Energy Transduction in Oxygenic Photosynthesis: An Overview of Structure and Mechanism

Ort, Donald R.

doi:10.1007/978-3-642-70936-4_4

Cited by 22 publications

(24 citation statements)

References 149 publications

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“…In the presence of DCMU, PSII-dependent reduction of the secondary quinone acceptor, QB, is prevented ( 17). Thus, the physiological electron donor, water, can no longer serve as the electron donor to PSI and an artificial donor must be used to allow PSI electron transport.…”

Section: Discussionmentioning

confidence: 99%

Electron Transport through photosystem I Stimulates Light Activation of Ribulose Bisphosphate Carboxylase/Oxygenase (Rubisco) by Rubisco Activase

Campbell¹,

Ogren²

1990

Plant Physiol.

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The activation state of ribulose bisphosphate carboxylase/ oxygenase (rubisco) in a lysed chloroplast system is increased by light in the presence of a saturating concentration of ATP and a physiological concentration of CO2 (10 micromolar). Electron transport inhibitors and artificial electron donors and acceptors were used to determine in which region of the photosynthetic electron transport chain this light-dependent reaction occurred. In the presence of DCMU and methyl viologen, the artificial donors durohydroquinone and 2,6-dichlorophenolindophenol (DCPIP) plus ascorbate both supported light activation of rubisco at saturating ATP concentrations. No light activation occurred when DCPIP was used as an acceptor with water as electron donor in the presence of ATP and dibromothymoquinone, even though photosynthetic electron transport was observed. Nigericin completely inhibited the light-dependent activation of rubisco. Based on these results, we conclude that stimulation of light activation of rubisco by rubisco activase requires electron transport through PSI but not PSII, and that this light requirement is not to supply the ATP needed by the rubisco activase reaction. Furthermore, a pH gradient across the thylakoid membrane appears necessary for maximum light activation of rubisco even when ATP is provided exogenously.Light activation ofrubisco' has been demonstrated in leaves (15,18,25,29) and intact chloroplasts (2, 10). The light responses of the activation of rubisco, the enzyme which catalyzes photosynthetic CO2 assimilation (16), and the rate of CO2 assimilation have been shown to proceed in tandem (18,25,29). Several enzymes involved in C3 photosynthetic carbon metabolism in addition to rubisco are also light activated, but unlike rubisco these enzymes are activated by the ferredoxin/thioredoxin system (5). Rubisco, in vivo, is activated by the rubisco activase system in an ATP-dependent process ( 19). Before the discovery of rubisco activase (24), the role of light in rubisco activation was proposed to be the establishment of an alkaline pH and an increase in Mg2' Previous studies (23,27,30) suggested a role for thylakoid membranes in the activation ofrubisco, and early experiments with the rubisco activase system (20, 24) required the presence ofthylakoid membranes and light in reconstituted chloroplast assays. However, considering the rubisco activase requirement for ATP, the role of the thylakoid membrane seemed to be defined. Recently we demonstrated a requirement for light and photosynthetic electron transport for full activation of rubisco by rubisco activase in lysed chloroplasts at physiological concentrations of C02, even though ATP was supplied exogenously at saturating concentrations (6). Activation occurred in the presence of MV, indicating the ferredoxin/ thioredoxin system was not involved. These results implied a more direct involvement of the thylakoid membrane in the activation of rubisco in vivo.The objectives of the present study were to determine the location of the light-...

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Section: Discussionmentioning

confidence: 99%

Electron Transport through photosystem I Stimulates Light Activation of Ribulose Bisphosphate Carboxylase/Oxygenase (Rubisco) by Rubisco Activase

Campbell¹,

Ogren²

1990

Plant Physiol.

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“…At low ATP/ADP ratios, flexible Q-cycle activity may make the participation of auxiliary ATP-producing reactions in C3 photosynthesis unnecessary (17,18). It should be noted that photosynthesis does not demand a high ratio of NADPH to NADP or a high phosphorylation potential of the chloroplast adenylate system.…”

Section: Consequences Of the H+/atp Ratio In Atp Synthesismentioning

confidence: 99%

“…According to Rich (19), Q-cycle coupling is obligatory. Others consider it to be facultative, with decreasing coupling efficiency at increasing light intensities (17,18). Crowther and Hind (4) proposed a modified Q-cycle, with electron input from the reducing side of PSI.…”

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confidence: 99%

Concerning a Dual Function of Coupled Cyclic Electron Transport in Leaves

Heber¹,

Walker²

1992

Plant Physiol.

