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1.
N 5 N 10-Methylenetetrahydromethanopterin reductase was purified 13-fold to apparent homogeneity from methanol grown Methanosarcina barkeri . The colourless enzyme was found to be composed of four identical subunits of apparent molecular mass 36 kDa. It catalysed the reduction of methylenetetrahydromethanopterin ( K m=15 μM) to methyltetrahydromethanopterin with reduced coenzyme F420 ( K m=12 μM) at a specific rate ( V max) of 2200 μmol min−1· mg protein−1 ( K cat=1320 s−1). With respect to coenzyme specificity, molecular properties and catalytic mechanism the enzyme was found to be similar to CH2=H4MPT reductase of Methanobacterium thermoautotrophicum which phylogenetically is only distantly related to M. barkeri .  相似文献   

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Abstract A mass spectrometer with membrane inlet was used to study methanol metabolism by Methanosarcina barkeri strain MS. The addition of methanol to methanol grown culture samples in the mass spectrometer vessel stimulated methanogenesis and hydrogen production. The apparent K s for methanol was determined as 0.5 mM and the V max as 8.14 mmol g (dry weight) h−1. The V max for methane production was fairly constant during growth of the culture on methanol implying that growth is tightly coupled to methanogenesis. The addition of methanol to culture samples in the mass spectrometer vessel stimulated methanogenesis with no lag which indicated that methanogenesis can be uncoupled from growth. Exposure of the culture sample in the mass spectrometer vessel to an atmosphere of 2 kPa oxygen for 80 min resulted in a decrease in the rate of methanogenesis from methanol but on returning the atmosphere to nitrogen the addition of further methanol stimulated methanogenesis. The effect of other inhibitors of methanogenesis (2-bromoethane sulphonate and monensin); K j values 21.5 μM and 0.3 mM, respectively) were also studied.  相似文献   

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Abstract In cell suspensions of the methanogenic bacterium strain Gö1 or Methanosarcina barkeri H2 formation from methanol in the presence of 2-bromoethanesulfonic acid (BES) was strictly dependent on sodium ions; apparent K S for Na+, 1.3±0.3 mM.H2 formation was inhibited by the uncoupler tetrachlorosalicylanilide (TCS), but this inhibition could be temporarily overcome, when a sodium pulse (100 mM) was given to the cell suspension. On the other hand, H2 formation from formaldehyde in the presence of BES (rate: 300 nmol H2/h·mg protein as compared to 25 nmol H2/h·mg protein from methanol) was not sodium-dependent, not TCS-sensitive and not inhibited by addition of monensin. H2 formation was accompanied by CO2 formation in stoichiometric amounts, 3 H2:1 CO2 for methanol and 2 H2:1 CO2 for formaldehyde oxidation.  相似文献   

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Abstract both growth and methanogenesis of Methanosarcina barkeri are completely inhibited by sodium dodecylbenzene sulphonate at between 15 and 20 mg·1−1. At lower concentrations growth of cultures was delayed, but no uncoupling of methanogenesis from growth was observed. Higher concentrations of detergent (50 mg·1−1) produced marked alterations in the surface structures of organisms observed in scanning electron micrographs. Thus levels of a detergent common in anaerobic sewage treatment plants can inhibit methanogenesis, the terminal stage in the anaerobic digestion process.  相似文献   

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During growth of Methanosarcina barkeri strain Fusaro on a mixture of trimethylamine and acetate, methane production and acetate consumption were biphasic. In the first phase trimethylamine (33 mmol x l-1) was depleted and some acetate (11–14 from 50 mmol x l-1) was metabolized simultaneously. In the second phase the remaining acetate was cleaved stoichiometrically into CH4 and CO2. Kinetic experiments with (2-14C)acetate revealed that only 2.5% of the methane produced in the first phase originated from acetate: 18% of the acetate metabolized was cleaved into CH4 and CO2, 23% of the acetate was oxidized, and 55% was assimilated. Methane produced from CD3–COOH in the first phase consisted of CD2H2 and CD3H in a ratio of 1:1.  相似文献   

