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11.
The final steps in the synthesis of acetyl-CoA by CO dehydrogenase (CODH) have been studied by following the exchange reaction between CoA and the CoA moiety of acetyl-CoA. This reaction had been studied earlier (Pezacka, E., and Wood, H. G. (1986) J. Biol. Chem. 261, 1609-1615 and Ramer, W. E., Raybuck, S. A., Orme-Johnson, W. H., and Walsh, C. T. (1989) Biochemistry 28, 4675-4680). The CoA/acetyl-CoA exchange activity was determined at various controlled redox potentials and was found to be activated by a one-electron reduction with half-maximum activity occurring at -486 mV. There is approximately 2000-fold stimulation of the exchange by performing the reaction at -575 mV relative to the rate at -80 mV. Binding of CoA to CODH is not sensitive to the redox potential; therefore, the reductive activation affects some step other than association/dissociation of CoA. We propose that a metal center on CODH with a midpoint reduction potential of less than or equal to -486 mV is activated by a one-electron reduction to cleave the carbonyl-sulfur bond and/or bind the acetyl group of acetyl-CoA. Based on a comparison of the redox dependence of this reaction with that for methylation of CODH (Lu, W-P., Harder, S. R., and Ragsdale, S. W. (1990) J. Biol. Chem. 265, 3124-3133) and CO2 reduction and formation of the Ni-Fe-C EPR signal (Lindahl, P. A., Münck, E., and Ragsdale, S. W. (1990) J. Biol. Chem. 265, 3873-3879), we propose that the assembly of the acetyl group of acetyl-CoA, i.e. binding the methyl group of the methylated corrinoid/iron-sulfur protein, binding CO, and methyl migration to form the acetyl-CODH intermediate, occur at the novel Ni-Fe3-4-containing site in CODH. CO has two effects on the CoA/acetyl-CoA exchange: it activates the reaction due to its reductive capacity and its acts as a noncompetitive inhibitor. We also discovered that the CoA/acetyl-CoA exchange was inhibited by nitrous oxide via an oxidative mechanism. In the presence of a low-potential electron donor, CODH becomes a nitrous oxide reductase which catalytically converts N2O to N2. This study combined with earlier results (Lu, W-P., Harder, S. R., and Ragsdale, S. W. (1990) J. Biol. Chem. 265, 3124-3133) establishes that the two-subunit form of CODH is completely active in all reactions known to be catalyzed by CODH. 相似文献
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Mutations in the conserved C-terminal sequence in thyroid hormone receptor dissociate hormone-dependent activation from interference with AP-1 activity. 总被引:4,自引:2,他引:2 下载免费PDF全文
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Lexical gap in cQA search, resulted by the variability of languages, has been recognized as an important and widespread phenomenon. To address the problem, this paper presents a question reformulation scheme to enhance the question retrieval model by fully exploring the intelligence of paraphrase in phrase-level. It compensates for the existing paraphrasing research in a suitable granularity, which either falls into fine-grained lexical-level or coarse-grained sentence-level. Given a question in natural language, our scheme first detects the involved key-phrases by jointly integrating the corpus-dependent knowledge and question-aware cues. Next, it automatically extracts the paraphrases for each identified key-phrase utilizing multiple online translation engines, and then selects the most relevant reformulations from a large group of question rewrites, which is formed by full permutation and combination of the generated paraphrases. Extensive evaluations on a real world data set demonstrate that our model is able to characterize the complex questions and achieves promising performance as compared to the state-of-the-art methods. 相似文献
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Influence of GATC sequences on Escherichia coli DNA mismatch repair in vitro. 总被引:6,自引:4,他引:2 下载免费PDF全文
A L Lu 《Journal of bacteriology》1987,169(3):1254-1259
The effect of the number and position of DNA adenine methylation (dam) sites, i.e., d(GATC) sequences, on mismatch repair in Escherichia coli was investigated. The efficiency of repair was measured in an in vitro assay which used an f1 heteroduplex containing a G/T mismatch within the single EcoRI site. Both an increase in the number of dam sites and a shortened distance between dam site and mismatched site increased the efficiency of mismatch repair. The sequences adjacent to d(GATC) also affected the efficiency of methylation-directed mismatch repair. Furthermore, heteroduplexes with one extra dam site located close to either the 5' or 3' end of the excised base increased the repair efficiency to about the same extent. The findings suggest that the mismatch repair pathway has no preferred polarity. 相似文献
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~(14)C 追踪试验结果表明,白兰瓜幼果中输入的~(14)C-葡萄糖,50%以上转化为稀酸水解和稀酸不水解的结构物质;果实发育后期,输入后48小时,在果肉和种子中分别只有18%和32%的~(14)C 参入结构物质。根据醇溶性糖的纸层析鉴定,幼果薄片渗入的~(14)C-葡萄糖仅转化为果糖,而发育后期果实则更多转化为蔗糖。显然,幼果的代谢模式是使物质和能量导向结构物质的形成;而后期果实生长已基本停止,物质代谢的方向又转向蔗糖合成的轨道上来。蔗糖合成底物试验结果表明,供给幼果不同底物都只有很低的蔗糖合成活性;发育后期果实供给UDPG+F-6-P 底物时可测出较高的蔗糖合成活性,初步推测白兰瓜中蔗糖合成主要是通过蔗糖磷酸酯合成酶来实现的。 相似文献