Proton conductance through phospholipid bilayers: water wires or weak acids?

Temperature-dependent regulation of electron transport and ATP synthesis in chloroplasts in vitro and in silico

The significance of temperature-dependent regulation of photosynthetic apparatus (PSA) is determined by the fact that plant temperature changes with environmental temperature. In this work, we present a brief overview of temperature-dependent regulation of photosynthetic processes in class B chloroplasts (thylakoids) and analyze these processes using a computer model that takes into account the key stages of electron and proton transport coupled to ATP synthesis. The rate constants of partial reactions were parametrized on the basis of experimental temperature dependences of partial photosynthetic processes: (1) photosystem II (PSII) turnover and plastoquinone (PQ) reduction, (2) the plastoquinol (PQH₂) oxidation by the cytochrome (Cyt) b₆f complex, (3) the ATP synthase activity, and (4) the proton leak from the thylakoid lumen. We consider that PQH₂ oxidation is the rate-limiting step in the intersystem electron transport. The parametrization of the rate constants of these processes is based on earlier experimental data demonstrating strong correlations between the functional and structural properties of thylakoid membranes that were probed with the lipid-soluble spin labels embedded into the membranes. Within the framework of our model, we could adequately describe a number of experimental temperature dependences of photosynthetic reactions in thylakoids. Computer modeling of electron and proton transport coupled to ATP synthesis supports the notion that PQH₂ oxidation by the Cyt b₆f complex and proton pumping into the lumen are the basic temperature-dependent processes that determine the overall electron flux from PSII to molecular oxygen and the net ATP synthesis upon variations of temperature. The model describes two branches of the temperature dependence of the post-illumination reduction of [Formula: see text] characterized by different activation energies (about 60 and ≤ 3.5 kJ mol^-1). The model predicts the bell-like temperature dependence of ATP formation, which arises from the balance of several factors: (1) the thermo-induced acceleration of electron transport through the Cyt b₆f complex, (2) deactivation of PSII photochemistry at sufficiently high temperatures, and (3) acceleration of the passive proton outflow from the thylakoid lumen bypassing the ATP synthase complex. The model describes the temperature dependence of experimentally measured parameter P/2e, determined as the ratio between the rates of ATP synthesis and pseudocyclic electron transport (H₂O → PSII → PSI → O₂).

Covalent modification of phosphatidylethanolamine by 4-hydroxy-2-nonenal increases sodium permeability across phospholipid bilayer membranes

Reactive aldehydes (RAs), such as 4-hydroxy-2-nonenal (HNE) and 4-oxo-2-nonenal (ONE), produced by cells under conditions of oxidative stress, were shown to react with phosphatidylethanolamine (PE) in biological and artificial membranes. They form RA-PE adducts, which affect the function of membrane proteins by modifying various biophysical properties of the membrane. The ratio of protein to lipid in biological membranes is different, but can reach 0.25 in the membranes of oligodendrocytes. However, the impact of RA-PE adducts on permeability (P) of the neat lipid phase and molecular mechanism of their action are poorly understood. In this study, we showed that HNE increased the membrane P for ions, and in particular for sodium. This effect depended on the presence of DOPE, and was not recorded for the more toxic compound, ONE. Molecular dynamics simulations suggested that HNE-PE and ONE-PE adducts anchored different positions in the lipid bilayer, and thus changed the membrane lipid area and bilayer thickness in different ways. Sodium permeability, calculated in the presence of double HNE-PE adducts, was increased by three to four orders of magnitude when compared to P_Na in adduct - free membranes. A novel mechanism by which HNE alters permeability of the lipid membrane may explain the multiple toxic or regulative effects of HNE on the function of excitable cells, such as neurons, cardiomyocytes and neurosensory cells under conditions of oxidative stress.

