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The respiratory and photosynthetic electron transport chains of the two facultative phototrophs Rhodobacter(Rb.) sphaeroides and Rb. Capsulatus are arranged in such a way to be spatially segregated in separate regions of the internal membrane system (CM and ICM). The CM part contains the majority of the oxidative redox components which are therefore in redox non-equilibrium with most of the photochemical RCs; conversely, the major part of the photosynthetic carriers (including RCs, Cyt c2 Cyt cy and Cyt bc1 complex) are located in the ICM part of the membrane. This spatial level of organization is paralleled by an arrangement of these photosynthetic elements in supramolecular complexes in order to allow a fast and efficient cyclic electron transfer by limiting the diffusion of the reactants. However, these two levels of arrangement are not present in all types of photosynthetic bacteria. Indeed, species like Blastochloris viridis or Rubrivivax gelatinosus, contain a large excess of RCs over the Cyt bc1 complexes so that the formation of superscomplexes is stoichiometrically hindered. Further, in obligate aerobic phototrophs such as for example Rosebacter denitirficans, the Qa is fully reduced under anaerobic conditions and this might be due to the lack of both quinol oxidase and ICM system. The expression of photosynthetic and respiratory components is controlled by the oxygen tension and by the redox state of the system. This genetic coordination mechanism does not necessary require a direct interaction of the two sets of components in line with their different spatial membrane location. The signals to which the system responds originate from either specific respiratory components, e.g. cbb3 oxidase, as in the case of oxygen sensing, or from redox carriers involved in both oxidative and photosynthetic ET, as for redox sensing. Although the genetic control of the supramolecular arrangement of the ETCs is, at present largely undefined, the working scheme presented here, suggests a tentative framework of genetic regulatory connections in Rb.capsulatus and/or Rb.sphaeroides. [ABSTRACT FROM AUTHOR]

Cyanobacterial respiration has some unique features. Cyanobacteria contain two independent respiratory chains, one in the cytoplasmic membrane and one in the intracytoplasmic membranes (also called thylakoids). The latter is intimately linked with the photosynthetic electron transport chain and some components, e.g. the quinone pool, the cytochrome b6f complex, and cytochrome c6, are shared by both photosynthesis and respiration. However, all components of the respiratory electron transport chain might have some—possibly regulatory—function in photosynthesis. In recent years the total genomic sequences of several cyanobacteria have been invaluable tools for understanding cyanobacterial respiration. Especially in Synechocystis sp. Strain PCC 6803 the majority of the components of the respiratory chains were finallly identified. Respiratory electron transport is highly branched, with several routes of electron input and several routes to the final electron acceptor, which is always dioxygen. The single component common to all respiratory electron pathways is the quinone pool. Furthermore, total genomic sequencing has shown that different cyanobacteria may contain different elecron transport components. Several strains, especially (but not exclusively) those that are capable of cell differentiation, contain more than one copy of genes for some respiratory electron transport chain components. The different functions of these genes and their gene products have only partially been elucidated. Several components of the respiratory chains are subject to complex gene regulations, about which information is only starting to appear. Some proteins, e.g. cytochrome cM, and the dehydrogenase DrgA, are likely to be involved in respiratory electron transport, but their positions in the electron transport chain (i.e. their in vivo electron donors and acceptors) have not yet been fully clarified. The electron transport chain of the cytoplasmic membrane is not yet well characterized in any strain. [ABSTRACT FROM AUTHOR]

