates. A large number of respiratory pathways can be established by combining different electron donors and acceptors. The respiratory dehydrogenases use quinones as the electron acceptors that are oxidized by the terminal reductase and oxidases. The enzymes vary largely with respect to their composition, architecture, membrane topology, and the mode of energy conservation. Most of the energy-conserving dehydrogenases (FdnGHI, HyaABC, HybCOAB, and others) and the terminal reductases (CydAB, NarGHI, and others) form a proton potential (Δp) by a redox-loop mechanism. Two enzymes (NuoA-N and CyoABCD) couple the redox energy to proton translocation by proton pumping. A large number of dehydrogenases and terminal reductases do not conserve the redox energy in a proton potential. For most of the respiratory enzymes, the mechanism of proton potential generation is known or can be predicted. The H+/2e- ratios for most respiratory chains are in the range from 2 to 6 H+/2e-. The energetics of the individual redox reactions and the respiratory chains is described and related to the H+/2e- ratios.

The Aerobic and Anaerobic Respiratory Chain of Escherichia coli and Salmonella enterica: Enzymes and Energetics

Escherichia coli contains a versatile respiratory chain which oxidizes ten different electron donor substrates and transfers the electrons to terminal reductases or oxidases for the reduction of six different electron acceptors. Salmonella is able to use even two more electron acceptors. The variation is further increased by the presence of isoenzymes for some substrates. Various respiratory pathways can be established by combining the oxidation of different electron donors and acceptors which are linked by respiratory quinones. The enzymes vary largely with respect to architecture, membrane topology, and mode of energy conservation. Most of the energy-conserving dehydrogenases (e.g., FdnGHI, HyaABC, and HybCOAB) and of the terminal reductases (CydAB, NarGHI, and others) form a proton potential (Δp) by a redox loop mechanism. Only two enzymes (NuoA-N and CyoABCD) couple the redox energy to proton translocation by proton pumping. A large number of dehydrogenases (e.g., Ndh, SdhABCD, and GlpD) and of terminal reductases (e.g., FrdABCD and DmsABC) do not conserve the redox energy in a proton potential. For most of the respiratory enzymes, the mechanism of proton potential generation is known from structural and biochemical studies or can be predicted from sequence information. The H+/2e- ratios of proton translocation for most respiratory chains are in the range from 2 to 6 H+/2e-. The energetics of the individual redox reactions and of the respiratory chains is described. In contrast to the knowledge on enzyme function are physiological aspects of respiration such as organization and coordination of the electron transport and the use of alternative respiratory enzymes, not well characterized.

The di-heme family of respiratory complex II enzymes

The di-heme family of succinate:quinone oxidoreductases is of particular interest, because its members support electron transfer across the biological membranes in which they are embedded. In the case of the di-heme-containing succinate:menaquinone reductase (SQR) from Gram-positive bacteria and other menaquinone-containing bacteria, this results in an electrogenic reaction. This is physiologically relevant in that it allows the transmembrane electrochemical proton potential Δp to drive the endergonic oxidation of succinate by menaquinone. In the case of the reverse reaction, menaquinol oxidation by fumarate, catalysed by the di-heme-containing quinol:fumarate reductase (QFR), evidence has been obtained that this electrogenic electron transfer reaction is compensated by proton transfer via a both novel and essential transmembrane proton transfer pathway ("E-pathway"). Although the reduction of fumarate by menaquinol is exergonic, it is obviously not exergonic enough to support the generation of a Δp. This compensatory "E-pathway" appears to be required by all di-heme-containing QFR enzymes and results in the overall reaction being electroneutral. In addition to giving a brief overview of progress in the characterization of other members of this diverse family, this contribution summarizes key evidence and progress in identifying constituents of the "E-pathway" within the framework of the crystal structure of the QFR from the anaerobic epsilon-proteobacterium Wolinella succinogenes at 1.78Å resolution. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease.