The Effect of Buffers on Weak Acid Uptake by Vesicles

The assessment of weak acid membrane permeability (P_m) frequently involves large unilamellar vesicles. It relies on measurements of the intravesicular pH drop, ΔpH_in, in response to a sudden augmentation of external acid concentration. However, ΔpH_in may be primarily governed by non-instantaneous protonation and deprotonation reactions of (i) the acid itself, (ii) the buffer molecules, and (iii) the fluorescent pH reporter dye. Moreover, buffer concentration and acid gradient also serve as determinants of ΔpH_in, as we show here. The uniexponential time constant (τ) of ΔpH_in(t) is an invalid measure of P_m as Arrhenius plots of P_m and τ reveal different activation energies for acid influx. We calculate P_m by fitting a mathematical model to experimental stopped-flow traces. The model takes into account not only the time course of total internal buffer capacity but also (i) water self-dissociation, (ii) volume changes due to acid induced osmotic water flow, and (iii) the spontaneous membrane proton leak. It allows extracting a P_m of 30.8 ± 3.5 μm/s for formic acid for 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) vesicles.

Dietary lipids from an evolutionary perspective: sources, structures and functions

Lipids are a complex group of biomolecules whose precise functions remain poorly understood. As a result of this poor understanding, it is difficult to make mechanistically based recommendations for appropriate dietary intakes. It is equally difficult to develop methods that are capable of diagnosing functional impairments because of insufficiencies or excesses in particular fatty acids. Lipids are abundant building blocks of cellular membranes, supply components for lipid particle assembly and substrates for metabolic fuel, and provide a precursor pool for an astonishingly diverse range of signalling molecules. In each of these broad functions, the functional consequences of different structures of fatty acids are not fully understood. According to research on membrane functions through early evolution, docosahexaenoic acid provides two biophysical properties to membranes - accelerating the lateral motion of lipids and proteins within the plane of the membrane and simultaneously slowing the rate of diffusion/leakage of charged species across the plane of the membrane. The range of fatty acid structures used as substrates for assembly of either lipoproteins or milk fat globules is broad, yet the functional consequences of differences are not known. Different lipids signal into a remarkable range of biological processes. Saturated and monounsaturated fatty acids are becoming recognized as signal molecules in their own right. The complex composition of human milk lipids implies that diets with a diversity of fatty acids in complex lipid forms and structures is more beneficial than a narrow range of any particular group of fatty acids.

Gene expression in vivo shows that Helicobacter pylori colonizes an acidic niche on the gastric surface

Helicobacter pylori is a gastric-dwelling pathogen responsible, with acid secretion, for peptic ulcer and a 20-fold increase in the risk of gastric cancer. Several transcriptomes have been described after short-term exposure to acidity in vitro, but there are no data identifying the effects of chronic gastric exposure on bacterial gene expression. Comparison of the in vivo to the in vitro transcriptome at pH 7.4 identified several groups of genes of known function that increased expression >2-fold, and three of these respond both to acidity in vitro and to gastric infection. Almost all known acid acclimation genes are highly up-regulated. These include ureA, ureB, and rocF and the pH-gated urea channel, ureI. There is also up-regulation of two groups of motility and chemotaxis genes and for pathogenicity island genes, especially cagA, a predictor for pathogenicity. Most of these genes interact with HP0166, the response element of the pH-sensing two-component histidine kinase, HP0165/HP0166, ArsRS. Based on the pH profile of survival of ureI deletion mutants in vitro and their inability to survive in gastric acidity, the habitat of the organism at the gastric surface is acidic with a pH < or = 4.0. Hence, the pH of the habitat of H. pylori on the surface of the stomach largely determines the regulation of these specific groups of genes.

Lipid raft components cholesterol and sphingomyelin increase H+/OH- permeability of phosphatidylcholine membranes