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Abstract Cell extracts prepared from Methanosarcina barkeri converted acetate into methane and carbon dioxide under a hydrogen atmosphere. Methanogenesis by cell extracts required acetate and ATP and, the in vitro rate was 5 to 10% of the rate of methanogenesis observed during exponential growth of cells on acetate. Methane and carbon dioxide produced by cell extracts originated predominantly from the methyl and carboxyl groups of acetate, respectively, in a manner consistent with that observed in whole cells. Acetate degradation activity was detected in the soluble (150000 × g supernatant) fraction and not in the membrane fraction. These results are discussed in relation to a proposed model for ATP generation from acetate that involves both membrane-bound and soluble enzymatic components such as CO dehydrogenase.  相似文献   

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Cell extracts (100,000×g) of acetate grown Methanosarcina barkeri (strain MS) catalyzed CH4 and CO2 formation from acetyl-CoA with specific activities of 50 nmol·min-1·mg protein-1. CH4 formation was found to be dependent on tetrahydromethanopterin (H4MPT) (apparent K M=4 μM), coenzyme M (H-S-CoM), and 7-mercaptoheptanoylthreonine phosphate (H-S-HTP=component B) rather than on methanofuran (MFR) and coenzyme F420 (F420). Methyl-H4MPT was identified as an intermediate. This compound accumulated when H-S-CoM and H-S-HTP were omitted from the assays. These and previous results indicate that methanogenesis from acetate proceeds via acetyl phosphate, acetyl-CoA, methyl-H4MPT, and CH3-S-CoM as intermediates. The disproportionation of formaldehyde to CO2 and CH4 was also studied. This reaction was shown to be dependent on H4MPT, MFR, F420, H-S-CoM, and H-S-HTP.  相似文献   

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Abstract Washed cells of Peptostreptococcus productus (strain Marburg), which were incubated in the presence of CO/CO2/N2 (50%/ 17%/ 33%; 200 kPa) catalyzed the synthesis of acetate from carbon monoxide. The rate of acetate formation from CO was stimulated more than threefold by the addition of sodium (10 mM); potassium did not effect acetate synthesis. The degree of stimulation was dependent on the sodium concentration; the dependence followed simple Michaelis-Menten kinetics. The apparent K m for sodium was determined to be about 2 mmol/1. Sodium also stimulated acetate synthesis from H2 plus CO2. In the absence of added sodium the formation of formate as an intermediate in methyl group synthesis was stimulated. It is suggested that the sodium dependent reaction(s) is one (or more) of the reactions involved in methyl group synthesis from CO2.  相似文献   

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A model is presented which quantifies a possible role for the carbonic anhydrase in the mitochondrial matrix of Chlamydomonas reinhardtii which incorporates the observation that the expression of this enzyme is increased under growth conditions in which the expression of the carbon dioxide-concentrating mechanism is increased. It is assumed that the inorganic carbon enters the cytosol from the medium, and leaves the cytosol to the plastids, as HCO3 and that there is negligible carbonic anhydrase activity in the cytosol. The role of the mitochondrial carbonic anhydrase is suggested to be the conversion to HCO3 of the CO2 produced in the mitochondria in the light from tricarboxylic acid cycle activity and from decarboxylation of glycine in any photorespiratory carbon oxidation cycle activity which is not suppressed by the carbon concentrating mechanism. If there is a HCO3 channel in the inner mitochondrial membrane then almost all of the inorganic carbon leaves the mitochondria as HCO3, thus limiting the potential for CO2 leakage through the plasmalemma. This mechanism could increase inorganic C supply to ribulose bisphosphate carboxylase-oxygenase by some 10% at the energetic expense of less than 1% of the total ATP generation by plastids plus mitochondria.  相似文献   

