1,720,974 research outputs found
Impact of osmolytes on buoyancy of marine phytoplankton
No full textMarine phytoplanktonic cells can achieve neutral buoyancy only if the excess density of their relatively heavy structural materials (proteins, carbohydrates, silicate) is compensated for by the incorporation of materials that have densities less than seawater. We have calculated densities and osmotic concentrations for several marine algae, based on published values of structural materials and concentrations of inorganic ions and other osmolytes. The calculations, incorporating the partial molal volume, molecular mass, concentrations and osmotic coefficients, indicate that most published listings of intracellular osmolytes in marine algae are insufficient to provide the turgor known to exist. Similarly, the density of phytoplanktonic cells, calculated on the basis of known or estimated concentrations of cellular components, generally exceeds the density of seawater, which would cause negative buoyancy (sinking) throughout. We use models of osmotic concentration and cellular density in which we supplement known concentrations of osmolytes with proxy osmolytes. In particular, concentrations of some 100 mol m(-3) of quaternary ammonium derivatives can explain the deficits of both osmotic concentration and buoyancy
Current-voltage-time records of ion translocating enzymes
No full textMembrane currents, as non-linear functions of membrane voltage, V, and time, t, can be recorded quickly by triangular V protocols. From the differences, dI(V,t), of these relationships upon addition of a putative substrate of a charge-translocating membrane protein, the I(V,t) relationships of the transporter itself can be determined. These relationships likely comprise a steady-state component, I-a(V), of the active transporter, and a dynamic component, p(a)(V,t), Of its V- and time-dependent activity, p(a). Here, the steady-state component is modeled by a central reaction cycle, which senses a fraction delta(tr) of the total V, whereas 1-delta(tr) can be assigned to an inner and outer pore section with delta(i) and delta(o), respectively (delta(i)+delta(tr)+delta(o)= 1). For the enzymatic cycle, fast binding/debinding is assumed, plus V-sensitive and insensitive reaction steps which may become rate limiting for charge translocation. At given substrate concentrations, I-a(V) is defined by eight independent system parameters, including a coefficient for the barrier shape of charge translocation. In ordinary cases, the behavior of p(a)(V,t) can be described by two rate constants (for activation and inactivation) and their respective V-sensitivity coefficients. Here, the effects of the individual system parameters on I(V,t) from triangular V-clamp experiments are investigated systematically. The results are illustrated by panels of typical curve shapes for non-gated and gated transporters to enable a first classification of mechanisms. We demonstrate that all system parameters can be determined fairly well by fitting the model to "experimental" data of known origin. Applicability of the model to channels, pumps and cotransporters is discussed
Three types of membrane excitations in the marine diatom Coscinodiscus wailesii
No full textThree types of electrical excitation have been investigated in the marine diatom Coscinodiscus wailesii. I: Depolarization-triggered, transient Cl- conductance, G(Cl)(t), followed by a transient, voltage-gated K+ conductance, G(K), with an active state a and two inactive states i(1) and i(2) in series (a-i(1)-i(2)). II: Similar C-Cl(t) as in Type-I but triggered by hyperpolarization; a subsequent increase of G(K) in this type is indicated but not analyzed in detail. III: Hyperpolarization-induced transient of a voltage-gated activity of an electrogenic pump (i(2)-a-i(3)), followed by G(Cl)(t) as in Type-IT excitations. Type-III with pump Sating is novel as such, G(Cl)(t) in all types seems to reflect the mechanism of InsP(3)(-); and Ca2+-mediated G(Cl)(t) in the action potential in Chara (Biskup et al., 1999). The nonlinear current-voltage-time relationships of Type-I and Type-III excitations have been recorded under voltage-clamp using single saw-tooth command voltages (voltage range: -200 to +50 mV, typical slope: +/-1 Vs(-1)). Fits of the: corresponding models to the experimental data provided numerical values of the model parameters. The statistical significance of these solutions is investigated. We suggest that the original function of electrical excitability of biological membranes is related to osmoregulation which has persisted through evolution in plants, whereas the familiar and osmotically neutral action potentials in animals have evolved later towards the novel function of rapid transmission of information over long distances