Hydrogen respiration can be considered either as (i) the oxidation of H2 to H+, with the electrons released being channeled into a membrane-bound, respiratory electron transport chain or (ii) as the reduction of H+ to H2 in the terminal reaction of an anaerobic, low-potential electron transport system. In both cases, the redox reaction involving H2 is catalyzed by a hydrogenase enzyme (H2ase) and electron transport to or from H2 is coupled to the vectorial translocation of H+ across a membrane, leading to the conservation of energy in the form of a protonmotive force. H2ases are a diverse group of enzymes, and can be classified into 3 groups according to their mental content, i.e. [NiFe]-H2ases, [Fe]-H2ases and iron-sulfur cluster-free H2ases. The [NiFe]-H2ases can be subdivided into 4 subgroups: (1) the uptake [NiFe]-H2ases; (2) the cytoplasmic H2 sensors and the cyanobacterial uptake [NiFe]-H2ases; (3) the bidirectional cytoplasmic [NiFe]-H2ases; (4) the H2 - evolving, energy -converting [NiFe]-H2ases. Unlike the [NiFe]-H2 ases, the [Fe] - H2ases form a homogeneous group and and are primarily involved in H2 evolution. The classification of H2ases on the basis of phylogenetic analysis, the correlation of the different phylogenetic groupings with H2ase function, the possible role of the [Fe] - H2ases in the genesis of the eukaryotic cell, the structural and functional relationships of H2ases subunits with those of Complex I of the respiratory electron transport chain, are all discussed. The diversity of H2ases may seem surprising in view of the simplicity of the reaction catalyzed (H2 ↔ 2H+ + 2e−) but can be understood in terms of the wide range of physiological roles that they play. In addition to the direct roles in respiration described above, work in recent years has revealed that H2ases may be involved in regulating gene expression, e.g. by acting as H2 -sensors, or may interact directly with membrane-bound electron transport systems in order to maintain redox poise, particularly in some photosynthetic micro-organisms, such as cyanobacteria and green algae. In addition, some H2-evolving H2ases thought to be involved in purely fermentative process have been shown to play a role in membrane-linked energy conservation, through the generation of a protonmotive force. This diversity of metabolic roles is reflected in a diversity of regulatory mechanisms, and our current knowledge of H2ases gene regulation is extensively described. Most studies of regulation have been carried out on members of the Proteobacteria, in which H2ases genes are frequently clustered, but these studies are now being extended to members of the Archaea. The purpose of this chapter is to emphasize recent advances that have greatly increased our knowledge of H2ases and H2ases function, to underline the importance of H2 metabolism in both prokaryotic and eukaryotic organisms, and to point out that, nevertheless, much remains to be discovered about the structure, function and regulation of this interesting class of enzymes. [ABSTRACT FROM AUTHOR]

Certain anaerobic prokaryotes grow by oxidative phosphorylation with elemental sulfur as the terminal electron acceptor instead of O2 (sulfur respiration or anaerobic respiration with sulfur). The mechanism of sulfur respiration has been investigated in great detail only in Wolinella (W.) succinogenes. This ε-proteobacterium uses, as terminal electron acceptor, polysulfide which is formed abiotically from elemental sulfur and sulfide. Polysulfide reduction is facilitated by a periplasmic sulfur-binding protein whose structure was determined. Polysulfide respiration with H2 or formate as electron donor is catalyzed by the membrane of W.succinogenes and is coupled to apparent proton translocation across the membrane. The eletrochemical proton potential (Δp) thus generated and the corresponding H+/e- ratio were determined to be 170 mV (negative inside) and 0.5, respectively. The electron transport chain catalyzing polysulfide respiration by H2 or formate consists of polysulfide reductase, methyl-menaquinone and hydrogenase or formate dehydrogenase. Each enzyme consists of two different hydrophilic and a hydrophobic subunit which anchors the enzyme in the membrane. The membrane anchors of hydrogenase and formate dehydrogenase are similar di-heme cytochromes b which carry the site of quinone reduction. Methyl-menaquinone is thought to be bound to the membrane anchor of polylsulfide reductase and to accept electrons from the cytochromes b of the dehydrogenases. Proteoliposomes containing polysulfide reductase, methy-menaquinone, and formate dehydrogenase isolated from w.succinogenes catalyze polysulfide reduction by formate which is coupled to Δp generation. The coupling mechanism is discussed. [ABSTRACT FROM AUTHOR]

Possible mechanisms by which energy may be conserved from the aerobic oxidation of ferrous iron by Thiobaccillus ferrooxidans are discussed. A rationale based on a consideration of the thermodynamic constraints of the system and an analysis of sequence information of the respiratory cytochrome oxidase is applied. Limitations are imposed on possible models and it is suggested that Tb.ferrooxidans cytochrome oxidase may be uniquely partially decoupled from proton-pumping. [ABSTRACT FROM AUTHOR]

