Further Studies of Proton Translocations in Chloroplasts After Single-Turnover Flashes. III. Conditions for the Operation of an Apparent Q-Cycle in Thylakoids

1985 ◽  
Vol 12 (4) ◽  
pp. 387 ◽  
Author(s):  
AB Hope ◽  
L Handley ◽  
DB Mathews
Keyword(s):  
Electric Field ◽  
Electron Acceptor ◽  
Electron Flow ◽  
Cytochrome B6 ◽  
Single Turnover ◽  
Ph Values ◽  
Maximal Stimulation ◽  
Q Cycle ◽  
Proton Uptake ◽  
Transverse Electron

The proton-to-electron ratio in pea thylakoids, considering proton uptake with ferricyanide as electron acceptor, was reconfirmed as 1 in periods of single-turnover (<0.5 �s, 2-3 mJ) flashes delivered at 5-50 Hz and at pH values 6.4-8.3. Addition of valinomycin in the presence of K+ increased proton uptake in a way depending on [val], flash frequency and flash number. A maximal stimulation by valinomycin of up to about 1.8× controls was observed in fresh preparations. Half-maximal stimulation was caused by c. 2 nM valinomycin at 10-20 Hz, at c. 3 Hz with 10 nM valinomycin, and after c. five flashes at 50 Hz with 10 nM valinomycin. The results are discussed in terms of recent models for the Q-cycle. It is suggested that such a cycle operates in chloroplasts only when the intramembrane electric field induced in a series of flashes is kept small by the presence of valinomycin. Preliminary observations of 'P518', the thylakoid component probably indicating the electric field, are consistent with this idea. This field may control the transverse electron flow between the two cytochrome b6 molecules in the b/f complexes.

Further Studies of Proton Translocations in Chloroplasts After Single-Turnover Flashes. I. Proton Uptake

1983 ◽  
Vol 10 (5) ◽  
pp. 363 ◽  
Author(s):  
AB Hope ◽  
DB Matthews
Keyword(s):  
Ionic Strength ◽  
Electron Flow ◽  
External Solution ◽  
Half Time ◽  
Cycle Model ◽  
Single Turnover ◽  
Q Cycle ◽  
Proton Uptake ◽  
Factor Α ◽  
Light Flashes

Damped, binary oscillations were observed in proton uptake by class C pea chloroplasts given a train of light flashes. The oscillation at pH 7.8 is predictable if the species accepting protons is either the doubly reduced secondary acceptor B�- or a plastoquinone PQ- from the pool, if 0.29 of the secondary acceptor is B- in dark-adapted chloroplasts and if a miss factor α = 0.12 governs the amval of electrons at B or B- after a flash. The rate of proton uptake was measured with varied pH, ionic strength and temperature. The half-time was 95 ms at pH 7.8 and 21°C. Using double flashes separated by variable intervals showed that the species able to accept protons was generated (t½) about 0.8 ms after a flash. The results are consistent with protons from the external solution reacting relatively slowly with univalent anions, which have earlier promptly supplied protons to B2- or PQ2-. Under conditions of cyclic and of non-cyclic electron flow, H+/e- stoichiometries were 1.1 and 1.0 respectively, and so the results do not support a Q-cycle model for pea chloroplasts.

Further Studies of Proton Translocations in Chloroplasts After Single-Turnover Flashes. II. Proton Deposition

1984 ◽  
Vol 11 (4) ◽  
pp. 267 ◽  
Author(s):  
AB Hope ◽  
DB Matthews
Keyword(s):  
Electron Acceptor ◽  
Water Oxidation ◽  
Electron Flow ◽  
Neutral Red ◽  
S Phase ◽  
Cyclic Electron Flow ◽  
Slow Phase ◽  
Single Turnover ◽  
Q Cycle ◽  
Average Size

The deposition of protons in the inside spaces of pea class C chloroplasts was studied by means of the acidification of neutral red measured spectrophotometrically, with the outside space buffered. Careful kinetic analysis of such signals revealed three components, during non-cyclic electron flow induced by single-turnover flashes. These components included a 'slow' phase not emphasized in previous studies. The half-times of these phases were: 'Fast', < 1 ms (not resolved); 'Intermediate', 13-25 ms with added electron acceptor or 4 ms without; and 'Slow', 70-90 ms. Under conditions for cyclic electron flow only the I phase remained; it was the same magnitude as the I phase in non-cyclic flow, and its half-time was c. 3 ms. The F phase, which is usually attributed to protons from the oxidation of water, increased in average size with number of flashes (taken four flashes at a time) and was not fully patent until more than 20 flashes. The size of the I phase, which is usually attributed to protons from the oxidation of plastohydroquinone, when measured in a sequence of flashes to dark-adapted suspensions under non- cyclic conditions, had a binary oscillation in phase with the oscillation in proton uptake reported previously. It was concluded that protons leave PQH2 two at a time on alternate flashes. The S phase (average in 10 test flashes) was reduced by fast preflashes; an origin near photosystem II is suggested. The S phase may imply a small pool of proton-sequestering ability near the water oxidation site, or a number of other possibilities. In steady-state conditions, the ratio of the protons from PQH2 to those from water was 1.0 under all conditions examined except in the absence of added electron acceptor, when it was as high as 1.6. This was the only condition apparently indicating a Q-cycle, with infrequent single-turnover flashes.

