Electron flow through plastoquinone and cytochromes b6 and f in chloroplasts.

Velthuys, B.R.

doi:10.1073/pnas.76.6.2765

Cited by 60 publications

(9 citation statements)

References 25 publications

(27 reference statements)

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“…We have proposed above that the oxidoreductase complex reacts in a secondorder process with ubiquinol from the pool, through a Q-cycle mechanism. As was first pointed out by Garland et al [53], and more recently noted by Malviya et al [54], Van Ark et al [55], Slater [56] and Velthuys [57], a consequence of this postulate is that the complex, in order to complete a turnover, must oxidize two equivalents of QH 2 and reduce one equivalent of Q. In line with this suggestion we make the following postulates (continuing the lettering from the previous list).…”

Section: Turnover Of the Chain In The Absence Of Antimycinsupporting

confidence: 63%

The role of the quinone pool in the cyclic electron-transfer chain of Rhodopseudomonas sphaeroides A modified Q-cycle mechanism

Crofts

Meinhardt

Jones

et al. 1983

Biochimica et Biophysica Acta (BBA) - Bioenergetics

402

299

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Abstract(1) The role of the ubiquinone pool in the reactions of the cyclic electron-transfer chain has been investigated by observing the effects of reduction of the ubiquinone pool on the kinetics and extent of the cytochrome and electrochromic carotenoid absorbance changes following flash illumination. (2) In the presence of antimycin, flash-induced reduction of cytochrome b-561 is dependent on a coupled oxidation of ubiquinol. The ubiquinol oxidase site of the ubiquinol:cytochrome c 2 oxidoreductase catalyses a concerted reaction in which one electron is transferred to a high-potential chain containing cytochromes c 1 and c 2 , the Rieske-type iron-sulfur center, and the reaction center primary donor, and a second electron is transferred to a lowpotential chain containing cytochromes b-566 and b-561. (3) The rate of reduction of cytochrome b-561 in the presence of antimycin has been shown to reflect the rate of turnover of the ubiquinol oxidase site. This diagnostic feature has been used to measure the dependence of the kinetics of the site on the ubiquinol concentration. Over a limited range of concentration (0-3 mol ubiquinol/ mol cytochrome b-561), the kinetics showed a second-order process, first order with respect to ubiquinol from the pool. At higher ubiquinol concentrations, other processes became rate determining, so that above approx. 25 mol ubiquinol/mol cytochrome b-561, no further increase in rate was seen. (4) The kinetics and extents of cytochrome b-561 reduction following a flash in the presence of antimycin, and of the antimycin-sensitive reduction of cytochrome c 1 and c 2 , and the slow phase of the carotenoid change, have been measured as a function of redox potential over a wide range. The initial rate for all these processes increased on reduction of the suspension over the range between 180 and 100 mV (pH 7). The increase in rate occurred as the concentration of ubiquinol in the pool increased on reduction, and could be accounted for in terms of the increased rate of ubiquinol oxidation. It is not necessary to postulate the presence of a tightly bound quinone at this site with altered redox properties, as has been previously assumed. (5) The antimycinsensitive reactions reflect the turnover of a second catalytic site of the complex, at which cytochrome b-561 is oxidized in an electrogenic reaction. We propose that ubiquinone is reduced at this site with a mechanism similar to that of the two-electron gate of the reaction center. We suggest that antimycin binds at this site, and displaces the quinone species so that all reactions at the site are inhibited. (6) In coupled chromatophores, the turnover of the ubiquinone reductase site can be measured by the antimycin-sensitive slow phase of the electrochromic carotenoid change. At redox potentials higher than 180 mV, where the pool is completely oxidized, the maximal extent of the slow phase is half that at 140 mV, where the pool contains approx. 1 mol ubiquinone/ mol cytochrome b-561 before the flash. At both potentials, cytochrome b-561 be...

