177,421 research outputs found
H-2-FORMING N-5,N-10-METHYLENETETRAHYDROMETHANOPTERIN DEHYDROGENASE FROM METHANOBACTERIUM-THERMOAUTOTROPHICUM CATALYZES A STEREOSELECTIVE HYDRIDE TRANSFER AS DETERMINED BY 2-DIMENSIONAL NMR-SPECTROSCOPY
No full text5,6,7,8-Tetrahydromethanopterin is a coenzyme playing a key role in the energy metabolism of methanogenic archaea. In Methanobacterium thermoautotrophicum, the reduction of N5,N-10-methenyl-5,6,7,8-tetrahydromethanopterin at C(14a) with H-2 to N5,N-10-methylene-5,6,7,8-tetrahydromethanopterin can be catalyzed by H-2-forming methylenetetrahydromethanopterin dehydrogenase, a new hydrogenase present in most methanogenic archaea, which is unique because it does not contain nickel or iron/sulfur clusters. In this work, the stereochemistry of this enzymatic hydride-transfer reaction is elucidated by means of a series of heteronuclear two-dimensional NMR experiments. It is found that the hydride from H-2 is transferred by the enzyme into the rel-(pro-R) position of the C(14a) methylene group of the reaction product N5,N-10-methylene-5,6,7,8-tetrahydromethanopterin. NMR experiments are described that show that the hydrogen nucleus of the hydride transferred to the oxidized coenzyme partially originates from water. The stereochemical course of this reaction is the same as that for direct hydride transfer. It is demonstrated that the diastereotopic atoms at C(14a) of the reaction product epimerize in an uncatalyzed reaction under the conditions of operation of the enzyme (k = 0.01 s-1 at 58-degrees-C and pH 6.5). On the basis of the known relative configuration of the pterin moiety of 5,6,7,8-tetrahydromethanopterin [Schleucher, J., Schworer, B., Zirngibl, C., Koch, U., Weber, W., Egert, E., Thauer, R. K., & Griesinger, C. (1992) FEBS Lett. 314, 440-444], the absolute configuration of this moiety is tentatively assigned to be (6S,7S, 11R) on the basis of a comparison of the CD spectra of N5,N-10-methenyl-5,6,7,8-tetrahydromethanopterin and its analog N5,N-10-methenyl-5,6,7,8-tetrahydrofolate. Given this absolute configuration of the pterin moiety, the rel-(pro-R) stereochemistry of the C(14a) methylene proton corresponds to the absolute (pro-R) stereochemistry
The managerial sources of corporate social responsibility : the spread of global standards
No full textWhy and under which conditions do companies voluntarily adopt high social and environmental standards? Christian Thauer looks inside the firm to illustrate the internal drivers of the social conduct of business. He argues that corporate social responsibility (CSR) assists decision-makers to resolve managerial dilemmas. Drawing on transaction cost economics, he asks why and which dilemmas bring CSR to the fore. In this context he describes a managerial dilemma as a situation where the execution of management's decisions transforms the mode of cooperation within the organization from a hierarchy to one in which managers become dependent on, and vulnerable to, the behavior of subordinates. Thauer provides empirical illustration of his theory by examining automotive and textile factories in South Africa and China. Thauer demonstrates that CSR is often driven by internal management problems rather than by the external pressures that corporations confront.-- 1. Introduction
-- 2. A theory of internal drivers of corporate social responsibility
-- 3. Corporate social responsibility: an inside-view approach and perspective
-- 4. Internal driver 1: the human resources dilemma
-- 5. Internal drivers 2 and 3: the technological specialization and foreign direct investment dilemmas
-- 6. Internal driver 4: the brand reputation dilemma
-- 7. Conclusion: internal drivers, corporate social responsibility and the spread of global standards.Published version of EUI PhD thesis, 201
The managerial sources of corporate social responsibility the spread of global standards
No full text"Why and under which conditions do companies voluntarily adopt high social and environmental standards? Christian Thauer looks inside the firm to illustrate the internal drivers of the social conduct of business. He argues that corporate social responsibility (CSR) assists decision-makers to resolve managerial dilemmas".
