1,721,223 research outputs found
Investigations into the mechanism of porphobilinogen deaminase
No full textPorphobilinogen deaminase has been purified and crystallized from an overproducing recombinant strain of Escherichia coli harbouring a hemC containing plasmid which has permitted the isolation of milligram quantities of the enzyme. Determination of the molecular weight of the enzyme by SDS gel electrophoresis (35,000) and gel filtration (32,500) agrees with the gene derived molecular weight of 33,857. The enzyme has a of 19fJ.M, an isoelectric point of 4.5 and an N terminal sequence NH2- MLDNVLRIAT.The enzyme is shown to contain a novel dipyrromethane cofactor which is linked covalently to the protein. The structure of the cofactor is proposed on the basis of its reaction with Ehrlich's reagent and from its chemical properties. Specific labelling of the dipyrromethane cofactor by growth of the E. coli in the presence of 5-amino[5- 14Cjlaevulinic acid demonstrated that the cofactor is not subject to catalytic turnover. The structure of the cofactor was further confirmed as a dipyrromethane made up of two linked pyrrole rings by [13C] n.m.r. studies after the deaminase was specifically labelled with [13C] by growth of the bacteria on 5-amino[5-13C]laevulinic acid. The chemical shift data indicate that one of the pyrrole rings of the cofactor is covalently attached to the deaminase enzyme through a cysteine residue. Evidence from protein chemistry studies confirm that cysteine-242 is the covalent binding site for the cofactor. The formation of the dipyrromethane cofactor of E. coli was shown to depend on the presence of 5-aminolaevulinic acid. A hemA" mutant formed inactive porphobilinogen deaminase when grown in the absence of 5-aminolaevulinic acid since this strain was unable to biosynthesise the dipyrromethane cofactor. The mutant formed normal levels of deaminase, however, when grown in the presence of 5 aminolaevulinic acid. Porphobilinogen, the substrate, interacts with the free a-position of the dipyrromethane cofactor to give three stable enzyme-intermediate complexes, ES, ES2 and ES3. Experiments with regiospecifically labelled intermediate complexes have shown that, in the absence of further substrate molecules, the complexes are interconvertible by the exchange of the terminal pyrrole ring of the complex. The formation of enzyme-intermediate complexes is accompanied by the exposure of a cysteine residue suggesting that substantial conformational changes occur on binding substrate. Experiments with the a-substituted substrate analogue, abromoporhobilinogen, have provided further evidence that the cofactor is responsible for the covalent binding of the substrate at the catalytic site. Based on these findings it has been possible, for the first time, to construct a mechanistic scheme for the deaminase reaction involving a single active site which is able to catalyse the addition or removal of either NH3 or H2O. The role of the cofactor as both a primer and as a means forregulating the number of substrates bound in each catalytic cycle is discussed.<br/
Madeline Kahn, Lesley Ann Warren, Martin Mull, Eileen Brennan, Christopher Lloyd, and Michael McKean in CLUE, 1985
No full textMadeline Kahn, Lesley Ann Warren, Martin Mull, Eileen Brennan, Christopher Lloyd, and Michael McKean in a scene from CLUE, 1985. 8x10 b&w photographic print
Madeline Kahn, Tim Curry, Eileen Brennan, Lesley Ann Warren, Martin Mull, Christopher Lloyd, and Michael McKean in CLUE, 1985
No full textMadeline Kahn, Tim Curry, Eileen Brennan, Lesley Ann Warren, Martin Mull, Christopher Lloyd, and Michael McKean in a scene from CLUE, 1985. 8x10 b&w photographic print
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
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