The electrochemical proton gradient generated by light in membrane vesicles and chromatophores from Rhodopseudomonas sphaeroides

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

Heterogeneity of photosynthetic membranes from Rhodobacter capsulatus: size dispersion and ATP synthase distribution

The density distribution of photosynthetic membrane vesicles (chromatophores) from Rhodobacter capsulatus has been studied by isopicnic centrifugation. The average vesicle diameters, examined by electron microscopy, varied between 61 and 72 nm in different density fractions (70 nm in unfractionated chromatophores). The ATP synthase catalytic activities showed maxima displaced toward the higher density fractions relative to bacteriochlorophyll, resulting in higher specific activities in those fractions (about threefold). The amount of ATP synthase, measured by quantitative Western blotting, paralleled the catalytic activities. The average number of ATP synthases per chromatophore, evaluated on the basis of the Western blotting data and of vesicle density analysis, ranged between 8 and 13 (10 in unfractionated chromatophores). Poisson distribution analysis indicated that the probability of chromatophores devoid of ATP synthase was negligible. The effects of ATP synthase inhibition by efrapeptin on the time course of the transmembrane electric potential (evaluated as carotenoid electrochromic response) and on ATP synthesis were studied comparatively. The ATP produced after a flash and the total charge associated with the proton flow coupled to ATP synthesis were more resistant to efrapeptin than the initial value of the phosphorylating currents, indicating that several ATP synthases are fed by protons from the same vesicle.

Unreliability of carotenoid electrochromism for the measure of electrical potential differences induced by ATP hydrolysis in bacterial chromatophores

ATP hydrolysis induces the activation of the proton ATPase in chromatophores of Rhodobacter capsulatus supplemented with nigericine and 50 mM K+ (i.e. when delta pH < 0.2 units). The value of transmembrane electric potential (delta phi) driving this activation was measured using three different approaches: carotenoid electrochromism, uptake of SCN- and responses of the dye oxonol VI. The value of delta phi calculated from the SCN- uptake, on the basis of an internal volume determined experimentally, was about 140 mV, while that indicated by the electrochromic signal ranged between 35 and 70 mV. Only the value indicated by SCN- distribution is consistent with the energetic requirement for the activation of H(+)-ATPase.

Active transport in phototrophic bacteria

Phototrophic bacteria utilize light-driven, cyclic electron flow to pump protons out of their cytoplasm, creating an electrochemical proton gradient, ΔμH+, outside acid and positive. These bacteria exchange external protons for internal cations (Na(+), K(+) and Ca(+2)), allowing the cells to maintain a nearly constant internal pH while maintaining the electrical component of ΔμH+. Na(+)/H(+) exchange also establishes an electrochemical Na(+) gradient. Phototrophic bacteria are able to utilize these electrochemical gradients as energy sources for the uptake of a wide variety of metabolites (e.g., sugars, organic acids and amino acids) via metabolite/cation symports.

Calibration of the response of 9-amino acridine fluorescence to transmembrane pH differences in bacterial chromatophores

The spectral characteristics of absorption and fluorescence emission of 9-amino acridine are not altered by the interaction with bacterial chromatophores, except for the attenuation of both the absorption and emission following the formation of a protonic gradient. The lifetime of fluorescence of the dye is significantly affected in the presence of membranes, and even more following illumination. The shortening of the lifetime induced by light is reversible and prevented by nigericin and K+. The onset kinetics of the fluorescence quenching following the generation of an artificial transmembrane pH difference is temperature dependent, with an activation energy of 17 +/- 3 kcal/mol. The effect of pH on the rate constants is consistent with a model assuming that the diffusion of the unprotonated species is the limiting step in the quenching phenomenon. The response of 9-amino acridine to artificially imposed delta pH's has been utilized as a calibration method for the measurements of the light-induced protonic gradient. The apparent inner volume of chromatophores, evaluated from the extraplation of the response at delta pH = 0, was found to be much larger (15- to 40-fold) than the true osmotic volume, indicating that most of the dye is bound to the membrane when accumulated into the inner lumen.

On the extent of localization of the energized membrane state in chromatophores from Rhodopseudomonas capsulata N22

1. The principle of the double-inhibitor titration method for assessing competing models of electron transport phosphorylation is expounded. 2. This principle is applied to photophosphorylation by chromatophores from Rhodopseudomonas capsulata N22. 3. It is found that, in contrast to the predictions of the chemiosmotic coupling model, free energy transfer is confined to individual electron transport chain and ATP synthase complexes. 4. This conclusion is not weakened by arguments concerning, the degree of uncoupling in the native chromatophore preparation or the relative number of electron transport chain and ATP synthase complexes present. 5. Photophosphorylation is completely inhibited by the uncoupler SF 6847 at a concentration corresponding to 0.31 molecules per electron transport chain. 6. The apparent paradox is solved by the proposal, consistent with the available evidence on the mode of action of uncouplers, that uncoupler binding causes a co-operative conformation transition in the chromatophore membrane, which leads to uncoupling and which is not present in the absence of uncoupler.