An ancient pathway combining carbon dioxide fixation with the generation and utilization of a sodium ion gradient for ATP synthesis
No full textSynthesis of acetate from carbon dioxide and molecular hydrogen is considered to be the first carbon assimilation pathway on earth. It combines carbon dioxide fixation into acetyl-CoA with the production of ATP via an energized cell membrane. How the pathway is coupled with the net synthesis of ATP has been an enigma. The anaerobic, acetogenic bacterium Acetobacterium woodii uses an ancient version of this pathway without cytochromes and quinones. It generates a sodium ion potential across the cell membrane by the sodium-motive ferredoxin:NAD oxidoreductase (Rnf). The genome sequence of A. woodii solves the enigma: it uncovers Rnf as the only ion-motive enzyme coupled to the pathway and unravels a metabolism designed to produce reduced ferredoxin and overcome energetic barriers by virtue of electron-bifurcating, soluble enzymes
European Journal of Biochemistry
No full textIn Methanosarcina barkeri, methanogenesis from methanol is initiated by the formation of methyl-coenzyme M from methanol and coenzyme M. This methyl transfer reaction is catalyzed by two enzymes, designated methyltransferases 1 (MT1) and 2 (MT2). Transferase MT1, which is composed of a 50-kDa subunit, MtaB, and a 27-kDa corrinoid-harbouring subunit, MtaC, has been shown recently to catalyze the methylation of free cob(I)alamin with methanol [Sauer, K., Harms, U. & Thauer, R. K. (1997) Eur. J. Biochem. 243, 670?677]. We report here that this reaction is catalyzed by subunit MtaB overproduced in Escherichia coli. MtaB also catalyzed the formation of methanol from methylcobalamin and H2O, the hydrolysis being associated with a free-energy change ?G0? of approximately +7.0 kJ/mol. MtaB was found to contain 1 mol zinc, and its activity to be zinc dependent (pKZn2+= 9.3). The zinc dependence of the MT2 (MtaA)-catalyzed reaction is also described (pKZn2+= 9.6)
European Journal of Biochemistry
No full textThe energy-conserving corrinoid-containing MtrA-H complex from Methanobacterium thermoautotrophicum is composed of eight different subunits of which MtrA harbors the corrinoid prosthetic group. EPR spectroscopic evidence has recently been presented for a histidine residue as a cobalt ligand of the cobamide [Harms, U. & Thauer, R. K. (1996a) Eur J. Biochem. 241, 149?154]. This active site histidine was now identified by site-directed mutagenesis to be His84 in the MtrA sequence that contains three histidines. This result was substantiated by sequence comparison of MtrA froin M. thermoautotrophicum, Methanococcus jannaschii, and Methanopyrus kandleri and of MtxA from Methanosarcina barkeri showing that only His84 is conserved. For comparison, the DNA sequences of the mtrEDCBAGH operon in M. kundleri and of the mtxXAH operon in M. barkeri were determined
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
Full text linkBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
Re-face stereospecificity of NADP-dependent methylenetetrahydromethanopterin dehydrogenase from Methylobacterium extorquens AM 1 as determined by NMR spectroscopy.
No full textAbstractMtdA catalyzes the dehydrogenation of N5,N10-methylenetetrahydromethanopterin (methylene-H4MPT) with NADP+ as electron acceptor. In the reaction two prochiral centers are involved, C14a of methylene-H4MPT and C4 of NADP+, between which a hydride is transferred. The two diastereotopic protons at C14a of methylene-H4MPT and at C4 of NADPH can be seen separately in 1H-NMR spectra. This fact was used to determine the stereospecificity of the enzyme. With (14aR)-[14a-2H1]-[14a-13C]methylene-H4MPT as the substrate, it was found that the pro-R hydrogen of methylene-H4MPT is transferred by MtdA into the pro-R position of NADPH
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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