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51.
Structure of the detoxification catalyst mercuric ion reductase from Bacillus sp. strain RC607 总被引:10,自引:0,他引:10
Several hundred million tons of toxic mercurials are dispersed in the biosphere. Microbes can detoxify organo-mercurials and mercury salts through sequential action of two enzymes, organomercury lyase and mercuric ion reductase (MerA). The latter, a homodimer with homology to the FAD-dependent disulphide oxidoreductases, catalyses the reaction NADPH + Hg(II)----NADP+ + H+ + Hg(0), one of the very rare enzymic reactions with metal substrates. Human glutathione reductase serves as a reference molecule for FAD-dependent disulphide reductases and between its primary structure and that of MerA from Tn501 (Pseudomonas), Tn21 (Shigella), p1258 (Staphylococcus) and Bacillus, 25-30% of the residues have been conserved. All MerAs have a C-terminal extension about 15 residues long but have very varied N termini. Although the enzyme from Streptomyces lividans has no addition, from Pseudomonas aeruginosa Tn501 and Bacillus sp. strain RC607 it has one and two copies respectively of a domain of 80-85 residues, highly homologous to MerP, the periplasmic component of proteins encoded by the mer operon. These domains can be proteolytically cleaved off without changing the catalytic efficiency. We report here the crystal structure of MerA from the Gram-positive bacterium Bacillus sp. strain RC607. Analysis of its complexes with nicotinamide dinucleotide substrates and the inhibitor Cd(II) reveals how limited structural changes enable an enzyme to accept as substrate what used to be a dangerous inhibitor. Knowledge of the mode of mercury ligation is a prerequisite for understanding this unique detoxification mechanism. 相似文献
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Tad M. Schmaltz 《Studies in history and philosophy of science》2003,34(4):737-762
There is considerable debate among scholars over whether Descartes allowed for genuine body–body interaction. I begin by considering Michael Della Rocca’s recent claim that Descartes accepted such interaction, and that his doctrine of the creation of the eternal truths indicates how this interaction could be acceptable to him. Though I agree that Descartes was inclined to accept real bodily causes of motion, I differ from Della Rocca in emphasizing that his ontology ultimately does not allow for them. This is not the end of the story however, since two of Descartes’s successors offered incompatible ways of developing his conflicted account of motion. I contrast the occasionalist view of Nicolas Malebranche that changes in motion derive directly from divine volitions with the non-occasionalist claim of Pierre-Sylvain Regis that such changes derive from a nature distinct from God. In light of Della Rocca’s interpretation, it is noteworthy that the issue of eternal truths is relevant to both alternative accounts. Indeed, Regis took the doctrine that such truths are created to provide crucial support for his alternative to an occasionalist account of body–body interaction. What does not help Della Rocca, however, is that Regis’s view of motion requires a fundamental revision of Descartes’s ontology. 相似文献
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Li XIE Lihua XIE 《系统科学与复杂性》2007,20(2):262-272
We consider the stability of a random Riccati equation with a Markovian binary jump coefficient. More specifically, we are concerned with the boundedness of the solution of a random Riccati difference equation arising from Kalman filtering with measurement losses. A sufficient condition for the peak covariance stability is obtained which has a simpler form and is shown to be less conservative in some cases than a very recent result in existing literature. Furthermore, we show that a known sufficient condition is also necessary when the observability index equals one. 相似文献
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Lanju XU 《系统科学与复杂性》2007,20(1):30-40
In the present paper, we focus on constructive spherical codes. By employing algebraic geometry codes, we give an explicit construction of spherical code sequences. By making use of the idea involved in the proof of the Gilbert-Varshamov bound in coding theory, we construct a spherical code sequence in exponential time which meets the best-known asymptotic bound by Shamsiev and Wyner. 相似文献
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