Energy coupling of facilitated transport of inorganic ions in Rhodopseudomonas sphaeroides

Within the scope of a study on the effects of changes in medium composition on the proton motive force in Rhodopseudomonas sphaeroides, the energy coupling of sodium, phosphate, and potassium (rubidium) transport was investigated. Sodium was transported via an electroneutral exchange system against protons. The system functioned optimally at pH 8 and was inactive below pH 7. The driving force for the phosphate transport varied with the external pH. At pH 8, Pi transport was dependent exclusively on delta psi (transmembrane electrical potential), whereas at pH 6 only the delta pH (transmembrane pH gradient) component of the proton motive force was a driving force. Potassium (rubidium) transport was facilitated by a transport system which catalyzed the electrogenic transfer of potassium (rubidium) ions. However, in several aspects the properties of this transport system were different from those of a simple electrogenic potassium ionophore such as valinomycin: (i) accumulated potassium leaked very slowly out of cells in the dark; and (ii) the transport system displayed a threshold in the delta psi, below which potassium (rubidium) transport did not occur.

The measurement of membrane potential during photosynthesis and during respiration in intact cells of Rhodopseudomonas capsulata by both electrochromism and by permeant ion redistribution

1. The membrane potential in intact cells of Rhodopseudomonas capsulata during photosynthesis and during dark respiration has been measured by two independent methods. 2. The light-induced and O2-induced shifts in the carotenoid absorption spectrum were measured in the intact cells. The shift was calibrated with K+-diffusion potentials in chromatophores derived from those cells. The light-induced and O2-induced membrane potentials were -290 mV and -230 mV respectively. 3. The energized uptake of butyltriphenylphosphonium ions was measured in the same batch of cells. The light-induced and O2-induced membrane potentials calculated from the Nernst equation were -160 mV and -120 mV respectively. 4. It is concluded that the two kinds of probe measure the electric potentials across different domains of the cytoplasmic membrane, but it is difficult to reconcile the existence of such domains with simple electrical analogues of the membrane and aqueous phases.

ATP-driven active transport in right-side-out bacterial membrane vesicles

Membrane vesicles from Salmonella typhimurium induced for phosphoglycerate transport, were loaded with pyruvate kinase and ADP by lysing spheroplasts under appropriate conditions. Vesicles so prepared catalyze active transport of proline and serine in the presence of phosphoenolpyruvate; this activity is abolished by the protonophore carbonyl cyanide-m-chlorophenylhydrazone and by the H+-ATPase inhibitor N,N' dicyclohexylcarbodiimide but not by anoxia or cyanide. In contrast, D-lactate-driven active transport is abolished by the hydrazone and by anoxia or cyanide but not by the carbodiimide. Moreover, phosphoenolpyruvate does not drive transport effectively in vesicles that lack the phosphoglycerate transport system. The results are consistent with an overall mechanism in which phosphoenolpyruvate gains access to the interior of the vesicles by means of the phosphoglycerate transporter and is then acted on by pyruvate kinase to phosphorylate ADP. ATP formed inside of the vesicles is then hydrolyzed by the H+-ATPase, leading to the generation of a proton electrochemical gradient that drives H+/solute symport. By using pBR322 as vector and Escherichia coli as host, a fragment of S. typhimurium DNA coding for the phosphoglycerate transport system has been cloned. E. coli membrane vesicles containing the phosphoglycerate transport system also catalyze transport in the presence of phosphoenolpyruvate when they are loaded with pyruvate kinase and ADP.

Clarification of factors influencing the nature and magnitude of the protonmotive force in bovine heart submitochondrial particles

