Proton to electron stoichiometry in electron transport of spinach thylakoids

Optimal efficiency of the Q-cycle mechanism around physiological temperatures from an open quantum systems approach

The Q-cycle mechanism entering the electron and proton transport chain in oxygenic photosynthesis is an example of how biological processes can be efficiently investigated with elementary microscopic models. Here we address the problem of energy transport across the cellular membrane from an open quantum system theoretical perspective. We model the cytochrome \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${b}_{6}\,f$$\end{document}b6f protein complex under cyclic electron flow conditions starting from a simplified kinetic model, which is hereby revisited in terms of a Markovian quantum master equation formulation and spin-boson Hamiltonian treatment. We apply this model to theoretically demonstrate an optimal thermodynamic efficiency of the Q-cycle around ambient and physiologically relevant temperature conditions. Furthermore, we determine the quantum yield of this complex biochemical process after setting the electrochemical potentials to values well established in the literature. The present work suggests that the theory of quantum open systems can successfully push forward our theoretical understanding of complex biological systems working close to the quantum/classical boundary.

Photosynthesis-related quantities for education and modeling

A quantitative understanding of the photosynthetic machinery depends largely on quantities, such as concentrations, sizes, absorption wavelengths, redox potentials, and rate constants. The present contribution is a collection of numbers and quantities related mainly to photosynthesis in higher plants. All numbers are taken directly from a literature or database source and the corresponding reference is provided. The numerical values, presented in this paper, provide ranges of values, obtained in specific experiments for specific organisms. However, the presented numbers can be useful for understanding the principles of structure and function of photosynthetic machinery and for guidance of future research.

Continuous ECS-indicated recording of the proton-motive charge flux in leaves

Technical features and examples of application of a special emitter-detector module for highly sensitive measurements of the electrochromic pigment absorbance shift (ECS) via dual-wavelength (550-520 nm) transmittance changes (P515) are described. This device, which has been introduced as an accessory of the standard, commercially available Dual-PAM-100 measuring system, not only allows steady-state assessment of the proton motive force (pmf) and its partitioning into ΔpH and ΔΨ components, but also continuous recording of the overall charge flux driven by photosynthetic light reactions. The new approach employs a double-modulation technique to derive a continuous signal from the light/dark modulation amplitude of the P515 signal. This new, continuously measured signal primarily reflects the rate of proton efflux via the ATP synthase, which under quasi-stationary conditions corresponds to the overall rate of proton influx driven by coupled electron transport. Simultaneous measurements of charge flux and CO2 uptake as a function of light intensity indicated a close to linear relationship in the light-limited range. A linear relationship between these two signals was also found for different internal CO2 concentrations, except for very low CO2, where the rate of charge flux distinctly exceeded the rate of CO2 uptake. Parallel oscillations in CO2 uptake and charge flux were induced by high CO2 and O2. The new device may contribute to the elucidation of complex regulatory mechanisms in intact leaves.

Mathematical review of the energy transduction stoichiometries of C(4) leaf photosynthesis under limiting light

A generalized model for electron (e(-) ) transport limited C(4) photosynthesis of NAD-malic enzyme and NADP-malic enzyme subtypes is presented. The model is used to review the thylakoid stoichiometries in vivo under strictly limiting light conditions, using published data on photosynthetic quantum yield and on photochemical efficiencies of photosystems (PS). Model review showed that cyclic e(-) transport (CET), rather than direct O(2) photoreduction, most likely contributed significantly to the production of extra ATP required for the C(4) cycle. Estimated CET, and non-cyclic e(-) transport supporting processes like nitrogen reduction, accounted for ca. 45 and 7% of total photosystem I (PSI) e(-) fluxes, respectively. The factor for excitation partitioning to photosystem II (PSII) was ca. 0.4. Further model analysis, in terms of the balanced NADPH: ATP ratio required for metabolism, indicated that: (1) the Q-cycle is obligatory; (2) the proton: ATP ratio is 4; and (3) the efficiency of proton pumping per e(-) transferred through the cytochrome b(6) /f complex is the same for CET and non-cyclic pathways. The analysis also gave an approach to theoretically assess CO(2) leakiness from bundle-sheath cells, and projected a leakiness of 0.07-0.16. Compared with C(3) photosynthesis, the most striking C(4) stoichiometry is its high fraction of CET.