Diversity and evolution of bioenergetic systems involved in microbial nitrogen compound transformations

Nitrogen is an essential element of life that needs to be assimilated in its most reduced form, ammonium. On the other hand, nitrogen exists in a multitude of oxidation states and, consequently, nitrogen compounds (NCs) serve as electron donor and/or acceptors in many catabolic pathways including various forms of microbial respiration that contribute to the global biogeochemical nitrogen cycle. Some of these NCs are also known as reactive nitrogen species able to cause nitrosative stress because of their high redox reactivity. The best understood processes of the nitrogen cycle are denitrification and ammonification (both beginning with nitrate reduction to nitrite), nitrification (aerobic oxidation of ammonium and nitrite) and anaerobic ammonium oxidation (anammox). This review presents examples of the diverse architecture, either elucidated or anticipated, and the high degree of modularity of the corresponding respiratory electron transport processes found in Bacteria and Archaea, and relates these to their respective bioenergetic mechanisms of proton motive force generation. In contrast to the multiplicity of enzymes that catalyze NC transformations, the number of proteins or protein modules involved in connecting electron transport to and from these enzymes with the quinone/quinol pool is comparatively small. These quinone/quinol-reactive protein modules consist of cytochromes b and c and iron-sulfur proteins. Conclusions are drawn towards the evolutionary relationships of bioenergetic systems involved in NC transformation and deduced aspects of the evolution of the biogeochemical nitrogen cycle are presented. This article is part of a Special Issue entitled: The evolutionary aspects of bioenergetic systems.

Succinate dehydrogenase functioning by a reverse redox loop mechanism and fumarate reductase in sulphate-reducing bacteria

Sulphate- or sulphur-reducing bacteria with known or draft genome sequences (Desulfovibrio vulgaris, Desulfovibrio desulfuricans G20, Desulfobacterium autotrophicum [draft], Desulfotalea psychrophila and Geobacter sulfurreducens) all contain sdhCAB or frdCAB gene clusters encoding succinate : quinone oxidoreductases. frdD or sdhD genes are missing. The presence and function of succinate dehydrogenase versus fumarate reductase was studied. Desulfovibrio desulfuricans (strain Essex 6) grew by fumarate respiration or by fumarate disproportionation, and contained fumarate reductase activity. Desulfovibrio vulgaris lacked fumarate respiration and contained succinate dehydrogenase activity. Succinate oxidation by the menaquinone analogue 2,3-dimethyl-1,4-naphthoquinone depended on a proton potential, and the activity was lost after degradation of the proton potential. The membrane anchor SdhC contains four conserved His residues which are known as the ligands for two haem B residues. The properties are very similar to succinate dehydrogenase of the Gram-positive (menaquinone-containing) Bacillus subtilis, which uses a reverse redox loop mechanism in succinate : menaquinone reduction. It is concluded that succinate dehydrogenases from menaquinone-containing bacteria generally require a proton potential to drive the endergonic succinate oxidation. Sequence comparison shows that the SdhC subunit of this type lacks a Glu residue in transmembrane helix IV, which is part of the uncoupling E-pathway in most non-electrogenic FrdABC enzymes.