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Section: Turnover Of the Chain In The Absence Of Antimycinsupporting

confidence: 63%

The role of the quinone pool in the cyclic electron-transfer chain of Rhodopseudomonas sphaeroides A modified Q-cycle mechanism

Crofts

Meinhardt

Jones

et al. 1983

Biochimica et Biophysica Acta (BBA) - Bioenergetics

402

299

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“…The observed redox dependence of the onset of the reduction phases was consistent with this model. Furthermore, the velocities of the rereduction phases were in line with this process being limited by turnover of the QO site (Velthuys, 1979;Crofts et al, 1983). The model outlined above is shown in Figure 7.…”

Section: Discussionmentioning

confidence: 87%

Membrane-Bound c-Type Cytochromes in Heliobacillus mobilis. In Vivo Study of the Hemes Involved in Electron Donation to the Photosynthetic Reaction Center

Nitschke¹,

Liebl²,

Matsuura³

et al. 1995

Biochemistry

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The amount of heme per photosynthetic reaction center (RC) was examined in whole cells of Heliobacillus mobilis, and a stoichiometry of 5-6 hemes c and 1-3 hemes b per RC was found. Virtually the full complement of heme was seen to be functionally connected to the pool of electron donors to the photosynthetic RC. The kinetic parameters of electron transfer between reduced c-type hemes and the photooxidized primary donor P798+ were studied in whole cells and membrane fragments. The in vivo half-times of electron donation (50% with t 1/2 = 110 microseconds, 50% with t 1/2 = 600 microseconds) were seen to slow down to half-times in the range of several and several tens of milliseconds following disruption of cells. A severe conformational alteration or a change in the identity of the donating heme is discussed. Redox titrations of the flash-induced absorption changes performed on whole cells in the presence of mediators yielded the following redox midpoint potentials: P798, Em = +240 mV; heme c553, Em = +190, +170, and +90 mV for the heme components oxidized after the first, second, and third flash, respectively. The results demonstrate that the pool of c553 hemes donating electrons to the RC is heterogeneous and that it consists of either several distinguishable cytochromes or multiheme cytochromes or both. The number of hemes reduced and the kinetics of heme rereduction after flash-induced oxidation were found to depend strongly on the degree of anaerobicity in the interior compartment of the cell. A model rationalizing the obtained results in terms of a set of differing redox components is proposed.

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“…There is also evidence that under certain conditions the Cyt b(,-f complex can support a Q-cycle mechanism (Velthuys, 1979;Crowther & Hind, 1980;Hurt & Hauska, 1982) whereby two H^ are transported across the membrane for every electron transferred to photo-oxidized P700 instead of one as indicated in Fig. 3 {but see also Olsen, Telfer & Barber, 1980).…”

Section: Organization Of Membrane Componentsmentioning

confidence: 99%

Photosynthetic electron transport in relation to thylakoid membrane composition and organization

Arc

1983

Plant Cell & Environment

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A review is given of the organization and properties of thylakoid membrane proteins and lipids as a basis for understanding the factors which regulate the light reactions of photosynthesis. Particular emphasis is placed on the lateral organization of the major intrinsic multipeptide complexes and on the importance of diffusional processes in controlling the kinetics of electron transport and the distribution of light energy between photosystems 1 and 2.

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Electron flow through plastoquinone and cytochromes b6 and f in chloroplasts.

Cited by 60 publications

References 25 publications

The role of the quinone pool in the cyclic electron-transfer chain of Rhodopseudomonas sphaeroides A modified Q-cycle mechanism

The role of the quinone pool in the cyclic electron-transfer chain of Rhodopseudomonas sphaeroides A modified Q-cycle mechanism

Membrane-Bound c-Type Cytochromes in Heliobacillus mobilis. In Vivo Study of the Hemes Involved in Electron Donation to the Photosynthetic Reaction Center

Photosynthetic electron transport in relation to thylakoid membrane composition and organization

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