The magnitude of the protonmotive force, and its division between pH gradient and membrane potential components has been further characterised in submitochondrial particles. In a reaction medium containing sucrose for osmotic support and 4-(2-hydroxyethyl)-1-piperazineethanesulfonate (Hepes) as buffer, with succinate as substrate, the total protonmotive force reached a maximum value of 245 mV. The presence of Cl- enhanced the pH gradient with a partial but not fully compensating decrease in the membrane potential. When submitochondrial particles were suspended in a medium of low osmolarity consisting of phosphoric acid neutralised with Tris, again with succinate as substrate, the protonmotive force was lower and did not exceed 185 mV, and the pH gradient component was equivalent to 25 mV or less. The final phosphorylation potential, delta Gp, (formula: see text); maintained by the particles was higher in the phosphate/Tris medium (46--47.7 kJ mol-1) than in the sucrose/Hepes/KCl medium (43.7 kJ mol-1). Thus, comparison of the phosphorylation potential with the protonmotive force would suggest that the mechanistic stoichiometry H+/ATP (H+ translocated per molecule of ATP synthesied) for the ATPase enzyme is 3 in the former medium and 2 in the latter, which might be taken to indicate two different types of mechanism required for ATP synthesis. However it is questioned whether a comparison of the protonmotive force with delta Gp in terms of equilibrium thermodynamics ought not to be complemented by analysis in terms of linear non-equilibrium thermodynamics. The latter treatment shows that it is possible to estimate only a value for the product of a phenomenological stoichiometry and the degree of coupling, which can be variable, but not the mechanistic stoichiometry. This treatment can also rationalise the observation of the higher delta Gp in reaction conditions where the lower values for delta p are estimated. Irrespective of possible explanations, the data show how an unprejudiced choice of reaction conditions can lead to different conclusions about the relationship between the phosphorylation potential and the protonmotive force.

Generation of an electrochemical proton gradient by lactate efflux in membrane vesicles of Escherichia coli

The 'energy-recycling model' [Michels et al. (1979) FEMS Microbiol. Lett. 5, 357-364] postulates the generation of an electrochemical gradient across the bacterial cytoplasmic membrane by carrier-mediated efflux of metabolic endproducts in symport with protons. Experimental evidence for this model is presented. In membrane vesicles from Escherichia coli ML 308-255 L-lactate translocation (both uptake and efflux) is carrier-mediated. The H+/L-lactate stoichiometry varies, depending on the external pH, between 1 and 2. This change in stoichiometry is most likely the result of a protonation of the lactate carrier protein. This process has a pK of 6.75. L-Lactate efflux from membrane vesicles, loaded with 50 mM potassium L-lactate, results at an external pH of 6.6 in an 11-fold accumulation of proline inside the vesicles. This accumulation is completely inhibited by the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone. The uptake of proline is not the result of a potassium or an osmotic gradient. At an external pH of 6.6 efflux of L-lactate from the vesicles leads to the generation of an electrical potential across the membrane of -55 mV, as is demonstrated from the accumulation of the lipophilic cation tetraphenylphosphonium.

The induction kinetics of bacterial photophosphorylation. Threshold effects by the phosphate potential and correlation with the amplitude of the carotenoid absorption band shift

1. ATP synthesis (monitored by luciferin-luciferase) can be elicited by a single turnover flash of saturating intensity in chromatophores from Rhodopseudomonas capsulata, Kb1. The ATP yield from the first to the fourth turnover is strongly influenced by the phosphate potential: at high phosphate potential (-11.5 kcal/mol) no ATP is formed in the first three turnovers while at lower phosphate potential (-8.2 kcal/mol) and the yield in the first flash is already one half of the maximum, which is reached after 2-3 turnovers. 2. The response to ionophores indicates that the driving force for ATP synthesis in the first 20 turnovers is mainly given by a membrane potential. The amplitude of the carotenoid band shift shows that during a train of flashes an increasing delta psi is built up, which reaches a stationary level after a few turnovers; at high phosphate potential, therefore, more turnovers of the same photosynthetic unit are required to overcome an energetic threshold. 3. After several (six to seven) flashes the ATP yield becomes constant, independently from the phosphate potential; the yield varies, however, as a function of dark time (td) between flashes, with an optimum for td = 160-320 ms. 4. The decay kinetics of the high energy state generated by a long (125 ms) flash have been studied directly measuring the ATP yield produced in post-illumination by one single turnover flash, under conditions of phosphate potential (-10 kcal/mol), which will not allow ATP formation by one single turnover. The high energy state decays within 20 s after the illumination. The decay rate is strongly accelerated by 10(-8) M valinomycin. 5. Under all the experimental conditions described, the amplitude of the carotenoid signal correlates univocally with the ATP yield per flash, demonstrating that this signal monitores accurately an energetic state of the membrane directly involved in ATP synthesis. 6. Although values of the carotenoid signal much larger than the minimal threshold are present, relax slowly, and contribute to the energy input for phosphorylation, no ATP is formed unless electron flow is induced by a single turnover flash. 7. The conclusions drawn are independent from the assumption that a delta psi between bulk phases is evaluable from the carotenoid signal.

