A critical examination of the supposed variable proton stoichiometry of the chloroplast cytochrome bf complex

Flux balance analysis of cyanobacteria reveals selective use of photosynthetic electron transport components under different spectral light conditions

Cyanobacteria acclimate and adapt to changing light conditions by controlling the energy transfer between photosystem I (PSI) and II (PSII) and pigment composition. Photosynthesis is driven by balancing the excitation between PSI and PSII. To predict the detailed electron transfer flux of cyanobacteria, we refined the photosynthesis-related reactions in our previously reconstructed genome-scale model. Two photosynthetic bacteria, Arthrospira and Synechocystis, were used as models. They were grown under various spectral light conditions and flux balance analysis (FBA) was performed using photon uptake fluxes into PSI and PSII, which were converted from each light spectrum by considering the photoacclimation of pigments and the distribution ratio of phycobilisome to PSI and PSII. In Arthrospira, the FBA was verified with experimental data using six types of light-emitting diodes (White, Blue, Green, Yellow, Red1, and Red2). FBA predicted the cell growth of Synechocystis for the LEDs, excepting Red2. In an FBA simulation, cells used respiratory terminal oxidases and two NADH dehydrogenases (NDH-1 and NDH-2) to balance the PSI and PSII excitations depending on the light conditions. FBA simulation with our refined model functionally implicated NDH-1 and NDH-2 as a component of cyclic electron transport in the varied light environments.

Analyzing both the fast and the slow phases of chlorophyll a fluorescence and P700 absorbance changes in dark-adapted and preilluminated pea leaves using a Thylakoid Membrane model

The dark-to-light transitions enable energization of the thylakoid membrane (TM), which is reflected in fast and slow (OJIPSMT or OABCDE) stages of fluorescence induction (FI) and P700 oxidoreduction changes (ΔA810). A Thylakoid Membrane model (T-M model), in which special emphasis has been placed on ferredoxin-NADP+-oxidoreductase (FNR) activation and energy-dependent qE quenching, was applied for quantifying the kinetics of FI and ΔA810. Pea leaves were kept in darkness for 15 min and then the FI and ΔA810 signals were measured upon actinic illumination, applied either directly or after a 10-s light pulse coupled with a subsequent 10-s dark interval. On the time scale from 40 µs to 30 s, the parallel T-M model fittings to both FI and ΔA810 signals were obtained. The parameters of FNR activation and the buildup of qE quenching were found to differ for dark-adapted and preilluminated leaves. At the onset of actinic light, photosystem II (PSII) acceptors were oxidized (neutral) after dark adaptation, while the redox states with closed and/or semiquinone QA(-)QB(-) forms were supposedly generated after preillumination, and did not relax within the 10 s dark interval. In qE simulations, a pH-dependent Hill relationship was used. The rate constant of heat losses in PSII antenna kD(t) was found to increase from the basic value kDconst, at the onset of illumination, to its maximal level kDvar due to lumenal acidification. In dark-adapted leaves, a low value of kDconst of ∼ 2 × 106 s-1 was found. Simulations on the microsecond to 30 s time scale revealed that the slow P-S-M-T phases of the fluorescence induction were sensitive to light-induced FNR activation and high-energy qE quenching. Thus, the corresponding time-dependent rate constants kD(t) and kFNR(t) change substantially upon the release of electron transport on the acceptor side of PSI and during the NPQ development. The transitions between the cyclic and linear electron transport modes have also been quantified in this paper.

Thylakoid membrane model of the Chl a fluorescence transient and P700 induction kinetics in plant leaves

A new Thylakoid model is presented, which describes in detail the electron/proton transfer reactions between membrane protein complexes including photosystems II and I (PSII, PSI), cytochrome (Cyt) b 6 f, mobile plastoquinone PQ pool in the thylakoid membrane, plastocyanin in lumen and ferredoxin in stroma, reduction of NADP via FNR and cyclic electron transfer. The Thylakoid model parameters were fitted both to Chl fluorescence induction data (FI) and oxido-reductions of P700 (ΔA 810) measured from 20 μs up to 20 s in pea leaves. The two-wave kinetics of FI and ΔA 810 (O(JI)PSM and OABCDE) were described quantitatively, provided that the values of membrane electrochemical potential components ΔΨ(t), pHL(t)/pHS(t) are in physiologically relevant ranges. The time courses on the time scale from nanoseconds to tens of seconds of oxido-reduction changes of ET components as well as concentrations of proton/ions (K+, Cl-) were calculated. We assume a low constant FNR activity over this period. Charge movements across the thylakoid membrane by passive leakage and active ATPase transport and proton buffer reactions are simulated. The dynamics of charge fluxes during photosynthetic induction under low light (PFD 200 μmol photons m-2 s-1) were analyzed. The initial wave of P700 oxidation within 20 ms during independent operation of PSI and PSII was followed after 50 ms by PSI donor-side reduction from reduced PQ pool via Cyt b 6 f site. The Cyt b 6 f reactions contribute to the stabilization of fluxes in the time range 1 s < t < 10 s. The detailed analysis of Chl a fluorescence at the PSM stage (t > 10 s) would need the investigation of FNR activation effect in order to explain the transitions between cyclic and linear electron transport.

