1,720,993 research outputs found
PH DEPENDENCE OF THE VARIOUS PHASES OF THE PROTON PUMP OF CYTOCHROME C OXIDASE
No full textCytochrome c oxidase(COX)catalyses the reduction of dioxygen to water by ferrocytochrome c.This reaction is coupled to the translocation of up to 1H+/e-
4H+/O2 across the coupling membrane from the inner to the outer aqueous phase (1,2).
The catalytic cycle of COX can be divided into two phases: a reductive phase and an oxidative phase.A study is presented on the pHdependence of proton transfer of COX reconstituted in vesicles,associated with the oxidation phase,the reduction phase and the oxidation-rereduction rapid transitio
Allosteric cooperativity in proton energy conversion in A1-type cytochrome c oxidase
No full textCytochrome c oxidase (CcO), the CuA, heme a, heme a3, CuB enzyme of respiratory chain, converts the free energy released by aerobic cytochrome c oxidation into a membrane electrochemical proton gradient (∆μH+). ∆μH+ derives from the membrane anisotropic arrangement of dioxygen reduction to two water molecules and transmembrane proton pumping from a negative (N) space to a positive (P) space separated by the membrane. Spectroscopic, potentiometric analyses and X-ray crystallography, characterize allosteric cooperativity of dioxygen binding and reduction with protonmotive conformational states of CcO. These studies show that allosteric cooperativity stabilizes the favourable conformational state for conversion of redox energy into a transmembrane ∆μH+. (i) Dioxygen binding to Fea3 at BNC selects a CcO state in which the conformation of the subunit I-helix X, next in sequence to the hemes a and a3 histidine axial ligands, opens access of pumped protons from the N space to a Mg2+-storage site at the opposite (P) membrane side. (ii) Sequential electron transfer at CuA and heme a is associated with additional CcO conformational change in which protons are translocated from the Mg2+-site to the propionate(s) of heme a. From there protons are expelled in the P space upon heme a oxidation by electrostatic repulsion along an hydrogen bond network of amino acid residues and/or structured water molecules
The proton/electron coupling ratio at heme a and CuA in bovine heart cytochrome c oxidase
No full textMeasurements of the H+/heme a, CuA ratios for proton−electron coupling at these centers (redox Bohr effect) in CO-inhibited cytochrome c oxidase purified from bovine heart mitochondria, both in the soluble state and reconstituted in liposomes, are presented. In the soluble oxidase, the H+/heme a, CuA ratios were experimentally determined upon oxidation by ferricyanide of these centers as well as upon their reduction by hexammineruthenium(II). These measurements showed that in order to obtain H+/heme a, CuA ratios approaching 1, one-step full oxidation of both metal centers by ferricyanide had to be induced by a stoicheiometric amount of the oxidant. Partial stepwise oxidation or reduction of heme a and CuA did produce H+/heme a, CuA ratios significantly lower or higher than 1, respectively. The experimental H+/heme a, CuA ratios measured upon stepwise reduction/oxidation of the metals were reproduced by mathematical simulation based on the coupling of oxido−reduction of both heme a and CuA to pK shifts of common acid−base groups. The vectorial nature of the proton−electron coupling at heme a/CuA was analyzed by measuring pH changes in the external bulk phase associated with oxido−reduction of these redox centers in the CO-inhibited oxidase reconstituted in liposomes. The results show that the proton release associated with the oxidation of heme a and CuA takes place in the external aqueous phase. Protons taken up by the oxidase upon rereduction of the centers derive, on the other hand, from the inner space. These results provide evidence supporting the view that cooperative proton−electron coupling at heme a/CuA is involved in the proton pump of the oxidase
Concerted involvement of cooperative proton-electron linkage and water production in the proton pump of cytochrome c oxidase
No full textIn cytochrome c oxidase, oxido-reductions of heme a/CuA and heme a3/CuB are cooperatively linked to proton transfer at acid/base groups in
the enzyme. H+/e− cooperative linkage at Fea3/CuB is envisaged to be involved in proton pump mechanisms confined to the binuclear center.
Models have also been proposed which involve a role in proton pumping of cooperative H+/e− linkage at heme a (and CuA). Observations will be
presented on: (i) proton consumption in the reduction of molecular oxygen to H2O in soluble bovine heart cytochrome c oxidase; (ii) proton
release/uptake associated with anaerobic oxidation/reduction of heme a/CuA and heme a3/CuB in the soluble oxidase; (iii) H+ release in the
external phase (i.e. H+ pumping) associated with the oxidative (R→O transition), reductive (O→R transition) and a full catalytic cycle
(R→O→R transition) of membrane-reconstituted cytochrome c oxidase. A model is presented in which cooperative H+/e− linkage at heme a/CuA
and heme a3/CuB with acid/base clusters, C1 and C2 respectively, and protonmotive steps of the reduction of O2 to water are involved in proton
pumping
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
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