Arsenic toxicity: the effects on plant metabolism

The two forms of inorganic arsenic, arsenate (AsV) and arsenite (AsIII), are easily taken up by the cells of the plant root. Once in the cell, AsV can be readily converted to AsIII, the more toxic of the two forms. AsV and AsIII both disrupt plant metabolism, but through distinct mechanisms. AsV is a chemical analog of phosphate that can disrupt at least some phosphate-dependent aspects of metabolism. AsV can be translocated across cellular membranes by phosphate transport proteins, leading to imbalances in phosphate supply. It can compete with phosphate during phosphorylation reactions, leading to the formation of AsV adducts that are often unstable and short-lived. As an example, the formation and rapid autohydrolysis of AsV-ADP sets in place a futile cycle that uncouples photophosphorylation and oxidative phosphorylation, decreasing the ability of cells to produce ATP and carry out normal metabolism. AsIII is a dithiol reactive compound that binds to and potentially inactivates enzymes containing closely spaced cysteine residues or dithiol co-factors. Arsenic exposure generally induces the production of reactive oxygen species that can lead to the production of antioxidant metabolites and numerous enzymes involved in antioxidant defense. Oxidative carbon metabolism, amino acid and protein relationships, and nitrogen and sulfur assimilation pathways are also impacted by As exposure. Readjustment of several metabolic pathways, such as glutathione production, has been shown to lead to increased arsenic tolerance in plants. Species- and cultivar-dependent variation in arsenic sensitivity and the remodeling of metabolite pools that occurs in response to As exposure gives hope that additional metabolic pathways associated with As tolerance will be identified.

The Q cycle of cytochrome bc complexes: a structure perspective

Aspects of the crystal structures of the hetero-oligomeric cytochrome bc(1) and b(6)f ("bc") complexes relevant to their electron/proton transfer function and the associated redox reactions of the lipophilic quinones are discussed. Differences between the b(6)f and bc(1) complexes are emphasized. The cytochrome bc(1) and b(6)f dimeric complexes diverge in structure from a core of subunits that coordinate redox groups consisting of two bis-histidine coordinated hemes, a heme b(n) and b(p) on the electrochemically negative (n) and positive (p) sides of the complex, the high potential [2Fe-2S] cluster and c-type heme at the p-side aqueous interface and aqueous phase, respectively, and quinone/quinol binding sites on the n- and p-sides of the complex. The bc(1) and b(6)f complexes diverge in subunit composition and structure away from this core. b(6)f Also contains additional prosthetic groups including a c-type heme c(n) on the n-side, and a chlorophyll a and β-carotene. Common structure aspects; functions of the symmetric dimer. (I) Quinone exchange with the bilayer. An inter-monomer protein-free cavity of approximately 30Å along the membrane normal×25Å (central inter-monomer distance)×15Å (depth in the center), is common to both bc(1) and b(6)f complexes, providing a niche in which the lipophilic quinone/quinol (Q/QH(2)) can be exchanged with the membrane bilayer. (II) Electron transfer. The dimeric structure and the proximity of the two hemes b(p) on the electrochemically positive side of the complex in the two monomer units allow the possibility of two alternate routes of electron transfer across the complex from heme b(p) to b(n): intra-monomer and inter-monomer involving electron cross-over between the two hemes b(p). A structure-based summary of inter-heme distances in seven bc complexes, representing mitochondrial, chromatophore, cyanobacterial, and algal sources, indicates that, based on the distance parameter, the intra-monomer pathway would be favored kinetically. (III) Separation of quinone binding sites. A consequence of the dimer structure and the position of the Q/QH(2) binding sites is that the p-side QH(2) oxidation and n-side Q reduction sites are each well separated. Therefore, in the event of an overlap in residence time by QH(2) or Q molecules at the two oxidation or reduction sites, their spatial separation would result in minimal steric interference between extended Q or QH(2) isoprenoid chains. (IV) Trans-membrane QH(2)/Q transfer. (i) n/p-side QH(2)/Q transfer may be hindered by lipid acyl chains; (ii) the shorter less hindered inter-monomer pathway across the complex would not pass through the center of the cavity, as inferred from the n-side antimycin site on one monomer and the p-side stigmatellin site on the other residing on the same surface of the complex. (V) Narrow p-side portal for QH(2)/Q passage. The [2Fe-2S] cluster that serves as oxidant, and whose histidine ligand serves as a H(+) acceptor in the oxidation of QH(2), is connected to the inter-monomer cavity by a narrow extended portal, which is also occupied in the b(6)f complex by the 20 carbon phytyl chain of the bound chlorophyll.

