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Query: EC:1.12.7.2 (hydrogenase)
 3,522 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The photosynthetic bacteria can evolve H2 in the light through a nitrogenase-mediated reaction. The nitrogenase enzyme in the photosynthetic bacteria is similar to other nitrogenases. It is made of two soluble components: a) the Fe protein (dinitrogenase reductase or Component II) which receives electrons from ferredoxin, and b) the Mo-Fe protein (dinitrogenase or Component I) on which the substrates (including protons) are reduced. In photosynthetic bacteria, the physiological regulation of nitrogenase activity involves inactivation by covalent modification of the nitrogenase Fe protein. This inactivation can be reversed by an activating factor (or activating enzyme) which is an extrinsic membrane protein. After an ammonia shock, both the Fe protein of nitrogenase, and the glutamine synthetase, become adenylylated in vivo. In the adenylylation state, glutamine synthetase has AMP moieties bound to the protein by phosphate linkage. In toluene-treated cells of Rhodopseudomas capsulata preincubated with radioactive ATP, labelled either by 14C on the adenine or by 32P on the P alpha of ATP and then submitted to an ammonia shock, the Fe protein becomes covalently labelled only with [14C]ATP ad not with [32P]alpha ATP, while glutamine synthetase becomes labelled with both radioactive ATP molecules. This indicates that a different type of linkage is involved in the binding of the modifying group to Fe protein and to glutamine synthetase. Like other N2 fixers, the photosynthetic bacteria also contain a hydrogenase. In R. capsulata, the hydrogenase is an intrinsic membrane protein which protrudes in the cytoplasmic space and is not accessible to anti-hydrogenase antibodies from the periplasmic side. The hydrogenase can transfer electrons from H2 to the electron transport chain. It functions physiologically as an uptake-hydrogenase and may contribute to the recycling of electrons to nitrogenase. In the presence of excess carbon compounds, its main role may be to maintain an anaerobic microenvironment for the nitrogenase. Ferredoxin has been isolated from photosynthetic bacteria. Rhodospirillum rubrum and Rhodopseudomonas capsulata each contain two different soluble ferredoxin molecules. Reduced Fd I from R. capsulata has been shown to donate its electrons to nitrogenase.
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PMID:H2 metabolism in photosynthetic bacteria and relationship to N2 fixation. 613 53

Tritrichomonas foetus mutants resistant to metronidazole lack the hydrogenosomal enzymes pyruvate: ferredoxin oxidoreductase and hydrogenase. Hydrogenosomes of these organisms did not oxidize pyruvate or produce ATP in its presence. Elimination of hydrogenosomal metabolism of pyruvate was compensated by an increased rate of glycolysis. The resistant mutants excreted no organic acids and H2 as metabolic end products. Glycolysis of the resistant T. foetus KV1-1MR-100 can be summarized as 1 mol glucose----2 mol ethanol + 2 mol CO2. The parent strain KV1, excreting H2, CO2 and acidic end products, converted about 10% of glucose to ethanol. Both strains produced ethanol from pyruvate through the action of two cytoplasmic enzymes: pyruvate decarboxylase and alcohol dehydrogenase. The specific activity of the former enzyme, catalyzing nonoxidative decarboxylation of pyruvate to acetaldehyde, was nearly seven times higher in the resistant than in the parent strain. Alcohol dehydrogenase reducing acetaldehyde to ethanol was specific to NADPH; it catalyzed the reverse reaction only slowly, and displayed similar activities in both resistant and sensitive trichomonads. Development of anaerobic metronidazole resistance in T. foetus depended on the loss of pyruvate:ferredoxin oxidoreductase as well as on the ability to increase alcoholic fermentation.
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PMID:Metabolic differences between metronidazole resistant and susceptible strains of Tritrichomonas foetus. 637 46