Respiratory reactions involving inorganic nitrogen species provide a rich variety of systems with which to study bacterial bioenergetics and biological chemistry. These respiratory reactions encompass the reduction of nitrate, nitrite, nitric oxide and nitrous oxide; in all cases catalysis involves redox chemistry that takes place at metal centers. These catalytic metal centers include mono-nuclear type II copper, c heme or d1 heme in nitrite reductases, the Mo-bis-molybdopterin guanine dinucleotide cofactor in nitrate reductases, a non heme Fe-heme Fe dincuclear center in nitric oxide reductases and a tetra-nuclear copper sulfide center in nitrous oxide reductase. Recent structures and new spectroscopic data for these enzymes have rapidly advanced our knowledge of the molecular and dynamic properties of the catalytic centers, as well as how electrons are transferred to them; models of catalysis can thus be proposed. Bioenergetic studies have also provided an understanding of the way in which these enzymes are coupled to energy-conserving electron transport pathways. [ABSTRACT FROM AUTHOR]

Methanotrophs are a unique bacterial group characterized by the ability to utilize methane as a sole carbon and energy source. Methanotrophs oxidize methane to CO2 via a series of two electron steps with methanol, formaldehyde, and formate as intermediates. In addition to the known growth substrates methanotrophs will oxidize a variety of multi-carbon compounds, molecular hydrogen, and ammonia. With the exception of methane, and in some cases, methanol, the oxidation of these other substrates do not support growth and have been termed co-oxidation because of the need to provide a second substrate, methane or methanol, to support growth. However, recent studies have demonstrated the oxidation of some non-growth substrates provide a secontary source of reducing equilavents to the cells. This review focuses on the electron flow between the enzymes that catalyze the oxidation of both growth and non-growth substrates, and the respiratory chains in methanotrophs. The properities of the individual oxidative enzymes and respiratory components are reviewed and working models presented for electron flow during methane and ammonia oxidation. [ABSTRACT FROM AUTHOR]

This chapter deals with the oxidation of ammonia (NH3 + 1.5 O2→ HNO2+H2O as a source of reducing power in the chemolithrotrophic bacterium Nitrosomonas europaea. Direct knowledge of the enzymes involved together with the sequence of the genome reveal core elements of a redox system unique to oxidation of ammonia to nitrite which feeds into a more traditional bacterial electron transport/terminal oxidase system. The apparently low stoichiometry of protons translocated per ammonia oxidized hints at the basis of the low growth yields of this bacterium. Remarkably, the putative complex of hydroxylamine oxidoreductase (HAO), cytochrome c554 (Cyt c554) and the membrane cytochrome cM552 (Cyt cM552), which catalyzes the oxidation of a molecule of hydroxylamine and transfer of four electrons to membrane ubiquinone would involve 16 c-hemes per catalytic site or 48 hemes for the hypothetical aggregate containing the trimeric HAO. The dehydrogenation catalyzed at the novel catalytic heme (heme P460) is unique by comparison with other known catalytic hemes which bind substrate to the iron; in all others electrons enter the system and reduce the substrate whereas the reverse is true with HAO. This mode of catalysis may be functionally related to a cross link which is found only in HAO; a covalent bond between a methyne carbon of heme P460 and a ring carbon of a peptide tyrosine. The dramatic crystal structures of HAO and Cyt c554 have provided insights into catalysis and electron transfer as well as illustrating evolutionary relationships which are not reflected in homology of amino acid sequence. Considering their relative spatial arrangement. The 4 hemes of Cyt c554 can be preciselyl superimposed with 4 of the hemes of HAO. Evidence suggests that they have a common ancestor and have preserved heme configurations even when sequence homology had been lost. The novel anaerobic oxidation of ammonia (NH3HNO2 → N2 +H2O) by a planctomycete bacterium and the oxidation of ammonia to nitrite in heterotrophic bacteria are described more briefly. [ABSTRACT FROM AUTHOR]