Photosynthetic control of electron transport and the regulation of gene expression

The term 'photosynthetic control' describes the short- and long-term mechanisms that regulate reactions in the photosynthetic electron transport (PET) chain so that the rate of production of ATP and NADPH is coordinated with the rate of their utilization in metabolism. At low irradiances these mechanisms serve to optimize light use efficiency, while at high irradiances they operate to dissipate excess excitation energy as heat. Similarly, the production of ATP and NADPH in ratios tailored to meet demand is finely tuned by a sophisticated series of controls that prevents the accumulation of high NAD(P)H/NAD(P) ratios and ATP/ADP ratios that would lead to potentially harmful over-reduction and inactivation of PET chain components. In recent years, photosynthetic control has also been extrapolated to the regulation of gene expression because mechanisms that are identical or similar to those that serve to regulate electron flow through the PET chain also coordinate the regulated expression of genes encoding photosynthetic proteins. This requires coordinated gene expression in the chloroplasts, mitochondria, and nuclei, involving complex networks of forward and retrograde signalling pathways. Photosynthetic control operates to control photosynthetic gene expression in response to environmental and metabolic changes. Mining literature data on transcriptome profiles of C(3) and C(4) leaves from plants grown under high atmospheric carbon dioxide (CO(2)) levels compared with those grown with ambient CO(2) reveals that the transition to higher photorespiratory conditions in C(3) plants enhances the expression of genes associated with cyclic electron flow pathways in Arabidopsis thaliana, consistent with the higher ATP requirement (relative to NADPH) of photorespiration.

Determining the limitations and regulation of photosynthetic energy transduction in leaves

The light-dependent production of ATP and reductants by the photosynthetic apparatus in vivo involves a series of electron and proton transfers. Consideration is given as to how electron fluxes through photosystem I (PSI), using absorption spectroscopy, and through photosystem II (PSII), using chlorophyll fluorescence analyses, can be estimated in vivo. Measurements of light-induced electrochromic shifts using absorption spectroscopy provide a means of analyzing the proton fluxes across the thylakoid membranes in vivo. Regulation of these electron and proton fluxes is required for the thylakoids to meet the fluctuating metabolic demands of the cell. Chloroplasts exhibit a wide and flexible range of mechanisms to regulate electron and proton fluxes that enable chloroplasts to match light use for ATP and reductant production with the prevailing metabolic requirements. Non-invasive probing of electron fluxes through PSI and PSII, and proton fluxes across the thylakoid membranes can provide insights into the operation of such regulatory processes in vivo.

Photosystem II cycle and alternative electron flow in leaves

Sunflower (Helianthus annuus L.) and tobacco (Nicotiana tabacum L.) were grown in the laboratory and leaves were taken from field-grown birch trees (Betula pendula Roth). Chlorophyll fluorescence, CO2 uptake and O2 evolution were measured and electron transport rates were calculated, J(C) from the CO2 uptake rate considering ribulose-1,5-bisphosphate (RuBP) carboxylation and oxygenation, J(O) from the O2 evolution rate, and J(F) from Chl fluorescence parameters. Mesophyll diffusion resistance, r(md), used for the calculation of J(C), was determined such that the in vivo Rubisco kinetic curve with respect to the carboxylation site CO2 concentration became a rectangular hyperbola with Km(CO2) of 10 microM at 22.5 degrees C. In sunflower, in the absence of external O2, J(O) = 1.07 J(C) when absorbed photon flux density (PAD) was varied, showing that the O2-independent components of the alternative electron flow to acceptors other than CO2 made up 7% of J(C). Under saturating light, J(F), however, was 20-30% faster than J(C), and J(F)-J(C) depended little on CO2 and O2 concentrations. The inter-relationship between J(F)-J(C) and non-photochemical quenching (NPQ) was variable, dependent on the CO2 concentration. We conclude that the relatively fast electron flow J(F)-J(C) appearing at light saturation of photosynthesis contains a minor component coupled with proton translocation, serving for nitrite, oxaloacetate and oxygen reduction, and a major component that is mostly cyclic electron transport around PSII. The rate of the PSII cycle is sufficient to release the excess excitation pressure on PSII significantly. Although the O2-dependent Mehler-type alternative electron flow appeared to be under the detection threshold, its importance is discussed considering the documented enhancement of photosynthesis by oxygen.

Consequences of the structure of the cytochrome b6f complex for its charge transfer pathways

At least two features of the crystal structures of the cytochrome b6f complex from the thermophilic cyanobacterium, Mastigocladus laminosus and a green alga, Chlamydomonas reinhardtii, have implications for the pathways and mechanism of charge (electron/proton) transfer in the complex: (i) The narrow 11 x 12 A portal between the p-side of the quinone exchange cavity and p-side plastoquinone/quinol binding niche, through which all Q/QH2 must pass, is smaller in the b6f than in the bc1 complex because of its partial occlusion by the phytyl chain of the one bound chlorophyll a molecule in the b6f complex. Thus, the pathway for trans-membrane passage of the lipophilic quinone is even more labyrinthine in the b6f than in the bc1 complex. (ii) A unique covalently bound heme, heme cn, in close proximity to the n-side b heme, is present in the b6f complex. The b6f structure implies that a Q cycle mechanism must be modified to include heme cn as an intermediate between heme bn and plastoquinone bound at a different site than in the bc1 complex. In addition, it is likely that the heme bn-cn couple participates in photosytem I-linked cyclic electron transport that requires ferredoxin and the ferredoxin: NADP+ reductase. This pathway through the n-side of the b6f complex could overlap with the n-side of the Q cycle pathway. Thus, either regulation is required at the level of the redox state of the hemes that would allow them to be shared by the two pathways, and/or the two different pathways are segregated in the membrane.