The hydrogenases of Geobacter sulfurreducens: a comparative genomic perspective

The hydrogenase content of the genome of Geobacter sulfurreducens, a member of the family Geobacteraceae within the delta-subdivision of the Proteobacteria, was examined and found to be distinct from that of Desulfovibrio species, another family of delta-Proteobacteria on which extensive research concerning hydrogen metabolism has been conducted. Four [NiFe]-hydrogenases are encoded in the G. sulfurreducens genome: two periplasmically oriented, membrane-bound hydrogenases, Hya and Hyb, and two cytoplasmic hydrogenases, Mvh and Hox. None of these [NiFe]-hydrogenases has a counterpart in Desulfovibrio species. Furthermore, the large and small subunits of Mvh and Hox appear to be related to archaeal and cyanobacterial hydrogenases, respectively. Clusters encoding [Fe]-hydrogenases and periplasmic [NiFeSe]-hydrogenases, which are commonly found in the genomes of Desulfovibrio species, are not present in the genome of G. sulfurreducens. Hydrogen-evolving Ech hydrogenases, which are present in the genomes of at least two Desulfovibrio species, were also absent from the G. sulfurreducens genome, despite the fact that G. sulfurreducens is capable of hydrogen production. Instead, the G. sulfurreducens genome contained a cluster encoding a multimeric Ech hydrogenase related (Ehr) complex that was similar in content to operons encoding Ech hydrogenases, but did not appear to encode a hydrogenase. Phylogenetic analysis revealed that the G. sulfurreducens ehr cluster is part of a family of related clusters found in both the Archaea and Bacteria.

The hydE gene is essential for the formation of Wolinella succinogenes NiFe-hydrogenase

Wolinella succinogenes grows by anaerobic respiration using hydrogen gas as electron donor. The hydE gene is located on the genome downstream of the structural genes encoding the membrane-bound NiFe-hydrogenase complex (HydABC) and a putative protease (HydD) possibly involved in hydrogenase maturation. Homologs of hydE are found in the vicinity of NiFe-hydrogenase-encoding genes on the genomes of several other proteobacteria. A hydE deletion mutant of W. succinogenes does not catalyze hydrogen oxidation with various electron acceptors. The hydrogenase iron-sulfur subunit HydA is absent in mutant cells whereas the apparently processed NiFe subunit (HydB) is located exclusively in the soluble cell fraction. It is suggested that HydE is involved in the maturation and/or stability of HydA or the HydAB complex in some, but not all bacteria containing NiFe-hydrogenases.

Characterization of the menaquinone reduction site in the diheme cytochrome b membrane anchor of Wolinella succinogenes NiFe-hydrogenase

The majority of bacterial membrane-bound NiFe-hydrogenases and formate dehydrogenases have homologous membrane-integral cytochrome b subunits. The prototypic NiFe-hydrogenase of Wolinella succinogenes (HydABC complex) catalyzes H2 oxidation by menaquinone during anaerobic respiration and contains a membrane-integral cytochrome b subunit (HydC) that carries the menaquinone reduction site. Using the crystal structure of the homologous FdnI subunit of Escherichia coli formate dehydrogenase-N as a model, the HydC protein was modified to examine residues thought to be involved in menaquinone binding. Variant HydABC complexes were produced in W. succinogenes, and several conserved HydC residues were identified that are essential for growth with H2 as electron donor and for quinone reduction by H2. Modification of HydC with a C-terminal Strep-tag II enabled one-step purification of the HydABC complex by Strep-Tactin affinity chromatography. The tagged HydC, separated from HydAB by isoelectric focusing, was shown to contain 1.9 mol of heme b/mol of HydC demonstrating that HydC ligates both heme b groups. The four histidine residues predicted as axial heme b ligands were individually replaced by alanine in Strep-tagged HydC. Replacement of either histidine ligand of the heme b group proximal to HydAB led to HydABC preparations that contained only one heme b group. This remaining heme b could be completely reduced by quinone supporting the view that the menaquinone reduction site is located near the distal heme b group. The results indicate that both heme b groups are involved in electron transport and that the architecture of the menaquinone reduction site near the cytoplasmic side of the membrane is similar to that proposed for E. coli FdnI.