H+/OH- permeation through lipid bilayers occurs at anomalously high rates and the determinants of proton flux through membranes are poorly understood. Since all life depends on proton gradients, it is important to develop a greater understanding of proton leak phenomena. We have used stopped-flow fluorimetry to probe the influence of two lipid raft components, chol (cholesterol) and SM (sphingomyelin), on H+/OH- and water permeability. Increasing the concentrations of both lipids in POPC (palmitoyl-2-oleoyl phosphatidylcholine) liposomes decreased water permeability in a concentration-dependent manner, an effect that correlated with increased lipid order. Surprisingly, proton flux was increased by increasing the concentration of chol and SM. The chol effect was complex with molar concentrations of 17.9, 33 and 45.7% giving 2.8-fold (P<0.01), 2.2-fold (P<0.001) and 5.1-fold (P<0.001) increases in H+/OH- permeability from a baseline of 2.4x10(-2) cm/s. SM at 10 mole% effected a 2.8-fold increase (P<0.01), whereas 20 and 30 mole% enhanced permeability by 3.6-fold (P<0.05) and 4.1-fold respectively (P<0.05). Supplementing membranes containing chol with SM did not enhance H+/OH- permeability. Of interest was the finding that chol addition to soya-bean lipids decreased H+/OH- permeability, consistent with an earlier report [Ira and Krishnamoorthy (2001) J. Phys. Chem. B 105, 1484-1488]. We speculate that the presence of proton carriers in crude lipid extracts might contribute to this result. We conclude that (i) chol and SM specifically and independently increase rates of proton permeation in POPC bilayers, (ii) domains enriched in these lipids or domain interfaces may represent regions with high H+/OH- conductivity, (iii) H+/OH- fluxes are not governed by lipid order and (iv) chol can inhibit or promote H+/OH- permeability depending on the total lipid environment. Theories of proton permeation are discussed in the light of these results.

Annexin A4 reduces water and proton permeability of model membranes but does not alter aquaporin 2-mediated water transport in isolated endosomes

Annexin A4 (Anx4) belongs to a ubiquitous family of Ca2+-dependent membrane-binding proteins thought to be involved in membrane trafficking and membrane organization within cells. Anx4 localizes to the apical region in epithelia; however, its physiological role is unclear. We show that Anx4 exhibited binding to liposomes (phosphatidylcholine:phosphatidylserine, 1:1) in the presence of Ca2+ and binding was reversible with EDTA. Anx4 binding resulted in liposome aggregation and a reduction in membrane water permeability of 29% (P < 0.001) at 25 degrees C. These effects were not seen in the presence of Ca2+ or Anx4 alone and were reversible with EDTA. Measurements of membrane fluidity made by monitoring fluorescence anisotropy of 2-(12-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)dodecanoyl-1-hexadecanoyl-sn-glycero-3-phosphocholine (NBD-HPC) demonstrated that Anx4 binding rigidified the outer leaflet of the bilayer (P < 0.001), thus providing a molecular explanation for the inhibition of water flux. To determine whether Anx4 would produce similar effects on physiological membranes we constructed liposomes which recapitulated the lipid composition of the inner leaflet of the MDCK apical membrane. These membranes exhibited reductions to water permeability upon Anx4 binding (19.5% at 25 degrees C, 31% at 37 degrees C; P < 0.01 and P < 0.001, respectively) and to proton permeability (15% at 25 degrees C, 19.5% at 37 degrees C; P < 0.05). Since our in vitro experiments indicated an effect on membrane permeability, we examined localization of Anx4 in the kidney collecting duct, a region of the nephron responsible for concentrating urine through water reabsorbtion. Anx4 was shown to colocalize apically with aquaporin 2 (AQP2) in collecting duct epithelia. To test for the existence of a functional interaction between Anx4 and AQP2 we isolated AQP2-containing endosomes and exposed them to Anx4/Ca2+. Water flux rates were unchanged, indicating Anx4 does not directly regulate AQP2. We conclude that Anx4 can alter the physical properties of membranes by associating with them and regulate passive membrane permeability to water and protons. These properties represent important new functions for Anx4.