Measurement of the ΔpH and electric field developed across Arabidopsis thylakoids in the light

Measurement of the different components of the proton motive force (pmf) gives information about the coupling of proton movement within thylakoids to chemiosmotic processes such as photophosphorylation and protein transport, as well as that relating to the overall quality of a thylakoid preparation. The techniques to assess the pmf have been known for many years, as they have been applied to the most popular model plants for photosynthetic research. The emergence of Arabidopsis thaliana as the prominent model plant in developmental and genetics research prompted us to apply these techniques to thylakoids isolated from Arabidopsis chloroplasts. We describe here two spectroscopic techniques to measure the transmembrane pH gradient and electric field developed in the light in Arabidopsis thylakoids.

From sunlight to phytomass: on the potential efficiency of converting solar radiation to phyto-energy

The relationship between solar radiation capture and potential plant growth is of theoretical and practical importance. The key processes constraining the transduction of solar radiation into phyto-energy (i.e. free energy in phytomass) were reviewed to estimate potential solar-energy-use efficiency. Specifically, the out-put:input stoichiometries of photosynthesis and photorespiration in C(3) and C(4) systems, mobilization and translocation of photosynthate, and biosynthesis of major plant biochemical constituents were evaluated. The maintenance requirement, an area of important uncertainty, was also considered. For a hypothetical C(3) grain crop with a full canopy at 30°C and 350 ppm atmospheric [CO(2) ], theoretically potential efficiencies (based on extant plant metabolic reactions and pathways) were estimated at c. 0.041 J J(-1) incident total solar radiation, and c. 0.092 J J(-1) absorbed photosynthetically active radiation (PAR). At 20°C, the calculated potential efficiencies increased to 0.053 and 0.118 J J(-1) (incident total radiation and absorbed PAR, respectively). Estimates for a hypothetical C(4) cereal were c. 0.051 and c. 0.114 J J(-1), respectively. These values, which cannot be considered as precise, are less than some previous estimates, and the reasons for the differences are considered. Field-based data indicate that exceptional crops may attain a significant fraction of potential efficiency.

The chloroplast Tat pathway transports substrates in the dark

Photosynthetic electron transport pumps protons into the thylakoid lumen, creating an electrochemical potential called the protonmotive force (PMF). The energy of the thylakoid PMF is utilized by such machinery as the chloroplast F(0)F(1)-ATPase as well as the chloroplast Tat (cpTat) pathway (a protein transporter) to do work. The bulk phase thylakoid PMF decays rapidly after the termination of actinic illumination, and it has been well established via potentiometric measurements that there is no detectable electrical or chemical potential in the thylakoid after a brief time in the dark. Yet, we report herein that cpTat transport can occur for long periods in the dark. We show that the thylakoid PMF is actually present long after actinic illumination of the thylakoids ceases and that this energy is present in physiologically useful quantities. Consistent with previous studies, the dark-persisting thylakoid potential is not detectable by established indicators. We propose that cpTat transport in the dark is dependent on a pool of protons in the thylakoid held out of equilibrium with those in the bulk aqueous phase.

