Energy thresholds for ATP synthesis in chloroplasts

Chloroplast ATP synthase: From structure to engineering

F-type ATP synthases are extensively researched protein complexes because of their widespread and central role in energy metabolism. Progress in structural biology, proteomics, and molecular biology has also greatly advanced our understanding of the catalytic mechanism, post-translational modifications, and biogenesis of chloroplast ATP synthases. Given their critical role in light-driven ATP generation, tailoring the activity of chloroplast ATP synthases and modeling approaches can be applied to modulate photosynthesis. In the future, advances in genetic manipulation and protein design tools will significantly expand the scope for testing new strategies in engineering light-driven nanomotors.

Evaluating the contribution of plant metabolic pathways in the light to the ATP:NADPH demand using a meta-analysis of isotopically non-stationary metabolic flux analyses

Balancing the ATP: NADPH demand from plant metabolism with supply from photosynthesis is essential for preventing photodamage and operating efficiently, so understanding its drivers is important for integrating metabolism with the light reactions of photosynthesis and for bioengineering efforts that may radically change this demand. It is often assumed that the C3 cycle and photorespiration consume the largest amount of ATP and reductant in illuminated leaves and as a result mostly determine the ATP: NADPH demand. However, the quantitative extent to which other energy consuming metabolic processes contribute in large ways to overall ATP: NADPH demand remains unknown. Here, we used the metabolic flux networks of numerous recently published isotopically non-stationary metabolic flux analyses (INST-MFA) to evaluate flux through the C3 cycle, photorespiration, the oxidative pentose phosphate pathway, the tricarboxylic acid cycle, and starch/sucrose synthesis and characterize broad trends in the demand of energy across different pathways and compartments as well as in the overall ATP:NADPH demand. These data sets include a variety of species including Arabidopsis thaliana, Nicotiana tabacum, and Camelina sativa as well as varying environmental factors including high/low light, day length, and photorespiratory levels. Examining these datasets in aggregate reveals that ultimately the bulk of the energy flux occurred in the C3 cycle and photorespiration, however, the energy demand from these pathways did not determine the ATP: NADPH demand alone. Instead, a notable contribution was revealed from starch and sucrose synthesis which might counterbalance photorespiratory demand and result in fewer adjustments in mechanisms which balance the ATP deficit.

Chloroplast ion homeostasis - what do we know and where should we go?

Plant yields heavily depend on proper macro- and micronutrient supply from the soil. In the leaf cells, nutrient ions fulfill specific roles in biochemical reactions, especially photosynthesis housed in the chloroplast. Here, a well-balanced ion homeostasis is maintained by a number of ion transport proteins embedded in the envelope and thylakoid membranes. Ten years ago, the first alkali metal transporters from the K+ EFFLUX ANTIPORTER family were discovered in the model plant Arabidopsis. Since then, our knowledge about the physiological importance of these carriers and their substrates has greatly expanded. New insights into the role of alkali ions in plastid gene expression and photoprotective mechanisms, both prerequisites for plant productivity in natural environments, were gained. The discovery of a Cl- channel in the thylakoid and several additional plastid alkali and alkali metal transport proteins have advanced the field further. Nevertheless, scientists still have long ways to go before a complete systemic understanding of the chloroplast's ion transportome will emerge. In this Tansley review, we highlight and discuss the achievements of the last decade. More importantly, we make recommendations on what areas to prioritize, so the field can reach the next milestones. One area, laid bare by our similarity-based comparisons among phototrophs is our lack of knowledge what ion transporters are used by cyanobacteria to buffer photosynthesis fluctuations.

Electrometric and Electron Paramagnetic Resonance Measurements of a Difference in the Transmembrane Electrochemical Potential: Photosynthetic Subcellular Structures and Isolated Pigment-Protein Complexes

A transmembrane difference in the electrochemical potentials of protons (ΔμH+) serves as a free energy intermediate in energy-transducing organelles of the living cell. The contributions of two components of the ΔμH+ (electrical, Δψ, and concentrational, ΔpH) to the overall ΔμH+ value depend on the nature and lipid composition of the energy-coupling membrane. In this review, we briefly consider several of the most common instrumental (electrometric and EPR) methods for numerical estimations of Δψ and ΔpH. In particular, the kinetics of the flash-induced electrometrical measurements of Δψ in bacterial chromatophores, isolated bacterial reaction centers, and Photosystems I and II of the oxygenic photosynthesis, as well as the use of pH-sensitive molecular indicators and kinetic data regarding pH-dependent electron transport in chloroplasts, have been reviewed. Further perspectives on the application of these methods to solve some fundamental and practical problems of membrane bioenergetics are discussed.

Dynamic response of photorespiration in fluctuating light environments

Photorespiration is a dynamic process that is intimately linked to photosynthetic carbon assimilation. There is a growing interest in understanding carbon assimilation during dynamic conditions, but the role of photorespiration under such conditions is unclear. In this review, we discuss recent work relevant to the function of photorespiration under dynamic conditions, with a special focus on light transients. This work reveals that photorespiration is a fundamental component of the light induction of assimilation where variable diffusive processes limit CO2 exchange with the atmosphere. Additionally, metabolic interactions between photorespiration and the C3 cycle may help balance fluxes under dynamic light conditions. We further discuss how the energy demands of photorespiration present special challenges to energy balancing during dynamic conditions. We finish the review with an overview of why regulation of photorespiration may be important under dynamic conditions to maintain appropriate fluxes through metabolic pathways related to photorespiration such as nitrogen and one-carbon metabolism.

ATP synthesis in an ancient ATP synthase at low driving forces

SignificanceThe ATP synthases of many anaerobic archaea have an unusual motor subunit c that otherwise is only found in eukaryotic V₁V_O ATPases. The evolutionary switch from synthase to hydrolase is thought to be caused by a doubling of the rotor subunit c, followed by a loss of the ion binding site. By purification and reconstitution of an ATP synthase with a V-type c subunit, we have unequivocally demonstrated, against expectations, the capability of such an enzyme to synthesize ATP at physiological relevant driving forces of 90 to 150 mV. This is the long-awaited answer to an eminent question in microbial energetics and physiology, especially for life near the thermodynamic limit of ATP synthesis.

