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 21,279 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The distinct roles of the two magnesium ions essential to the activity of D-xylose isomerase from Streptomyces olivochromogenes were examined. The enzyme-magnesium complex was isolated, and the stoichiometry of cation binding determined by neutron activation analysis to be 2 mol of magnesium per mole of enzyme. A plot of Mg2+ added versus Mg2+ bound to enzyme is consistent with apparent KD values of < or = 0.5-1.0 mM for one Mg2+ and < or = 2-5 mM for the second. A site-directed mutant of D-xylose isomerase was designed to remove the tighter, tetracoordinated magnesium binding site (site 1, Mg-1); Glu180 was replaced with Lys180. The stoichiometry of metal binding to this mutant, E180K, is 1 mol of magnesium per mole of enzyme. Ring-opening assays with 1-thioglucose (H2S released upon ring opening) show E180K catalyzes the opening of the sugar ring at 20% the rate of the wild-type, but E180K does not catalyze isomerization of glucose to fructose. Thus, the magnesium bound to Glu180 is essential for isomerization but not essential for ring opening. The X-ray crystallographic structures of E180K in the absence of magnesium and in the presence and absence of 250 mM glucose were obtained to 1.8-A resolution and refined to R factors of 17.7% and 19.7%, respectively. The wild-type and both E180K structures show no significant structural differences, except the epsilon-amino group of Lys180, which occupies the position usually occupied by the Mg-1.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Role of the divalent metal ion in sugar binding, ring opening, and isomerization by D-xylose isomerase: replacement of a catalytic metal by an amino acid. 790 42

In cell suspension of Desulfovibrio desulfuricans B-1388, oxidation of CO as the only energy source is associated with reduction of SO42-. After a 2-h incubation of cells in 8% CO, 81% of the gas is converted. Oxidation of 1 mole CO results in formation of 0.23 mole H2S. Intracellular ATP content increases from 2.5 (control) to 8.3 nmoles/mg (during CO conversion). Dinitrophenol inhibits sulfate reduction and CO oxidation. CO dehydrogenase was detected in cytoplasmic and membrane cell fractions (59 and 34%, respectively).
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PMID:Energetic aspects of CO oxidation in desulfovibrio desulfuricans 986 47

Thermodynamic calculations provide the means to quantify the chemical disequilibrium inherent in the mixing of reduced hydrothermal fluids with seawater. The chemical energy available for metabolic processes in these environments can be evaluated by taking into account the pressure and temperature dependence of the apparent standard Gibbs free energies of reactions in the S-H2-H2O system together with geochemical constraints on pH, activities of aqueous sulfur species and fugacities of H2 and/or O2. Using present-day mixing of hydrothermal fluids and seawater as a starting point, it is shown that each mole of H2S entering seawater from hydrothermal fluids represents about 200,000 calories of chemical energy for metabolic systems able to catalyze H2S oxidation. Extrapolating to the early Earth, which was likely to have had an atmosphere more reduced than at present, shows that this chemical energy may have been a factor of two or so less. Nevertheless, mixing of hydrothermal fluids with seawater would have been an abundant source of chemical energy, and an inevitable consequence of the presence of an ocean on an initially hot Earth. The amount of energy available was more than enough for organic synthesis from CO2 or CO, and/or polymer formation, indicating that the vicinity of hydrothermal systems at the sea floor was an ideal location for the emergence of the first chemolithoautotrophic metabolic systems.
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PMID:Geochemical constraints on chemolithoautotrophic reactions in hydrothermal systems. 1153 67

Sulfur hexafluoride (SFd)-contained gas is a common pollutant emitted during the etching process used in the semiconductor industry. This study demonstrated the application of radio-frequency (RF) plasma in the decomposition of SF6. The decomposition fraction of SF6 [etaSF6 (C(in)-C(out))/C(in) x 100%] and the mole fraction profile of the products were investigated as functions of input power and feed O2/SF6 ratio in an SiO2 reactor. The species detected in both SF6/Ar and SF6/O2/ Ar RF plasmas were SiF4, SO2, Fe2, SO2F2, SOF2, SOF4, S2F10, S2OF10, S2O2F10, and SF4. The results revealed that at 40 W, etaSF6 exceeded 99%, and the reaction products were almost all converted into stable compounds such as SiF4, SO2, and F2 with or without the addition of oxygen. Sulfur oxyfluorides such as SO2F2, SOF2, SOF4, S2OF10, and S2O2F10 were produced only below 40 W. The results of this work can be used to design a plasma/chemical system for online use in a series of a manufacturing process to treat SF6-containing exhaust gases.
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PMID:Decomposition of SF6 in an RF plasma environment. 1246 13

