Steady-state current-voltage characteristics of amino acid transport in rhizoid cells of Riccia fluitans Is the carrier negatively charged?

The amino acid transport across the plasmalemma of Riccia fluitans rhizoid cells has been further characterized by means of current-voltage $\text{I?V}$) analysis. On the basis of two cyclic transport models which include six different carrier states, the question is raised, whether the electrochemical pH-gradient drives a negatively charged carrier or a positively charged alanine-proton-carrier complex across the membrane. $\text{I?V}$ analysis shows that (1) the typical $\text{I?V}$ characteristic of l-alanine transport follows a sigmoid curve, (2) maximal accumulation of l-alanine within the cytoplasm is reached after about 1 hour, (3) the electrically accessible cytoplasmic l-alanine concentration is limited to about 20 mM, and (4) the steady-state saturation current depends directly on external l-alanine concentration. It is concluded that (a) these results are consistent with the predictions of the models for a negatively charged carrier, and (b) that the rate-limiting step involves the translocation of the ternary complex.     

Effect of pH on the surface charge density of plant membranes. Comparison of microsomes and liposomes

The surface potential of microsomes of horse bean roots was compared to the one of liposomes prepared from the whole phospholipid extracts. The surface potential was determined from the affinity of the membranes for the anilinonaphthalene sulphonate dye. The effect of pH was studied at two KCl concentrations. It appeared from this comparison that the surface charge density was nearly the same on both materials in the neutral pH range. The isoelectric point was pH 1.7 for the liposomes and pH 4.0 for the microsomes. The implication of these observations is that the surface charge density of microsomes is nearly the same above the lipid and protein components of the membrane. This hypothesis was checked by measuring the activity of a microsomal enzyme with an anionic substrate, while modifying the net surface charge of the membrane. The biological significance of the results is discussed.     

Determination of the electric potential at the external and internal bilayer-aqueous interfaces of the human erythrocyte membrane using spin probes

Partitioning of oppositely charged amphipathic spin probes indicates that the electric potential at the external bilayer-aqueous interface of the human erythrocyte is insignificant, and that protruding sialic acids do not contribute to this potential. This potential at the surface is distinguished from the electrokinetic potential due to all charged groups within the hydrodynamic surface of shear. By contrast, using inside-out erythrocyte membrane vesicles, a substantial potential is observed at the cytoplasmic membrane surface. This can be attributed to the asymmetric distribution of acidic phospholipids on the two sides of the erythrocyte membrane bilayer.     

Systematic probing of an atomic charge set of sialic acid disaccharides for the rational molecular modeling of avian influenza virus based on molecular dynamics simulations

A systematic searching approach for an atomic charge set through molecular dynamics simulations is introduced to calculate a reasonable sialic acid carbohydrate conformation with respect to the experimentally observed structures. The present molecular dynamics simulation study demonstrated that B3LYP/6-31G is the most suitable basis set for the sialic acid disaccharides, attaining good agreement with experimental data.     

The effect of fixed charge density and cartilage swelling on mechanics of knee joint cartilage during simulated gait

The effect of swelling of articular cartilage, caused by the fixed charge density (FCD) of proteoglycans, has not been demonstrated on knee joint mechanics during simulated walking before. In this study, the influence of the depth-wise variation of FCD was investigated on the internal collagen fibril strains and the mechanical response of the knee joint cartilage during gait using finite element (FE) analysis. The FCD distribution of tibial cartilage was implemented from sodium (²³Na) MRI into a 3-D FE-model of the knee joint (“Healthy model”). For comparison, models with decreased FCD values were created according to the decrease in FCD associated with the progression of osteoarthritis (OA) (“Early OA” and “Advanced OA” models). In addition, a model without FCD was created (“No FCD” model). The effect of FCD was studied with five different collagen fibril network moduli of cartilage. Using the reference fibril network moduli, the decrease in FCD from “Healthy model” to “Early OA” and “Advanced OA” models resulted in increased axial strains (by +2 and +6%) and decreased fibril strains (by −3 and −13%) throughout the stance, respectively, calculated as mean values through cartilage depth in the tibiofemoral contact regions. Correspondingly, compared to the “Healthy model”, the removal of the FCD altogether in “NoFCD model” resulted in increased mean axial strains by +16% and decreased mean fibril strains by −24%. This effect was amplified as the fibril network moduli were decreased by 80% from the reference. Then mean axial strains increased by +6, +19 and +49% and mean fibril strains decreased by −9, −20 and −32%, respectively. Our results suggest that the FCD in articular cartilage has influence on cartilage responses in the knee during walking. Furthermore, the FCD is suggested to have larger impact on cartilage function as the collagen network degenerates e.g. in OA.     