Chloroplast pH Homeostasis for the Regulation of Photosynthesis

The pH of various chloroplast compartments, such as the thylakoid lumen and stroma, is light-dependent. Light illumination induces electron transfer in the photosynthetic apparatus, coupled with proton translocation across the thylakoid membranes, resulting in acidification and alkalization of the thylakoid lumen and stroma, respectively. Luminal acidification is crucial for inducing regulatory mechanisms that protect photosystems against photodamage caused by the overproduction of reactive oxygen species (ROS). Stromal alkalization activates enzymes involved in the Calvin-Benson-Bassham (CBB) cycle. Moreover, proton translocation across the thylakoid membranes generates a proton gradient (ΔpH) and an electric potential (ΔΨ), both of which comprise the proton motive force (pmf) that drives ATP synthase. Then, the synthesized ATP is consumed in the CBB cycle and other chloroplast metabolic pathways. In the dark, the pH of both the chloroplast stroma and thylakoid lumen becomes neutral. Despite extensive studies of the above-mentioned processes, the molecular mechanisms of how chloroplast pH can be maintained at proper levels during the light phase for efficient activation of photosynthesis and other metabolic pathways and return to neutral levels during the dark phase remain largely unclear, especially in terms of the precise control of stromal pH. The transient increase and decrease in chloroplast pH upon dark-to-light and light-to-dark transitions have been considered as signals for controlling other biological processes in plant cells. Forward and reverse genetic screening approaches recently identified new plastid proteins involved in controlling ΔpH and ΔΨ across the thylakoid membranes and chloroplast proton/ion homeostasis. These proteins have been conserved during the evolution of oxygenic phototrophs and include putative photosynthetic protein complexes, proton transporters, and/or their regulators. Herein, we summarize the recently identified protein players that control chloroplast pH and influence photosynthetic efficiency in plants.

Proton motive force in plant photosynthesis dominated by ΔpH in both low and high light

The proton motive force (pmf) across the thylakoid membrane couples photosynthetic electron transport and ATP synthesis. In recent years, the electrochromic carotenoid and chlorophyll absorption band shift (ECS), peaking ∼515 nm, has become a widely used probe to measure pmf in leaves. However, the use of this technique to calculate the parsing of the pmf between the proton gradient (ΔpH) and electric potential (Δψ) components remains controversial. Interpretation of the ECS signal is complicated by overlapping absorption changes associated with violaxanthin de-epoxidation to zeaxanthin (ΔA505) and energy-dependent nonphotochemical quenching (qE; ΔA535). In this study, we used Arabidopsis (Arabidopsis thaliana) plants with altered xanthophyll cycle activity and photosystem II subunit S (PsbS) content to disentangle these overlapping contributions. In plants where overlap among ΔA505, ΔA535, and ECS is diminished, such as npq4 (lacking ΔA535) and npq1npq4 (also lacking ΔA505), the parsing method implies the Δψ contribution is virtually absent and pmf is solely composed of ΔpH. Conversely, in plants where ΔA535 and ECS overlap is enhanced, such as L17 (a PsbS overexpressor) and npq1 (where ΔA535 is blue-shifted to 525 nm) the parsing method implies a dominant contribution of Δψ to the total pmf. These results demonstrate the vast majority of the pmf attributed by the ECS parsing method to Δψ is caused by ΔA505 and ΔA535 overlap, confirming pmf is dominated by ΔpH following the first 60 s of continuous illumination under both low and high light conditions. Further implications of these findings for the regulation of photosynthesis are discussed.

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.

Flexibility in the Energy Balancing Network of Photosynthesis Enables Safe Operation under Changing Environmental Conditions

Given their ability to harness chemical energy from the sun and generate the organic compounds necessary for life, photosynthetic organisms have the unique capacity to act simultaneously as their own power and manufacturing plant. This dual capacity presents many unique challenges, chiefly that energy supply must be perfectly balanced with energy demand to prevent photodamage and allow for optimal growth. From this perspective, we discuss the energy balancing network using recent studies and a quantitative framework for calculating metabolic ATP and NAD(P)H demand using measured leaf gas exchange and assumptions of metabolic demand. We focus on exploring how the energy balancing network itself is structured to allow safe and flexible energy supply. We discuss when the energy balancing network appears to operate optimally and when it favors high capacity instead. We also present the hypothesis that the energy balancing network itself can adapt over longer time scales to a given metabolic demand and how metabolism itself may participate in this energy balancing.

Optimization of ATP Synthase c-Rings for Oxygenic Photosynthesis

The conversion of sunlight into useable cellular energy occurs via the proton-coupled electron transfer reactions of photosynthesis. Light is absorbed by photosynthetic pigments and transferred to photochemical reaction centers to initiate electron and proton transfer reactions to store energy in a redox gradient and an electrochemical proton gradient (proton motive force, pmf), composed of a concentration gradient (ΔpH) and an electric field (Δψ), which drives the synthesis of ATP through the thylakoid FoF1-ATP synthase. Although ATP synthase structure and function are conserved across biological kingdoms, the number of membrane-embedded ion-binding c subunits varies between organisms, ranging from 8 to 17, theoretically altering the H+/ATP ratio for different ATP synthase complexes, with profound implications for the bioenergetic processes of cellular metabolism. Of the known c-ring stoichiometries, photosynthetic c-rings are among the largest identified stoichiometries, and it has been proposed that decreasing the c-stoichiometry could increase the energy conversion efficiency of photosynthesis. Indeed, there is strong evidence that the high H+/ATP of the chloroplast ATP synthase results in a low ATP/nicotinamide adenine dinucleotide phosphate (NADPH) ratio produced by photosynthetic linear electron flow, requiring secondary processes such as cyclic electron flow to support downstream metabolism. We hypothesize that the larger c subunit stoichiometry observed in photosynthetic ATP synthases was selected for because it allows the thylakoid to maintain pmf in a range where ATP synthesis is supported, but avoids excess Δψ and ΔpH, both of which can lead to production of reactive oxygen species and subsequent photodamage. Numerical kinetic simulations of the energetics of chloroplast photosynthetic reactions with altered c-ring size predicts the energy storage of pmf and its effects on the photochemical reaction centers strongly support this hypothesis, suggesting that, despite the low efficiency and suboptimal ATP/NADPH ratio, a high H+/ATP is favored to avoid photodamage. This has important implications for the evolution and regulation of photosynthesis as well as for synthetic biology efforts to alter photosynthetic efficiency by engineering the ATP synthase.