Control of cytochrome b6f at low and high light intensity and cyclic electron transport in leaves

The light-dependent control of photosynthetic electron transport from plastoquinol (PQH(2)) through the cytochrome b(6)f complex (Cyt b(6)f) to plastocyanin (PC) and P700 (the donor pigment of Photosystem I, PSI) was investigated in laboratory-grown Helianthus annuus L., Nicotiana tabaccum L., and naturally-grown Solidago virgaurea L., Betula pendula Roth, and Tilia cordata P. Mill. leaves. Steady-state illumination was interrupted (light-dark transient) or a high-intensity 10 ms light pulse was applied to reduce PQ and oxidise PC and P700 (pulse-dark transient) and the following re-reduction of P700(+) and PC(+) was recorded as leaf transmission measured differentially at 810-950 nm. The signal was deconvoluted into PC(+) and P700(+) components by oxidative (far-red) titration (V. Oja et al., Photosynth. Res. 78 (2003) 1-15) and the PSI density was determined by reductive titration using single-turnover flashes (V. Oja et al., Biochim. Biophys. Acta 1658 (2004) 225-234). These innovations allowed the definition of the full light response curves of electron transport rate through Cyt b(6)f to the PSI donors. A significant down-regulation of Cyt b(6)f maximum turnover rate was discovered at low light intensities, which relaxed at medium light intensities, and strengthened again at saturating irradiances. We explain the low-light regulation of Cyt b(6)f in terms of inactivation of carbon reduction cycle enzymes which increases flux resistance. Cyclic electron transport around PSI was measured as the difference between PSI electron transport (determined from the light-dark transient) and PSII electron transport determined from chlorophyll fluorescence. Cyclic e(-) transport was not detected at limiting light intensities. At saturating light the cyclic electron transport was present in some, but not all, leaves. We explain variations in the magnitude of cyclic electron flow around PSI as resulting from the variable rate of non-photosynthetic ATP-consuming processes in the chloroplast, not as a principle process that corrects imbalances in ATP/NADPH stoichiometry during photosynthesis.

Characterization of photosynthetic electron transport in bundle sheath cells of maize. I. Ascorbate effectively stimulates cyclic electron flow around PSI

Redox changes of the reaction-center chlorophyll of photosystem I (P700) and chlorophyll fluorescence yield were measured in bundle sheath strands (BSS) isolated from maize (Zea mays L.) leaves. Oxidation of P700 in BSS by actinic light was suppressed by nigericin, indicating the generation of a proton gradient across the thylakoid membranes of BSS chloroplasts. Methyl viologen, which transfers electrons from photosystem I (PSI) to O2, caused a considerable decrease in the reduction rate of P700+ in BSS after turning off actinic light, showing that electron flow from the acceptor side of PSI to stromal components is critical for this reduction. Ascorbate (Asc), and to a lesser extent malate (Mal), caused a lower level of P700+ in BSS under aerobic conditions in far-red light, implying electron donation from these substances to the intersystem carriers. When Asc or Mal was added to BSS during pre-illumination under anaerobic conditions in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU), the far-red-induced level of P700+ was lowered. The results suggest Asc and Mal can cause reduction of stromal donors, which in turn establishes conditions for rapid PSI-driven P700+ reduction. Addition of these metabolites also strongly stimulated the development of a proton gradient in thylakoids under aerobic conditions in the absence of DCMU, i.e. under conditions analogous to those in vivo. Ascorbate was a much more effective electron donor than Mal, suggesting it has a physiological role in activation of cyclic electron flow around PSI.

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.

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.

Photosynthetic electron sinks in transgenic tobacco with reduced amounts of Rubisco: little evidence for significant Mehler reaction

Transgenic tobacco (Nicotiana tabacum L. cv. W38) plants with an antisense gene directed against the mRNA of the small subunit of Rubisco were used to investigate the role of O2 as an electron acceptor during photosynthesis. The reduction in Rubisco has reduced the capacity for CO2-fixation in these plants without a similar reduction in electron transport capacity. Concurrent measurements of chlorophyll fluorescence and CO2 assimilation at different CO2 and O2 partial pressures showed close linear relationships between chloroplast electron transport rates calculated from chlorophyll fluorescence and those calculated from CO2-fixation. These relationships were similar for wild-type and transgenic plants, indicating that the reduced capacity for CO2 fixation in the transgenic plants did not result in extra electron transport not associated with the photosynthetic carbon reduction (PCR) or photorespiratory carbon oxidation (PCO) cycle. This was further investigated with mass spectrometric measurements of 16O2 and 18O2 exchange made concurrently with measurements of chlorophyll fluorescence. In all tobacco lines the rates of 18O2 uptake in the dark were similar to the 18O2 uptake rates at very high CO2 partial pressures in the light. Rates of oxygenase activity calculated from 18O2 uptake at the compensation point were linearly related to the Rubisco content of leaves. The ratios of oxygenase to carboxylase rates were calculated from measurements of 16O2 evolution and 18O2 uptake at the compensation point. These ratios were lower in the transgenic plants, consistent with their higher CO2 compensation points. It is concluded that although there may be some electron transport to O2 to balance conflicting demands of NADPH to ATP requirements, this flux must decrease in proportion with the reduced demand for ATP and NADPH consumption in the transgenic lines. The altered balance between electron transport and Rubisco capacity, however, does not result in rampant electron transport to O2 or other electron transport acceptors in the absence of PCR and PCO cycle activity.