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Methanol:coenzyme M methyltransferase is an enzyme complex composed of three subunits, MtaA, MtaB, and MtaC, found in methanogenic archaea and is needed for their growth on methanol ultimately producing methane. MtaABC catalyzes the energetically favorable methyl transfer from methanol to coenzyme M to form methyl coenzyme M. Here we demonstrate that this important reaction for possible production of methanol from the anaerobic oxidation of methane can be reversed in vitro. To this effect, we have expressed and purified the Methanosarcina barkeri MtaABC enzyme, and developed an in vitro functional assay that demonstrates MtaABC can catalyze the energetically unfavorable (ΔG° = 27 kJ/mol) reverse reaction starting from methyl coenzyme M and generating methanol as a product. Demonstration of an in vitro ability of MtaABC to produce methanol may ultimately enable the anaerobic oxidation of methane to produce methanol and from methanol alternative fuel or fuel‐precursor molecules. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 33:1243–1249, 2017  相似文献   

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There is approximately 50 times more inorganic carbon in the global ocean than in the atmosphere. On time scales of decades to millions of years, the interaction between these two geophysical fluids determines atmospheric CO2 levels. During glacial periods, for example, the ocean serves as the major sink for atmospheric CO2, while during glacial–interglacial transitions, it is a source of CO2 to the atmosphere. The mechanisms responsible for determining the sign of the net exchange of CO2 between the ocean and the atmosphere remain unresolved. There is evidence that during glacial periods, phytoplankton primary productivity increased, leading to an enhanced sedimentation of particulate organic carbon into the ocean interior. The stimulation of primary production in glacial episodes can be correlated with increased inputs of nutrients limiting productivity, especially aeolian iron. Iron directly enhances primary production in high nutrient (nitrate and phosphate) regions of the ocean, of which the Southern Ocean is the most important. This trace element can also enhance nitrogen fixation, and thereby indirectly stimulate primary production throughout the low nutrient regions of the central ocean basins. While the export flux of organic carbon to the ocean interior was enhanced during glacial periods, this process does not fully account for the sequestration of atmospheric CO2. Heterotrophic oxidation of the newly formed organic carbon, forming weak acids, would have hydrolyzed CaCO3 in the sediments, increasing thereby oceanic alkalinity which, in turn, would have promoted the drawdown of atmospheric CO2. This latter mechanism is consistent with the stable carbon isotope pattern derived from air trapped in ice cores. The oceans have also played a major role as a sink for up to 30% of the anthropogenic CO2 produced during the industrial revolution. In large part this is due to CO2 solution in the surface ocean; however, some, poorly quantified fraction is a result of increased new production due to anthropogenic inputs of combined N, P and Fe. Based on ‘circulation as usual’, models predict that future anthropogenic CO2 inputs to the atmosphere will, in part, continue to be sequestered in the ocean. Human intervention (large-scale Fe fertilization; direct CO2 burial in the deep ocean) could increase carbon sequestration in the oceans, but could also result in unpredicted environmental perturbations. Changes in the oceanic thermohaline circulation as a result of global climate change would greatly alter the predictions of C sequestration that are possible on a ‘circulation as usual’ basis.  相似文献   

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1 In a glasshouse experiment we studied the effect of raised CO2 concentration (720 p.p.m.) on CH4 emission at natural boreal peat temperatures using intact cores of boreal peat with living vascular plants and Sphagnum mosses. After the end of the growing season half of the cores were kept unnaturally warm (17–20 °C). The potential for CH4 production and oxidation was measured at the end of the emission experiment.
2 The vascular cores ('Sedge') consisted of a moss layer with sedges, and the moss cores (' Sphagnum ') of Sphagnum mosses (some sedge seedlings were removed by cutting). Methane efflux was 6–12 times higher from the Sedge cores than from the Sphagnum cores. The release of CH 4 from Sedge cores increased with increasing temperature of the peat and decreased with decreasing temperature. Methane efflux from Sphagnum cores was quite stable independent of the peat temperatures.
3 In both Sedge and Sphagnum samples, CO2 treatment doubled the potential CH4 production but had no effect on the potential CH4 oxidation. A raised concentration of CO2 increased CH4 efflux weakly and only at the highest peat temperatures (17–20 °C).
4 The results suggest that in cool regions, such as boreal wetlands, temperature would restrict decomposition of the extra substrates probably derived from enhanced primary production of mire vegetation under raised CO2 concentrations, and would thus retard any consequent increase in CH4 emission.  相似文献   