A novel membrane-bound respiratory complex from Desulfovibrio desulfuricans ATCC 27774

In the anaerobic respiration of sulfate, performed by sulfate-reducing prokaryotes, reduction of the terminal electron acceptor takes place in the cytoplasm. The membrane-associated electron transport chain that feeds electrons to the cytoplasmic reductases is still very poorly characterized. In this study we report the isolation and characterization of a novel membrane-bound redox complex from Desulfovibrio desulfuricans ATCC 27774. This complex is formed by three subunits, and contains two hemes b, two FAD groups and several iron-sulfur centers. The two hemes b are low-spin, with macroscopic redox potentials of +75 and -20 mV at pH 7.6. Both hemes are reduced by menadiol, a menaquinone analogue, indicating a function for this complex in the respiratory electron-transport chain. EPR studies of the as-isolated and dithionite-reduced complex support the presence of a [3Fe-4S](1+/0) center and at least four [4Fe-4S](2+/1+) centers. Cloning of the genes coding for the complex subunits revealed that they form a putative transcription unit and have homology to subunits of heterodisulfide reductases (Hdr). The first and second genes code for soluble proteins that have homology to HdrA, whereas the third gene codes for a novel type of membrane-associated protein that contains both a hydrophobic domain with homology to the heme b protein HdrE and a hydrophilic domain with homology to the iron-sulfur protein HdrC. Homologous operons are found in the genomes of other sulfate-reducing organisms and in the genome of the green-sulfur bacterium Chlorobium tepidum TLS. The isolated complex is the first example of a new family of respiratory complexes present in anaerobic prokaryotes.

Formate dehydrogenase--a versatile enzyme in changing environments

Several structures belonging to the large bis-molybdopterin guanine dinucleotide enzyme family have been published during the past four years. These include the structures of three formate dehydrogenases containing intrinsic selenocysteine residues - two soluble enzymes and one integral membrane protein. Together these have given detailed structural and mechanistic information about this family of enzymes.

Protonmotive force generation by a redox loop mechanism

Respiration involves the oxidation and reduction of substrate for the redox-linked formation of a protonmotive force (PMF) across the inner membrane of mitochondria or the plasma membrane of bacteria. A mechanism for PMF generation was first suggested by Mitchell in his chemiosmotic theory. In the original formulations of the theory, Mitchell envisaged that proton translocation was driven by a 'redox loop' between two catalytically distinct enzyme complexes. Experimental data have shown that this redox loop does not operate in mitochondria, but has been confirmed as an important mechanism in bacteria. The nitrate respiratory pathway in Escherichia coli is a paradigm for a protonmotive redox loop. The structure of one of the enzymes in this two-component system, formate dehydrogenase-N, has revealed the structural basis for the PMF generation by the redox loop mechanism and this forms the basis of this review.

[NiFe] hydrogenases from the hyperthermophilic bacterium Aquifex aeolicus: properties, function, and phylogenetics

Genes potentially coding for three distinct [NiFe] hydrogenases are present in the genome of Aquifex aeolicus. We have demonstrated that all three hydrogenases are expressed under standard growth conditions of the organism. Two hydrogenases were further purified to homogeneity. A periplasmically oriented hydrogenase was obtained in two forms, i.e., as a soluble enzyme containing only the two essential subunits and as a detergent-solubilized complex additionally containing a membrane-integral b-type cytochrome. The second hydrogenase purified was identified as a soluble cytoplasmic enzyme. The isolated enzymes were characterized with respect to biochemical/biophysical parameters, activity, thermostability, and substrate specificity. The phylogenetic positioning of all three hydrogenases was analyzed. A model for the metabolic roles of the three enzymes is proposed on the basis of the obtained results.

Wolinella succinogenes quinol:fumarate reductase-2.2-A resolution crystal structure and the E-pathway hypothesis of coupled transmembrane proton and electron transfer