Reduction of the thylakoid electron transport chain by stromal reductants--evidence for activation of cyclic electron transport upon dark adaptation or under drought

The reduction of P700(+), the primary electron donor of photosystem I (PSI), following a saturating flash of white light in the presence of the photosystem II (PSII) inhibitor 3-(3.4-dichlorophenyl)-1,1-dimethylurea (DCMU), was examined in barley plants exposed to a variety of conditions. The decay kinetic fitted to a double exponential decay curve, implying the presence of two distinct pools of PSI. A fast component, with a rate constant for decay of around 0.03-0.04 ms(-1) was observed to be sensitive to the duration of illumination. This rate constant was slower than, but comparable to, that observed in non-inhibited samples (i.e. where linear flow was active). It was substantially faster than values typically reported for experiments where PSII activity is inhibited. The magnitude of this component rose in leaves that were dark-adapted or exposed to drought. This component was assigned to PSI centres involved in cyclic electron transport. The remaining slowly decaying P700(+) population (rate constant of around 0.001-0.002 ms(-1)) was assigned to centres normally involved in linear electron transport (but inhibited here because of the presence of DCMU), or inactivated centres involved in the cyclic pathway. Processes that might regulate the relative flux through cyclic electron transport are discussed.

Light-induced proton slip and proton leak at the thylakoid membrane

A treatment of leaves of Spinacia oleracea L. with light or with the thiol reagent dithiothreitol in the dark led to partly uncoupled thylakoids. After induction in intact leaves, the partial uncoupling was irreversible at the level of isolated thylakoids. We distinguish between uncoupling by proton slip, which means a decrease of the H+/e(-) -ratio due to less efficient proton pumping, and proton leak as defined by enhanced kinetics of proton efflux. Proton slip and proton leak made about equal contributions to the total uncoupling. The enhanced proton efflux kinetics corresponded to reduction of subunit CF1-gamma of the ATP synthase as shown by fluorescence labeling of thylakoid proteins with the sulfhydryl probe 5-iodoacetamido fluorescein. The maximum value of the fraction of reduced CF1-gamma was only 36%, which indicates that in vivo the reduction of CF1-gamma could be limited by fast reoxidation and/or restricted accessibility of CF1-gamma to thioredoxin. Measurements of the ratio ATP/2e indicated that only the uncoupling related to less efficient proton pumping led to a decrease in the ATP yield.

Metabolic Flexibility of the Green Alga Chlamydomonas reinhardtii as Revealed by the Link between State Transitions and Cyclic Electron Flow

In this Review we focus on the conversion of linear photosynthetic electron transport from water to NADP to the cyclic pathway around Photosystem I in the green alga Chlamydomonas reinhardtii. We discuss the strict relationship that exists between the changes in pathways of electron transport and state transitions, i.e., the reversible functional association of light harvesting proteins with one of the two photosystems of oxygenic photosynthesis. Such a link has not been reported in the case of other photosynthetic organisms, where the state transitions do not affect the pathway of electron transport. Rather, they provide a tool to optimise the rate of linear flow. We propose a kinetic-structural model that explains the mechanism of this particular relationship in Chlamydomonas, and discuss the advantages that this peculiar situation gives to the energetic metabolism of this alga.

Endosymbiosis and the design of eukaryotic electron transport

The bioenergetic organelles of eukaryotic cells, mitochondria and chloroplasts, are derived from endosymbiotic bacteria. Their electron transport chains (ETCs) resemble those of free-living bacteria, but were tailored for energy transformation within the host cell. Parallel evolutionary processes in mitochondria and chloroplasts include reductive as well as expansive events: On one hand, bacterial complexes were lost in eukaryotes with a concomitant loss of metabolic flexibility. On the other hand, new subunits have been added to the remaining bacterial complexes, new complexes have been introduced, and elaborate folding patterns of the thylakoid and mitochondrial inner membranes have emerged. Some bacterial pathways were reinvented independently by eukaryotes, such as parallel routes for quinol oxidation or the use of various anaerobic electron acceptors. Multicellular organization and ontogenetic cycles in eukaryotes gave rise to further modifications of the bioenergetic organelles. Besides mitochondria and chloroplasts, eukaryotes have ETCs in other membranes, such as the plasma membrane (PM) redox system, or the cytochrome P450 (CYP) system. These systems have fewer complexes and simpler branching patterns than those in energy-transforming organelles, and they are often adapted to non-bioenergetic functions such as detoxification or cellular defense.