In this survey we describe the influence of hydrogen oxidation on the physiology of Rhizobium ORS 571. The presence of hydrogen is required for the synthesis of hydrogenase. Carbon substrates do not repress the synthesis of hydrogenase. The respiratory system contains cytrochromes of the b- and c-type. Cytochrome alpha 600 is present after growth at high oxygen tensions. The nature of the terminal oxidases functioning at low oxygen tensions has not been established yet----H+/O values with endogenous substrates are between 6 and 7. The results show the presence of two phosphorylation sites: site 1 (ATP/2e = 1.0) and site 2(ATP/2e = 1.33). By measuring molar growth yields it has been demonstrated that carbon-limited, nitrogen-fixing cultures obtain additional ATP from hydrogen oxidation, and that site 2 of oxidative phosphorylation is passed during hydrogen oxidation. A method is described to calculate ATP/N2 values (the total amount of ATP used by nitrogenase during the fixation of 1 mol N2) and H2/N2 ratios (mol hydrogen formed per mol N2 fixed) in aerobic organisms. For Rhizobium ORS 571 the ATP/N2 value is about 40 and the H2/N2 ratio is between 5 and 7.5. Cells obtained from oxygen-limited nitrogen-fixing cultures contain 30-40% poly-beta-hydroxybutyrate, which explains the high molar growth yields found. Hydrogen has not been detected in the effluent gas of these cultures, which may point to reoxidation of the hydrogen formed at nitrogen fixation. Calculations show that the effect of hydrogen reoxidation on the efficiency of nitrogen fixation (g N fixed X mol-1 substrate converted) is not very large and that the actual H2/N2 ratio is of much more importance. After addition of hydrogen to succinate-limited, ammonia-assimilating cultures, an initial increase of the Ysuccinate value (g dry wt X mol-1 succinate) is followed by a gradual decrease. This is accompanied by a large decrease of the YO2 value, and an increased permeability of the cytoplasmic membrane to protons. The results may be explained by a transition of the culture from an energy-limited state to a carbon-limited state.
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PMID:Hydrogen oxidation and nitrogen fixation in rhizobia, with special attention focused on strain ORS 571. 639 31

Methanol:5-hydroxybenzimidazolylcobamide methyltransferase from Methanosarcina barkeri has been purified to approximately 90% homogeneity by ion-exchange chromatography on DEAE-cellulose and QAE-A50 Sephadex columns. The molecular weight, estimated by gel electrophoresis, was found to be 122,000, and the enzyme contained two different subunits with molecular weights of 34,000 and 53,000, which indicates an alpha 2 beta structure. The enzyme contains three or four molecules of 5-hydroxybenzimidazolylcobamide, which could be removed by treatment of the enzyme with 2-mercaptoethanol or sodium dodecyl sulfate. In both cases the enzyme dissociated into its subunits. For stability, the enzyme required the presence of divalent cations such as Mg2+, Mn2+, Sr2+, Ca2+, or Ba2+. ATP, GTP, or CTP was needed in a reductive activation process of the enzyme. This activation was brought about by a mixture of H2, ferredoxin, and hydrogenase, but also by CO, which is thought to reduce the corrinoid chemically. The CO dehydrogenase-like activity of the methyltransferase is discussed.
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PMID:Purification and properties of methanol:5-hydroxybenzimidazolylcobamide methyltransferase from Methanosarcina barkeri. 643 59

The variation with redox potential of nitrogenase activity and the ratio of ATP hydrolyzed per two electrons transferred were measured using two systems: the dithionite/bisulfite couple at pH 7.4; and H2, hydrogenase, and ferredoxin at pH 8.5. In both cases, the variation in nitrogenase activity with redox potential followed a theoretical Nernst plot for a two-electron process with an apparent midpoint potential of about -470 mV. The ratio ATP/2e- was about 4 under highly reducing conditions. However, above the apparent midpoint potential, the ratio ATP/2e- increased drastically, reaching values as high as 20. These data imply that a low redox potential must be maintained for efficient nitrogen fixation in vitro and in vivo.
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PMID:Nitrogenase reduction by electron carriers: influence of redox potential on activity and the ATP/2e- ratio. 657 45

Electron transport from H2, NADPH, NADH and succinate to O2 or ferricytochrome c in respiratory particles isolated from Anacystis nidulans in which hydrogenase had been induced was abolished after extraction of the membranes with n-pentane; oxidation of ascorbate plus NNN'N'-tetramethyl-p-phenylenediamine remained unaffected. Incorporation of authentic ubiquinone-10, plastoquinone-9, menaquinone-7 and phylloquinone (in order of increasing efficiency) restored the electron-transport reactions. ATP-dependent reversed electron flow from NNN'N'-tetramethyl-p-phenylenediamine to NADP+ or, via the membrane-bound hydrogenase, to H+ was likewise abolished by pentane extraction and restored by incorporation of phylloquinone. Participation of the incorporated quinones in the respiratory electron-transport reactions of reconstituted particles was confirmed by measuring the degree of steady-state reduction of the quinones. Isolation and identification of the quinones present in native Anacystis membranes yielded mainly plastoquinone-9 and phylloquinone; neither menaquinone nor alpha-tocopherolquinone could be detected. Together with the results from reconstitution experiments this suggests that phylloquinone might function as the main respiratory quinone in Anacystis nidulans.
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PMID:Restoration of respiratory electron-transport reactions in quinone-depleted particle preparations from Anacystis nidulans. 676 34