The dependency of proton extrusion in the light on the developmental stage of the photosynthetic apparatus in Rhodospirillum rubrum

The rate of proton extrusion by whole cells of Rhodospirillum rubrum is constant on a bacteriochlorophyll basis only above cellular bacteriochlorophyll concentrations of about 10 nmol bacteriochlorophyll per mg cell protein. At specific bacteriochlorophyll cellular levels below this value, the rate of proton extrusion per bacteriochlorophyll increases. Correspondingly, membrane preparations isolated from these cells exhibit increases in the rate of proton uptake on a pigment basis. Concomitant with variations in the rates of proton extrusion by whole cells, light energy fluxes for saturating this process also vary. A fair proportionality between maximum rates of proton extrusion of whole cells and the bacteriochlorophyll cellular levels above 10 nmol per mg protein indicates that the degree of continuity of intracytoplasmic membranes and of the cytoplasmic membrane remains largely constant.

Kinetic and steady-state investigations of solute accumulation in bacterial membranes by continuously monitoring the radioactivity in the effluent of flow-dialysis experiments

The flow-dialysis technique for studies of solute accumulation by membrane preparations has been made more suitable for routine measurements by recording continuously the radioactivity in the effluent of a flow-dialysis vessel with a homogeneous flow-monitoring device for beta-emitters. This modification not only decreases the time and cost of a flow-dialysis experiment but also allows the investigator to react directly on the outcome of his experiments. Analysis of the kinetics of this automated flow-dialysis system shows that this technique can also be used for the determination of the rate of uptake of solutes into bacterial membranes. This has been confirmed in Rhodopseudomonas sphaeroides by comparing the results of Rb+ and inorganic phosphate uptake studies performed by automated flow-dialysis and by the conventional filtration procedure. This application has the limitation that solute uptake has to proceed linearly for a period of about five times the half-time of the response of the flow-dialysis system. The two described applications make automated flow-dialysis very well-suited for experiments on the bioenergetics and regulation of solute uptake into bacterial membranes. Both driving force and rate of solute uptake can now be determined in one experiment.

Measurements of membrane potentials in Escherichia coli K-12 inner membrane vesicles with the safranine method

The use of safranine, a positively-charged dye, as a probe for the determination of membrane potentials in Escherichia coli vesicles has been studied. 1. Shifts in the spectrum of safranine were observed during induction of potassium ion diffusion potentials with valinomycin or during oxidation of formate by vesicles prepared from cells of E. coli K-12 or ML 308-225 subjected to anaerobic growth with nitrate. The extent of the valinomycin-dependent spectral change correlated linearly with the magnitude of the K+ equilibrium potential, as calculated from the Nernst equation, from 50 to 160 mV (interior negative). The formate-induced changes could also be calibrated by increasing the concentration of potassium in the presence of valinomycin, after the formation of formate-dependent responses. In this case, results identical to those obtained with the first method were obtained. 2. O2 or nitrate-dependent oxidation of formate resulted in a membrane potential of the order of 170 mV. The oxidation of ascorbate-reduced N-methylphenazonium methosulphate resulted in a potential of similar magnitude, but anaerobically with nitrate only a small but definite potential was formed. 3. The water-soluble quinones, duroquinone and menadione, could produce membrane potentials when used in their oxidized or reduced forms in the presence of formate or nitrate (or oxygen). 2-Hydroxy-1,4-naphthoquinone was not only ineffective but was found to be inhibitory. 4. N,N'-dicyclohexylcarbodiimide at suitable concentrations increased the rate of formation and the extent of membrane potentials induced by respiration or by artificial means.

Reassembly of protein-lipid complexes into large bilayer vesicles: perspectives for membrane reconstitution

Protein-lipid complexes in apolar solvents reassemble into large bilayer protein-lipid vesicles (PLVs) with diameters of several micrometers. PLVs form spontaneously upon hydration of the protein-lipid complex residue after solvent removal. This procedure has been applied to the following membrane proteins: bovine and squid rhodopsin, reaction centers from Rhodopseudomonas sphaeroides, beef heart cytochrome c oxidase, and acetylcholine receptors from Torpedo californica. PLVs have a large internal aqueous space (e.g., 790 mul/mg of lipid for cattle rhodopsin vesicles). Freeze-fracture replicas of PLVs revealed that both internal and external leaflets contained numerous intramembranous particles with diameters between 80 and 120 A, depending on the specific protein incorporated in the membrane. The optical spectral properties of rhodopsin and reaction centers in PLVs were similar to those recorded in the respective natural membrane. Furthermore, bovine rhodopsin in PLVs was chemically regenerable with 9-cis-retinal. Actinic illumination induced proton efflux from reaction center vesicles that was abolished by proton ionophores. Therefore, this method is suitable for the incorporation of some membrane proteins in their functional state. PLVs were penetrated with microelectrodes and visualized by the injection of a fluorescent dye. Preliminary electrical recordings were obtained by sealing PLVs to a hole in a septum separating two aqueous compartments. These studies suggest that PLVs assembled by this procedure permit the simultaneous analysis of reconstituted membranes by chemical, optical, and electrical techniques.