Photosystem I-abhängige Phosphorylierung isolierter Chloroplasten mit Ascorbat/2.6-Dichlorphenolindophenol als Elektronendonator und Methylviologen als Elektronenakzeptor / Photosystem I Dependent Phosphorylation of Isolated Chloroplasts with Ascorbate/2,6-Dichlorophenolindophenol as Electron Donor and Methylviologen as Electron Acceptor

1972 ◽  
Vol 27 (4) ◽  
pp. 445-455 ◽  
Author(s):  
Heinrich Strotmann ◽  
Christa Von Gösseln
Keyword(s):  
Electron Transport ◽  
Linear System ◽  
Photosystem I ◽  
Electron Acceptor ◽  
Electron Flow ◽  
Phosphorylation Site ◽  
Cyclic System ◽  
Atp Production ◽  
Proton Uptake ◽  
Isolated Chloroplasts

Photosystem I related phosphorylation of isolated chloroplasts was investigated with special reference to the stoichiometry between ATP production and electron transprt (ATP: 2e⊖). The system studied contained DCMU to inhibit electron flow from photosystem II, ascorbate and DPIP to supply electrons to photosystem I, and methylviologen as electron acceptor. The following results were obtained:1. Basal electron transport is stimulated by the addition of the phosphorylating system, indicating that phosphorylation is really coupled to non-cyclic electron flow. The ratio ATP: 2e⊖ is 1, when the increase of electron flow obtained by the addition of ADP and phosphate is correlated to phosphorylation. This ratio is constant upon varying several parameters including DPIP concentration and light intensity.2. In the absence of methylviologen a DPIP catalyzed cyclic phosphorylation takes place (cf. I. c.7, 11, 12). Phosphorylation is not increased by the addition of methylviologen, indicating that both, the cyclic DPIP mediated and the non-cyclic system are coupled to the same phosphorylation site and limited by the same reaction step.3. In the absence of oxygen a methylviologen supported cyclic phosphorylation occurs. Comparing optimum rates, phosphorylation under these conditions is about twice as high as in the noncyclic system. Therefore we conclude that two phosphorylation sites are involved in methylviologen catalyzed cyclic electron transport. This system is sensitive against trypsin treatment of the chloroplasts, whereas the linear system is not.4. The two cyclic systems as well as the non-cyclic system are coupled to reversible proton uptake. Furthermore the linear system exhibits an irreversible uptake of hydrogen ions, which is stoichiometric to electron flow. From the reversible and the irreversible components of the pH changes the ratio of the proton pump to electron transprt can be calculated. Under steady state conditions the ration H⨁ : e⊖ approaches 1.

Further Studies of Proton Translocations in Chloroplasts After Single-Turnover Flashes. IV. Effects of Cytochrome b/f Complex Inhibitors on Proton Uptake and Cytochrome b6 Turnover

1987 ◽  
Vol 14 (1) ◽  
pp. 47 ◽  
Author(s):  
AB Hope ◽  
S Birch ◽  
DB Matthews
Keyword(s):  
Electron Transport ◽  
Photosystem Ii ◽  
Reduction Rate ◽  
Maximum Effect ◽  
Phenol Red ◽  
Cytochrome B6 ◽  
Oxidation Rates ◽  
Single Turnover ◽  
Proton Uptake ◽  
Reduction And Oxidation