Extracellular Subunit Interactions Control Transitions between Functional States of Acid-sensing Ion Channel 1a

Acid-sensing ion channels (ASICs) are neuronal, voltage-independent Na⁺ channels that are transiently activated by extracellular acidification. They are involved in pain sensation, the expression of fear, and in neurodegeneration after ischemic stroke. Our study investigates the role of extracellular subunit interactions in ASIC1a function. We identified two regions involved in critical intersubunit interactions. First, formation of an engineered disulfide bond between the palm and thumb domains leads to partial channel closure. Second, linking Glu-235 of a finger loop to either one of two different residues of the knuckle of a neighboring subunit opens the channel at physiological pH or disrupts its activity. This suggests that one finger-knuckle disulfide bond (E235C/K393C) sets the channel in an open state, whereas the other (E235C/Y389C) switches the channel to a non-conducting state. Voltage-clamp fluorometry experiments indicate that both the finger loop and the knuckle move away from the β-ball residue Trp-233 during acidification and subsequent desensitization. Together, these observations reveal that ASIC1a opening is accompanied by a distance increase between adjacent thumb and palm domains as well as a movement of Glu-235 relative to the knuckle helix. Our study identifies subunit interactions in the extracellular loop and shows that dynamic changes of these interactions are critical for normal ASIC function.     

Structure and desensitization of AMPA receptor complexes with type II TARP γ5 and GSG1L

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Gating of Single N-type Calcium Channels Recorded from Bullfrog Sympathetic Neurons

For many neurons, N-type calcium channels provide the primary pathway for calcium influx during an action potential. We investigated the gating properties of single N-type calcium channels using the cell-attached patch technique. With 100 mM Ba²⁺ in the pipet, mean N-channel open probability (P _o, measured over 100 ms) increased with depolarization, but the range at a single voltage was large (e.g., P _o at +40 mV ranged from 0.1 to 0.8). The open dwell time histograms were generally well fit by a single exponential with mean open time (τ_o) increasing from 0.7 ms at +10 mV to 3.1 ms at +40 mV. Shut time histograms were well fit by two exponentials. The brief shut time component (τ_sh1 = 0.3 ms) did not vary with the test potential, while the longer shut time component (τ_sh2) decreased with voltage from 18.9 ms at +10 mV to 2.3 ms at +40 mV. Although N-channel P _o during individual sweeps at +40 mV was often high (∼0.8), mean P _o was reduced by null sweeps, low P _o gating, inactivation, and slow activation. The variability in mean P _o across patches resulted from differences in the frequency these different gating processes were expressed by the channels. Runs analysis showed that null sweeps tended to be clustered in most patches, but that inactivating and slowly activating sweeps were generally distributed randomly. Low P _o gating (P _o = 0.2, τ_o = 1 ms at +40 mV) could be sustained for ∼1 min in some patches. The clustering of null sweeps and sweeps with low P _o gating is consistent with the idea that they result from different modes of N-channel gating. While P _o of the main N-channel gating state is high, the net P _o is reduced to a maximum value of close to 0.5 by other gating processes.     

Selective Blocking Effects of Tropisetron and Atropine on Recombinant Glycine Receptors

Some serotonin 5-HT3 receptor ligands of tropeine structure have been recently shown to modulate ionophore function and binding of glycine receptors. This led us to study the effects of the tropeines tropisetron and atropine on recombinant human glycine receptors transiently expressed in Xenopus oocytes by using whole-cell voltage-clamp electrophysiology. Glycine currents were inhibited by atropine in an apparently competitive manner and with considerable selectivity of the tropeines for alpha2 versus alpha1 subunits. Coexpression of beta with alpha subunits and replacement of the N-terminal region of the alpha1 subunits by the corresponding beta segment resulted in similar increases in the inhibitory potencies. Our data suggest common sites of the tropeines for inhibition on the N-terminal region of glycine receptors. The point mutations R271K and R271L of the alpha1 subunit decreased, whereas a T112A substitution increased, the inhibition constants (Ki) of the tropeines. These changes in the Ki values of the tropeines were associated with opposite changes in the EC50 of glycine. Selectivities for the tropeines versus glycine (EC50/Ki) varied within three orders of magnitude. These results, when expressed in terms of free energy changes, can be interpreted according to a two-state receptor model.     

Nonaromatic Green‐Solvent‐Processable,Dopant‐Free,and Lead‐Capturable Hole Transport Polymers in Perovskite Solar Cells with High Efficiency

With the recent developments in the efficiency of perovskite solar cells (PSCs), diverse functionalities are necessary for next‐generation charge‐transport layers. Specifically, the hole‐transport layer (HTL) in the various synthesized materials modified with functional groups is explored. A novel donor–acceptor type polymer, alkoxy‐PTEG, composed of benzo[1,2‐b:4,5:b′]dithiophene and tetraethylene glycol (TEG)‐substituted 2,1,3‐benzothiadiazole is reported. The alkoxy‐PTEG exhibits high solubility even in nonaromatic solvents, such as 3‐methylcyclohexanone (3‐MC), and can prevent possible lead leakage via chelation. The optical and electronic properties of alkoxy‐PTEG are thoroughly analyzed. Finally, a dopant‐free alkoxy‐PTEG device processed with 3‐MC exhibits 19.9% efficiency and a device with 2‐methyl anisole, which is a reported aromatic food additive, exhibits 21.2% efficiency in a tin oxide planar structure. The PSC device shows 88% stability after 30 d at ambient conditions (40–50% relative humidity and room temperature). In addition, nuclear magnetic resonance reveals that TEG groups can chelate lead ions with moderate strength (K_binding = 2.76), and this strength is considered to be nondestructive to the perovskite lattice to prevent lead leakage. This is the first report to consider lead leakage and provide solutions to reduce this problem.