Far-Red Light Accelerates Photosynthesis in the Low-Light Phases of Fluctuating Light

It is well known that far-red light (FR; >700 nm) drives PSI photochemistry, but its effect on photosynthetic performance has received little attention. In this study, the effects of the addition of FR to red fluctuating light (FL) have on photosynthesis were examined in the leaves of Arabidopsis thaliana. Light-activated leaves were illuminated with FL [alternating high light/low light (HL/LL) at 800/30 μmol m-2 s-1] for 10-15 min without or with FR at intensities that reflected natural conditions. The CO2 assimilation rates upon the transition from HL to LL were significantly greater with FR than without FR. The enhancement of photosynthesis by FR was small under the steady-state conditions and in the HL phases of FL. Proton conductivity through the thylakoid membrane (gH+) in the LL phases of FL, estimated from the dark relaxation kinetics of the electrochromic absorbance shift, was greater with FR than without FR. The relaxation of non-photochemical quenching (NPQ) in the PSII antenna system and the increase in PSII photochemistry in the LL phases accelerated in the presence of FR. Similar FR-effects in FL were confirmed in typical sun and shade plants. On the basis of these results, we concluded that FR exerted beneficial effects on photosynthesis in FL by exciting PSI and accelerating NPQ relaxation and PSII-yield increase. This was probably because of the increased gH+, which would reflect faster ΔpH dissipation and ATP synthesis.

Regulation of cyclic electron flow by chloroplast NADPH-dependent thioredoxin system

Linear electron transport in the thylakoid membrane drives photosynthetic NADPH and ATP production, while cyclic electron flow (CEF) around photosystem I only promotes the translocation of protons from stroma to thylakoid lumen. The chloroplast NADH dehydrogenase-like complex (NDH) participates in one CEF route transferring electrons from ferredoxin back to the plastoquinone pool with concomitant proton pumping to the lumen. CEF has been proposed to balance the ratio of ATP/NADPH production and to control the redox poise particularly in fluctuating light conditions, but the mechanisms regulating the NDH complex remain unknown. We have investigated potential regulation of the CEF pathways by the chloroplast NADPH-thioredoxin reductase (NTRC) in vivo by using an Arabidopsis knockout line of NTRC as well as lines overexpressing NTRC. Here, we present biochemical and biophysical evidence showing that NTRC stimulates the activity of NDH-dependent CEF and is involved in the regulation of generation of proton motive force, thylakoid conductivity to protons, and redox balance between the thylakoid electron transfer chain and the stroma during changes in light conditions. Furthermore, protein-protein interaction assays suggest a putative thioredoxin-target site in close proximity to the ferredoxin-binding domain of NDH, thus providing a plausible mechanism for redox regulation of the NDH ferredoxin:plastoquinone oxidoreductase activity.

Hacking the thylakoid proton motive force for improved photosynthesis: modulating ion flux rates that control proton motive force partitioning into Δψ and ΔpH

There is considerable interest in improving plant productivity by altering the dynamic responses of photosynthesis in tune with natural conditions. This is exemplified by the 'energy-dependent' form of non-photochemical quenching (q_E), the formation and decay of which can be considerably slower than natural light fluctuations, limiting photochemical yield. In addition, we recently reported that rapidly fluctuating light can produce field recombination-induced photodamage (FRIP), where large spikes in electric field across the thylakoid membrane (Δψ) induce photosystem II recombination reactions that produce damaging singlet oxygen (¹O₂). Both q_E and FRIP are directly linked to the thylakoid proton motive force (pmf), and in particular, the slow kinetics of partitioning pmf into its ΔpH and Δψ components. Using a series of computational simulations, we explored the possibility of 'hacking' pmf partitioning as a target for improving photosynthesis. Under a range of illumination conditions, increasing the rate of counter-ion fluxes across the thylakoid membrane should lead to more rapid dissipation of Δψ and formation of ΔpH. This would result in increased rates for the formation and decay of q_E while resulting in a more rapid decline in the amplitudes of Δψ-spikes and decreasing ¹O₂ production. These results suggest that ion fluxes may be a viable target for plant breeding or engineering. However, these changes also induce transient, but substantial mismatches in the ATP : NADPH output ratio as well as in the osmotic balance between the lumen and stroma, either of which may explain why evolution has not already accelerated thylakoid ion fluxes. Overall, though the model is simplified, it recapitulates many of the responses seen in vivo, while spotlighting critical aspects of the complex interactions between pmf components and photosynthetic processes. By making the programme available, we hope to enable the community of photosynthesis researchers to further explore and test specific hypotheses.This article is part of the themed issue 'Enhancing photosynthesis in crop plants: targets for improvement'.

Limitations to photosynthesis by proton motive force-induced photosystem II photodamage

The thylakoid proton motive force (pmf) generated during photosynthesis is the essential driving force for ATP production; it is also a central regulator of light capture and electron transfer. We investigated the effects of elevated pmf on photosynthesis in a library of Arabidopsis thaliana mutants with altered rates of thylakoid lumen proton efflux, leading to a range of steady-state pmf extents. We observed the expected pmf-dependent alterations in photosynthetic regulation, but also strong effects on the rate of photosystem II (PSII) photodamage. Detailed analyses indicate this effect is related to an elevated electric field (Δψ) component of the pmf, rather than lumen acidification, which in vivo increased PSII charge recombination rates, producing singlet oxygen and subsequent photodamage. The effects are seen even in wild type plants, especially under fluctuating illumination, suggesting that Δψ-induced photodamage represents a previously unrecognized limiting factor for plant productivity under dynamic environmental conditions seen in the field.