The effects of the substances 2-n-heptyl- and 2-n-nonyl-4-hydroxyquinoline N-oxide (HQNO, NQNO), and antimycin A (AMA) on proton uptake stimulated by a 10-20 Hz train of single-turnover flashes given to pea thylakoids were investigated. Electron transport was from H2O to ferricyanide ('oxidising conditions') and the pH indicator of proton uptake was phenol red. All three of HQNO, NQNO and AMA inhibited proton uptake in control conditions, with concentrations (c½) for half-maximum effect of 1.7, 0.1 and 5 �M, respectively. The valinomycin-stimulated proton uptake, which has been attributed to Q-cycle activity in thylakoids, was more sensitive to HQNO and NQNO, with c½ of 0.6 and < 0.05 �M respectively. AMA had the same or less relative effect on proton uptake in the presence of valinomycin as in its absence. In oxidising conditions the maximum extent of flash-induced cytochrome (cyt) b6 reduction was 7-9% of the total present (which was 2 molecules/620 chlorophylls), as an average during 10 flashes, valinomycin being always added to reduce interference from the electrochromic effect. The average half- time for this reduction was 3.4 ms, while that for oxidation was 420 ms. The amount of cyt b6 reduced was increased by NQNO to a maximum of 14-19%, the c½ being 0.05 �M. Reduction and oxidation rates were both diminished by NQNO. In reducing conditions [electrons from duroquinol to methyl viologen, 3-(3,4-dichlorophenyl)-l,l- dimethylurea added to inhibit photosystem II], the cytochrome b6 was 12-16% reduced during flashes at 0.5-1 Hz, with half-times of 3.1 and 21 ms for reduction and oxidation, respectively. NQNO increased the percentage reduced to a maximum of 34-45, with a c½ of 0.05 �M. The diminution of the oxidation rate of cyt b6 was similarly related to [NQNO] but that of the reduction rate had a c½ of -1 �M. The findings on proton uptake are seen as consistent with HQNO and NQNO inhibiting at the Qc sites on cyt b/f complexes, at QB sites near photosystem II with less specificity and possibly at Q2 sites during the first few turnovers. Data for AMA indicated that it does not inhibit at Qc. Electron transport from H2O to methyl purple was more sensitive to NQNO for the first few turnovers (c½ 0.1 �M) than in the steady state (c½ - 1 �M).

Electron and Proton Transfers around the b/f Complex in Chloroplasts: Modelling the Constraints on Q-cycle Activity

1988 ◽  
Vol 15 (4) ◽  
pp. 567 ◽  
Author(s):  
AB Hope ◽  
DB Matthews
Keyword(s):  
Reduction Potential ◽  
Slow Phase ◽  
Single Turnover ◽  
Reduction Potentials ◽  
Q Cycle ◽  
Proton Transfers ◽  
Proton Uptake ◽  
Time Standard ◽  
Electron Transfers ◽  
Stimulation Of

The requirements for the operation of a Q-cycle in thylakoids are discussed. A computer model is described in which the state of reduction of components of the b/f complex is followed, single turnover at a time. Standard reduction potentials from the literature were assigned to the cytochrome b563 molecules; those for the second electron oxidation of plastoquinol at p-sites, and for the reduction of plastoquone at n-sites, were found by optimising the predicted stimulation of proton uptake by valinomycin. The stimulation has been attributed to uptake by doubly reduced plastoquinone at b/f complexes, a process thought to continue only if the membrane potential (ΔV) is kept low by ionophores. ΔV was simulated in the model via the electrochromic signal; its effect on electron transfers in the b/f complex was incorporated by modifying the reduction potentials. The extent of valinomycin stimulation of proton uptake, its dependence on [valinomycin] and flash frequency, the slow phase of the electrochromic signal and the extent of cytochrome b reduction were predicted by the model when the standard reduction potential for the p-site was set at -0.05 to -0.08 V. with that for the n-site at about 0 V.

Effects of Difenzoquat on Photoreactions and Respiration in Wheat (Triticum aestivum) and Wild Oat (Avena fatua)

Weed Science ◽  
1983 ◽  
Vol 31 (5) ◽  
pp. 693-699 ◽  
Author(s):  
Blaik P. Halling ◽  
Richard Behrens
Keyword(s):  
Triticum Aestivum ◽  
Electron Transport ◽  
Electron Acceptor ◽  
Photosynthetic Electron Transport ◽  
Significant Inhibition ◽  
Avena Fatua ◽  
Wild Oat ◽  
Leaf Chlorosis ◽  
Acceptor Activity ◽  
Proton Uptake

Experiments were conducted with isolated protoplasts of wild oat (Avena fatuaL. # AVEFA) and isolated chloroplasts of wild oat and wheat (Triticum aestivumL.), to determine if the methyl sulfate salt of difenzoquat (1,2-dimethyl-3,5-diphenyl-1H-pyrazolium) might influence photoreactions in these species. Difenzoquat did not affect CO2fixation, uncoupled electron transport, or proton uptake. At concentrations of 0.5 mM and 1 mM, difenzoquat caused a slight, but statistically significant, inhibition of photophosphorylation. Experiments assaying coupled electron transport indicated that inhibition of photophosphorylation occurred not through uncoupling, but by an energy-transfer inhibition. This same effect was observed in isolated mitocondria of both species, with about 50% inhibition of state 3 respiration rates occurring with 10 μM difenzoquat. However, no important differentials were observed in the relative susceptibilities of wheat and wild oat mitochondria. Difenzoquat also functioned as a weak autooxidizing electron acceptor in photosynthetic electron transport. Therefore, difenzoquat-induced leaf chlorosis and necrosis may result from a bipyridilium-type electron acceptor activity if sufficient herbicide is absorbed.

