1,720,958 research outputs found
Allosteric regulation of catalytic activity: Escherichia coli aspartate transcarbamoylase versus yeast chorismate mutase
No full textAllosteric regulation of key metabolic enzymes is a fascinating field to study the structure-function relationship of induced conformational changes of proteins. In this review we compare the principles of allosteric transitions of the complex classical model aspartate transcarbamoylase (ATCase) from Escherichia coli, consisting of 12 polypetides, and the less complicated chorismate mutase derived from baker's yeast, which functions as a homodimer. Chorismate mutase presumably represents the minimal oligomerization state of a cooperative enzyme which still can be either activated or inhibited by different heterotropic effectors. Detailed knowledge of the number of possible quaternary states and a description of molecular triggers for conformational changes of model enzymes such as ATCase and chorismate mutase shed more and more light on allostery as an important regulatory mechanism of any living cell. The comparison of wild-type and engineered mutant enzymes reveals that current textbook models for regulation do not cover the entire picture needed to describe the function of these enzymes in detail
Refined molecular hinge between allosteric and catalytic domain determines allosteric regulation and stability of fungal chorismate mutase
No full textThe yeast chorismate mutase is regulated by tyrosine as feedback inhibitor and tryptophan as crosspathway activator. The monomer consists of a catalytic and a regulatory domain covalently linked by the loop L220s (212-226), which functions as a molecular hinge. Two monomers form the active dimeric enzyme stabilized by hydrophobic interactions in the vicinity of loop L220s. The role of loop L220s and its environment for enzyme regulation, dimerization, and stability was analyzed. Substitution of yeast loop L220s in place of the homologous loop from the corresponding and similarly regulated Aspergillus enzyme (and the reverse substitution) changed tyrosine inhibition to activation. Yeast loop L220s substituted into the Aspergillus enzyme resulted in a tryptophan-inhibitable enzyme. Monomeric yeast chorismate mutases could be generated by substituting two hydrophobic residues in and near the hinge region. The resulting Thr-212-->Asp-Phe-28-->Asp enzyme was as stable as wild type, but lost allosteric regulation and showed reduced catalytic activity. These results underline the crucial role of this molecular hinge for inhibition, activation, quaternary structure, and stability of yeast chorismate mutase.NIGMS NIH HHS [R01 GM006920, GM06920
Chorismate mutase of Thermus thermophilus is a monofunctional AroH class enzyme inhibited by tyrosine
No full textaroG, encoding the monofunctional chorismate mutase (TtCM) of the thermophilic gram-negative bacterium Thermus thermophilus, was cloned and its gene product characterized. TtCM was purified to homogeneity on an SDS polyacrylamide gel as a His-fusion protein with a deduced molecular mass of 15.8 kDa. The enzyme belongs to the rare group of AroH-type chorismate mutases which are mainly found in gram-positive bacteria of the Bacillus/ Clostridia group and have recently also been described for gram-negative organisms. The native molecular mass is consistent with a pseudo-alpha/beta barrel enzyme that is organized as a trimer. Comparison of the enzyme's structure with that of its mesophilic counterpart from Bacillus revealed an increase in hydrophilicity on the protein's surface, greater hydrophobicity in cavities within the protein, and greater restriction of conformational freedom, features that contribute to the thermal stability of this chorismate mutase. The kinetic data show Michaelis-Menten substrate saturation with a Km of 290 microM, and a kcat/ Km value of 180 s(-1) mM(-1). TtCM was inhibited by tyrosine with a Ki =34 microM, possibly in a competitive manner
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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