The structure of the respiratory membrane protein complex quinol:fumarate reductase (QFR) from Wolinella succinogenes has been determined by X-ray crystallography at 2.2-A resolution [Nature 402 (1999) 377]. Based on the structure of the three protein subunits A, B, and C and the arrangement of the six prosthetic groups (a covalently bound FAD, three iron-sulfur clusters, and two haem b groups), a pathway of electron transfer from the quinol-oxidising dihaem cytochrome b in the membrane to the site of fumarate reduction in the hydrophilic subunit A has been proposed. The structure of the membrane-integral dihaem cytochrome b reveals that all transmembrane helical segments are tilted with respect to the membrane normal. The "four-helix" dihaem binding motif is very different from other dihaem-binding transmembrane four-helix bundles, such as the "two-helix motif" of the cytochrome bc(1) complex and the "three-helix motif" of the formate dehydrogenase/hydrogenase group. The gamma-hydroxyl group of Ser C141 has an important role in stabilising a kink in transmembrane helix IV. By combining the results from site-directed mutagenesis, functional and electrochemical characterisation, and X-ray crystallography, a residue was identified which was found to be essential for menaquinol oxidation [Proc. Natl. Acad. Sci. U. S. A. 97 (2000) 13051]. The distal location of this residue in the structure indicates that the coupling of the oxidation of menaquinol to the reduction of fumarate in dihaem-containing succinate:quinone oxidoreductases could in principle be associated with the generation of a transmembrane electrochemical potential. However, it is suggested here that in W. succinogenes QFR, this electrogenic effect is counterbalanced by the transfer of two protons via a proton transfer pathway (the "E-pathway") in concert with the transfer of two electrons via the membrane-bound haem groups. According to this "E-pathway hypothesis", the net reaction catalysed by W. succinogenes QFR does not contribute directly to the generation of a transmembrane electrochemical potential.

Enzymology and bioenergetics of respiratory nitrite ammonification

Nitrite is widely used by bacteria as an electron acceptor under anaerobic conditions. In respiratory nitrite ammonification an electrochemical proton potential across the membrane is generated by electron transport from a non-fermentable substrate like formate or H(2) to nitrite. The corresponding electron transport chain minimally comprises formate dehydrogenase or hydrogenase, a respiratory quinone and cytochrome c nitrite reductase. The catalytic subunit of the latter enzyme (NrfA) catalyzes nitrite reduction to ammonia without liberating intermediate products. This review focuses on recent progress that has been made in understanding the enzymology and bioenergetics of respiratory nitrite ammonification. High-resolution structures of NrfA proteins from different bacteria have been determined, and many nrf operons sequenced, leading to the prediction of electron transfer pathways from the quinone pool to NrfA. Furthermore, the coupled electron transport chain from formate to nitrite of Wolinella succinogenes has been reconstituted by incorporating the purified enzymes into liposomes. The NrfH protein of W. succinogenes, a tetraheme c-type cytochrome of the NapC/NirT family, forms a stable complex with NrfA in the membrane and serves in passing electrons from menaquinol to NrfA. Proteins similar to NrfH are predicted by open reading frames of several bacterial nrf gene clusters. In gamma-proteobacteria, however, NrfH is thought to be replaced by the nrfBCD gene products. The active site heme c group of NrfA proteins from different bacteria is covalently bound via the cysteine residues of a unique CXXCK motif. The lysine residue of this motif serves as an axial ligand to the heme iron thus replacing the conventional histidine residue. The attachment of the lysine-ligated heme group requires specialized proteins in W. succinogenes and Escherichia coli that are encoded by accessory nrf genes. The proteins predicted by these genes are unrelated in the two bacteria but similar to proteins of the respective conventional cytochrome c biogenesis systems.

Reconstitution of coupled fumarate respiration in liposomes by incorporating the electron transport enzymes isolated from Wolinella succinogenes

