How well does cholesteryl hemisuccinate mimic cholesterol in saturated phospholipid bilayers?

Journal of Molecular Modeling - We studied the doping effects on the electronic and structural properties of graphene upon interaction with phenol. Calculations were performed within the periodic...     

Proton translocation in corn coleoptiles: ATPase or redox chain?

The O₂ dependence of net H⁺ efflux of maize coleoptiles has been investigated. Below 100 M O₂, H⁺ efflux in young (1 cm long) coleoptiles is markedly decreased while old (7 cm long) coleoptiles show a decline only at 10 M O₂. Old coleoptiles show the same decrease in net H⁺ efflux as young ones if treated with fusicoccin. The ratio of alteration of CO₂ production to the change in net proton efflux is about 1:1 at 40–80 M O₂ but not at 10 M O₂. An influx can be observed at 10 M O₂ in young as well as in old coleoptiles if the H⁺ concentration is held at values below pH 6.5. Lower O₂ concentrations lead to an increase of net H⁺ efflux, which might be caused by leaching of organic acids resulting from anaerobic processes, but CO₂ production is not significantly changed at these values. It is proposed that more than one system is responsible for proton translocation across the plasmalemma. One of the systems has a high sensitivity to reduced O₂ concentration which is within the same range as the high K_m of the alternative path.Abbreviation FC fusicoccin     

Molecular dynamics simulation of cation–phospholipid clustering in phospholipid bilayers: Possible role in stalk formation during membrane fusion

In this study, we performed all-atom long-timescale molecular dynamics simulations of phospholipid bilayers incorporating three different proportions of negatively charged lipids in the presence of K⁺, Mg^2 +, and Ca^2 + ions to systemically determine how membrane properties are affected by cations and lipid compositions. Our simulations revealed that the binding affinity of Ca^2 + ions with lipids is significantly stronger than that of K⁺ and Mg^2 + ions, regardless of the composition of the lipid bilayer. The binding of Ca^2 + ions to the lipids resulted in bilayers having smaller lateral areas, greater thicknesses, greater order, and slower rotation of their lipid head groups, relative to those of corresponding K⁺- and Mg^2 +-containing systems. The Ca^2 + ions bind preferentially to the phosphate groups of the lipids. The complexes formed between the cations and the lipids further assembled to form various multiple-cation-centered clusters in the presence of anionic lipids and at higher ionic strength—most notably for Ca^2 +. The formation of cation–lipid complexes and clusters dehydrated and neutralized the anionic lipids, creating a more-hydrophobic environment suitable for membrane aggregation. We propose that the formation of Ca^2 +–phospholipid clusters across apposed lipid bilayers can work as a “cation glue” to adhere apposed membranes together, providing an adequate configuration for stalk formation during membrane fusion.     

Salicylic acids: Local,systemic or inter-systemic regulators?

Salicylic acid is well known phytohormone, emerging recently as a new paradigm of an array of manifestations of growth regulators. The area unleashed yet encompassed the applied agriculture sector to find the roles to strengthen the crops against plethora of abiotic and biotic stresses. The skipped part of integrated picture, however, was the evolutionary insight of salicylic acid to either allow or discard the microbial invasion depending upon various internal factors of two interactants under the prevailing external conditions. The metabolic status that allows the host invasion either as pathogenesis or symbiosis with possible intermediary stages in close systems has been tried to underpin here.     

Proton conductance by the gramicidin water wire. Model for proton conductance in the F1F0 ATPases?

The gramicidin channel contains a single strand of water molecules associated through hydrogen bonds. Previous work has shown that channels of similar size are formed by association of transmembrane alpha helices of synthetic leucine-serine peptides. Both types of channels translocate protons with considerable selectivity relative to other cations, and it has been proposed that the selectivity arises by proton "hopping" along hydrogen-bonded chains of water, whereas other cations must cross by ordinary diffusion processes. It is possible that a similar mechanism underlies proton transport in the Fo subunit of the F1F0 ATP synthase. Using the gramicidin channel as a model, we have tested whether a single strand of water is kinetically competent to translocate protons at a rate sufficient to support known rates of ATP synthesis. We found that the gramicidin channel saturates at approximately 530 pS of protonic current in 4 M HCl, more than sufficient for typical ATP synthesis rates. It follows that proton diffusion to a putative channel in Fo, rather than the channel itself, may limit ATP synthesis rates.     