Homeostasis of adenylate status during photosynthesis in a fluctuating environment

This review describes and assesses pathways likely to influence and stabilize the ATP/reductant balance during whole cell photosynthesis. The sole reductive step of the Calvin cycle occurs during the conversion of 3-phosphoglycerate to triose phosphate. Photophosphorylation linked to this reaction can undoubtedly supply most of the ATP required by the Calvin cycle and other chloroplastic reactions. Small but crucial contributions must come from several other pathways, some of which involve co-operation between the chloroplast and the rest of the cell. Extrachloroplastic compartments can contribute to chloroplastic ATP requirements by supplying ATP directly or, probably more significantly, by accepting reducing equivalents and so supporting ATP synthesis within the chloroplast.

Proton to electron stoichiometry in electron transport of spinach thylakoids

According to the concept of the Q-cycle, the H+/e- ratio of the electron transport chain of thylakoids can be raised from 2 to 3 by means of the rereduction of plastoquinone across the cytochrome b6f complex. In order to investigate the H+/e- ratio we compared stationary rates of electron transport and proton translocation in spinach thylakoids both in the presence of the artificial electron acceptor ferricyanide and in the presence of the natural acceptor system ferredoxin+NADP. The results may be summarised as follows: (1) a variability of the H+/e- ratio occurs with either acceptor. H+/e- ratios of 3 (or even higher in the case of the natural acceptor system, see below) are decreased towards 2 if strong light intensity and low membrane permeability are employed. Mechanistically this could be explained by proton channels connecting the plastoquinol binding site alternatively to the lumenal or stromal side of the cytochrome b6f complex, giving rise to a proton slip reaction at high transmembrane DeltapH. In this slip reaction protons are deposited on the stromal instead of the lumenal side. In addition to the pH effect there seems to be a contribution of the redox state of the plastoquinone pool to the control of proton translocation; switching over to stromal proton deposition is favoured when the reduced state of plastoquinone becomes dominant. (2) In the presence of NADP a competition of both NADP and oxygen for the electrons supplied by photosystem I takes place, inducing a general increase of the H+/e- ratios above the values obtained with ferricyanide. The implications with respect to the adjustment of a proper ATP/NADPH ratio for CO2 reduction are discussed.

Models of CO2 concentrating mechanisms in microalgae taking into account cell and chloroplast structure

Detailed mathematical models have been developed for the functioning of CO2 concentration mechanisms in microalgae. The models treat a microalgal cell as several compartments: pyrenoid, chloroplast stroma, cytoplasm and periplasmic space. Cases for both the active bicarbonate transport through the plasmalemma and the passive CO2 diffusion through it with the subsequent concentrating of CO2 inside the chloroplast are analyzed. CO2 evolution from bicarbonate inside the pyrenoid is modeled. The great diffusion resistance for CO2 flux from the pyrenoid is caused by a starch envelope and the concentric thylakoid membranes surrounding it. The role of carbonic anhydrase in the periplasmic space, cytoplasm and inside the chloroplast is evaluated numerically. The models also offer an explanation for the absence of 'short-circuited' inorganic carbon fluxes between the external medium and the cytoplasm under active bicarbonate transport through the plasmalemma and in the presence of carbonic anhydrase in the cytoplasm. If the cytoplasm is driven from the space between a chloroplast envelope and plasmalemma upon the microalgae adaptation to low concentration of the dissolved inorganic carbon, the inorganic carbon leak might be avoided. The models reproduce accurately the majority of known experimental data. The high efficiency of CO2 concentrating mechanisms in microalgae can be explained by a considerable diffusion resistance for CO2 flux from the pyrenoid and by the effective scavenging of CO2 leaking outward from the chloroplast to cytoplasm and from cell to periplasmic space.

SOME NEW STRUCTURAL ASPECTS AND OLD CONTROVERSIES CONCERNING THE CYTOCHROME b6f COMPLEX OF OXYGENIC PHOTOSYNTHESIS

The cytochrome b6f complex functions in oxygenic photosynthetic membranes as the redox link between the photosynthetic reaction center complexes II and I and also functions in proton translocation. It is an ideal integral membrane protein complex in which to study structure and function because of the existence of a large amount of primary sequence data, purified complex, the emergence of structures, and the ability of flash kinetic spectroscopy to assay function in a readily accessible ms-100 mus time domain. The redox active polypeptides are cytochromes f and b6 (organelle encoded) and the Rieske iron-sulfur protein (nuclear encoded) in a mol wt = 210,000 dimeric complex that is believed to contain 22-24 transmembrane helices. The high resolution structure of the lumen-side domain of cytochrome f shows it to be an elongate (75 A long) mostly beta-strand, two-domain protein, with the N-terminal alpha-amino group as orthogonal heme ligand and an internal linear 11-A bound water chain. An unusual electron transfer event, the oxidant-induced reduction of a significant fraction of the p (lumen)-side cytochrome b heme by plastosemiquinone indicates that the electron transfer pathway in the b6f complex can be described by a version of the Q-cycle mechanism, originally proposed to describe similar processes in the mitochondrial and bacterial bc1 complexes.