Limitations to photosynthesis by proton motive force-induced photosystem II photodamage

The thylakoid proton motive force (pmf) generated during photosynthesis is the essential driving force for ATP production; it is also a central regulator of light capture and electron transfer. We investigated the effects of elevated pmf on photosynthesis in a library of Arabidopsis thaliana mutants with altered rates of thylakoid lumen proton efflux, leading to a range of steady-state pmf extents. We observed the expected pmf-dependent alterations in photosynthetic regulation, but also strong effects on the rate of photosystem II (PSII) photodamage. Detailed analyses indicate this effect is related to an elevated electric field (Δψ) component of the pmf, rather than lumen acidification, which in vivo increased PSII charge recombination rates, producing singlet oxygen and subsequent photodamage. The effects are seen even in wild type plants, especially under fluctuating illumination, suggesting that Δψ-induced photodamage represents a previously unrecognized limiting factor for plant productivity under dynamic environmental conditions seen in the field.

DOI:http://dx.doi.org/10.7554/eLife.16921.001

pH-dependent regulation of electron transport and ATP synthesis in chloroplasts

This review is focused on pH-dependent mechanisms of regulation of photosynthetic electron transport and ATP synthesis in chloroplasts. The light-induced acidification of the thylakoid lumen is known to decelerate the plastoquinol oxidation by the cytochrome b 6 f complex, thus impeding the electron flow between photosystem II and photosystem I. Acidification of the lumen also triggers the dissipation of excess energy in the light-harvesting antenna of photosystem II, thereby protecting the photosynthetic apparatus against a solar stress. After brief description of structural and functional organization of the chloroplast electron transport chain, our attention is focused on the nature of the rate-limiting step of electron transfer between photosystem II and photosystem I. In the context of pH-dependent mechanism of photosynthetic control in chloroplasts, the mechanisms of plastoquinol oxidation by the cytochrome b 6 f complex have been considered. The light-induced alkalization of stroma is another factor of pH-dependent regulation of electron transport in chloroplasts. Alkalization of stroma induces activation of the Bassham-Benson-Calvin cycle reactions, thereby promoting efflux of electrons from photosystem I to NADP(+). The mechanisms of the light-induced activation of ATP synthase are briefly considered.

Kinetic equivalence of transmembrane pH and electrical potential differences in ATP synthesis

ATP synthase is the key player of Mitchell's chemiosmotic theory, converting the energy of transmembrane proton flow into the high energy bond between ADP and phosphate. The proton motive force that drives this reaction consists of two components, the pH difference (ΔpH) across the membrane and transmembrane electrical potential (Δψ). The two are considered thermodynamically equivalent, but kinetic equivalence in the actual ATP synthesis is not warranted, and previous experimental results vary. Here, we show that with the thermophilic Bacillus PS3 ATP synthase that lacks an inhibitory domain of the ε subunit, ΔpH imposed by acid-base transition and Δψ produced by valinomycin-mediated K(+) diffusion potential contribute equally to the rate of ATP synthesis within the experimental range examined (ΔpH -0.3 to 2.2, Δψ -30 to 140 mV, pH around the catalytic domain 8.0). Either ΔpH or Δψ alone can drive synthesis, even when the other slightly opposes. Δψ was estimated from the Nernst equation, which appeared valid down to 1 mm K(+) inside the proteoliposomes, due to careful removal of K(+) from the lipid.

Measurement of chloroplast ATP synthesis activity in Arabidopsis

There are numerous options for monitoring ATP synthesis in chloroplasts using isolated thylakoid membranes, intact chloroplasts, and even whole leaves. Currently, the most commonly used method employs isolated thylakoids coupling the synthesis of ATP to light emission from luciferin in a reaction catalyzed by luciferase. The luciferin-luciferase assay can be highly sensitive and is a direct measure of ATP. Another direct measurement of ATP is the incorporation of 32P into ATP, which, while more technically difficult, has the advantage over the luciferin-luciferase assay of being able to distinguish newly synthesized from total ATP. The phosphorylation of ADP results in a net decrease in pKa (acid disassociation constant) between the reactants and the product ATP, resulting in an increase in the pH of the assay media, which can be used as a convenient, continuous measurement of ATP synthesis. The formation of ΔμH+ across the thylakoid membrane and its concomitant dissipation as ATP is synthesized can be measured by an electrochromic absorption band shift (ECS) of thylakoid pigments measured at 518 nm (Witt, Biochim. Biophys. Acta 505:355-427, 1979; Petty and Jackson, Biochim. Biophys. Acta: Bioenergetics 547:463-473, 1979). The first-order decay time of the ESC can be used to estimate the rate of ATP synthesis providing a noninvasive, indirect method for measuring ATP synthase activity that can be used with intact leaves.

Functional and topological aspects of pH-dependent regulation of electron and proton transport in chloroplasts in silico

In this work, we summarize results of computer simulation of electron and proton transport processes coupled to ATP synthesis in chloroplasts performed within the frames of a mathematical model developed as a system of differential equations for concentrations of electron carriers and hydrogen ion inside and outside the granal and stromal thylakoids. The model takes into account topological peculiarities and lateral heterogeneity of the chloroplast lamellar system. This allowed us to analyze the influence of restricted diffusion of protons inside small compartments of a chloroplast (e.g., in the narrow inter-thylakoid gap) on electron transport processes. The model adequately describes two modes of pH-dependent feedback control of electron transport associated with: (i) the acidification of the thylakoid lumen, which causes the slowing down of plastoquinol oxidation and stimulates an increase in dissipation of excess energy in PS2, and (ii) the alkalization of stroma, inducing the activation of the BBC (Bassham-Benson-Calvin) cycle and intensified consumption of ATP and NADPH. The influence of ATP on electron transport is mediated by modulation of the thylakoid membrane conductivity to protons through the ATP synthase complexes. We also analyze the contribution of alternative electron transport pathways to the maintenance of optimal balance between the energy donating and energy consuming stages of the light-induced photosynthetic processes.

