A microchemiosmotic interpretation of energy-dependent processes in biomembranes based on the photosynthetic behaviour of thylakoids

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.

Effects of dark- and light-induced proton gradients in thylakoids on the Q and B thermoluminescence bands

Thermoluminescence experiments have been carried out to study the effect of a transmembrane proton gradient on the recombination properties of the S2 and S3 states of the oxygen evolving complex with QA (-) and QB (-), the reduced electron acceptors of Photosystem II. We first determined the properties of the S2QA (-) (Q band), S2QB (-) and S3QB (-) (B bands) recombinations in the pH range 5.5 to 9.0, using uncoupled thylakoids. The, a proton gradient was created in the dark, using the ATP-hydrolase function of ATPases, in coupled unfrozen thylakoids. A shift towards low temperature of both Q and B bands was observed to increase with the magnitude of the proton gradient measured by the fluorescence quenching of 9-aminoacridine. This downshift was larger for S3QB (-) than for S2QB (-) and it was suppressed by nigericin, but not by valinomycin. Similar results were obtained when a proton gradient was formed by photosystem I photochemistry. When Photosystem II electron transfer was induced by a flash sequence, the reduction of the plastoquinone pool also contributed to the downshift in the absence of an electron acceptor. In leaves submitted to a flash sequence above 0°C, a downshift was also observed, which was supressed by nigericin infiltration. Thus, thermoluminescence provides direct evidence on the enhancing effect of lumen acidification on the S3→S2 and S2→S1 reverse-transitions. Both reduction of the plastoquinone pool and lumen acidification induce a shift of the Q and B bands to lower temperature, with a predominance of lumen acidification in non-freezing, moderate light conditions.

Flow-force relationships in lettuce thylakoids. 2. Effect of the uncoupler FCCP on local proton resistances at the ATPase level

The relationship between the steady-state proton gradient (delta pH) and the rate of phosphorylation was investigated in thylakoids under various conditions. Under partial uncoupling by carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP), the rate of ATP synthesis was reduced by less than expected from the decrease of delta pH. This was observed in the case of the pyocyanine-mediated cyclic electron flow around photosystem 1, but not with the H2O-->photosystem 2-->cytochrome b6f-->photosystem 1-->methyl viologen system. In state 4, a unique relation was found between delta pH and the "phosphate potential", delta Gp, regardless of whether the energy level was controlled by light input or FCCP. The anomalous effect of FCCP on the rate of ATP synthesis disappeared when the ATPase was partially blocked by the reversible inhibitor venturicidin, but not in the presence of tentoxin, an irreversible inhibitor. These results are consistent with the existence of a small kinetic barrier for protons, limiting their access to the ATPase. This resistance would be collapsed by FCCP.

Flow-force relationships in lettuce thylakoids. 1. Strict control of electron flow by internal pH

The regulation by the proton gradient of the electron flow from water to ferricyanide was investigated in thylakoids extracted from lettuce leaves. When the transmembrane proton current was varied by an uncoupler or by the ATP synthase activity, a unique relationship was found between the rate of ferricyanide reduction and the proton gradient, restricted here to its delta pH component. This behavior was conserved in CF1-depleted thylakoids where the passive proton flow was varied by the concentration of an Fo inhibitor or by the concentration of an uncoupler after 100% inhibition of Fo. This shows that under our experimental conditions no direct proton transfer exists in steady state between the site of regulation of the redox chain and the ATPase. Studies at two different pH's indicate that the internal pH, and not the transmembrane pH difference, controls the electron transfer between PS2 and PS1. Modeling the data suggests that a single deprotonation step is kinetically limiting.

Energy coupling, membrane lipids and structure of thylakoids of Lupin plants submitted to water stress