Complex of human apolipoprotein C-1 with phospholipid: thermodynamic or kinetic stability?

Thermal unfolding of discoidal complexes of apolipoprotein (apo) C-1 with dimyristoyl phosphatidylcholine (DMPC) reveals a novel mechanism of lipoprotein stabilization that is based on kinetics rather than thermodynamics. Far-UV CD melting curves recorded at several heating/cooling rates from 0.047 to 1.34 K/min show hysteresis and scan rate dependence characteristic of slow nonequilibrium transitions. At slow heating rates, the apoC-1 unfolding in the complexes starts just above 25 degrees C and has an apparent melting temperature T(m) approximately 48 +/- 1.5 degrees C, close to T(m) = 51 +/- 1.5 degrees C of free protein. Thus, DMPC binding may not substantially increase the low apparent thermodynamic stability of apoC-1, DeltaG(25 degrees C) < 2 kcal/mol. The scan rate dependence of T(m) and Arrhenius analysis of the kinetic data suggest an activation enthalpy E(a) = 25 +/- 5 kcal/mol that provides the major contribution to the free energy barrier for the protein unfolding on the disk, DeltaG > or = 17 kcal/mol. Consequently, apoC-1/DMPC disks are kinetically but not thermodynamically stable. To explore the origins of this kinetic stability, we utilized dynode voltage measured in CD experiments that shows temperature-dependent contribution from UV light scattering of apoC-1/DMPC complexes (d approximately 20 nm). Correlation of CD and dynode voltage melting curves recorded at 222 nm indicates close coupling between protein unfolding and an increase in the complex size and/or lamellar structure, suggesting that the enthalpic barrier arises from transient disruption of lipid packing interactions upon disk-to-vesicle fusion. We hypothesize that a kinetic mechanism may provide a general strategy for lipoprotein stabilization that facilitates complex stability and compositional variability in the absence of high packing specificity.     

Speciation: more likely through a genetic or through a learned habitat preference?

A problem in understanding sympatric speciation is establishing how reproductive isolation can arise when there is disruptive selection on an ecological trait. One of the solutions that has been proposed is that a habitat preference evolves, and that mates are chosen within the preferred habitat. We present a model where the habitat preference can evolve either by means of a genetic mechanism or by means of learning. Employing an adaptive-dynamical analysis, we show that evolution proceeds either to a single population of specialists with a genetic preference for their optimal habitat, or to a population of generalists without a habitat preference. The generalist population subsequently experiences disruptive selection. Learning promotes speciation because it increases the intensity of disruptive selection. An individual-based version of the model shows that, when loci are completely unlinked and learning confers little cost, the presence of disruptive selection most probably leads to speciation via the simultaneous evolution of a learned habitat preference. For high costs of learning, speciation is most likely to occur via the evolution of a genetic habitat preference. However, the latter only happens when the effect of mutations is large, or when there is linkage between genes coding for the different traits.     

Limitation of stomatal conductance by hydraulic traits: sensing or preventing xylem cavitation?

We tested the hypothesis that hydraulic conductance per unit leaf surface area of plant shoots (K_SL) determines the maximum diurnal stomatal conductance (g_L) that can be reached by plants growing in the field. A second hypothesis was tested that some xylem cavitation cannot be avoided by transpiring plants and might act as a signal for regulating g_L. Eleven woody species were studied, differing from each other with respect to taxonomy, wood anatomy and leaf habit. Maximum diurnal g_L, transpiration rate (E_L), pre-dawn and minimum diurnal leaf water potential (O_pd and O_min, respectively) were measured in the field. The critical O level at which stem cavitation was triggered (O_cav) was measured on detached branches, using the acoustic method. A high-pressure flow meter was used to measure maximum K_SL of 1-year-old shoots. Both g_L and E_L were positively related to K_SL. The whole-plant hydraulic conductance per unit leaf area (K_WL) of all the species studied, calculated as the ratio of E_L to (O (=O_pd-O_min) was closely related to K_SL. In every case, O_min (ranging between -0.85 and -1.35 MPa in the different species) dropped to the O_cav range or was <O_cav (ranging between -0.71 and -1.23 MPa), thus suggesting that some cavitation-induced embolism could not be avoided. The possibility is discussed that some cavitation-induced reduction in K_SL is the signal for stomatal closure preventing runaway embolism. The lack of correlation of g_L to O_cav is discussed in terms of the inconsistency of O_cav as an indicator of the vulnerability of plants to cavitation. No differences in hydraulic traits were observed between evergreen and deciduous species.     