Inhibition of oxygen evolution by ivalin

1994 ◽  
Vol 72 (2) ◽  
pp. 177-181 ◽  
Author(s):  
Ernesto Bernal-Morales ◽  
Alfonso Romo De Vivar ◽  
Bertha Sanchez ◽  
Martha Aguilar ◽  
Blas Lotina-Hennsen
Keyword(s):  
Electron Transport ◽  
Photosystem Ii ◽  
Sesquiterpene Lactone ◽  
Oxygen Evolution ◽  
Electron Flow ◽  
Atp Synthesis ◽  
Naturally Occurring ◽  
Transport Inhibitor ◽  
Proton Uptake ◽  
Key Words Oxygen

The inhibition of ATP synthesis, proton uptake, and electron transport (basal, phosphorylating, and uncoupled) from water to methylviologen by ivalin (a naturally occurring sesquiterpene lactone in Zaluzania triloba and Iva microcephala) indicates that it acts as electron transport inhibitor. Since photosystem I and electron transport from DPC to QA were not affected, while the electron flow of uncoupled photosystem II from H2O to DAD and from water to silicomolybdate was inhibited, we concluded that the site of inhibition of ivalin is located at the oxygen evolution level. Key words: oxygen evolution, ivalin, photosynthesis, sesquiterpene lactone.

scholarly journals Electron transfer via cytochrome b6f complex displays sensitivity to Antimycin A upon STT7 kinase activation.

2022 ◽  
Author(s):  
Felix Buchert ◽  
Martin Scholz ◽  
Michael Hippler
Keyword(s):  
Electron Transfer ◽  
Electron Flow ◽  
Anaerobic Treatment ◽  
State Transitions ◽  
Antimycin A ◽  
Cytochrome B6f Complex ◽  
Cytochrome B6f ◽  
Q Cycle ◽  
B6f Complex

The cytochrome b6f complex (b6f) has been initially considered as the ferredoxin-plastoquinone reductase (FQR) during cyclic electron flow (CEF) with photosystem I that is inhibited by antimycin A (AA). The binding of AA to the b6f Qi-site is aggravated by heme-ci, which challenged the FQR function of b6f during CEF. Alternative models suggest that PROTON GRADIENT REGULATION5 (PGR5) is involved in a b6f-independent, AA-sensitive FQR. Here, we show in Chlamydomonas reinhardtii that the b6f is conditionally inhibited by AA in vivo and that the inhibition did not require PGR5. Instead, activation of the STT7 kinase upon anaerobic treatment induced the AA sensitivity of b6f which was absent in stt7-1. However, a lock in State 2 due to persisting phosphorylation in the phosphatase double mutant pph1;pbcp did not increase AA sensitivity of electron transfer. The latter required a redox poise, supporting the view that state transitions and CEF are not coercively coupled. This suggests that the b6f-interacting kinase is required for structure-function modulation of the Qi-site under CEF favoring conditions. We propose that PGR5 and STT7 independently sustain AA-sensitive FQR activity of the b6f. Accordingly, PGR5-mediated electron injection into an STT7-modulated Qi-site drives a Mitchellian Q cycle in CEF conditions.

How does dissipation work in the electron diffusion region of asymmetric magnetic reconnection

2020 ◽  
Author(s):  
Michael Hesse ◽  
Cecilia Norgren ◽  
Paul Tenfjord ◽  
James Burch ◽  
Yi-Hsin Liu ◽  
...  
Keyword(s):  
Electric Field ◽  
Magnetic Reconnection ◽  
Stagnation Point ◽  
Electric Fields ◽  
Current Density ◽  
Electron Flow ◽  
Diffusion Region ◽  
Density Maximum ◽  
Density Peak ◽  
Electron Diffusion

&lt;p&gt;At some level, magnetic reconnection functions by means of a balance between current dissipation, and current maintenance due to the reconnection electric field. While this dissipation is well understood process in symmetric magnetic reconnection, the way nonideal electric fields interact with the current density in asymmetric reconnection is still unclear. In symmetric reconnection, the current density maximum, the X point location, and the nonideal electric field determined by the divergence of the electron pressure tensor usually coincide. In asymmetric reconnection, however, the electric field at the X point can be partly provided by bulk inertia terms, implying that the X point cannot be the dominant location of dissipation. On the other hand, we know that the nongyrotropic pressure-based electric field must dominate at the stagnation point of the in-plane electron flow, and that electron distributions here feature crescents. The further fact that the current density peak is shifted off the position of the X point indicates that there may be a relation between this current density enhancement, the location of the stagnation point, and the electron nongyrotropies. In this presentation we report on further progress investigating the physics of the electron diffusion region in asymmetric reconnection with a focus on how to explain the dissipation operating under these conditions.&amp;#160;&lt;/p&gt;

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