Bioenergetic properties of thylakoids from plants submitted to a water stress stress (watering stopped for 6-15 days) have been measured in two lupin genotypes characterized as resistant or susceptible to drought. This energy coupling was assessed by flow-force relationships relating the phosphorylation rate to the magnitude of the proton gradient % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqef0uAJj3BZ9Mz0bYu% H52CGmvzYLMzaerbd9wDYLwzYbItLDharqqr1ngBPrgifHhDYfgasa% acOqpw0xe9v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8Wq% Ffea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8frFve9Fve9Ff0dme% GabaqaaiGacaGaamqadaabaeaafiaakabbaaa6daaahjxzL5gapeqa% aiabgs5aenaaxacabaGaeqiVd0galeqabaGaaiOFaaaakmaaBaaale% aacaWGibWaaWbaaWqabeaacqGHRaWkaaaaleqaaaaa!4D55!\[\Delta \mathop \mu \limits^\~ _{H^ + } \]. The fluorescent probe 9-aminoacridine was used to express, as a ΔpH, the whole % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqef0uAJj3BZ9Mz0bYu% H52CGmvzYLMzaerbd9wDYLwzYbItLDharqqr1ngBPrgifHhDYfgasa% acOqpw0xe9v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8Wq% Ffea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8frFve9Fve9Ff0dme% GabaqaaiGacaGaamqadaabaeaafiaakabbaaa6daaahjxzL5gapeqa% aiabgs5aenaaxacabaGaeqiVd0galeqabaGaaiOFaaaakmaaBaaale% aacaWGibWaaWbaaWqabeaacqGHRaWkaaaaleqaaaaa!4D55!\[\Delta \mathop \mu \limits^\~ _{H^ + } \] by calibrating fluorescence quenching against the phosphate potential ΔGp in 'state 4', i.e., when ATP synthesis is strictly balanced by its hydrolysis. This calibration procedure was shown to be unaffected by treatments. At equal energization (iso-ΔpH), ATP synthesis was halved by a medium stress and disappeared for a more severe stress, whereas ΔpH at equal energy input (light) declined only under a severe drought. For an identical ΔpH, PS 1-driven phosphorylation is always more efficient than PS 2, both in control and stressed plants. Thus, uncoupling is not the cause of the phosphorylation decline; moreover, retention of a 'micro-chemiosmotic' type of coupling implies that the distribution of photosystems and ATPases is unchanged. Parallel to these functional alterations, the lipid content of thylakoids dramatically dropped. As galactolipids fell strongly, neutral lipids rose slightly. Fatty acids decreased then increased with stress, yet phosphorylation did not recover in the latter case and membrane permeability to protons remained unaffected. Overall, these observations suggest a preserved thylakoid structure and this was indeed observed on electron micrographs, even for a severe stress. Therefore, the membrane integrity is probably preserved more by the protein network than by the lipid matrix and the loss of the phosphorylating activity mainly reflects a loss of ATPases or at least their inactivation, possibly due to their altered lipid environment. Finally, from the bioenergetic point of view, the susceptible genotype was unexpectedly less affected by drought than the resistant.

Dependence of kinetic parameters of chloroplast ATP synthase on external pH, internal pH, and delta pH

ATP synthesis by the membrane-bound chloroplast ATPase in the oxidized state of its gamma disulfide bridge was studied as a function of the ADP concentration, delta pH, and external pH values, under conditions where delta pH was clamped and delocalized. At a given pH, the rate of phosphorylation at saturating ADP concentration (Vmax) and the Michaelis constant Km (ADP) depend strictly on delta pH, irrespective of the way the delta pH is generated: there evidently is no specific interaction between the redox carriers and the ATPase. It was also shown that both Km (ADP) and Vmax depend on delta pH, not on the external or internal pH. This suggests that internal proton binding and external proton release are concerted, so that net proton translocation is an elementary step of the phosphorylation process. These results appear to be consistent with a modified "proton substrate" model, provided the delta G0 of the condensation reaction within the catalytic site is low. At least one additional assumption, such as a shift in the pK of bound phosphate or the existence of an additional group transferring protons from or to reactants, is nevertheless required to account for the strict delta pH dependence of the rate of ATP synthesis. A purely "conformational" model, chemically less explicit, only requires constraints on the pK's of the groups involved in proton translocation.

Lateral proton conduction at a lipid/water interface. Effect of lipid nature and ionic content of the aqueous phase

Fast lateral proton conduction was observed along the lipid/water interface using a fluorescence technique. This conduction can be detected for a large number of lipids, both phospholipids and glycolipids. The efficiency of the proton transfer is dependent on the molecular packing of the host lipid at a given surface pressure. The proton conduction which is present in the liquid expanded state is abolished by the transition to the liquid condensed state. The proton transfer is affected slightly by the ionic content of the aqueous subphase except in the case of calcium which can inhibit the conduction along phosphatidylglyceroethanolamine. We suggest that the transfer of the protons occurs along a bidimensional hydrogen-bond network formed from the polar head groups, their water molecules of hydration and the water molecules which are intercalated between the lipid molecules.

Integrated functioning of the chloroplast coupling factor

This review is focused on some functional characteristics of the chloroplast coupling factor. The structure of the enzyme and the putative role of its subunits are recalled. An attempt is made to discriminate the driving force and the activator effects of the electrochemical proton gradient. Respective roles of delta pH, delta phi, external and internal pH are discussed with regard to mechanistic implications. The hypothesis of a functional switch of the enzyme between two states with better efficiency either in ATP synthesis or in ATP hydrolysis is also examined. A brief survey is made on some problems complicating quantitative studies of energy coupling, such as localized chemiosmosis, delta pH and delta phi computations, and scalar ATPases. The main data on the enzyme activation and the energy-dependent release of tightly bound nucleotides are summarized. The arguments for and against the catalytic competence of theses nucleotides are reviewed. Lastly, some prevailing models of the catalytic mechanism are presented. The relevance of nucleotides binding change events in this process is discussed.