Nitrogen fixation allows a diverse array of bacteria, either free-living in the environment or in symbiosis with plants, to grow in areas where fixed N is deficient. This confers them a large advantage over their non-N2 fixing competitors. Nevertheless, N2 fixation is an energy-demanding process, so that energy-generating respiration is oftentimes closely associated with efficient N2 fixing systems. The root nodule bacteria have many (O2-binding) terminal oxidases that differ in expression depending on whether the bacterium exists in free-living or in the symbiotic state. Many of the aerobic N2 fixers contain a very high O2 affinity terminal oxidase, like one that can function at free O2 levels as low as 7nM when the bacteria are surrounded in leghemoglobin within legume root nodules. The previously described ‘uncoupled' nature of some oxidases of N2 fixing bacteria may not be the case, from more recent studies. The N2 fixing enzyme, nitrogenase is labile to O2 exposure; this creates a physiological problem for aerobic diazotrophs. The many strategies to overcome this problem must ensure a steady supply of ATP during nitrogenase function yet permit the enzyme to be protected from O2 inactivation. Strategies include living in aggregated or within viscous slimy sheaths, or in O2-restricted root nodule barriers or forming specialized cells lacking oxygenic O2-evolving photosynthetic machinery. The temporal separation of N2 fixation from O2 evolving photosynthesis is a mechanism used by some marine filamentous cyanobacteria. Employing vigorous respiratory activity at the membrane so that the inside of the cell is nearly anaerobic, or the use of a small redox-active iron-sulfur protein that binds to and protects nitrogenase from O2 inactivation are well studied mechanisms. Most of the protection mechanisms, even the expression of terminal oxidase activities are generally co-regulated with N2 fixation genes, and some of these protection systems may concomitantly combat oxidative stress. This additional stress is related to reactive oxygen species produced from N2 fixation related proteins. [ABSTRACT FROM AUTHOR]

Acetic acid bacteria are obligate aerobes, and well known to have a strong ability to oxidize ethanol, sugars or sugar alcohols to produce the corresponding sugar acids. These oxidation reactions of sugars and alcohols in acetic acid bacteria are uniquely carried out by primary dehydrogenases located in the periplasmic side of the cytoplasmic membrane and linked to the terminal ubiquinol oxidase(s) via ubiquinone in the respiratory chain. The terminal ubiquinol oxidases working in the respiratory chain of acetic acid bacteria could be classified into four different types, cytochrome o, cytochrome a1, cytochrome d, and CN-resistant bypass oxidase, which are not unique but found in other bacterial species; whereas the primary dehydrogenases working in the periplasmic sugar alcohol respirations include many unique quinoproteins and quinoprotein-cytochrome c complexes, and flavoprotein-cytochrome c complexes. Such periplasmic sugar and alcohol respirations of acetic acid bacteria are involved in the accumulation of the oxidized products of sugars or alcohols, but not in the assimilation of these substrates, at least in their early growth phase. These sugar and alcohol respirations seem not to generate so much energy, but instead to work for rapid oxidation to produce a large amount of oxidized products, which are usually very hard to be utilized by other organisms, or confer very harmful conditions for other organisms to keep alive. Thus, the specific respirations of acetic acid bacteria seem to have evolved to their own living strategy for competing with other microorganisms. [ABSTRACT FROM AUTHOR]

The microaerophilic, human gastro-intestinal pathogens, Campylobacter jejuni and Helicobacter pylori are closely related phylogentically, yet distinct in some major aspects of their physiology, especially with regard to electron transport. C. jejuni is a more versatile and metabolically active pathogen, with a complete citri-acid cycle, and a complex and highly branched respiratory chain allowing the use of a variety of electron donors such as formate, hydrogen, D-lactate, succinate, malate, NAD(P)H and alternative electron acceptors to oxygen, including fumarate, nitrate, nitrite, N- or S-oxides and hydrogen peroxide. H. pylori is a more specialized pathogen, largely restricted to the human stomach, with an incomplete citric-acid cycle and a simpler respiratory chain. Alternative electron acceptors to oxygen include fumarate, hydrogen peroxide and possibly N- or S-oxides. Both organisms contain a novel type of complex I, which lacks the two subunits NQO1 (NuoE) and NQO2 (NuoF), responsible for NADH binding and initial electron transfer reactions in other bacteria. The nature of the electron donor to complex I has yet to be identified. In both bacteria, reducing equivalents are transferred to the sole quinone, menaquinone-6. Menoquinaol reduces the cytochrome bc1 complex, which in turn reduces periplasmic cytochrome c. Unusually for a bacterium, H. pylori contains only a single terminal oxidase (a cb-type cytochrome c oxidase). However, C. jejuni also possesses a bd-like quinol oxidase in addition ot cb-type oxidase, allowing an additional but less coupled pathway of electron transfer to oxygen. [ABSTRACT FROM AUTHOR]