Hydrogenase and fumarate reductase isolated from Wolinella succinogenes were incorporated into liposomes containing menaquinone. The two enzymes were found to be oriented solely to the outside of the resulting proteoliposomes. The proteoliposomes catalyzed fumarate reduction by H2 which generated an electrical proton potential (Delta(psi) = 0.19 V, negative inside) in the same direction as that generated by fumarate respiration in cells of W. succinogenes. The H+/e ratio brought about by fumarate reduction with H2 in proteoliposomes in the presence of valinomycin and external K+ was approximately 1. The same Delta(psi) and H+/e ratio was associated with the reduction of 2,3-dimethyl-1,4-naphthoquinone (DMN) by H2 in proteoliposomes containing menaquinone and hydrogenase with or without fumarate reductase. Proteoliposomes containing menaquinone and fumarate reductase with or without hydrogenase catalyzed fumarate reduction by DMNH2 which did not generate a Delta(psi). Incorporation of formate dehydrogenase together with fumarate reductase and menaquinone resulted in proteoliposomes catalyzing the reduction of fumarate or DMN by formate. Both reactions generated a Delta(psi) of 0.13 V (negative inside). The H+/e ratio of formate oxidation by menaquinone or DMN was close to 1. The results demonstrate for the first time that coupled fumarate respiration can be restored in liposomes using the well characterized electron transport enzymes isolated from W. succinogenes. The results support the view that Delta(psi) generation is coupled to menaquinone reduction by H2 or formate, but not to menaquinol oxidation by fumarate. Delta(psi) generation is probably caused by proton uptake from the cytoplasmic side of the membrane during menaquinone reduction, and by the coupled release of protons from H2 or formate oxidation on the periplasmic side. This mechanism is supported by the properties of two hydrogenase mutants of W. succinogenes which indicate that the site of quinone reduction is close to the cytoplasmic surface of the membrane.

Molecular basis of proton motive force generation: structure of formate dehydrogenase-N

The structure of the membrane protein formate dehydrogenase-N (Fdn-N), a major component of Escherichia coli nitrate respiration, has been determined at 1.6 angstroms. The structure demonstrates 11 redox centers, including molybdopterin-guanine dinucleotides, five [4Fe-4S] clusters, two heme b groups, and a menaquinone analog. These redox centers are aligned in a single chain, which extends almost 90 angstroms through the enzyme. The menaquinone reduction site associated with a possible proton pathway was also characterized. This structure provides critical insights into the proton motive force generation by redox loop, a common mechanism among a wide range of respiratory enzymes.

The function of methyl-menaquinone-6 and polysulfide reductase membrane anchor (PsrC) in polysulfide respiration of Wolinella succinogenes

Wolinella succinogenes grows by oxidative phosphorylation with polysulfide as terminal electron acceptor and either H2 or formate as electron donor (polysulfide respiration). The function of the respiratory chains catalyzing these reactions was investigated. Proteoliposomes containing polysulfide reductase (Psr) and either hydrogenase or formate dehydrogenase isolated from the membrane fraction of Wolinella succinogenes catalyzed polysulfide respiration, provided that methyl-menaquinone-6 isolated from W. succinogenes was also present. The specific activities of electron transport were commensurate with those of the bacterial membrane fraction. Using site-directed mutagenesis, certain residues were substituted in PsrC, the membrane anchor of polysulfide reductase. Replacement of Y23, D76, Y159, D218, E225 or R305 caused nearly full inhibition of polysulfide respiration without affecting the activity of Psr, which was still bound to the membrane. These residues are predicted to be located in hydrophobic helices of PsrC, or next to them. Substitution of 13 other residues of PsrC either caused partial inhibition ofblankpolysulfide respiration or had no effect. The function of methyl-menaquinone-6, which is thought to be bound to PsrC, is discussed.

Behaviour of topological marker proteins targeted to the Tat protein transport pathway

The Escherichia coli Tat system mediates Sec-independent export of protein precursors bearing twin arginine signal peptides. Formate dehydrogenase-N is a three-subunit membrane-bound enzyme, in which localization of the FdnG subunit to the membrane is Tat dependent. FdnG was found in the periplasmic fraction of a mutant lacking the membrane anchor subunit FdnI, confirming that FdnG is located at the periplasmic face of the cytoplasmic membrane. However, the phenotypes of gene fusions between fdnG and the subcellular reporter genes phoA (encoding alkaline phosphatase) or lacZ (encoding beta-galactosidase) were the opposite of those expected for analogous fusions targeted to the Sec translocase. PhoA fusion experiments have previously been used to argue that the peripheral membrane DmsAB subunits of the Tat-dependent enzyme dimethyl sulphoxide reductase are located at the cytoplasmic face of the inner membrane. Biochemical data are presented that instead show DmsAB to be at the periplasmic side of the membrane. The behaviour of reporter proteins targeted to the Tat system was analysed in more detail. These data suggest that the Tat and Sec pathways differ in their ability to transport heterologous passenger proteins. They also suggest that caution should be observed when using subcellular reporter fusions to determine the topological organization of Tat-dependent membrane protein complexes.