270

159

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Coupled cyclic electron transport is assigned a role in the protection of leaves against photoinhibition in addition to its role in ATP synthesis. In leaves, as in reconstituted thylakoid systems, cyclic electron transport requires 'poising," i.e. availability of electrons at the reducing side of photosystem I (PSI) and the presence of some oxidized plastoquinone between photosystem 11 (PSII) and PSI. Under self-regulatory poising conditions that are established when carbon dioxide limits photosynthesis at high light intensities, and particularly when stomata are partially or fully closed as a result of water stress, coupled cyclic electron transport controls linear electron transport by helping to establish a proton gradient large enough to decrease PSII activity and electron flow to PSI. This brings electron donation by PSII, and electron consumption by available electron acceptors, into a balance in which PSI becomes more oxidized than it is during fast carbon assimilation. Avoidance of overreduction of the electron transport chain is a prerequisite for the efficient protection of the photosynthetic apparatus against photoinactivation.Despite several decades of intensive research, it is still not entirely clear how in leaves the reactions of the photosynthetic electron transport chain, which yield reductant (in the form of NADPH) and phosphate energy (in form of ATP), are geared to the reactions of stromal enzymes. These use this "assimilatory power' (1) from the chloroplast stroma to the intrathylakoid space of the chloroplasts, and that the free energy of the proton gradient thus formed is used for ATP synthesis, neither the stoichiometry of H+/e coupling nor that of ATP synthesis are known for certain. During linear electron transport from water to NADP, two protons are released inside the thylakoids per H20 oxidized by PSII and at least another two by the subsequent oxidation of the plastoquinol formed as a consequence of the transfer of the two electrons of water to plastoquinone. Unfortunately, it is still not quite clear how the Cyt b/f complex mediates oxidation of plastoquinol. If it passes electrons of this two-electron carrier straight on to PSI, which lifts them to the redox levels of Fd and NADP, the H+/e ratio of linear electron transport is 2. If only one electron from the plastoquinol is transferred to PSI and the other one is shuttled back to plastoquinone in the loop of a Mitchellian Q-cycle, the H+/e ratio is 3. According to Rich (19), Q-cycle coupling is obligatory. Others consider it to be facultative, with decreasing coupling efficiency at increasing light intensities (17, 18). Crowther and Hind (4) proposed a modified Q-cycle, with electron input from the reducing side of PSI. CONSEQUENCES OF THE H+/ATP RATIO

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“…A central question is the possible involvement of the Q-cycle, a cyclic electron flow inside the cytochrome (cyt) b 6 /f complex (Mitchell, 1975(Mitchell, , 1977 able to translocate additional H ϩ and therefore provide extra ATP. However, the obligatory character or the flexibility of the Q-cycle during CO 2 fixation remains a matter of debate (Davenport and McCarty, 1984;Ort, 1986;Heber and Walker, 1992;Cramer et al, 1996). Other mechanisms, like cooperation with mitochondrial respiration (Krö mer, 1995;Hoefnagel et al, 1998) and Mehler reactions (also known as waterwater cycle) (Schreiber and Neubauer, 1990) have also been suggested to re-equilibrate the chloroplastic ATP to NADPH ratio by generating extra-ATP, but their contribution during CO 2 fixation remains to be established.…”

mentioning

confidence: 99%

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

et al. 2001

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) 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 CO 2 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 Ϫ and wild type (WT). When leaf discs were treated with 5 m 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 Ϫ . Under non-photorespiratory conditions (2% O 2 , 2,500 L L Ϫ1 CO 2 ), 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 CO 2 assimilation in ndhB Ϫ . The effect of antimycin A on ndhB Ϫ could not be mimicked by myxothiazol, an inhibitor of the mitochondrial cytochrome bc 1 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 Ϫ plants under normal conditions is explained by the complement of these two pathways in the supply of extra-ATP for photosynthesis.During oxygenic photosynthesis of C 3 plants, both photosystem II (PSII) and photosystem I (PSI) cooperate to achieve NADP ϩ reduction using water as an electron donor and generate a trans-membrane proton gradient driving ATP synthesis. Although NADP ϩ reduction is recognized to be dependent on the activity of both photosystems through electron transport reactions of the "Z" scheme (Hill and Bendall, 1960;Redding et al., 1999), it has early been reported from studies on isolated thylakoids that ATP could be produced by the sole PSI through cyclic electron transfer reactions (Arnon, 1959). The cyclic electron flow around PSI has been extensively studied in thylakoids and/or chloroplasts of C 3 plants (for review, see Fork and Herbert, 1993; Bendall and Manasse, 1995). This mechanism has been suggested to provide ATP for a variety of cellular processes, including stress adaptation (Havaux et al., 1991) and CO 2 fixation (Furbank and Horton, 1987; Herbert et al., 1990). During photosynthetic CO 2 fixation, both NADPH and ATP are used to regenerate ribulose-1,5-bisphosphate and allow functioning of the photosynthetic carbon reduction cycle (Calvin cycle). In the absence of Q cycle, when one NADPH is produced by linear electron transport reactions, four H ϩ are released in ...

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Energy Transduction in Oxygenic Photosynthesis: An Overview of Structure and Mechanism

Cited by 22 publications

References 149 publications

Electron Transport through photosystem I Stimulates Light Activation of Ribulose Bisphosphate Carboxylase/Oxygenase (Rubisco) by Rubisco Activase

Electron Transport through photosystem I Stimulates Light Activation of Ribulose Bisphosphate Carboxylase/Oxygenase (Rubisco) by Rubisco Activase

Concerning a Dual Function of Coupled Cyclic Electron Transport in Leaves

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

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