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The mechanism of proton translocation driven by the respiratory nitrate reductase complex of Escherichia coli.

ABSTRACT: Low concentrations (1-50mum) of ubiquinol(1) were rapidly oxidized by spheroplasts of Escherichia coli derepressed for synthesis of nitrate reductase using either nitrate or oxygen as electron acceptor. Oxidation of ubiquinol(1) drove an outward translocation of protons with a corrected -->H(+)/2e(-) stoichiometry [Scholes & Mitchell (1970) J. Bioenerg.1, 309-323] of 1.49 when nitrate was the acceptor and 2.28 when oxygen was the acceptor. Proton translocation driven by the oxidation of added ubiquinol(1) was also observed in spheroplasts from a double quinone-deficient mutant strain AN384 (ubiA(-)menA(-)), whereas a haem-deficient mutant, strain A1004a, did not oxidize ubiquinol(1). Proton translocation was not observed if either the protonophore carbonyl cyanide m-chlorophenylhydrazone or the respiratory inhibitor 2-n-heptyl-4-hydroxyquinoline N-oxide was present. When spheroplasts oxidized Diquat radical (DQ(+)) to the oxidized species (DQ(++)) with nitrate as acceptor, nitrate was reduced to nitrite according to the reaction: [Formula: see text] and nitrite was further reduced in the reaction: [Formula: see text] Nitrite reductase activity (2) was inhibited by CO, leaving nitrate reductase activity (1) unaffected. Benzyl Viologen radical (BV(+)) is able to cross the cytoplasmic membrane and is oxidized directly by nitrate reductase to the divalent cation, BV(++). In the presence of CO, this reaction consumes two protons: [Formula: see text] The consumption of these protons could not be detected by a pH electrode in the extra-cellular bulk phase of a suspension of spheroplasts unless the cytoplasmic membrane was made permeable to protons by the addition of nigericin or tetrachlorosalicylanilide. It is concluded that the protons of eqn. (3) are consumed at the cytoplasmic aspect of the cytoplasmic membrane. Diquat radical, reduced N-methylphenazonium methosulphate and its sulphonated analogue N-methylphenazonium-3-sulphonate (PMSH) and ubiquinol(1) are all oxidized by nitrate reductase via a haem-dependent, endogenous quinone-independent, 2-n-heptyl-4-hydroxyquinoline N-oxide-sensitive pathway. Approximate-->H(+)/2e(-) stoichiometries were zero with Diquat radical, an electron donor, 1.0 with reduced N-methylphenazonium methosulphate or its sulphonated analogue, both hydride donors, and 2.0 with ubiquinol(1) (QH(2)), a hydrogen donor. It is concluded that the protons appearing in the medium are derived from the reductant and the observed-->H(+)/2e(-) stoichiometries are accounted for by the following reactions occurring at the periplasmic aspect of the cytoplasmic membrane.: [Formula: see text]