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.

Spin-probes designed for measuring the intrathylakoid pH in chloroplasts

Nitroxide radicals are widely used as molecular probes in different fields of chemistry and biology. In this work, we describe pH-sensitive imidazoline- and imidazolidine-based nitroxides with pK values in the range 4.7-7.6 (2,2,3,4,5,5-hexamethylperhydroimidazol-1-oxyl, 4-amino-2,2,5,5-tetramethyl-2,5-dihydro-1H-imidazol-1-oxyl, 4-dimethylamino-2,2-diethyl-5,5-dimethyl-2,5-dihydro-1H-imidazol-1-oxyl, and 2,2-diethyl-5,5-dimethyl-4-pyrrolidyline-1-yl-2,5-dihydro-1H-imidazol-1-oxyl), which allow the pH-monitoring inside chloroplasts. We have demonstrated that EPR spectra of these spin-probes localized in the thylakoid lumen markedly change with the light-induced acidification of the thylakoid lumen in chloroplasts. Comparing EPR spectrum parameters of intrathylakoid spin-probes with relevant calibrating curves, we could estimate steady-state values of lumen pHin established during illumination of chloroplasts with continuous light. For isolated bean (Vicia faba) chloroplasts suspended in a medium with pHout=7.8, we found that pHin approximately 5.4-5.7 in the state of photosynthetic control, and pHin approximately 5.7-6.0 under photophosphorylation conditions. Thus, ATP synthesis occurs at a moderate acidification of the thylakoid lumen, corresponding to transthylakoid pH difference DeltapH approximately 1.8-2.1. These values of DeltapH are consistent with a point of view that under steady-state conditions the proton gradient DeltapH is the main contributor to the proton motive force driving the operation of ATP synthesis, provided that stoichiometric ratio H+/ATP is n> or =4-4.7.

The thylakoid proton motive force in vivo. Quantitative, non-invasive probes, energetics, and regulatory consequences of light-induced pmf

Endogenous probes of light-induced transthylakoid proton motive force (pmf), membrane potential (Deltapsi) and DeltapH were used in vivo to assess in Arabidopsis the lumen pH responses of regulatory components of photosynthesis. The accumulation of zeaxanthin and protonation of PsbS were found to have similar pK(a) values, but quite distinct Hill coefficients, a feature allowing high antenna efficiency at low pmf and fine adjustment at higher pmf. The onset of "energy-dependent' exciton quenching (q(E)) occurred at higher lumen pH than slowing of plastoquinol oxidation at the cytochrome b(6)f complex, presumably to prevent buildup of reduced electron carriers that can lead to photodamage. Quantitative comparison of intrinsic probes with the electrochromic shift signal in situ allowed quantitative estimates of pmf and lumen pH. Within a degree of uncertainly of approximately 0.5 pH units, the lumen pH was estimated to range from approximately 7.5 (under weak light at ambient CO(2)) to approximately 5.7 (under 50 ppm CO(2) and saturating light), consistent with a 'moderate pH' model, allowing antenna regulation but preventing acid-induced photodamage. The apparent pK(a) values for accumulation of zeaxanthin and PsbS protonation were found to be approximately 6.8, with Hill coefficients of about 4 and 1 respectively. The apparent shift between in vitro violaxanthin deepoxidase protonation and zeaxanthin accumulation in vivo is explained by steady-state competition between zeaxanthin formation and its subsequent epoxidation by zeaxanthin epoxidase. In contrast to tobacco, Arabidopsis showed substantial variations in the fraction of pmf (0.1-0.7) stored as Deltapsi, allowing a more sensitive qE response, possible as an adaptation to life at lower light levels.

Storage of light-driven transthylakoid proton motive force as an electric field (Deltapsi) under steady-state conditions in intact cells of Chlamydomonas reinhardtii

Proton motive force (pmf) is physiologically stored as either a DeltapH or a membrane potential (Deltapsi) across bacterial and mitochondrial energetic membranes. In the case of chloroplasts, previous work (Cruz et al. 2001, Biochemistry 40: 1226-1237) indicates that Deltapsi is a significant fraction of pmf, in vivo, and in vitro as long as the activities of counterions are relatively low. Kinetic analysis of light-induced changes in the electrochromic shift (ECS) in intact leaves was consistent with these observations. In this work, we took advantage of the spectroscopic properties of the green alga, Chlamydomonas reinhardtii, to demonstrate that light-driven Deltapsi was stored in vivo over the hours time scale. Analysis of the light-induced ECS kinetics suggested that the steady-state Deltapsi in 400 micromol photons m(-2) s(-1) red light was between 20 and 90 mV and that this represented about 60% of the light-induced increase in pmf. By extrapolation, it was surmised that about half of total (basal and light-induced) pmf is held as Deltapsi. It is hypothesized that Deltapsi is stabilized either by maintaining low chloroplast ionic strength or by active membrane ion transporters. In addition to the strong implications for regulation of photosynthesis by the xanthophyll cycle, these results imply that pmf partitioning is important across a wide range of species.

Balancing the central roles of the thylakoid proton gradient

The photosynthetic electron transfer chain generates proton motive force (pmf), composed of both electric field (Deltapsi) and concentration (DeltapH) gradients. Both components can drive ATP synthesis, whereas the DeltapH component alone can trigger feedback regulation of the antenna. It has often been suggested that a relatively large pmf is needed to sustain the energetic contributions of the ATP synthase reaction (DeltaG(ATP)), and that the Deltapsi component is dissipated during illumination, leading to an acidic lumen in the light. We suggest that this is incompatible with the stabilities of lumenal components and the observed activation of downregulation. Recent work on the chloroplast ATP synthase suggests that a more moderate pmf can sustain DeltaG(ATP). In addition, in vivo probes suggest that a substantial fraction of pmf can be stored as Deltapsi. Together, these factors should allow sufficient DeltaG(ATP) to maintain lumen pH in a range where lumenal enzyme activities are nearly optimal, and where the level of NPQ is regulated.