An evolutionary explanation is sought for the fact that ATP is needed for N2 fixation in spite of the exergonicity of the process. After a survey of the state of knowledge about the thermodynamics of N2 fixation in fermenters, photosynthesizers and respirers it is suggested that nitrogenase, which still shows ATP-dependent hydrogenase activity, evolved from an ATP-requiring hydrogenase that lacked nitrogenase activity. The hydrogenase action in the Archaean, reducing, biosphere may have needed ATP to ensure expulsion of H2. Extant non-nitrogenase hydrogenases have lost the dependence on ATP. Because of its complexity, nitrogenase could not rid itself of the ATP dependence or of hydrogenase activity, both wasteful. Presumably all hydrogenases evoled from ferredoxin-like Fe-S proteins.
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PMID:Evolutionary considerations on the thermodynamics of nitrogen fixation. 677 99

The soluble, NAD+-reducing hydrogenase in intact cells of Alcaligenes eutrophus was inactivated by oxygen when electron donors such as hydrogen or pyruvate were available. The sole presence of either oxygen or oxidizable substrates did not lead to inactivation of the enzyme. Inactivation occurred similarly under autotrophic growth conditions with hydrogen, oxygen and carbon dioxide. The inactivation followed first order reaction kinetics, and the half-life of the enzyme in cells exposed to a gas atmosphere of hydrogen and oxygen (8:2, v/v) at 30 degrees C was 1.5h. The process of inactivation did not require ATP-synthesis. There was no experimental evidence that the inactivation is a reversible process catalyzed by a regulatory protein. The possibility is discussed that the inactivation is due to superoxide radical anions (O2-) produced by the hydrogenase itself.
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PMID:In vivo inactivation of soluble hydrogenase of Alcaligenes eutrophus. 678 48

Two H2 uptake-negative (Hup-) Rhizobium japonicum mutants were obtained that also lacked symbiotic N2 fixation (acetylene reduction) activity. One of the mutants formed green nodules and was deficient in heme. Hydrogen oxidation activity in this mutant could be restored by the addition of heme plus ATP to crude extracts. Bacteroid extracts from the other mutant strain lacked hydrogenase activity and activity for both of the nitrogenase component proteins. Hup+ revertants of the mutant strains regained both H2 uptake ability and nitrogenase activity.
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PMID:Nif- Hup- mutants of Rhizobium japonicum. 687 48

The enzyme which oxidizes alpha-keto[1-14C]isocaproate to 14CO2 is activated by incubation of adipose tissue segments with insulin. A 3-fold reduction in the apparent Km of the enzyme for alpha-ketoisocaproate was observed when homogenates of adipose tissue segments treated with insulin were compared to homogenates of control tissues. The enzyme was assayed at various times after homogenization of adipose tissue segments. Relatively small changes were observed in the activity from control or insulin-treated tissues for 30 min after homogenization. The persistence of the insulin effect after homogenization suggests that insulin may cause a covalent modification of the enzyme. The possibility that alpha-ketoisocaproate is oxidized by pyruvate dehydrogenase, which is also stimulated by insulin, is unlikely since the enzyme responsible for oxidation of 14C-labeled branched chain alpha-keto acids can be inactivated by heat at a rate distinct from that of pyruvate dehydrogenase. Moreover, unlabeled branched chain alpha-keto acids inhibit the oxidation of alpha-keto[1-14C]isocaproate but not that of [1-14C]pyruvate. Branched chain alpha-keto acid hydrogenase can be activated by incubation of adipose tissue homogenates in the presence of magnesium chloride and in the absence of ATP. The addition of ATP plus an ATP-regenerating system reverses the activation of the enzyme. The apparent Km of the enzyme is reduced and the Vmax is increased by incubation of tissue extracts under appropriate conditions.
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PMID:Insulin regulation of branched chain alpha-keto acid dehydrogenase in adipose tissue. 699 67


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