Flavocytochrome c-sulfide dehydrogenases (FCSDs) are complexes of a flavoprotein with a c-type cytochrome performing hydrogen sulfide-dependent cytochrome c reduction in vitro. The amino acid sequence analysis revealed that the phylogenetic relationship of different flavoproteins reflected the relationship of sulfur-oxidizing bacteria. The flavoprotein SoxF of Paracoccus pantotrophus is 29-67% identical to the flavoprotein subunit of FCSD of phototrophic sulfur-oxidizing bacteria. Purification of SoxF yielded a homogeneous emerald-green monomeric protein of 42 797 Da. SoxF catalyzed sulfide-dependent horse heart cytochrome c reduction at the optimum pH of 6.0 with a k(cat) of 3.9 s(-1), a K(m) of 2.3 microM for sulfide, and a K(m) of 116 microM for cytochrome c, as determined by nonlinear regression analysis. The yield of 1.9 mol of cytochrome c reduced per mole of sulfide suggests sulfur or polysulfide as the product. Sulfide dehydrogenase activity of SoxF was inhibited by sulfur (K(i) = 1.3 microM) and inactivated by sulfite. Cyanide (1 mM) inhibited SoxF activity at pH 6.0 by 25% and at pH 8.0 by 92%. Redox titrations in the infrared spectral range from 1800 to 1200 cm(-1) and in the visible spectral range from 400 to 700 nm both yielded a midpoint potential for SoxF of -555 +/- 10 mV versus Ag/AgCl at pH 7.5 and -440 +/- 20 mV versus Ag/AgCl at pH 6.0 (-232 mV versus SHE') and a transfer of 1.9 electrons. Electrochemically induced FTIR difference spectra of SoxF as compared to those of free flavin in solution suggested a strong cofactor interaction with the apoprotein. Furthermore, an activation/variation of SoxF during the redox cycles is observed. This is the first report of a monomeric flavoprotein with sulfide dehydrogenase activity.
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PMID:Sulfide dehydrogenase activity of the monomeric flavoprotein SoxF of Paracoccus pantotrophus. 1554 40

Ce(IV)-loaded Y-zeolites (CeY) were prepared for selective removal of the trace amount of organic sulfur compounds from hydrodesulfurization (HDS)-treated diesel oil. The CeY samples can be obtained from NH4-Y-zeolite (NH4Y) using liquid-phase ion-exchange and solid-state ion-exchange methods. The ion-exchange reactions, structures, and selective adsorptions of organic sulfur compounds of the CeY samples were investigated using XRD, IR, XPS, TEM, and GC sulfur analyzer. The organic sulfur compound uptakes strongly depend on the amount and the valency of Ce in the zeolite structure. Ce(IV) shows much higher adsorptive ability than Ce(III). A CeY-S sample prepared by solid-state ion-exchange reaction of NH4Y and Ce(NO3)3 with Ce/NH4 mole ratio of 0.63 at 250 degrees C showed a maximum sulfur uptake from a model solution of HDS-treated gasoline containing thiophene [S = 5 ppm (ppm = mg/L)]. A desulfurization from a HDS-treated diesel oil containing organic sulfur compounds (S = 1.87 ppm) and H2S (S = 0.73 ppm) was investigated with a combination of the CeY-S and a CuO adsorbent for removal of H2S by a batch method. The sulfur content was reduced to below 0.01 ppm for the first time. This method provides a promising desulfurization process to prepare a clean fuel for fuel cells.
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PMID:Preparation of cerium-loaded Y-zeolites for removal of organic sulfur compounds from hydrodesulfurizated gasoline and diesel oil. 1645 14

CS2 removal was obtained by coupling catalytic hyidation on bi-functional catalyst. On the hydrolysis active sites, CS2 is hydrolyzed to H2S, while on the oxidation active sites, H2S is oxidized to elemental S or sulfuric acid deposited on the porous support. The above process can be expressed as follows: CS2 -->(H2O)COS -->(H2O) -->H2S -->(O2)--> S/SO4(2-). H2S oxidation eliminates its prohibition on CS2 hydrolysis so that the rate of coupling removal CS2 is 5 times higher than that of CS2 hydrolysis. The same active energy of hydrolysis and coupling reaction also indicates that H2S oxidation does not change the reaction mechanism of CS2 hydrolysis. Temperature has obvious effect on the process while the mole ratio of O2 concentration to CS2 concentration (O/S) does not, especially in excess of 2.5. The formation of sulfuric acid on the catalyst surface poisons hydrolysis active sites and causes the decrease of left OH(-1) concentration on the catalysts surface. Lower temperature is suggested for this bi-functional catalyst owing to the low yield ratio of S/SO4(2-).
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PMID:Coupling catalytic hydrolysis and oxidation for CS2 removal. 1857 28