Anti-predatory chemical defenses of ascidians: secondary metabolites or inorganic acids?

Both secondary metabolites and inorganic acids have been hypothesized to protect adult ascidians from predation, raising the possibility of alternative defensive strategies in these sessile, soft-bodied, benthic invertebrates. The objective of this investigation was to determine if ascidian species from the Western Atlantic have these chemical defenses against fish predators, and if so, to determine their location within the body of the ascidian. The palatability of crude organic extracts of whole ascidians, as well as the dissected tunics, viscera, and gonads (when possible) were determined at natural volumetric concentrations using laboratory feeding assays with the bluehead wrasse, Thalassoma bifasciatum. Acidified food pellets were also assayed to determine the effect of lowered pH on predation. Sixteen of the 17 species tested had deterrent organic extracts from some region of the body (Aplidium constellatum, Aplidium stellatum, Ascidia interrupta, Ascidia nigra, Botrylloides sp., Clavellina picta, Didemnum candidum, Didemnum vanderhosti, Diplosoma listerianum, Ecteinascidia turbinata, Eudistoma capsulatum, Eudistoma hepaticum, Rhopalaea abdominalis, Styela plicata, Symplegma rubra, and Trididemnum solidum). The location of the deterrent secondary metabolites was isolated in the gonad in all three solitary species, raising the possibility that these defenses are passed on to eggs or larvae. Nine ascidian species sequestered acid in their tunics (A. interrupta, A. nigra, A. stellatum, D. candidum, D. vanderhosti, E. capsulatum, E. hepaticum, R. abdominalis, and T. solidum) at levels that were effective in deterring fish predation (pH≤3.0). Only one species (Botrylloides nigrum) had neither chemical defense. Results of this study indicate that there is not a clear trade-off between the presence of secondary metabolites and inorganic acid defenses in ascidians, suggesting that these defenses are redundant, or that alternative chemical defenses may have evolved for different predators or for different stages in the life history of the ascidians producing them.     

Conjugated linoleic acids: are they beneficial or detrimental to health?

Conjugated linoleic acids (CLAs) comprise a family of positional and geometric isomers of linoleic acid (18:2n − 6; LA) that are formed by biohydrogenation and oxidation processes in nature. The major dietary sources of these unusual fatty acids are foods derived from ruminant animals, in particular dairy products. The main form of CLA, cis-9, trans-11-18:2, can be produced directly by bacterial hydrogenation in the rumen or by delta-9 desaturation of the co-product vaccenic acid (trans-11-18:1) in most mammalian tissues including man. The second most abundant isomer of CLA is the trans-10, cis-12-18:2 form. Initially identified in grilled beef as a potential anti-carcinogen a surprising number of health benefits have subsequently been attributed to CLA mixtures and more recently to the main individual isoforms. It is also clear from recent studies that the two main isoforms can have different effects on metabolism and cell functions and can act through different cell signalling pathways. The majority of studies on body compositional effects (i.e. fat loss, lean gain), on cancer and cardiovascular disease attenuation, on insulin sensitivity and diabetes and on immune function have been conducted with a variety of animal models. Observations clearly emphasise that differences exist between mammalian species in their response to CLAs with mice being the most sensitive. Recent studies indicate that some but not all of the effects observed in animals also pertain to human volunteers. Reports of detrimental effects of CLA intake appear to be largely in mice and due mainly to the trans-10, cis-12 isomer. Suggestions of possible deleterious effects in man due to an increase in oxidative lipid products (isoprostanes) with trans-10, cis-12 CLA ingestion require substantiation. Unresponsiveness to antioxidants of these non-enzymatic oxidation products casts some doubt on their physiological relevance. Recent reports, albeit in the minority, that CLAs, particularly the trans-10, cis-12 isomer, can elicit pro-carcinogenic effects in animal models of colon and prostate cancer and can increase prostaglandin production in cells also warrant further investigation and critical evaluation in relation to the many published anti-cancer and anti-prostaglandin effects of CLAs.     