Characterisation of a H2O 2-oxidisable cytochrome b-559 in intact chloroplasts with a new type of LED Array Spectrophotometer

Cytochrome (cyt) b-559 absorbance changes in intact chloroplasts were deconvoluted using a previously described LED-Array-Spectrophotometer (Klughammer et al. (1990), Photosynth Res 25: 317-327). When intact chloroplasts were isolated in the presence of ascorbate, approx. 15% of the total cyt b-559 could be transiently oxidised by 200 μM H2O2 in the dark. This fraction displays low-potential properties, as it can be also oxidised by menadione in the presence of 5 mM ascorbate. Heat pretreatment increased the size of this fraction by a factor of 3-4. Low concentrations of cyanide (in the μM range) prolonged the oxidation time while high concentrations suppressed the oxidation (I50=1.5 mM KCN). The former KCN-effect relates to inhibition of ascorbate dependent H2O2-reduction which is catalysed by ascorbate peroxidase, whereas the latter effect reflects competition between H2O2 and CN(-) for the same binding site at the cytochrome heme. In the light, much lower concentrations of H2O2 were required to obtain oxidation, the amplitude depending on light intensity and on the concentration of the added H2O2, but never exceeding approx. 15% of the total cyt b-559. In the light, but not in the dark, H2O2 also induced the transient oxidation of a cyt f fraction similar in size to the H2O2-oxidisable cyt b-559 fraction. In this case, H2O2 serves as an acceptor of Photosystem I in conjunction with the ascorbate peroxidase detoxification system. Light can also induce oxidation of a 15% cyt b-559 fraction without H2O2-addition, if nitrite is present as electron acceptor and the chloroplasts are depleted of ascorbate. It is concluded that light-induced cyt b-559 oxidation in vivo is likely to be restricted to the H2O2-oxidisable cyt b-559 LP fraction and is normally counteracted by ascorbate.

Quantitative evaluation of the role of a putative CO2-scavenging entity in the cyanobacterial CO2-concentrating mechanism

This paper assesses the contribution of a postulated CO2-scavenging system to the efficient operation of the CO2-concentrating mechanism (CCM) in cyanobacteria. A quantitative model for the CCM is presented which incorporates an energy-dependent carbonic anhydrase-like entity located at or near the inner surface of the plasma membrane. This entity, which converts CO2 to HCO3- against the thermodynamic potential, scavenges CO2 leaking outward from the carboxysomes, and, further, converts CO2 entering from the medium to HCO3-, thus maintaining an inward diffusion gradient along which CO2 enters passively. The model resembles our earlier models in postulating that CO2 and HCO3- are not at equilibrium throughout the greater part of the cell, and that CO2 is generated in high concentration at carbonic anhydrase sites within the carboxysomes. The model further takes into account the concentric thylakoid membranes which surround the carboxysomes, and events in the periplasmic space and the unstirred layer surrounding the cell. Implications of the predicted steady state fluxes of CO2 and HCO3-, and of their steady state concentrations in various cellular compartments, are discussed. The plasma membrane carbonic anhydrase-like activity lowers the photosynthetic Km for external Ci, as well as decreasing the inorganic C 'leak', but it may not save on energy expenditure.

Coregulation of electron transport through PS I by Cyt b 6 f, excitation capture by P700 and acceptor side reduction. Time kinetics and electron transport requirement

Regulation of electron transport rate through Photosystem I (PS I) was investigated in intact sunflower leaves. The rate constant of electron donation via the cytochrome b 6 f complex (kq, s(-1)) was obtained from the postillumination P700(+) reduction rate, measured as the exponential decay of the light-dark difference (D830) of the 830 nm transmission signal. D830 corresponding to maximum oxidisable P700 (D830m) was obtained by applying white light flashes of different intensity and extrapolating the plot of the quantum yield Y vs. D830 to the axis of abscissae (Y->0). Maximum quantum yield of PS I at completely reduced P700 (Ym) was obtained by extrapolating the same plot to the axis of ordinates (D830->0). Regulation of kq, D830m and Ym under rate-limiting CO2 and O2 concentrations applied after air (21% O2, 310 ppm CO2) was investigated. The amplitude of the downregulation of kq (photosynthetic control) was maximal when electron transport rate (ETR) was limited to about 3 nmol cm(-2) s(-1) and decreased when ETR was higher or lower. Downregulation did not occur in the absence of CO2 and O2. These gases acted only as substrates of ribulosebisphosphate carboxylase-oxygenase, no high-affinity reaction of O2 leading to enhanced photosynthetic control (e.g. Mehler reaction) was detected. After the transition, D830m at first decreased and then increased again, showing that the reduction of the PS I acceptor side disappeared as a result of the downregulation of kq. The variation of Ym had two reasons, PS I acceptor side reduction and variable excitation capture efficiency by P700. It is concluded that electron transport through PS I is coregulated by the rate of plastoquinol oxidation at Cyt b 6 f, excitation capture efficiency by P700, and by acceptor side reduction.