Increased sensitivity of photosynthesis to antimycin A induced by inactivation of the chloroplast ndhB gene. Evidence for a participation of the NADH-dehydrogenase complex to cyclic electron flow around photosystem I

Tobacco (Nicotiana tabacum var Petit Havana) ndhB-inactivated mutants (ndhB-) obtained by plastid transformation (E.M. Horvath, S.O. Peter, T. Joët, D. Rumeau, L. Cournac, G.V. Horvath, T.A. Kavanagh, C. Schäfer, G. Peltier, P. MedgyesyHorvath [2000] Plant Physiol 123: 1337-1350) were used to study the role of the NADH-dehydrogenase complex (NDH) during photosynthesis and particularly the involvement of this complex in cyclic electron flow around photosystem I (PSI). Photosynthetic activity was determined on leaf discs by measuring CO2 exchange and chlorophyll fluorescence quenchings during a dark-to-light transition. In the absence of treatment, both non-photochemical and photochemical fluorescence quenchings were similar in ndhB- and wild type (WT). When leaf discs were treated with 5 microM antimycin A, an inhibitor of cyclic electron flow around PSI, both quenchings were strongly affected. At steady state, maximum photosynthetic electron transport activity was inhibited by 20% in WT and by 50% in ndhB-. Under non-photorespiratory conditions (2% O2, 2,500 microL x L(-1) CO2), antimycin A had no effect on photosynthetic activity of WT, whereas a 30% inhibition was observed both on quantum yield of photosynthesis assayed by chlorophyll fluorescence and on CO2 assimilation in ndhB-. The effect of antimycin A on ndhB- could not be mimicked by myxothiazol, an inhibitor of the mitochondrial cytochrome bc1 complex, therefore showing that it is not related to an inhibition of the mitochondrial electron transport chain but rather to an inhibition of cyclic electron flow around PSI. We conclude to the existence of two different pathways of cyclic electron flow operating around PSI in higher plant chloroplasts. One of these pathways, sensitive to antimycin A, probably involves ferredoxin plastoquinone reductase, whereas the other involves the NDH complex. The absence of visible phenotype in ndhB- plants under normal conditions is explained by the complement of these two pathways in the supply of extra-ATP for photosynthesis.

Identification of a Ca2+/H+ antiport in the plant chloroplast thylakoid membrane

To assess the availability of Ca2+ in the lumen of the thylakoid membrane that is required to support the assembly of the oxygen-evolving complex of photosystem II, we have investigated the mechanism of 45Ca2+ transport into the lumen of pea (Pisum sativum) thylakoid membranes using silicone-oil centrifugation. Trans-thylakoid Ca2+ transport is dependent on light or, in the dark, on exogenously added ATP. Both light and ATP hydrolysis are coupled to Ca2+ transport through the formation of a transthylakoid pH gradient. The H+-transporting ionophores nigericin/K+ and carbonyl cyanide 3-chlorophenylhydrazone inhibit the transport of Ca2+. Thylakoid membranes are capable of accumulating up to 30 nmol Ca2+ mg-1 chlorophyll from external concentrations of 15 μM over the course of a 15-min reaction. These results are consistent with the presence of an active Ca2+/H+ antiport in the thylakoid membrane. Ca2+ transport across the thylakoid membrane has significant implications for chloroplast and plant Ca2+ homeostasis. We propose a model of chloroplast Ca2+ regulation whereby the activity of the Ca2+/H+ antiporter facilitates the light-dependent uptake of Ca2+ by chloroplasts and reduces stromal Ca2+ levels.

Light-driven production of ATP catalysed by F0F1-ATP synthase in an artificial photosynthetic membrane

Energy-transducing membranes of living organisms couple spontaneous to non-spontaneous processes through the intermediacy of protonmotive force (p.m.f.)--an imbalance in electrochemical potential of protons across the membrane. In most organisms, p.m.f. is generated by redox reactions that are either photochemically driven, such as those in photosynthetic reaction centres, or intrinsically spontaneous, such as those of oxidative phosphorylation in mitochondria. Transmembrane proteins (such as the cytochromes and complexes I, III and IV in the electron-transport chain in the inner mitochondrial membrane) couple the redox reactions to proton translocation, thereby conserving a fraction of the redox chemical potential as p.m.f. Many transducer proteins couple p.m.f. to the performance of biochemical work, such as biochemical synthesis and mechanical and transport processes. Recently, an artificial photosynthetic membrane was reported in which a photocyclic process was used to transport protons across a liposomal membrane, resulting in acidification of the liposome's internal volume. If significant p.m.f. is generated in this system, then incorporating an appropriate transducer into the liposomal bilayer should make it possible to drive a non-spontaneous chemical process. Here we report the incorporation of F0F1-ATP synthase into liposomes containing the components of the proton-pumping photocycle. Irradiation of this artificial membrane with visible light results in the uncoupler- and inhibitor-sensitive synthesis of adenosine triphosphate (ATP) against an ATP chemical potential of approximately 12 kcal mol(-1), with a quantum yield of more than 7%. This system mimics the process by which photosynthetic bacteria convert light energy into ATP chemical potential.

The H+/ATP coupling ratio of the ATP synthase from thiol-modulated chloroplasts and two cyanobacterial strains is four

In this paper the authors emphasise that the proton translocating ATP synthase from thiol-modulated chloroplasts and two cyanobacterial strains has a coupling ratio of 4 protons per ATP synthesised or hydrolysed. This ratio is determined by several thermodynamic studies at equilibrium between phosphate potential (Delta Gp) and proton gradient (Delta(mu)H+), and is confirmed by measurement of proton flux during ATP hydrolysis. Ratios lower than 4 H+/ATP that have been published in the past have predominantly been determined with the oxidised chloroplast enzyme. Errors in these measurements will be discussed.