In vivo changes of the oxidation-reduction state of NADP and of the ATP/ADP cellular ratio linked to the photosynthetic activity in Chlamydomonas reinhardtii

The ATP/ADP and NADP/NADPH ratios have been measured in whole-cell extract of the green alga Chlamydomonas reinhardtii, to understand their availability for CO(2) assimilation by the Calvin cycle in vivo. Measurements were performed during the dark-light transition of both aerobic and anaerobic cells, under illumination with saturating or low light intensity. Two different patterns of behavior were observed: (a) In anaerobic cells, during the lag preceding O(2) evolution, ATP was synthesized without changes in the NADP/NADPH ratio, consistently with the operation of cyclic electron flow. (b) In aerobiosis, illumination increased the ATP/ADP ratio independently of the intensity used, whereas the amount of NADPH was decreased at limiting photon flux and regained the dark-adapted level under saturating photon flux. Moreover, under these conditions, the addition of low concentrations of uncouplers stimulated photosynthetic O(2) evolution. These observations suggest that the photosynthetic generation of reducing equivalents rather than the rate of ATP formation limits the photosynthetic assimilation of CO(2) in C. reinhardtii cells. This situation is peculiar to C. reinhardtii, because neither NADPH nor ATP limited this process in plant leaves, as shown by their increase upon illumination in barley (Hordeum vulgare) leaves, independent of light intensity. Experiments are presented and were designed to evaluate the contribution of different physiological processes that might increase the photosynthetic ATP/NADPH ratio-the Mehler reaction, respiratory ATP supply following the transfer of reducing equivalents via the malate/oxaloacetate shuttle, and cyclic electron flow around PSI-to this metabolic situation.

Energetics of protein transport across biological membranes. a study of the thylakoid DeltapH-dependent/cpTat pathway

Among the pathways for protein translocation across biological membranes, the DeltapH-dependent/Tat system is unusual in its sole reliance upon the transmembrane pH gradient to drive protein transport. The free energy cost of protein translocation via the chloro-plast DeltapH-dependent/Tat pathway was measured by conducting in vitro transport assays with isolated thylakoids while concurrently monitoring energetic parameters. These experiments revealed a substrate-specific energetic barrier to cpTat-mediated transport as well as direct utilization of protons from the gradient, consistent with a H+/protein antiporter mechanism. The magnitude of proton flux was assayed by four independent approaches and averaged 7.9 x 10(4) protons released from the gradient per transported protein. This corresponds to a DeltaG transport of 6.9 x 10(5) kJ.mol protein translocated(-1), representing the utilization of an energetic equivalent of 10(4) molecules of ATP. At this cost, we estimate that the DeltapH-dependent/cpTat pathway utilizes approximately 3% of the total energy output of the chloroplast.

Introduction of a carboxyl group in the loop of the F0 c-subunit affects the H+/ATP coupling ratio of the ATP synthase from Synechocystis 6803