Membrane dipole potential modulates proton conductance through gramicidin channel: movement of negative ionic defects inside the channel

The effect of membrane dipole potential on gramicidin channel activity in bilayer lipid membranes (BLMs) was studied. Remarkably, it appeared that proton conductance of gramicidin A (gA) channels responded to modulation of the dipole potential oppositely as compared with gA alkali metal cation conductance. In particular, the addition of phloretin, known to reduce the membrane dipole potential, resulted in a decrease in gA proton conductance, on one hand, and an increase in gA alkali metal conductance, on the other hand, whereas 6-ketocholestanol, the agent raising the membrane dipole potential, provoked an increase in gA proton conductance as opposed to a decrease in the alkali metal cation conductance. The peculiarity of the 6-ketocholestanol effect consisted in its dependence on the H(+) concentration. The experiments with the impermeant dipolar compound, phloridzin, showed that the response of proton transport through gramicidin channels to varying the membrane dipole potential did not change qualitatively if the dipole potential of only one monolayer or both monolayers of the BLM was altered. In contrast to gA proton conductance, the single-channel lifetime changed similarly with varying the membrane dipole potential, regardless of the kind of permeant cations (protons or potassium ions). The results of this study could be tentatively accounted for by an assumption that one of the rate-limiting steps of proton conduction through gramicidin channels represents, in fact, movement of negatively charged species (negative ionic defects) across a membrane.

Role of leaflet asymmetry in the permeability of model biological membranes to protons, solutes, and gases

Bilayer asymmetry in the apical membrane may be important to the barrier function exhibited by epithelia in the stomach, kidney, and bladder. Previously, we showed that reduced fluidity of a single bilayer leaflet reduced water permeability of the bilayer, and in this study we examine the effect of bilayer asymmetry on permeation of nonelectrolytes, gases, and protons. Bilayer asymmetry was induced in dipalmitoylphosphatidylcholine liposomes by rigidifying the outer leaflet with the rare earth metal, praseodymium (Pr3+). Rigidification was demonstrated by fluorescence anisotropy over a range of temperatures from 24 to 50 degrees C. Pr3+-treatment reduced membrane fluidity at temperatures above 40 degrees C (the phase-transition temperature). Increased fluidity exhibited by dipalmitoylphosphatidylcholine liposomes at 40 degrees C occurred at temperatures 1-3 degrees C higher in Pr3+-treated liposomes, and for both control and Pr3+-treated liposomes permeability coefficients were approximately two orders of magnitude higher at 48 degrees than at 24 degrees C. Reduced fluidity of one leaflet correlated with significantly reduced permeabilities to urea, glycerol, formamide, acetamide, and NH3. Proton permeability of dipalmitoylphosphatidylcholine liposomes was only fourfold higher at 48 degrees than at 24 degrees C, indicating a weak dependence on membrane fluidity, and this increase was abolished by Pr3+. CO2 permeability was unaffected by temperature. We conclude: (a) that decreasing membrane fluidity in a single leaflet is sufficient to reduce overall membrane permeability to solutes and NH3, suggesting that leaflets in a bilayer offer independent resistances to permeation, (b) bilayer asymmetry is a mechanism by which barrier epithelia can reduce permeability, and (c) CO(2) permeation through membranes occurs by a mechanism that is not dependent on fluidity.

Deuterium isotope effects on permeation and gating of proton channels in rat alveolar epithelium

The voltage-activated H+ selective conductance of rat alveolar epithelial cells was studied using whole-cell and excised-patch voltage-clamp techniques. The effects of substituting deuterium oxide, D2O, for water, H2O, on both the conductance and the pH dependence of gating were explored. D+ was able to permeate proton channels, but with a conductance only about 50% that of H+. The conductance in D2O was reduced more than could be accounted for by bulk solvent isotope effects (i.e., the lower mobility of D+ than H+), suggesting that D+ interacts specifically with the channel during permeation. Evidently the H+ or D+ current is not diffusion limited, and the H+ channel does not behave like a water-filled pore. This result indirectly strengthens the hypothesis that H+ (or D+) and not OH- is the ionic species carrying current. The voltage dependence of H- channel gating characteristically is sensitive to pH0 and pHi and was regulated by pD0 and pDi in an analogous manner. shifting 40 mV/U change in the pD gradient. The time constant of H+ current activation was about three times slower (T(act) was larger) in D2O than in H2O. The size of the isotope effect is consistent with deuterium isotope effects for proton abstraction reactions, suggesting that H+ channel activation requires deprotonation of the channel. In contrast, deactivation (T(tail)) was slowed only by a factor < or = 1.5 in D2O. The results are interpreted within the context of a model for the regulation of H+ channel gating by mutually exclusive protonation at internal and external sites (Cherny, V.V., V.S. Markin, and T.E. DeCoursey. 1995. J. Gen. Physiol. 105:861-896). Most of the kinetic effects of D2O can be explained if the pKa of the external regulatory site is approximately 0.5 pH U higher in D2O.