Detecting localized proton currents in photophosphorylation by procaine inhibition of the transthylakoid pH-gradient

The relationship between the transthylakoid pH-gradient, delta pH, and the velocity of photophosphorylation, Vp, in thylakoid membranes from spinach was investigated using the local anesthetic amine procaine as inhibitor of delta pH. When delta pH was driven by Photosystem (PS) II+I-dependent electron flow, passing through the cytochrome b6/f complex, inhibition by procaine was accompanied by an increase of ATP formation. It appeared that procaine allowed for values of Vp similar to those in controls (without procaine) at a significantly lower delta pH than in the controls. In contrast, when delta pH was driven by cyclic electron flow around PS I or by PS-II+I-dependent electron flow via a bypass around the cytochrome b6/f complex, or by PS II alone, procaine simultaneously caused an inhibition of delta pH and a decrease of ATP formation. Inhibition of delta pH by procaine did not induce an electrical membrane potential gradient that otherwise may have energetically compensated for the observed decline of delta pH. The electron flow capacity was unaffected by procaine. However, inhibition of delta pH did not significantly relax pH-dependent control of electron flux. Procaine accelerated ATP hydrolysis by pre-activated thylakoid ATPase to rates which were observed in the presence of uncouplers and had no direct effect on the activation state of the ATPase. The shift in the relationship between delta pH and Vp towards lower delta pH persisted in thermodynamic equilibrium between the phosphorylation potential and delta pH. The data indicated that the unconventional effect of procaine on photophosphorylation may be related to effects on proton translocation at the cytochrome b6/f complex and that a localized protonic coupling may occur between cytochrome b6/f and thylakoid-ATP-synthase complexes.

Evidence that localized energy coupling in thylakoids can continue beyond the energetic threshold onset into steady illumination

Energy transduction from proton gradients into ATP formation in chloroplast thylakoids has been hypothesized to be driven equally efficiently by localized domain delta mu H+ or by a delocalized delta mu H+ (Beard, W. A. and Dilley, R. A. (1988) J. Bioenerg. Biomembr. 20, 129-154). An important question is whether the apparent localized protonmotive force energy coupling mode can be observed only in the dark-to-light transient in the flash excitation protocol commonly used, or whether the localized energy coupling gradient can be maintained under conditions of continuous illumination ATP formation. The assay in the previous work was to use permeable amines, added to thylakoids in the dark, and observe the effect of the amine on the length of the energization lag (number of single-turnover flashes) required to initiate ATP formation in the dark-to-light transition. Amine buffers delayed the ATP onset in high-salt-stored membranes but did not delay the onset with low salt-stored membranes. This work tested whether permeable amines show the different effects in low- or high-salt-stored thylakoids which had attained a steady-state ATP formation rate (in continuous light) for 20-40 s prior to adding the amine. Hydroxyethylmorpholine was the preferred amine for such experiments, a suitable choice inasmuch as it behaves similarly to pyridine in the flash-induced ATP formation onset experiments, but it permeates more rapidly than pyridine and it has a higher pKa, which enhances its buffering effects. With high-salt-stored thylakoids, 0.5 or 1.0 mM hydroxyethylmorpholine added after 40 s of continuous illumination caused a marked, but transient, slowing of the ATP formation rate, but little or no slowing of the rate was observed with low-salt-stored thylakoids (at similar phosphorylation rates for the two thylakoid samples). Those data indicate that in continuous illumination conditions the proton gradient driving ATP formation in thylakoids from the low-salt-stored treatment did not equilibrate with the lumen, but in thylakoids stored in high-salt the delta mu H+ freely equilibrated with the lumen. That suggestion was supported by measurement of the luminal pH under coupling conditions by the [14C]methylamine distribution method using low- or high-salt-stored thylakoids. Further supportive evidence was obtained from measuring the effect of permeable amine buffers on H+ uptake under coupled and basal conditions with both types of thylakoid.(ABSTRACT TRUNCATED AT 400 WORDS)

Active transport, ion movements, and pH changes : II. Changes of pH and ATP synthesis

Electron transport in chloroplasts takes place across the thylakoid membrane in such a way as to redistribute ions. This redistribution can cause transmembrane electric fields and transmembrane hydrogen ion activity differences, events that are often correlated with the ability of the membrane to phosphorylate ADP. Analysis of the chemistry responsible for the phosphorylation is difficult because there seems to be no single satisfactory description of energized state of the thylakoid vesicles responsible for the photophosphorylation, the state depending on the experimental protocol employed. Under some conditions, acidification of the lumen confers on the vesicles the ability to synthesize ATP. Thus, when electron transport or preincubation in acid causes exogenous protonated buffers to accumulate in the lumen, ATP can be made in amounts commensurate with the accumulation if the pH of the medium is raised. When permeant exogenous buffers are absent an ability to make ATP also develops during prior electron transport, presumably because of protonation of membrane components. The nature of the energized state responsible for post-illumination phosphorylation in the latter instance is unclear. The energized state probably cannot then be simply a general delocalized ionic disequilibrium because of the precisely exponential nature of its decay with time after the light is off. The nature of the energized state which drives prompt phosphorylation in single stage experiments is even more puzzling. It may not depend on the kind of ion fluxes that result in a reversible pH rise in the medium. Certainly phosphorylation can begin at high efficiency when any measurable acidification of the vesicle lumen is prevented, even under conditions where the presence of a membrane potential is unlikely.