The proton translocation stoichiometry (H+/ATP ratio) was investigated in membrane vesicles from a Synechocystis 6803 mutant in which the serine at position 37 in the hydrophilic loop of the c-subunit from the wild type was replaced by a negatively charged glutamic acid residue (strain plc37). At this position the c-subunit of chloroplasts and the cyanobacterium Synechococcus 6716 already contains glutamic acid. H+/ATP ratios were determined with active ATP synthase in thermodynamic equilibrium between phosphate potential (deltaGp) and the proton gradient (deltamuH+) induced by acid-base transition. The mutant displayed a significantly higher H+/ATP ratio than the control strain (wild type with kanamycin resistance) at pH 8 (4.3 vs. 3.3); the higher ratio also being observed in chloroplasts and Synechococcus 6716. Furthermore, the pH dependence of the H+/ATP of strain plc37 resembles that of Synechococcus 6716. When the pH was increased from 7.6 to 8.4, the H+/ATP of the mutant increased from 4.2 to 4.6 whereas in the control strain the ratio decreased from 3.8 to 2.8. Differences in H+/ATP between the mutant and the control strain were confirmed by measuring the light-induced phosphorylation efficiency (P/2e), which changed as expected, i.e., the P/2e ratio in the mutant was significantly less than that in the wild type. The need for more H+ ions used per ATP in the mutant was also reflected by the significantly lower growth rate of the mutant strain. The results are discussed against the background of the present structural and functional models of proton translocation coupled to catalytic activity of the ATP synthase.

pH-dependent Ca2+ binding to the F0 c-subunit affects proton translocation of the ATP synthase from Synechocystis 6803

It was shown before (Wooten, D. C., and Dilley, R. A. (1993) J. Bioenerg. Biomembr. 25, 557-567; Zakharov, S. D., Li, X., Red'ko, T. P., and Dilley, R. A. (1996) J. Bioenerg. Biomembr. 28, 483-493) that pH dependent reversible Ca2+ binding near the N- and C-terminal end of the 8 kDa subunit c modulates ATP synthesis driven by an applied pH jump in chloroplast and E. coli ATP synthase due to closing a "proton gate" proposed to exist in the F0 H+ channel of the F0F1 ATP synthase. This mechanism has further been investigated with the use of membrane vesicles from mutants of the cyanobacterium Synechocystis 6803. Vesicles from a mutant with serine at position 37 in the hydrophilic loop of the c-subunit replaced by the charged glutamic acid (strain plc 37) has a higher H+/ATP ratio than the wild type and therefore shows ATP synthesis at low values of deltamuH+. The presence of 1 mM CaCl2 during the preparation and storage of these vesicles blocked acid-base jump ATP formation when the pH of the acid side (inside) was between pH 5.6 and 7.1, even though the deltapH of the acid-base jump was thermodynamically in excess of the necessary energy to drive ATP formation at an external pH above 8.28. That is, in the absence of added CaCl2, ATP formation did occur under those conditions. However, when the base stage pH was 7.16 and the acid stage below pH 5.2, ATP was formed when Ca2+ was present. This is consistent with Ca2+ being displaced by H+ ions from the F0 on the inside of the thylakoid membrane at pH values below about 5.5. Vesicles from a mutant with the serine of position 3 replaced by a cysteine apparently already contain some bound Ca2+ to F0. Addition of 1 mM EGTA during preparation and storage of those vesicles shifted the otherwise already low internal pH needed for onset of ATP synthesis to higher values when the external pH was above 8. With both strains it was shown that the Ca2+ binding effect on acid-base induced ATP synthesis occurs above an internal pH of about 5.5. These results were corroborated by 45Ca2+-ligand blot assays on organic solvent soluble preparations containing the 8 kDa F0 subunit c from the S-3-C mutant ATP synthase, which showed 5Ca2+ binding as occurs with the pea chloroplast subunit III. The phosphorylation efficiency (P/2e), at strong light intensity, of Ca2+ and EGTA treated vesicles from both strains were almost equal showing that Ca2+ or EGTA have no other effect on the ATP synthase such as a change in the proton to ATP ratio. The results indicate that the Ca2+ binding to the F0 H+ channel can block H+ flux through the channel at pH values above about 5.5, but below that pH protons apparently displace the bound Ca2+, opening the CF0 H+ channel between the thylakoid lumen and H+ conductive channel.