Competition between electron acceptors in photosynthesis: Regulation of the malate valve during CO2 fixation and nitrite reduction

For maximal rates of CO2 assimilation in isolated intact spinach chloroplasts the generation of the adequate NADPH/ATP ratio is achieved either by cyclic electron flow around photosystem I or by linear electron transport to oxaloacetate, nitrite or oxygen (Mehler-reaction). The interrelationships between these poising mechanisms turn out to be strictly hierarchical. In the presence of antimycin A, an inhibitor of ferredoxin-dependent cyclic electron transport, the reduction of both, oxaloacetate and nitrite, but not that of oxygen restores CO2 fixation. When oxaloacetate and nitrite are added at low concentrations simultaneously during steady-state CO2 fixation, the reduction of nitrite is clearly preferred over the reduction of oxaloacetate, but CO2 fixation is not influenced. Nitrite reduction is not decreased upon addition of oxaloacetate, but vice versa. This is due to the regulation of NADP-malate dehydrogenase activation by electron pressure via the ferredoxin/thioredoxin system on the one hand, and by the NADPH/(NADP+NADPH) ratio (anabolic reduction charge, ARC) on the other hand. Thus the closing of the 'malate valve' prevents drainage of reducing equivalents from the chloroplast (1) when a low ARC indicates a high demand for NADPH in the stroma and (2) when nitrite reduction reduces the electron pressure at ferredoxin. The 'malate valve' is opened when cyclic electron transport is inhibited by antimycin A. Under these conditions the rate of malate formation is higher than in the absence of the inhibitor even in the presence of oxaloacetate, thus indicating that the regulation of the 'malate valve' functions at various redox states of the acceptor side of Photosystem I.

Rates of vectorial proton transport supported by cyclic electron flow during oxygen reduction by illuminated intact chloroplasts

The light-dependent quenching of 9-aminoacridine fluorescence was used to monitor the state of the transthylakoid proton gradient in illuminated intact chloroplasts in the presence or absence of external electron acceptors. The absence of appreciable light-dependent fluorescence quenching under anaerobic conditions indicated inhibition of coupled electron transport in the absence of external electron acceptors. Oxygen relieved this inhibition. However, when DCMU inhibited excessive reduction of the plastoquinone pool in the absence of oxygen, coupled cyclic electron transport supported the formation of a transthylakoid proton gradient even under anaerobiosis. This proton gradient collapsed in the presence of oxygen. Under aerobic conditions, and when KCN inhibited ribulose bisphosphate carboxylase and ascorbate peroxidase, fluorescence quenching indicated the formation of a transthylakoid proton gradient which was larger with oxygen in the Mehler reaction as electron acceptor than with methylviologen at similar rates of linear electron transport. Apparently, cyclic electron transport occured simultaneously with linear electron transport, when oxygen was available as electron acceptor, but not when methylviologen accepted electrons from Photosystem I. The ratio of cyclic to linear electron transport could be increased by low concentrations of DCMU. This shows that even under aerobic conditions cyclic electron transport is limited in isolated intact chloroplasts by excessive reduction of electron carriers. In fact, P700 in the reaction center of Photosystem I remained reduced in illuminated isolated chloroplasts under conditions which resulted in extensive oxidation of P700 in leaves. This shows that regulation of Photosystem II activity is less effective in isolated chloroplasts than in leaves. Assuming that a Q-cycle supports a H(+)/e ratio of 3 during slow linear electron transport, vectorial proton transport coupled to Photosystem I-dependent cyclic electron flow could be calculated. The highest calculated rate of Photosystem I-dependent proton transport, which was not yet light-saturated, was 330 μmol protons (mg chlorophyll h)(-1) in intact chloroplasts. If H(+)/e is not three but two proton transfer is not 330 but 220 μmol (mg Chl H)(-1). Differences in the regulation of cyclic electron transport in isolated chloroplasts and in leaves are discussed.

Selective interaction of valinomycin/K+ with the cytochrome bf complex of chloroplasts. Synergistic effect with MOA stilbene on extent of cytochrome b563 reduction in continuous light

Valinomycin/K+ is shown to selectively interact at sub-micromolar concentrations with the cytochrome bf complex in thylakoid membranes, inducing a red shift of the ferrohaem b absorbance alpha-band, a slow down of post-illumination b-reoxidation and a corresponding increase of b-reduction level in continuous light. These effects of valinomycin/K+ are not related to its field dissipating action, as they are not affected by nonactin. Presence of K+ is required. Phenomenologically the valinomycin/K+ effects are similar to those caused by 10 times higher MOA stilbene nonactin. Presence of K+ is required. Phenomenologically the valinomycin/K+ effects are similar to those caused by 10 times higher MOA stilbene concentrations. However, synergism is observed between the two inhibitors, suggesting different modes of action. When both inhibitors are combined more than one haem b can be reduced by illumination.

A proposed pathway of proton translocation through the bc complexes of mitochondria and chloroplasts

The cytochrome bc complexes of the electron transport chain from a wide variety of organisms generate an electrochemical proton gradient which is used for the synthesis of ATP. Proton translocation studies with radiolabeled N,N'-dicyclohexylcarbodiimide (DCCD), the well-established carboxyl-modifying reagent, inhibited proton-translocation 50-70% with minimal effect on electron transfer in the cytochrome bc1 and cytochrome bf complexes reconstituted into liposomes. Subsequent binding studies with cytochrome bc1 and cytochrome bf complexes indicate that DCCD specifically binds to the subunit b and subunit b6, respectively, in a time and concentration dependent manner. Further analyses of the results with cyanogen bromide and protease digestion suggest that the probable site of DCCD binding is aspartate 160 of yeast cytochrome b and aspartate 155 or glutamate 166 of spinach cytochrome b6. Moreover, similar inhibition of proton translocating activity and binding to cytochrome b and cytochrome b6 were noticed with N-cyclo-N-(4-dimethylamino-napthyl)carbodiimide (NCD-4), a fluorescent analogue of DCCD. The spin-label quenching experiments provide further evidence that the binding site for NCD-4 on helix cd of both cytochrome b and cytochrome b6 is localized near the surface of the membrane but shielded from the external medium.