ATP formation onset lag and post-illumination phosphorylation initiated with single-turnover flashes. II. Two modes of post-illumination phosphorylation driven by either delocalized or localized proton gradient coupling

Two modes of chloroplast membrane post-illumination phosphorylation were detected, using the luciferin-luciferase ATP assay, one of which was not influenced by added permeable buffer (pyridine). That finding provides a powerful new tool for studying proton-membrane interactions during energy coupling. When ADP and Pi were added to the thylakoid suspension after a train of flashes [similar to the traditional post-illumination phosphorylation protocol (termed PIP- here)], the post-illumination ATP yield was influenced by pyridine as expected, in a manner consistent with the ATP formation, in part, being driven by protons present in the bulk inner aqueous phase, i.e., through a delocalized protonmotive force. However, when ADP and Pi were present during the flash train (referred to as PIP+), and ATP formation occurred during the flash train, the post-illumination ATP yield was unaffected by the presence of pyridine, consistent with the hypothesis that localized proton gradients were driving ATP formation. To test this hypothesis further, the pH and flash number dependence of the PIP- and PIP+ ATP yields were measured, the results being consistent with the above hypothesis of dual compartment origins of protons driving post-illumination ATP formation. Measuring proton accumulation during the attainment of the threshold energization level when no delta psi component was allowed to form (+ valinomycin, K+), and testing for pyridine effects on the proton uptake, reveals that the onset of ATP formation requires the accumulation of about 60 nmol H+ (mg Chl)-1. Between that level and about 110-150 nmol H+ (mg Chl)-1, the accumulation appears to be absorbed by localized-domain membrane buffering groups, the protons of which do not equilibrate readily with the inner aqueous (lumen) phase. Post-illumination phosphorylation driven by the dissipation of the domain protons was not affected by pyridine (present in the lumen), even though the effective pH in the domains must have been well into the buffering range of the pyridine. That finding provides additional insight into the localized domains, namely that protons can be absorbed by endogenous low pK buffering groups, and released at a low enough pH (less than or equal to 5.7 when the external pH was 8, less than or equal to 4.7 at pH 7 external) to drive significant ATP formation when no further proton production occurs due to the redox turnovers.(ABSTRACT TRUNCATED AT 400 WORDS)

THE MINIMUM PHOTON REQUIREMENT FOR PHOTOSYNTHESIS: AN ANALYSIS OF THE DATA OF WARBURG & BURK (1950) AND YUAN, EVANS & DANIELS (1955)

Variations in the apparent photon requirement for photosynthesis (Φ^-1 _{co 2} ) or (Φ^-1 _{co 2} ) in the data of Warburg & Burk (1950) and Yuan, Evans & Daniels (1955) can be ascribed to changes in O₂ uptake and energy-dependent processes which result in aberrant photon requirements in organisms subjected to non-optimal conditions. The increase in Φ^-1 _{co 2} with increases in the gas exchange quotient (γ) in the observations of Yuan et al. (1955) is consistent with increases in photorespiratory production of glycollate, whilst changes in Φ^-1 _{co 2} and Φ^-1 _{co 2} in the results of Warburg & Burk (1950) can be explained by a variable Kok effect associated with nitrate assimilation at low light levels. When these O₂ and energy-dependent processes are minimal, the lowest values should be observed. The minimum value obtained when Chlorella is photosynthesizing under optimal conditions is 6 mol photons mol^-1 O₂ . These results provide direct independent evidence for a photon requirement for photosynthesis of less than 8 mol photons mol^-1 O₂ . Such a value is not consistent with the Z scheme of photosynthesis.

Effects of the sesquiterpene lactone tetraesters thapsigargicin and thapsigargin, from roots of Thapsia garganica L., on isolated spinach chloroplasts

The effect of thapsigargicin and thapsigargin, extracted from the roots of Thapsia garganica L., on isolated photosynthetic membranes (thylakoids) and intact chloroplasts from spinach leaves (Spinacia oleracea L.) was investigated. Both sesquiterpene lactone tetraesters impair membranes and organelles in an identical, chlorophyll-dependent manner. In thylakoids these compounds primarily act as inhibitors of photophosphorylation. At lower sesquiterpene lactone tetraester/chlorophyll ratios, cyclic and non-cyclic photophosphorylation, ADP-stimulated electron transport and the photosynthetic control ratio progressively decreased with increasing concentrations of thapsigargicin and thapsigargin, whereas the state 4 electron flow, the coupling efficiency of photophosphorylation, the light-induced proton gradient, and the H+ flux across the membranes remained nearly unaffected. Half-maximal inhibition of photophosphorylation was obtained with 4-5 X 10(-7) moles sesquiterpene lactone tetraesters per mg chlorophyll. At higher sesquiterpene lactone tetraester/chlorophyll ratios, uncoupling of photophosphorylation from electron transport occurred. This was evident from stimulation of the state 4 electron flow, decline in the ADP/2e- ratio, increase in proton permeability and decrease in delta pH, whereas the uncoupled electron transport was only little impaired. In intact chloroplasts inhibition of HCO-3, 3-phosphoglycerate and oxaloacetate reduction by thapsigargicin and thapsigargin was not caused by inactivation of the photochemical reactions of the thylakoid membranes but were rather due to alterations in the permeability properties of the chloroplast envelope. This was concluded from similarities in the kinetics of these reactions. It is suggested that the highly lipid soluble sesquiterpene lactone tetraesters effectively disrupt the lipid-protein associations of biomembranes.

The relationship between light-induced increases in the H+ conductivity of thylakoid membranes and activity of the coupling factor

In the absence of a transmembrane electric field, about 15 saturating single-turnover flashes are required for chloroplast thylakoid membranes to accumulate the 80 mmol H+ X mol chlorophyll-1 that are necessary to form a delta pH sufficiently large to initiate net ATP synthesis. Lowering the number of turnovers of proton-producing redox components by decreasing the flash intensity increased the number of flashes required for the onset of ATP formation. Thus, regardless of the intensity, the accumulation of the same number of hydrogen ions was needed for phosphorylation to begin. Since the size of the threshold input was constant over a very wide range of proton accumulation rates, it follows that there were no significant proton leakages during the filling of the pool to its threshold level. However, non-productive leaks were initiated once phosphorylation began since progressively lower phosphorylation efficiencies were observed at lower and lower flash intensities. It is difficult to explain this observation except in terms of competing, non-phosphorylating hydrogen ion fluxes only when the threshold accumulation had been reached. We observed an increase in the permeability of thylakoid membrane to hydrogen ions that correlated with indications of coupling factor activity: the onset of ATP synthesis, the release of tightly bound ADP and, in dithiothreitol treated membranes, the initiation of ATPase activity. Our data support the notion that the dependence of coupling factor activation and deactivation on the delta pH accounts for the substantial changes in the ion conductivity that occur in thylakoid membranes.

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.