In vivo modulation of nonphotochemical exciton quenching (NPQ) by regulation of the chloroplast ATP synthase

Nonphotochemical quenching (NPQ) of excitation energy, which protects higher plant photosynthetic machinery from photodamage, is triggered by acidification of the thylakoid lumen as a result of light-induced proton pumping, which also drives the synthesis of ATP. It is clear that the sensitivity of NPQ is modulated in response to changing physiological conditions, but the mechanism for this modulation has remained unclear. Evidence is presented that, in intact tobacco or Arabidopsis leaves, NPQ modulation in response to changing CO(2) levels occurs predominantly by alterations in the conductivity of the CF(O)-CF(1) ATP synthase to protons (g(H)(+)). At a given proton flux, decreasing g(H)(+) will increase transthylakoid proton motive force (pmf), thus lowering lumen pH and contributing to the activation of NPQ. It was found that an approximately 5-fold decrease in g(H)(+) could account for the majority of NPQ modulation as atmospheric CO(2) was decreased from 2,000 ppm to 0 ppm. Data are presented that g(H)(+) is kinetically controlled, rather than imposed thermodynamically by buildup of DeltaG(ATP). Further results suggest that the redox state of the ATP synthase gamma-subunit thiols is not responsible for altering g(H)(+). A working model is proposed wherein g(H)(+) is modulated by stromal metabolite levels, possibly by inorganic phosphate.

Indications for chlororespiration in relation to light regime in the marine diatom Thalassiosira weissflogii

The marine diatom Thalassiosira weissflogii was cultured under a light regime simulating the daily rise and fall of the sun. The light regime caused a daily cycle in non-photochemical quenching. Remarkable were the changes in fluorescence directly after a light-to-dark transition that occurred in addition to the changes induced by non-photochemical quenching. A transient non-photochemical reduction of PQ and of Q(A) was indicated by a transient increase in apparent F(o) and by changes in the shape of the fluorescence induction curve. The observed changes developed approximately the first 100-120 s after a light-to-dark transition and could be reversed by the application of far-red illumination. Chlororespiration is thought to cause the reduction of PQ and, as the PQ-pool is in equilibrium with Q(A), also a reduction of Q(A). The function and ecological relevance of chlororespiration are discussed.

Rotation of the c subunit oligomer in fully functional F1Fo ATP synthase

The F(1)F(o)-type ATP synthase is the smallest motor enzyme known. Previous studies had established that the central gamma and epsilon subunits of the F(1) part rotate relative to a stator of alpha(3)beta(3) and delta subunits during catalysis. We now show that the ring of c subunits in the F(o) part moves along with the gamma and epsilon subunits. This was demonstrated by linking the three rotor subunits with disulfide bridges between cysteine residues introduced genetically at the interfaces between the gamma, epsilon, and c subunits. Essentially complete cross-linking of the gamma, epsilon, and c subunits was achieved by using CuCl(2) to induce oxidation. This fixing of the three subunits together had no significant effect on ATP hydrolysis, proton translocation, or ATP synthesis, and each of these functions retained inhibitor sensitivity. These results unequivocally place the c subunit oligomer in the rotor part of this molecular machine.

Rotation of the c subunit oligomer in fully functional F1Fo ATP synthase

The F(1)F(o)-type ATP synthase is the smallest motor enzyme known. Previous studies had established that the central gamma and epsilon subunits of the F(1) part rotate relative to a stator of alpha(3)beta(3) and delta subunits during catalysis. We now show that the ring of c subunits in the F(o) part moves along with the gamma and epsilon subunits. This was demonstrated by linking the three rotor subunits with disulfide bridges between cysteine residues introduced genetically at the interfaces between the gamma, epsilon, and c subunits. Essentially complete cross-linking of the gamma, epsilon, and c subunits was achieved by using CuCl(2) to induce oxidation. This fixing of the three subunits together had no significant effect on ATP hydrolysis, proton translocation, or ATP synthesis, and each of these functions retained inhibitor sensitivity. These results unequivocally place the c subunit oligomer in the rotor part of this molecular machine.