Electron transport and photophosphorylation by Photosystem I in vivo in plants and cyanobacteria

Recently, a number of techniques, some of them relatively new and many often used in combination, have given a clearer picture of the dynamic role of electron transport in Photosystem I of photosynthesis and of coupled cyclic photophosphorylation. For example, the photoacoustic technique has detected cyclic electron transport in vivo in all the major algal groups and in leaves of higher plants. Spectroscopic measurements of the Photosystem I reaction center and of the changes in light scattering associated with thylakoid membrane energization also indicate that cyclic photophosphorylation occurs in living plants and cyanobacteria, particularly under stressful conditions.In cyanobacteria, the path of cyclic electron transport has recently been proposed to include an NAD(P)H dehydrogenase, a complex that may also participate in respiratory electron transport. Photosynthesis and respiration may share common electron carriers in eukaryotes also. Chlororespiration, the uptake of O2 in the dark by chloroplasts, is inhibited by excitation of Photosystem I, which diverts electrons away from the chlororespiratory chain into the photosynthetic electron transport chain. Chlororespiration in N-starved Chlamydomonas increases ten fold over that of the control, perhaps because carbohydrates and NAD(P)H are oxidized and ATP produced by this process.The regulation of energy distribution to the photosystems and of cyclic and non-cyclic phosphorylation via state 1 to state 2 transitions may involve the cytochrome b 6-f complex. An increased demand for ATP lowers the transthylakoid pH gradient, activates the b 6-f complex, stimulates phosphorylation of the light-harvesting chlorophyll-protein complex of Photosystem II and decreases energy input to Photosystem II upon induction of state 2. The resulting increase in the absorption by Photosystem I favors cyclic electron flow and ATP production over linear electron flow to NADP and 'poises' the system by slowing down the flow of electrons originating in Photosystem II.Cyclic electron transport may function to prevent photoinhibition to the photosynthetic apparatus as well as to provide ATP. Thus, under high light intensities where CO2 can limit photosynthesis, especially when stomates are closed as a result of water stress, the proton gradient established by coupled cyclic electron transport can prevent over-reduction of the electron transport system by increasing thermal de-excitation in Photosystem II (Weis and Berry 1987). Increased cyclic photophosphorylation may also serve to drive ion uptake in nutrient-deprived cells or ion export in salt-stressed cells.There is evidence in some plants for a specialization of Photosystem I. For example, in the red alga Porphyra about one third of the total Photosystem I units are engaged in linear electron transfer from Photosystem II and the remaining two thirds of the Photosystem I units are specialized for cyclic electron flow. Other organisms show evidence of similar specialization.Improved understanding of the biological role of cyclic photophosphorylation will depend on experiments made on living cells and measurements of cyclic photophosphorylation in vivo.

The cytochrome bc 1 complexes of photosynthetic purple bacteria

Complete nucleotide sequences are now available for the pet (fbc) operons coding for the three electron carrying protein subunits of the cytochrome bc 1 complexes of four photosynthetic purple non-sulfur bacteria. It has been demonstrated that, although the complex from one of these bacteria may contain a fourth subunit, three subunit complexes appear to be fully functional. The ligands to the three hemes and the one [2Fe-2S] cluster in the complex have been identified and considerable progress has been made in mapping the two quinone-binding sites present in the complex, as well as the binding sites for quinone analog inhibitors. Hydropathy analyses and alkaline phosphatase fusion experiments have provided considerable insight into the likely folding pattern of the cytochrome b peptide of the complex and identification of the electrogenic steps associated with electron transport through the complex has allowed the orientation within the membrane of the electron-carrying groups of the complex to be modeled.

Photosystem I-dependent cyclic electron transport is important in controlling Photosystem II activity in leaves under conditions of water stress

Leaves of the C3 plant Brassica oleracea were illuminated with red and/or far-red light of different photon flux densities, with or without additional short pulses of high intensity red light, in air or in an atmosphere containing reduced levels of CO2 and/or oxygen. In the absence of CO2, far-red light increased light scattering, an indicator of the transthylakoid proton gradient, more than red light, although the red and far-red beams were balanced so as to excite Photosystem II to a comparable extent. On red background light, far-red supported a transthylakoid electrical field as indicated by the electrochromic P515 signal. Reducing the oxygen content of the gas phase increased far-red induced light scattering and caused a secondary decrease in the small light scattering signal induced by red light. CO2 inhibited the light-induced scattering responses irrespective of the mode of excitation. Short pulses of high intensity red light given to a background to red and/or far-red light induced appreciable additional light scattering after the flashes only, when CO2 levels were decreased to or below the CO2 compensation point, and when far-red background light was present. While pulse-induced light scattering increased, non-photochemical fluorescence quenching increased and F0 fluorescence decreased indicating increased radiationless dissipation of excitation energy even when the quinone acceptor QA in the reaction center of Photosystem II was largely oxidized. The observations indicate that in the presence of proper redox poising of the chloroplast electron transport chain cyclic electron transport supports a transthylakoid proton gradient which is capable of controlling Photosystem II activity. The data are discussed in relation to protection of the photosynthetic apparatus against photoinactivation.

