1,720,955 research outputs found
Mechanistic aspects of thiamine biosynthesis in escherichia coli
No full textThe biosynthesis of the vitamin thiamine is laden with interesting chemistry. In anaerobic bacteria (such as Escherichia coli) a reactive intermediate, dehydroglycine, required for the biosynthesis of the thiazole moiety is derived from cleavage of the C??C? bond of tyrosine in a reaction catalysed by the radical S-adenosyl-L-methionine (SAM) enzyme, tyrosine lyase (ThiH). The aromatic by-product of this reaction in vivo was unequivocally characterised as p-cresol, a result which supported previous in vitro observations. Development of a reproducible activity assay for ThiH activity initiated detailed studies on the kinetics. ThiH, isolated either in a complex with thiazole synthase (ThiG), or as a monomer displayed pre-steady state burst phase kinetics. The SAM cleavage products (5‘-deoxyadenosine (DOA) and methionine) were identified as inhibitors of ThiH activity and this inhibition could be alleviated by addition of 5‘-methylthioadenosine / S-adenosylhomocysteine nucleosidase (MTAN), which catalysed rapid hydrolysis of DOA to adenine and 5‘-deoxyribose. The addition of MTAN increased the activity of the ThiGH complex but drastically increased the amount of observed uncoupled cleavage of SAM. The in vitro products of tyrosine cleavage (glyoxylate and ammonium ions) were also identified as inhibitors of the ThiH mediated tyrosine cleavage reaction. However, reductive cleavage of SAM continued in an uncoupled manner. Experiments with tyrosine analogues showed that 4-hydroxyphenylpropionic acid compounds could support uncoupled SAM cleavage, but the C??C? bond cleavage reaction was dependent on a correctly orientated ?-amino functional group. A mechanistic model was constructed, based on the available experimental data, which explained the observed product inhibition, uncoupled turnover and complex formation between ThiH and ThiG. The model proposes that ThiH controls the tyrosine cleavage reaction in order to co-ordinate the generation of the intermediate, iv dehydroglycine, with its utilisation in the thiazole forming cylisation reaction, catalysed by Thi
Catalytic activity of the anaerobic tyrosine lyase required for thiamine biosynthesis in Escherichia coli
No full textThiazole synthase in Escherichia coli is an ?? heterodimer of ThiG and ThiH. ThiH is a tyrosine lyase that cleaves the C?–C? bond of tyrosine, generating p-cresol as a by-product, to form dehydroglycine. This reactive intermediate acts as one of three substrates for the thiazole cyclization reaction catalyzed by ThiG. ThiH is a radical S-adenosylmethionine (AdoMet) enzyme that utilizes a [4Fe-4S]+ cluster to reductively cleave AdoMet, forming methionine and a 5?-deoxyadenosyl radical. Analysis of the time-dependent formation of the reaction products 5?-deoxyadenosine (DOA) and p-cresol has demonstrated catalytic behavior of the tyrosine lyase. The kinetics of product formation showed a pre-steady state burst phase, and the involvement of DOA in product inhibition was identified by the addition of 5?-methylthioadenosine/S-adenosylhomocysteine nucleosidase to activity assays. This hydrolyzed the DOA and changed the rate-determining step but, in addition, substantially increased the uncoupled turnover of AdoMet. Addition of glyoxylate and ammonium inhibited the tyrosine cleavage reaction, but the reductive cleavage of AdoMet continued in an uncoupled manner. Tyrosine analogues were incubated with ThiGH, which showed a strong preference for phenolic substrates. 4-Hydroxyphenylpropionic acid analogues allowed uncoupled AdoMet cleavage but did not result in further reaction (C?–C? bond cleavage). The results of the substrate analogue studies and the product inhibition can be explained by a mechanistic hypothesis involving two reaction pathways, a product-forming pathway and a futile cycle
Thiamine biosynthesis in Escherichia coli: Identification of the intermediate and by-product derived from tyrosine
No full textIn anaerobic organisms such as E. coli the tyrosine lyase ThiH is essential for the biosynthesis of the thiazole moiety of the vitamin thiamine. ThiH is a member of the radical AdoMet family. The products formed by cleavage of tyrosine in vitro have been identified and suggest a radical-mediated cleavage resulting in p-cresol and dehydroglycine which is hydrolyzed to glyoxylate
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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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