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Uptake of CO2 by aquatic vegetation   总被引:5,自引:10,他引:5  
Abstract Photosynthesis by aquatic plants based on the supply of CO2 from air-equilibrated solutions may be limited by the low diffusion coefficient of CO2 in water. For plants in which the transport of CO2 from the bulk medium is by diffusion, and the initial carboxylation uses RUBISCO, CO2 supply can be increased by growth in habitats with fast water flow over the surface (reducing unstirred layer thickness), or with heterotrophically-augmented CO2 levels, including the direct use of sediment CO2. Many aquatic plants using RUBISCO as their initial carboxylase counter the limitations on CO2 supply via the operation of biophysical CO2 concentrating mechanisms which are based on active transport of HCO?3, CO2 or H+ at the plasmalemma, and use bulk-phase HCO?3 or CO2 as the C source. A final group of aquatic plants use biochemical CO2 concentrating mechanisms based on auxiliary carboxylation by PEPc: C4-like and Crassulacean Acid Metabolism–like processes are involved. These various mechanisms for increasing CO2 supply to RUBISCO also help to offset the low specific reaction rate of aquatic plant RUBISCOs at low [CO2] and low [CO2]: [CO2]. In addition to overcoming restrictions on CO2 supply, the various methods of increasing inorganic C availability may also be important in alleviating shortages of nitrogen or photons.  相似文献   

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Sensing of atmospheric CO2 by plants   总被引:3,自引:12,他引:3  
Abstract. Despite recent interest in the effects of high CO2 on plant growth and physiology, very little is known about the mechanisms by which plants sense changes in the concentration of this gas. Because atmospheric CO2 concentration is relatively constant and because the conductance of the cuticle to CO2 is low, sensory mechanisms are likely to exist only for intercellular CO2 concentration. Therefore, responses of plants to changes in atmospheric CO2 will depend on the effect of these changes on intercellular CO2 concentration. Although a variety of plant responses to atmospheric CO2 concentration have been reported, most of these can be attributed to the effects of intercellular CO2 on photosynthesis or stomatal conductance. Short-term and long-term effects of CO2 on photosynthesis and stomatal conductance are discussed as sensory mechanisms for responses of plants to atmospheric CO2. Available data suggest that plants do not fully realize the potential increases in productivity associated with increased atmospheric CO2. This may be because of genetic and environmental limitations to productivity or because plant responses to CO2 have evolved to cope with variations in intercellular CO2 caused by factors other than changes in atmospheric CO2.  相似文献   

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Abstract The experimental determination of cytoplasmic and vacuolar pH values is discussed. Despite variation in these values evidence indicates that intracellular pH values are normally regulated within narrow limits. The regulatory mechanisms proposed involve the metabolic consumption of OH& and the active efflux of H +. The evidence for intracellular pH modification in response to CO2 hydration and the production of HCO?3 and H+ is examined. Theoretical calculations and experimental data indicate that CO2 concentrations as high as 5% will lower intracellular pH. Conversely, variation in CO2 levels around atmospheric concentrations is unlikely to perturb intracellular pH. High CO2 levels are found in bulky tissues, and flooded root systems. Evidence is presented that the slow diffusion of dissolved CO2 compared to gaseous CO2 results in its accumulation. It is proposed that the accumulation of respiratory CO2 may reduce intracellular pH values when plant tissues, cells or protoplasts are maintained in a liquid culture medium. Finally, the possible role of dark CO2 fixation and organic acid synthesis in the regulation of intracellular pH is examined.  相似文献   

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