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.

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.

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.

scholarly journals PGR5 is required for efficient Q cycle in the cytochrome b6f complex during cyclic electron flow

2019 ◽  
Author(s):  
Felix Buchert ◽  
Laura Mosebach ◽  
Philipp Gäbelein ◽  
Michael Hippler
Keyword(s):  
Electron Transfer ◽  
Reductase Activity ◽  
Electron Flow ◽  
Cyclic Electron Flow ◽  
Redox Activity ◽  
Q Cycle ◽  
Redox Tuning ◽  
Photosynthetic Control ◽  
Photosynthetic Electron Transfer

AbstractProton Gradient Regulation 5 (PGR5) is involved in the control of photosynthetic electron transfer but its mechanistic role is not yet clear. Several models have been proposed to explain phenotypes such as a diminished steady state proton motive force (pmf) and increased photodamage of photosystem I (PSI). Playing a regulatory role in cyclic electron flow (CEF) around PSI, PGR5 contributes indirectly to PSI protection by enhancing photosynthetic control, which is a pH-dependent downregulation of electron transfer at the cytochrome b6f complex (b6f). Here, we re-evaluated the role of PGR5 in the green alga Chlamydomonas reinhardtii and conclude that pgr5 possesses a dysfunctional b6f. Our data indicate that the b6f low-potential chain redox activity likely operated in two distinct modes – via the canonical Q cycle during linear electron flow and via an alternative Q cycle during CEF, attributing a ferredoxin-plastoquinone reductase activity to the b6f. The latter mode allowed efficient oxidation of the low-potential chain in the WT b6f. A switch between the two Q cycle modes was dependent of PGR5 and relied on unknown stromal electron carrier(s), which were a general requirement for b6f activity. In CEF-favouring conditions the electron transfer bottleneck in pgr5 was the b6f and insufficient flexibility in the low-potential chain redox tuning might account for the mutant pmf phenotype and the secondary consequences. Models of our findings are discussed.

scholarly journals PGR5 is required for efficient Q cycle in the cytochrome b6f complex during cyclic electron flow

2020 ◽  
Vol 477 (9) ◽  
pp. 1631-1650 ◽  
Author(s):  
Felix Buchert ◽  
Laura Mosebach ◽  
Philipp Gäbelein ◽  
Michael Hippler
Keyword(s):  
Electron Transfer ◽  
Reductase Activity ◽  
Electron Flow ◽  
Current Model ◽  
Cyclic Electron Flow ◽  
Redox Activity ◽  
Cytochrome B6f Complex ◽  
Cytochrome B6f ◽  
Q Cycle ◽  
B6f Complex

Proton gradient regulation 5 (PGR5) is involved in the control of photosynthetic electron transfer, but its mechanistic role is not yet clear. Several models have been proposed to explain phenotypes such as a diminished steady-state proton motive force (pmf) and increased photodamage of photosystem I (PSI). Playing a regulatory role in cyclic electron flow (CEF) around PSI, PGR5 contributes indirectly to PSI protection by enhancing photosynthetic control, which is a pH-dependent down-regulation of electron transfer at the cytochrome b6f complex (b6f). Here, we re-evaluated the role of PGR5 in the green alga Chlamydomonas reinhardtii and conclude that pgr5 possesses a dysfunctional b6f. Our data indicate that the b6f low-potential chain redox activity likely operated in two distinct modes — via the canonical Q cycle during linear electron flow and via an alternative Q cycle during CEF, which allowed efficient oxidation of the low-potential chain in the WT b6f. A switch between the two Q cycle modes was dependent on PGR5 and relied on unknown stromal electron carrier(s), which were a general requirement for b6f activity. In CEF-favoring conditions, the electron transfer bottleneck in pgr5 was the b6f, in which insufficient low-potential chain redox tuning might account for the mutant pmf phenotype. By attributing a ferredoxin-plastoquinone reductase activity to the b6f and investigating a PGR5 cysteine mutant, a current model of CEF is challenged.

Physiological Role of Cyclic Electron Flow in Higher Plants

2012 ◽  
Vol 30 (1) ◽  
pp. 100
Author(s):  
Wei HUANG ◽  
Shi-Bao ZHANG ◽  
Kun-Fang CAO
Keyword(s):  
Higher Plants ◽  
Electron Flow ◽  
Physiological Role ◽  
Cyclic Electron Flow

[RuV(NCN-Me)(bpy)(O)]3+ mediated efficient photo-driven water oxidation

RSC Advances ◽  
2016 ◽  
Vol 6 (66) ◽  
pp. 61959-61965 ◽  
Author(s):  
Jully Patel ◽  
Karunamay Majee ◽  
Sumanta Kumar Padhi
Keyword(s):  
Electron Acceptor ◽  
Water Oxidation ◽  
Phosphate Buffer ◽  
Active Catalyst

The complex [Ru(NCN-Me)(bpy)H2O](PF6)2 acts as an active catalyst for the photo-driven oxidation of water, when employed with [Ru(bpy)3]2+ as photosensitizer and Na2S2O8 as sacrificial electron acceptor at pH 6.5 phosphate buffer, with a TON of 130.

Cyclic electron flow around photosystem I is enhanced at low pH

2014 ◽  
Vol 83 ◽  
pp. 194-199 ◽  
Author(s):  
Teena Tongra ◽  
Sudhakar Bharti ◽  
Anjana Jajoo
Keyword(s):  
Photosystem I ◽  
Electron Flow ◽  
Low Ph ◽  
Cyclic Electron Flow
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