Fumarate respiration of Wolinella succinogenes: enzymology, energetics and coupling mechanism

Wolinella succinogenes performs oxidative phosphorylation with fumarate instead of O2 as terminal electron acceptor and H2 or formate as electron donors. Fumarate reduction by these donors ('fumarate respiration') is catalyzed by an electron transport chain in the bacterial membrane, and is coupled to the generation of an electrochemical proton potential (Deltap) across the bacterial membrane. The experimental evidence concerning the electron transport and its coupling to Deltap generation is reviewed in this article. The electron transport chain consists of fumarate reductase, menaquinone (MK) and either hydrogenase or formate dehydrogenase. Measurements indicate that the Deltap is generated exclusively by MK reduction with H2 or formate; MKH2 oxidation by fumarate appears to be an electroneutral process. However, evidence derived from the crystal structure of fumarate reductase suggests an electrogenic mechanism for the latter process.

Functional role of polar amino acid residues in Na+/H+ exchangers

Na(+)/H(+) exchangers are a family of ubiquitous membrane proteins. In higher eukaryotes they regulate cytosolic pH by removing an intracellular H(+) in exchange for an extracellular Na(+). In yeast and Escherichia coli, Na(+)/H(+) exchangers function in the opposite direction to remove intracellular Na(+) in exchange for extracellular H(+). Na(+)/H(+) exchangers display an internal pH-sensitivity that varies with the different antiporter types. Only recently have investigations examined the amino acids involved in pH-sensitivity and in cation binding and transport. Histidine residues are good candidates for H(+)-sensing amino acids, since they can ionize within the physiological pH range. Histidine residues have been shown to be important in the function of the E. coli Na(+)/H(+) exchanger NhaA and in the yeast Na(+)/H(+) exchanger sod2. In E. coli, His(225) of NhaA may function to interact with, or regulate, the pH-sensory region of NhaA. In sod2, His(367) is also critical to transport and may be a functional analogue of His(225) of NhaA. Histidine residues are not critical for the function of the mammalian Na(+)/H(+) exchanger, although an unusual histidine-rich sequence of the C-terminal tail has some influence on activity. Other amino acids involved in cation binding and transport by Na(+)/H(+) exchangers are only beginning to be studied. Amino acids with polar side chains such as aspartate and glutamate have been implicated in transport activity of NhaA and sod2, but have not been studied in the mammalian Na(+)/H(+) exchanger. Further studies are needed to elucidate the mechanisms involved in pH-sensitivity and cation binding and transport by Na(+)/H(+) exchangers.