Apparent charge of binding site in ion-translocating enzymes: kinetic impact
No full textRecently, we presented a general scope for the nonlinear electrical properties of enzymes E which catalyze translocation of a substrate S with charge number z(S) through lipid membranes (Boyd et al. J. Membr. Biol. 195:1-12, 2003). In this study, the voltage sensitivity of the enzymatic reaction cycle has been assigned to one predominant reversible reaction step, i.e. the reorientation of either E or ES in the electric field, leaving the reorientation of the alternate state (ES or E) electro-neutral, respectively. With this simplification, the steady-state current-voltage relationships (IV) assumed saturation kinetics like in Michaelis-Menten systems. Here, we introduce an apparent charge number z(E) of the unoccupied binding site of the enzyme, which accounts for the impact of all charged residues in the vicinity of the physical binding site. With this more realistic concept, the occupied binding site assumes an apparent charge Of z(ES) = z(E) + z(S), and IV does not saturate any more in general, but exponentially approaches infinite or zero current for large voltage displacements from equilibrium. These nonlinear characteristics are presented here explicitly. They are qualtitatively explained in a mechanistic way, and are illustrated by simple examples. We also demonstrate that the correct determination of the model parameter from experimental data is still possible after incorporating z(E) and its corollaries into the previous model of enzyme-mediated ion translocation
Transinhibition and voltage-gating in a fungal nitrate transporter
No full textWe have applied enzyme kinetic analysis to electrophysiological steady-state data of Zhou et al. (Zhou, J.J., Trueman, L.J., Boorer, K.J., Theodoulou, F.L., Forde, B.G., Miller, A.J. 2000. A high-affinity fungal nitrate carrier with two transport mechanisms. J. Biol. Chem. 275:39894-9) and to new current-voltage-time records from Xenopus oocytes with functionally expressed NrtA (crnA) 2H(+)-NO3- symporter from Emericella (Aspergillus) nidulans. Zhou et al. stressed two Michaelis-Menten (MM) mechanisms to mediate the observed nitrate-induced currents, INO3-. We show that a single straightforward reaction cycle describes the data well, pointing out that during exposure to external substrate, S = (2H(+) + NO3-)(o), the product concentration inside, [P] = [H+](i)(2) . [NO3-](i). may rise substantially near the plasma membrane, violating the condition [P] much less than [S] for MM kinetics. Here, [P] and its changes during experimentation are treated explicitly. K-1/2 approximate to 20 muM for INO3- at pH(o) from Zhou et al. is confirmed. According to our analysis, NrtA operates between about 0.2 and 0.6 of the electrical distance in the membrane (outside 0, inside 1). In absence of thermodynamic gradients, the predominant orientation of the binding site(s) is probably inwards. The activity of the enzyme is sensitive to the transmembrane voltage, V, with an apparent gating charge of +1.0 +/- 0.5 for inactivation, and transition probabilities of 0.3-1.3 s(-1) at V = 0. This gating mode impedes loss of cellular NO3- during depolarization
Fast, Triangular Voltage Clamp for Recording and Kinetic Analysis of an Ion Transporter Expressed in Xenopus Oocytes
No full textAbstractWe present a procedure for determination of 11 system parameters of an ion transporter expressed in Xenopus oocytes. The experiments consist of fast triangular voltage-clamp experiments in the presence and absence of external substrate. A four-state enzymatic cycle operating between an external and an internal section of electrodiffusion is used for analysis. The explicit example treats experiments with the fungal 2H+-NO3− symporter EnNRT, a member of the major superfamily transporters. The results comprise a density of ≈150fmol functional transporter molecules per oocyte, a gross charge number zE≈−0.3 of the empty binding site of the enzyme, individual rate constants for reorientation of the empty and occupied binding site in the range of 5–500s−1, electrical access sections between bulk solutions and reaction cycle of ∼3% inside and 15% outside, an increase of internal NO3− at the plasma membrane from ∼0.5 to ∼2mM during exposure to external NO3−, and KD≈0.3μM3 inside and KD≈3μM3 outside in binding the triplicate substrate (2H++NO3−). The results compare well with the known structure of the lactose permease, another major superfamily transporter
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
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