Maintenance of membrane phospholipid asymmetry Lipid-cytoskeletal interactions or lipid pump?

Two models for the mechanism of maintenance of lipid asymmetry in erythrocytes are considered: binding of internal lipids to cytoskeletal proteins, and pumping of internal lipids from the outside to the inside of the cell. Analysis of the kinetics of lipid internalization suggests that the first model is more likely, and that the apparent pumping of lipids represents the activity of an ATP-dependent lipid flip/flop catalyst.     

Cooperation through indirect reciprocity: image scoring or standing strategy?

Theorists have only recently shown that cooperation through indirect reciprocity can evolve. The first modelling approach favoured a mechanism called image scoring. Helping someone increases one's image score, whereas refusing to help reduces it. The evolutionary outcome was a discriminator image scoring strategy that helps everybody who has, for example, a positive image score. Two experimental studies with humans found results that were compatible with discriminator image scoring. However, a new analysis of other theorists, based on another population structure, has cast doubts on the evolutionary stability of strategies using the recipient's score as a sole basis for decision. The new theoretical study confirmed that a strategy aiming at "good standing" has superior properties and easily beats image scoring. An individual loses good standing by failing to help a recipient in good standing, whereas failing to help recipients who lack good standing does not damage the standing of a potential donor (but would reduce his image score). The present empirical study with 23 groups of seven human subjects each was designed for distinguishing between the two proposed mechanisms experimentally. The results differed strongly from standing strategies, which might demand too much working memory capacity, but were compatible with image scoring or a similar strategy to a large extent. Furthermore, donors of constant "NO players" compensated for their refusing to help these players by being more generous to others.     

Holding on through DNA Replication: Histone Modification or Modifier?

Histone methylation is widely believed to contribute to epigenetic inheritance by persevering through DNA replication and subsequently templating methylation of daughter chromosome regions. However, a report in this issue (Petruk et?al.) suggests that chromatin association of the methytransferase complexes themselves persists through replication and re-establishes histone methylation.     

Animal or Plant: Which Is the Better Fog Water Collector?

Occasional fog is a critical water source utilised by plants and animals in the Namib Desert. Fog basking beetles (Onymacris unguicularis, Tenebrionidae) and Namib dune bushman grass (Stipagrostris sabulicola, Poaceae) collect water directly from the fog. While the beetles position themselves optimally for fog water collection on dune ridges, the grass occurs predominantly at the dune base where less fog water is available. Differences in the fog-water collecting abilities in animals and plants have never been addressed. Here we place beetles and grass side-by-side in a fog chamber and measure the amount of water they collect over time. Based on the accumulated amount of water over a two hour period, grass is the better fog collector. However, in contrast to the episodic cascading water run-off from the grass, the beetles obtain water in a steady flow from their elytra. This steady trickle from the beetles' elytra to their mouth could ensure that even short periods of fog basking--while exposed to predators--will yield water. Up to now there is no indication of specialised surface properties on the grass leafs, but the steady run-off from the beetles could point to specific property adaptations of their elytra surface.     

Keystone species and vulnerable species in ecological communities: strong or weak interactors?

The loss of a species from an ecological community can trigger a cascade of secondary extinctions. The probability of secondary extinction to take place and the number of secondary extinctions are likely to depend on the characteristics of the species that is lost--the strength of its interactions with other species--as well as on the distribution of interaction strengths in the whole community. Analysing the effects of species loss in model communities we found that removal of the following species categories triggered, on average, the largest number of secondary extinctions: (a) rare species interacting strongly with many consumers, (b) abundant basal species interacting weakly with their consumers and (c) abundant intermediate species interacting strongly with many resources. We also found that the keystone status of a species with given characteristics was context dependent, that is, dependent on the structure of the community where it was embedded. Species vulnerable to secondary extinctions were mainly species interacting weakly with their resources and species interacting strongly with their consumers.