Fatty acids as modulators of cytochrome c oxidase in proteoliposomes

The control of cytochrome c oxidase turnover in proteoliposomes by membrane potential (delta psi) and by pH gradient (delta pH) is probably kinetic in nature, and inhibition by valinomycin and stimulation by nigericin indicate that delta pH exerts a greater influence than does an equivalent delta psi. Oleic acid at 100 microM removes all delta psi and delta pH control, whereas a similar concentration of palmitic acid increases turnover but does not completely abolish control. Valinomycin acts synergistically with both fatty acids, indicating that the latter can act as H+/K+ exchangers, but neither fatty acid alone markedly affects delta pH, showing that they cannot fully mimic nigericin. Oleate, but not palmitate, diminishes delta psi, and can move electrophoretically as oleate anion. Submicromolar palmitic acid concentrations partly stimulate turnover in delta psi- and delta pH-controlled proteoliposomes, as reported by Labonia, Muller and Azzi [(1988) Biochem. J. 254, 130-145], which might represent a direct effect on cytochrome c oxidase. The ubiquity of fatty acids in biological membranes suggests that these substances might be responsible for limiting respiratory control and enzyme activity in vivo.

Proton transport across transient single-file water pores in a lipid membrane studied by molecular dynamics simulations

To test the hypothesis that water pores in a lipid membrane mediate the proton transport, molecular dynamic simulations of a phospholipid membrane, in which the formation of a water pore is induced, are reported. The probability density of such a pore in the membrane was obtained from the free energy of formation of the pore, which was computed from the average force needed to constrain the pore in the membrane. It was found that the free energy of a single file of water molecules spanning the bilayer is 108(+/-10) kJ/mol. From unconstrained molecular dynamic simulations it was further deduced that the nature of the pore is very transient, with a mean lifetime of a few picoseconds. The orientations of water molecules within the pore were also studied, and the spontaneous translocation of a turning defect was observed. The combined data allowed a permeability coefficient for proton permeation across the membrane to be computed, assuming that a suitable orientation of the water molecules in the pore allows protons to permeate the membrane relatively fast by means of a wirelike conductance mechanism. The computed value fits the experimental data only if it is assumed that the entry of the proton into the pore is not rate limiting.

The voltage-activated hydrogen ion conductance in rat alveolar epithelial cells is determined by the pH gradient

Voltage-activated H+ currents were studied in rat alveolar epithelial cells using tight-seal whole-cell voltage clamp recording and highly buffered, EGTA-containing solutions. Under these conditions, the tail current reversal potential, Vrev, was close to the Nernst potential, EH, varying 52 mV/U pH over four delta pH units (delta pH = pHo - pHi). This result indicates that H+ channels are extremely selective, PH/PTMA > 10(7), and that both internal and external pH, pHi, and pHo, were well controlled. The H+ current amplitude was practically constant at any fixed delta pH, in spite of up to 100-fold symmetrical changes in H+ concentration. Thus, the rate-limiting step in H+ permeation is pH independent, must be localized to the channel (entry, permeation, or exit), and is not bulk diffusion limitation. The instantaneous current-voltage relationship exhibited distinct outward rectification at symmetrical pH, suggesting asymmetry in the permeation pathway. Sigmoid activation kinetics and biexponential decay of tail currents near threshold potentials indicate that H+ channels pass through at least two closed states before opening. The steady state H+ conductance, gH, as well as activation and deactivation kinetic parameters were all shifted along the voltage axis by approximately 40 mV/U pH by changes in pHi or pHo, with the exception of the fast component of tail currents which was shifted less if at all. The threshold potential at which H+ currents were detectably activated can be described empirically as approximately 20-40(pHo-pHi) mV. If internal and external protons regulate the voltage dependence of gH gating at separate sites, then they must be equally effective. A simpler interpretation is that gating is controlled by the pH gradient, delta pH. We propose a simple general model to account for the observed delta pH dependence. Protonation at an externally accessible site stabilizes the closed channel conformation. Deprotonation of this site permits a conformational change resulting in the appearance of a protonation site, possibly the same one, which is accessible via the internal solution. Protonation of the internal site stabilizes the open conformation of the channel. In summary, within the physiological range of pH, the voltage dependence of H+ channel gating depends on delta pH and not on the absolute pH.

