1,721,625 research outputs found
Characterisation of the key enzymes involved in vitamin B6 salvage pathway in Escherichia coli and humans
Full text linkThe catalytically active form of vitamin B6, pyridoxal 5'-phosphate (PLP), acts as a coenzyme in a variety of different enzymatic reactions. Organisms which are not able to synthesize PLP de novo acquire B6 vitamers from nutrients and interconvert them through a salvage pathway, which involves pyridoxine 5'-phosphate oxidase (PNPOx) and pyridoxal kinase (PDXK). PNPOx converts pyridoxine 5'-phosphate (PNP) and pyridoxamine 5'-phosphate (PMP) to PLP, using flavinmononucleotide (FMN) as coenzyme. Both Escherichia coli and human PNPOx are homodimers and, although these enzymes share only 39% of sequence identity, have very similar structural and functional properties. PNPOx plays a crucial role in the regulation of PLP metabolism. It has been proposed that PLP inhibits the catalytic activity of both E. coli and human PNPOx by binding at the active site and acting as a competitive inhibitor. However, PLP can also bind tightly at a secondary site. Our kinetics characterisation suggests that PLP inhibition results from binding of this vitamer at an allosteric site, in both E. coli and human enzymes. This interpretation was confirmed by the analysis of mutated forms of E. coli PNPOx, in which PLP binding at the active site is impaired. Crystallographic studies carried out by other authors on the E. coli PNPOx indicated a possible location of the secondary PLP binding site in two surface pockets of the protein, but site-directed mutants of amino acid residues putatively critical for this interaction showed that this hypothesis is wrong. Molecular docking analyses identified a possible alternative PLP binding site, which is a cleft on the protein surface mainly delimited by arginine residues and located near the subunit interface. Characterisation of mutant forms of this site and crystallographic studies suggested that this might be the allosteric PLP binding site.
Concerning human PNPOx, it is known that missense mutations in the gene encoding this enzyme lead to the onset of a rare neurological disease, the neonatal epileptic encephalopathy (NEE); however, the molecular reason of most PNPOx mutations remains to be established. We expressed PNPOx mutants as recombinant proteins in E. coli, purified and characterised them with respect to structural and functional properties, in order to better understand the molecular basis of the disease.
The other key enzyme involved in the salvage pathway is PDXK, which converts pyridoxal (PL), pyridoxamine (PM) and pyridoxine (PN) into PLP, PMP and PNP, respectively. In Drosophila, mutations in the dPdxk gene encoding PDXK cause chromosome aberrations (CABs) and increase glucose content in larval haemolymph. This observation suggests that PDXK mutations in humans may be involved in diseases such as cancer and diabetes. We analysed the effect of the expression of four PDXK human variants in Drosophila dPdxk mutants: three of them (D87H, V128I and H246Q) are listed in databases, and one (A243G) was found in a genetic screening of patients with gestational diabetes. None of the variants was able to completely rescue CABs and glucose content. Our biochemical analysis revealed reduced catalytic activity and different affinity of these variants for PLP precursors. Overall, our findings suggest that, when PLP levels are reduced by the presence of these PDXK variants, cancer and diabetes risk may be increased
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
The cosmic ray primary spectrum in the transition region between direct and indirect measurements (10 TeV - 10 PeV)
No full textThe cosmic ray spectrum between 10 TeV and 10 PeV is a key region to understand the origin and the acceleration processes of the galactic radiation. Around 100 TeV measurements performed by balloons and air shower arrays partially overlap. The experimental results recently obtained by both techniques are summarized. The picture that comes out from a common interpretation of the data is a steeper proton spectrum compared to the helium and CNO groups with a dominance of the helium component in the knee region. The agreeement on the proton spectrum by balloons and air shower arrays is remarkable
Adult neurogenesis in the short-lived teleost Nothobranchius furzeri: localization of neurogenic niches, molecular characterization and effects of aging
Full text linkWe studied adult neurogenesis in the short-lived annual fish Nothobranchius furzeri and quantified the effects of aging on the
mitotic activity of the neuronal progenitors and the expression of glial fibrillary acid protein (GFAP) in the radial glia. The distribution of neurogenic niches is substantially similar to that of zebrafish and adult stem cells generate neurons, which persist in the
adult brain. As opposed to zebrafish, however, the N. furzeri genome contains a doublecortin (DCX) gene. Doublecortin is transiently expressed by newly generated neurons in the telencephalon and optic tectum (OT). We also analyzed the
expression of the microRNA miR-9 and miR-124 and found that they have complementary expression domains: miR-9 is expressed
in the neurogenic niches of the telencephalon and the radial glia of the OT, while miR-124 is expressed in differentiated neurons. The main finding of this paper is the demonstration of an agedependent decay in adult neurogenesis. Using unbiased stereological estimates of cell numbers, we detected an almost fivefold decrease in the number of mitotically active cells in the OT
between young and old age. This reduced mitotic activity is paralleled by a reduction in DCX labeling. Finally, we detected a dramatic up-regulation of GFAP in the radial glia of the aged brain. This up-regulation is not paralleled by a similar up-regulation of S100B and Musashi-1, two other markers of the radial glia. In summary, the brain of N. furzeri replicates two typical hallmarks of
mammalian aging: gliosis and reduced adult neurogenesis
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
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