The practicability of lead removal from sulfate-rich wastewater through biological sulfate reduction process with hydrogen as electron donor was investigated. Sulfide, which was converted from sulfate by a sulfate-reducing bacteria (SRB) in a gas-lift reactor, was used to remove lead as lead sulfide precipitate. Furthermore, the toxicity of wastewater in terms of whole effluent toxicity (WET) before and after treatment was analyzed by using Microtox analyzer. The experiment was divided into three stages as follows: Stage I, startup and operation of sulfidogenic process fed with synthetic wastewater in a gas-lift reactor; Stage II, operation of sulfidogenic process fed with real wastewater in the same reactor and analysis of toxicity; and Stage III, separation of lead from wastewater. In stage I, the volumetric sulfate-sulfur loading rate was gradually increased from 1.0 g/L.d until no improvement of sulfide-sulfur production efficiency was evident at 2.58 g/L.d and maximum sulfide-sulfur concentration was set to 340 mg/L. In stage II, the results showed that the laboratory scale reactor could treat a real wastewater without inhibition or any remarkable problem. The produced sulfide-sulfur, 200 mg/L, was a little less in comparison with that of the previous stage. It could be due to the higher concentration of total dissolved solid (TDS). However, the sulfate concentration was still reduced by approximately 30%. The WET test by Microtox showed that toxicity was reduced more than 13 times. In stage III, the effluent from the reactor containing sulfide-sulfur of about 200 mg/L and lead-containing solution of 20 mg/L were fed with sulfide to lead ratio 3 moles: 1 mole into the precipitation chamber in which the optimum pH for lead sulfide precipitation of 8.0 was maintained. It was found that lead removal of 99% was attained.
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PMID:Lead removal and toxicity reduction from industrial wastewater through biological sulfate reduction process. 1878 Feb 20

WE CAN IDENTIFY THREE MAIN GROUPS OF CUTANEOUS PIGMENTED LESIONS THAT COULD BE REPRESENTED AS MELANOMA PRECURSORS: (a) congenital melanocytic nevi, (b) dysplastic or atypical nevi, and (c) acquired melanocytic nevi. The occurrence of melanoma in small and intermediate congenital melanocytic nevi is very uncommon, but there is a high risk in large congenital melanocytic nevi, in particular those arising in the so-called "bathing trunk" distribution. It is very important to distinguish the familial dysplastic nevus syndrome, which is a strong risk factor for cutaneous melanoma, from not familial (sporadic) dysplastic nevus, in which the risk for melanoma would depend on the total number of melanocytic nevi, phototype, and on the relationship to environmental factors.
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PMID:On the clinical significance of cutaneous melanoma's precursors. 2313 Feb 79

Methane is the main component of natural gas and biogas. As an abundant energy source, methane is crucial not only to meet current energy needs but also to achieve a sustainable energy future. Conversion of methane to liquid fuels provides energy-dense products and therefore reduces costs for storage, transportation, and distribution. Compared to thermochemical processes, biological conversion has advantages such as high conversion efficiency and using environmentally friendly processes. This paper is a comprehensive review of studies on three promising groups of microorganisms (methanotrophs, ammonia-oxidizing bacteria, and acetogens) that hold potential in converting methane to liquid fuels; their habitats, biochemical conversion mechanisms, performance in liquid fuels production, and genetic modification to enhance the conversion are also discussed. To date, methane-to-methanol conversion efficiencies (moles of methanol produced per mole methane consumed) of up to 80% have been reported. A number of issues that impede scale-up of this technology, such as mass transfer limitations of methane, inhibitory effects of H2S in biogas, usage of expensive chemicals as electron donors, and lack of native strains capable of converting methane to liquid fuels other than methanol, are discussed. Future perspectives and strategies in addressing these challenges are also discussed.
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PMID:Biological conversion of methane to liquid fuels: status and opportunities. 2528 83


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