SUBMITTER: Jones RW

PROVIDER: S-EPMC1162066 | biostudies-other | 1980 Jul

REPOSITORIES: biostudies-other

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Project description:Stoicheometries and rates of proton translocation associated with respiratory reduction of NO3- have been measured for spheroplasts of Escherichia coli grown anaerobically in the presence of NO3-. Observed stoicheiometries [leads to H+/NO3- ratio; P. Mitchell (1966) Chemiosmotic Coupling in Oxidative and Photosynthetic Phosphorylation, Glynn Research, Bodmin] were approx. 4 for L-malate oxidation and approx. 2 for succinate, D-lactate and glycerol oxidation. Measurements of the leads to H+/2e- ratio with formate as the reductant and oxygen or NO3- as the oxidant were complicated by pH changes associated with formate uptake and CO2 formation. Nevertheless, it was possible to conclude that the site of formate oxidation is on the inner aspect of the cytoplasmic membrane, that the leads to H+/O ratio for formate oxidation is approx. 4, and that the leads to H+/NO3- ratio is greater than 2. Measurements of the rate of NO3- penetration into osmotically sensitive spheroplasts demonstrated an electrogenic entry of NO3- anion. The permeability coefficient for nitrate entry at 30 degrees C was between 10(-9) and 10(-10) cm- s(-1). The calculated rate of nitrate entry at the concentration typically used for the assay of nitrate reductase (EC 1.7.99.4) activity was about 0.1% of that required to support the observed rate of nitrate reduction by reduced Benzyl Viologen. Measurements of the distribution of nitrate between the intracellular and extracellular spaces of a haem-less mutant, de-repressed for nitrate reductase but unable to reduce nitrate by the respiratory chain, showed that, irrespective of the presence or the absence of added glucose, nitrate was not concentrated intracellularly. Osmotic-swelling experiments showed that the rate of diffusion of azid anion across the cytoplasmic membrane is relatively low in comparison with the fast diffusion of hydrazoic acid. The inhibitory effect of azide on nitrate reductase was not altered by treatments that modify pH gradients across the cytoplasmic membrane. It is concluded that the nitrate-reducing azide-sensitive site of nitrate reductase is located on the outer aspect of the cytoplasmic membrane. The consequences of this location for mechanisms of proton translocation driven by nitrate reduction are discussed, and lead to the proposal that the nitrate reductase of the cytoplasmic membrane is vectorial, reducing nitrate on the outer aspect of the membrane with 2H+ and 2e- that have crossed from the inner aspect of the membrane.

Project description:Cellular respiration is a fundamental process undergone within energy-transducing membranes through the redox activity of multimeric protein assemblies, so-called respiratory complexes. In most cases, electron transport is ensured by lipophilic molecules, quinones, serving as electron shuttles. This redox activity is coupled to the net translocation of protons across the membrane thereby establishing an electrochemical gradient, the protonmotive force (pmf). Such a gradient powers the transport of molecules (such as proteins, ions or antibiotics) and ATP synthesis but was also shown to participate in protein localization in prokaryotes. Importantly, energy-transducing membranes show a high level of organization with heterogeneous distribution of respiratory complexes as observed across several bacterial lineages. Although electron transport in energy-transducing membranes is considered to be a kinetic process coupled to the diffusion of all reactants and in particular quinones, it is not clear to which extent subcellular distribution of respiratory complexes can have an influence on the overall kinetics. We previously evidenced polar clustering of nitrate reductase, NarGHI, an anaerobic respiratory complex in the gut bacterium Escherichia coli. Such spatial organization was shown to directly impact the electron flux of the associated respiratory chain. While dynamic localization of such complex plays an important role in controlling respiration, the mechanism by which it impacts the electron flux is not understood. Yet, under such conditions where nitrate is the sole terminal electron acceptor, functioning of the electron transport chain in enteric bacteria such as E. coli and Salmonella typhimurium is associated with the production of nitric oxide (NO) mainly resulting from the reduction of nitrite by nitrate reductase. As a consequence, both E. coli and S. typhimurium produce a multitude of enzymes controlling NO homeostasis, the primary source of nitrosative stress. Protein S-nitrosylation is a ubiquitous NO-dependent post-translational modification of cysteine that regulates protein structure and function. Recently, Seth et al demonstrated that, in absence of oxygen, NO oxidation is mainly catalyzed by the hybrid cluster protein Hcp allowing its further reaction with thiols of a broad spectrum of proteins. Among Hcp-dependent S-nitrosylated targets are the nitrate reductase complex, multiple metabolic enzymes and the transcription factor OxyR whose S-nitrosylation entails a distinct nitrosative stress regulon. Optimizing electron transport through clustering of nitrate reductase will lead to nitrite accumulation which may in turn be responsible for NO production. Here, the electron-donating respiratory complex, the formate dehydrogenase, FdnGHI was shown to cluster at the poles under nitrate respiring conditions. Its proximity with the nitrate reductase complex was confirmed by evaluating the interactome of the later under distinct metabolic conditions reported to impact its subcellular organization. All the identified partners were exclusively found when cells were grown under nitrate respiration. The proximity of two respiratory complexes provides mechanistic explanation on the importance of subcellular organization onto quinone pool turnover. Noteworthy is the identification of several components playing key roles in the protection against endogenous nitrosative stress.

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The mechanism of proton translocation driven by the respiratory nitrate reductase complex of Escherichia coli.

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