Photosystem I cyclic electron transport: Measurement of ferredoxin-plastoquinone reductase activity

Absorbance changes of ferredoxin measured at 463 nm in isolated thylakoids were shown to arise from the activity of the enzyme ferredoxin-plastoquinone reductase (FQR) in cyclic electron transport. Under anaerobic conditions in the presence of DCMU and an appropriate concentration of reduced ferredoxin, a light-induced absorbance decrease due to further reduction of Fd was assigned to the oxidation of the other components in the cyclic pathway, primarily plastoquinone. When the light was turned off, Fd was reoxidised and this gave a direct quantitative measurement of the rate of cyclic electron transport due to the activity of FQR. This activity was sensitive to the classical inhibitor of cyclic electron transport, antimycin, and also to J820 and DBMIB. Antimycin had no effect on Fd reduction although this was inhibited by stigmatellin. This provides further evidence that there is a quinone reduction site outside the cytochrome bf complex. The effect of inhibitors of ferredoxin-NADP(+) reductase and experiments involving the modification of ferredoxin suggest that there may be some role for the reductase as a component of FQR. Contrary to expectations, NADPH2 inhibited FQR activity; ATP and ADP had no effect.

MOA-stilbene: A new tool for investigation of the reactions of the chloroplast cytochrome bf complex

MOA-stilbene is known to be a specific inhibitor of the Qo site of mammalian cytochrome bc 1 complex. We show that it also binds to the chloroplast cytochrome bf complex. Binding to the reduced enzyme induces a red-shift of the Soret and visible absorption bands of the haems b. Steady state and single turnover experiments with thylakoid membranes show that MOA-stilbene promotes additional 'oxidant-induced reduction' of the b haems and slows their subsequent dark reoxidation. In single turnover experiments, the associated slow phase of the carotenoid bandshift at 518 nm is only partially decreased in apparent extent and rate. These and other effects are similar to those produced by NQNO, a Qi site effector, and by analogy indicate that MOA-stilbene should also be primarily a Qi-site effector of the cytochrome bf complex. MOA-stilbene has less effect on other parts of the photosynthetic chain. This confers an important advantage on MOA-stilbene in that its effects on the cytochrome bf complex can be studied by using Photosystem II to activate turnover. Myxothiazol displays effects on the cytochrome bf complex which are similar to, but much weaker than, those of MOA-stilbene.A Q cycle-based model of turnover of the cytochrome bf complex is presented, which can account for several unusual features of kinetic behaviour.

The physical state of ubiquinone-10, in pure form and incorporated into phospholipid bilayers. A Fourier-transform infrared spectroscopic study

Long-chain quinones are essential components of both bacterial and eukaryotic respiratory chains, and some of the main unsolved questions on energy transduction in membranes are complicated by the lack of consistent information on the physical state of the quinones in membrane bilayers. We have recorded, at various temperatures and under different conditions, the infrared spectra of ubiquinone-10 (the main species in mitochondria) and several analogues. The C = O stretching vibration band located at 1663-1670 cm-1 has been identified as the most sensitive one to phase and environmental changes. Three distinct phases have been characterized in which pure ubiquinone-10 may exist: crystalline (LC1), isotropic liquid (IL) and liquid crystalline (Lc). The only allowed thermotropic transitions are LC1----IL, IL----Lc and Lc----LC1. Our investigations with pure quinones provide a simpler and more detailed description of their phase changes than any of the previous studies and shed light on their behaviour in membranes. When incorporated into phospholipid bilayers, ubiquinone-10 appears to be removed from the aqueous environment and is found to exist, in the 4-70 degrees C range, in an isotropic liquid phase, in the form of small aggregates.

DCCD binds to cytochrome b6 of a cytochrome bf complex isolated from spinach chloroplasts and inhibits proton translocation

Radiolabeled N,N'-dicyclohexylcarbodiimide (DCCD) was bound selectively in a time- and concentration-dependent manner to cytochrome b6 of an enzymatically active cytochrome bf complex isolated from spinach chloroplasts. Maximum labeling of cytochrome b6 was observed with 30 nmol DCCD per nmol cytochrome b6 in the cytochrome bf complex incubated for 30-60 min at 12 degrees C. After incubation of the cytochrome bf complex with DCCD under these conditions, the rate of proton ejection in the complex reconstituted into liposomes was decreased approximately 65-70% when compared to controls; however, under these same conditions the rate of electron transfer through either the soluble bf complex or the complex reconstituted into liposomes was only decreased around 20%. These results suggest that the mechanism of proton translocation through the cytochrome bf complex of spinach chloroplasts is similar to that of the cytochrome bc1 complex from yeast mitochondria in which proton pumping but not electron transfer is also inhibited by DCCD (D. S. Beattie and A. Villalobo, 1982, J. Biol. Chem. 257, 14,745-14,752).

Glynn and the conceptual development of the chemiosmotic theory: a retrospective and prospective view

The origin and evolution of the chemiosmotic theory is described particularly in relation to Peter Mitchell's application of it to model oxidative phosphorylation. Much of the deployment, development and evaluation of the theory occurred at the independent laboratory of the Glynn Research Foundation; the value and future of such an institution is discussed. The role of models mediating between theories and phenomena is analyzed with regard to the growth of knowledge of chemiosmotic systems.

Slip and leak in mitochondrial oxidative phosphorylation

During oxidative phosphorylation by mammalian mitochondria part of the free energy stored in reduced substrates is dissipated and energy is released as heat. Here I review the mechanisms and the physiological significance of this phenomenon.