Kinetic modeling of the photosynthetic electron transport chain

A simulation model of the photosynthetic electron transport chain operating under steady state conditions is presented. The model enables the calculation of (1) the rates of electron transport and transmembrane proton translocation, (2) the proton/electron stoichiometry, (3) the number of electrons stored in the different redox centers and (4) the stationary transmembrane pH difference. Light intensity and proton permeability of the thylakoid membrane are varied in order to compare the predictions of the model with experimental data. The routes of electron transport and proton translocation are simulated by two coupled arithmetic loops. The first one represents the sequence of reaction steps making up the linear electron transport chain and the Q-cycle. This loop yields the electron flow rate and the proton/electron ratio. The second loop balances the H+ fluxes and yields the internal H+ concentration. The bifurcation of the electron transport pathways at the stage of plastoquinol oxidation is obligatory. The first electron enters always the linear branch and is transferred to photosystem I. The electron of the remaining semiquinone can enter the Q-cycle or, alternatively, the semiquinone can be lost from the cytochrome b6f complex. The competition between these two reactions explains the experimentally observed variability of the proton/electron ratio. We also investigated additional model variants, where the variation of the proton/electron stoichiometry is attributed to other loss reactions within the cytochrome b6f complex. However, the semiquinone detachment seems to be the best candidate for a satisfactory description of the experimental data. Additional calculations were done in order to assess the effects of the movement of the Rieske protein on linear electron transport; it was found that this conformational change does not limit the electron transport rate, if it occurs with a time constant of at least 1000 s(-1).

The proton to electron stoichiometry of steady-state photosynthesis in living plants: A proton-pumping Q cycle is continuously engaged

A noninvasive technique is introduced with which relative proton to electron stoichiometries (H(+)/e(-) ratios) for photosynthetic electron transfer can be obtained from leaves of living plants under steady-state illumination. Both electron and proton transfer fluxes were estimated by a modification of our previously reported dark-interval relaxation kinetics (DIRK) analysis, in which processes that occur upon rapid shuttering of the actinic light are analyzed. Rates of turnover of linear electron transfer through the cytochrome (cyt) b(6)f complex were estimated by measuring the DIRK signals associated with reduction of cyt f and P(700). The rates of proton pumping through the electron transfer chain and the CF(O)-CF(1) ATP synthase (ATPase) were estimated by measuring the DIRK signals associated with the electrochromic shifting of pigments in the light-harvesting complexes. Electron transfer fluxes were also estimated by analysis of saturation pulse-induced changes in chlorophyll a fluorescence yield. It was shown that the H(+)/e(-) ratio, with respect to both cyt b(6)f complex and photosystem (PS) II turnover, was constant under low to saturating illumination in intact tobacco leaves. Because a H(+)/e(-) ratio of 3 at a low light is generally accepted, we infer that this ratio is maintained under conditions of normal (unstressed) photosynthesis, implying a continuously engaged, proton-pumping Q cycle at the cyt b(6)f complex.

Lipidic cubic phase crystallization of bacteriorhodopsin and cryotrapping of intermediates: towards resolving a revolving photocycle

Bacteriorhodopsin is a small retinal protein found in the membrane of the halophilic bacterium Halobacterium salinarum, whose function is to pump protons across the cell membrane against an electrostatic potential, thus converting light into a proton-motive potential needed for the synthesis of ATP. Because of its relative simplicity, exceptional stability and the fundamental importance of vectorial proton pumping, bacteriorhodopsin has become one of the most important model systems in the field of bioenergetics. Recently, a novel methodology to obtain well-diffracting crystals of membrane proteins, utilizing membrane-like bicontinuous lipidic cubic phases, has been introduced, providing X-ray structures of bacteriorhodopsin and its photocycle intermediates at ever higher resolution. We describe this methodology, the new insights provided by the higher resolution ground state structures, and review the mechanistic implications of the structural intermediates reported to date. A detailed understanding of the mechanism of vectorial proton transport across the membrane is thus emerging, helping to elucidate a number of fundamental issues in bioenergetics.