Transport of C(4)-dicarboxylates in Wolinella succinogenes

C(4)-dicarboxylate transport is a prerequisite for anaerobic respiration with fumarate in Wolinella succinogenes, since the substrate site of fumarate reductase is oriented towards the cytoplasmic side of the membrane. W. succinogenes was found to transport C(4)-dicarboxylates (fumarate, succinate, malate, and aspartate) across the cytoplasmic membrane by antiport and uniport mechanisms. The electrogenic uniport resulted in dicarboxylate accumulation driven by anaerobic respiration. The molar ratio of internal to external dicarboxylate concentration was up to 10(3). The dicarboxylate antiport was either electrogenic or electroneutral. The electroneutral antiport required the presence of internal Na(+), whereas the electrogenic antiport also operated in the absence of Na(+). In the absence of Na(+), no electrochemical proton potential (delta p) was measured across the membrane of cells catalyzing fumarate respiration. This suggests that the proton potential generated by fumarate respiration is dissipated by the concomitant electrogenic dicarboxylate antiport. Three gene loci (dcuA, dcuB, and dctPQM) encoding putative C(4)-dicarboxylate transporters were identified on the genome of W. succinogenes. The predicted gene products of dcuA and dcuB are similar to the Dcu transporters that are involved in the fumarate respiration of Escherichia coli with external C(4)-dicarboxylates. The genes dctP, -Q, and -M probably encode a binding-protein-dependent secondary uptake transporter for dicarboxylates. A mutant (DcuA(-) DcuB(-)) of W. succinogenes lacking the intact dcuA and dcuB genes grew by nitrate respiration with succinate as the carbon source but did not grow by fumarate respiration with fumarate, malate, or aspartate as substrates. The DcuA(-), DcuB(-), and DctQM(-) mutants grew by fumarate respiration as well as by nitrate respiration with succinate as the carbon source. Cells of the DcuA(-) DcuB(-) mutant performed fumarate respiration without generating a proton potential even in the presence of Na(+). This explains why the DcuA(-) DcuB(-) mutant does not grow by fumarate respiration. Growth by fumarate respiration appears to depend on the function of the Na(+)-dependent, electroneutral dicarboxylate antiport which is catalyzed exclusively by the Dcu transporters. Dicarboxylate transport via the electrogenic uniport is probably catalyzed by the DctPQM transporter and by a fourth, unknown transporter that may also operate as an electrogenic antiporter.

The hydrogenase cytochrome b heme ligands of Azotobacter vinelandii are required for full H(2) oxidation capability

The hydrogenase in Azotobacter vinelandii, like other membrane-bound [NiFe] hydrogenases, consists of a catalytic heterodimer and an integral membrane cytochrome b. The histidines ligating the hemes in this cytochrome b were identified by H(2) oxidation properties of altered proteins produced by site-directed mutagenesis. Four fully conserved and four partially conserved histidines in HoxZ were substituted with alanine or tyrosine. The roles of these histidines in HoxZ heme binding and hydrogenase were characterized by O(2)-dependent H(2) oxidation and H(2)-dependent methylene blue reduction in vivo. Mutants H33A/Y (H33 replaced by A or Y), H74A/Y, H194A, H208A/Y, and H194,208A lost O(2)-dependent H(2) oxidation activity, H194Y and H136A had partial activity, and H97Y,H98A and H191A had full activity. These results suggest that the fully conserved histidines 33, 74, 194, and 208 are ligands to the hemes, tyrosine can serve as an alternate ligand in position 194, and H136 plays a role in H(2) oxidation. In mutant H194A/Y, imidazole (Imd) rescued H(2) oxidation activity in intact cells, which suggests that Imd acts as an exogenous ligand. The heterodimer activity, quantitatively determined as H(2)-dependent methylene blue reduction, indicated that the heterodimers of all mutants were catalytically active. H33A/Y had wild-type levels of methylene blue reduction, but the other HoxZ ligand mutants had significantly less than wild-type levels. Imd reconstituted full methylene blue reduction activity in mutants H194A/Y and H208A/Y and partial activity in H194,208A. These results indicate that structural and functional integrity of HoxZ is required for physiologically relevant H(2) oxidation, and structural integrity of HoxZ is necessary for full heterodimer-catalyzed H(2) oxidation.

Cytochrome c' from Rhodobacter capsulatus confers increased resistance to nitric oxide

We report the cloning and sequencing of the gene containing cytochrome c' (cycP) from the photosynthetic purple bacterium Rhodobacter capsulatus and the regions flanking that gene. Mutant strains unable to synthesize cytochrome c' had increased sensitivity to nitrosothiols and to nitric oxide (which binds to the heme moiety of cytochrome c').