The group of Gram-positive bacteria is a major phylum of prokaryotes, including several typical saprophytic aerobes. Their respiratory chains are apparently similar to those of eukaryotic mitochondria, but in several points are different from them. The respiratory chain of Gram-positives, like many bacteria, contains branched electron transfer pathways, usually 1-3 heme-Cu oxidases, but SoxB-type cytochrome c oxidases (cytochrome b(a/o)3) are working under air-limited conditions in thermophilic and alkaliphilic Bacillus species. On the other hand, SoxM-type quinol oxidase (cytochrome aa3) is mainly working in mesophilic Bacillus subtilis. Quinol-cytochorme c reductase of low G+C content Gram-positives (Firmicutes) is composed of a Rieske Fe-S subunit, a split cytochrome b (b6-type) and a novel cytochrome c1 subunit formed from ‘subunit IV' and a kind of class-I cytochromes c, while that of high G+C content Gram-positives (Actinobacteria) has a cytochrome b, a Fe-S protein with three transmembrane helices and a novel diheme cytochrome cc, without and small cytochromes c in the cell. Some of these characteristics are due to the lack of outer membranes of Gram-positives, in which any c-type cytochrome should be anchored to the cell membrane, and other are probably due to the evolutionary history of bacteria. [ABSTRACT FROM AUTHOR]

Oxygen originated in evolution within the prokaryotic kingdom and was acquired by eukaryotes later via endosymbiosis. Prokaryotes are divided into the domains of Bacteria and Archaea. The latter represent the so called third phylogenetic domain and are not only characterized by their unusual extremophilic life styles, but also by a variety of molecular pecularities including their bioenergetic systems. Though the prinicple of chemiosmotic energy conservation holds also for Archea, some primary energy transducing reations are restricted to this domain, as for example methoangenesis and rhodopsin dependent phototrophic energentics. Whereas the majority of known Archaea can thrive anaerobically, also a significant number of species is capable of obligate or facultative aerobic growth. The present chapter highlights the essential and archetypical differences with respect to orthodox pro- and eukaryotic respiratory systems. In particular the following aspects are addressed: 1. Several archaeal genomes have been completely analyzed, but protein chemical verification of the genomic data on electron transport systems lacks behing; 2. According to present knowledge, aerobic Archaea can oxide NADH by a type-II NADH dehyrogenase while an equivalent to complex-I has not been verified; 3. In several reactions the function of NAD is replaced by ferredoxins; 4. Other auxiliary cofactors are small copper proteins which may replace the function of c-type cytochromes, as well as unusual thiopheno-benzoquinones which act as membrane residing pools for reducing equivalents. Further, most a-type cytochromes contain an archetypical heme-AS which also replaces heme-b in several respiratory complexes of thermoacidophilic Archaea; 5. Archaeal complex-II equivalents contain modified S3 FeS-clusters as well as unusual cysteine-rich motifs in their small subunits which host a novel [2Fe2S] cluster and, in addition, can be devoid of their typical membrane anchors; 6. A novel type of Rieske/cytochrome complex, equivalent to respiratory complex-III, has been detected in thermoacidophilic Archaea; 7. Minimal respiratory chains can consist simply of a quinol oxidase and are contrasted by supercomplexes comprising a genetic and structural fusion of a whole respiratory chain into one unique protein assembly; 8. A novel regulatory adaptation toacidic environments is proposed based on the unique properties of a CuA containing terminal oxidase from an extreme thermoacidophile; 9. A revised view on the structural basis for proton pumping capabilities of terminal oxidases is provided. 10. The Conclusions paragraph addresses the necessary experimental approaches required to investigate the unsolved questions on archaeal respiratory systems. [ABSTRACT FROM AUTHOR]