Permeability and electrical properties of planar lipid membranes from thylakoid lipids

Electrical measurements were carried out on planar lipid membranes from thylakoid lipids. The specific capacitance of membranes formed from decane-containing monogalactosyldiacylglycerol (MGDG), which accounts for 57% of the total lipid content of thylakoids, showed that it adopted a bilayer structure. Solvent-free bilayers of MGDG were not formed, with very rare exceptions, indicating that decane is required to stabilize the planar conformation. However, this cone-shaped lipid produces bilayer structures in combination with other cylindrical thylakoid lipids even in the absence of organic solvent. We compared the properties of solvent-free and decane-containing bilayers from MGDG, soybean lecithin, and the quaternary mixture of lipids similar to that found in vivo. The conductance of decane-MGDG was 26 times higher than that of decane-lecithin. The flux through the decane-lecithin bilayer was found to be slightly dependent on pH, whereas the decane-MGDG membrane was not. The specific conductance of bilayers formed from the quaternary mixture of lipids was 5 to 10 times larger than lecithin (with alkane or not). Further experiments with bilayers made in the presence of a KCl gradient showed that decane-MGDG, decane-MGDG/DGDG/SQDG/PG, and solvent-free MGDG/DGDG/SQDG/PG were cation-selective. The permeability coefficient for potassium ranged from 4.9 to 8.3 x 10(-11) cm s-1. The permeability coefficient for protons in galactolipids, however, was determined to be about six orders of magnitude higher than the value for potassium ions. The HCl permeation mechanism through the lipid membranes was determined from diffusion potentials measured in HCl gradients. Our results suggest that HCl was not transported as neutral molecules. The data is discussed with regard to the function of galactolipids in the ion transport through thylakoid membranes.

Salicylates and proton transport through lipid bilayer membranes: a model for salicylate-induced uncoupling and swelling in mitochondria

Mechanisms of proton transport were investigated in phospholipid bilayer membranes exposed to salicylates and benzoates. Membranes were formed from diphytanoyl phosphatidylcholine in decane plus chlorodecane (50% vol/vol). Proton and anion conductances (GH and GA) were calculated from the total conductances and the H+ or A diffusion potentials produced by transmembrane H+ or A gradients. At low pH salicylate caused a GH which was proportional to the square of the total weak acid concentration, and GH was maximum when pH = pK. At neutral to alkaline pH salicylate caused a GA which was proportional to the first power of the salicylate concentration, and GA was independent of pH. Both GH and GA were inhibited by phloretin. The results suggest that salicylate acts as an HA2-type proton carrier at low pH and as a lipid-soluble anion at neutral pH. Salicylate has been implicated as a causal factor in Reye's syndrome, as well as in aspirin poisoning, and salicylate has been reported to increase the proton conductance of inner mitochondrial membranes. The present results suggest that in mitochondria salicylate increases passive proton uptake by a combination of HA influx (driven by the concentration gradient) and A efflux (driven by the voltage and concentration gradients). Model calculations suggest that over the range of therapeutic to toxic concentrations, salicylate causes net H+ influx sufficient to explain the reported "loose coupling," uncoupling and swelling of mitochondria. The relative ineffectiveness of aspirin and benzoate can be explained by their low A permeabilities, whereas the ineffectiveness of 2,6-dihydroxybenzoate can be explained by its low pK.