Photosynthetic Electron Transfer through the Cytochromeb6f Complex Can Bypass Cytochrome f

Javier G Fernández-Velasco,Arash Jamshidi,Xiao-Song Gong,Jianhui Zhou,Rosie Y Ueng

doi:10.1074/jbc.m102241200

Javier G Fernández-Velasco, Arash Jamshidi + Show 3 more

Open Access

https://doi.org/10.1074/jbc.m102241200

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Abstract

The cytochrome b(6)f complex is an obligatory electron transfer and proton-translocating enzyme in all oxygenic photosynthesis. Its operation has been described by the "Q-cycle." This model proposes that electrons are transferred from plastoquinol to plastocyanin (the reductant of P700 in Photosystem I) through, obligatorily in series, the iron-sulfur and the cytochrome f redox centers in the cytochrome b(6)f complex. However, here we demonstrate that (a) the iron-sulfur center-dependent reductions of plastocyanin and P700 are much faster than cytochrome f reduction, both in Chlamydomonas reinhardtii cytochrome f mutants and in the wild type, and (b) the steady-state photosynthetic electron transport does not correlate with strongly inhibited cytochrome f reduction kinetics in the mutants. Thus, cytochrome f is not an obligatory intermediate for electrons flowing through the cytochrome b(6)f complex. The oxidation equivalents from Photosystem I are delivered to the high potential chain of the cytochrome b(6)f complex both at the cytochrome f level and, independently, at another site connected to the quinol-oxidizing site, possibly the iron-sulfur center.

Highlights

Plants and algae transform light into high energy compounds by the combined operation of Photosystem II (PSII) and Photosystem I (PSI)1 in the chloroplast [1]
Here we demonstrate that (a) the iron-sulfur center-dependent reductions of plastocyanin and P700 are much faster than cytochrome f reduction, both in Chlamydomonas reinhardtii cytochrome f mutants and in the wild type, and (b) the steady-state photosynthetic electron transport does not correlate with strongly inhibited cytochrome f reduction kinetics in the mutants
The oxidation equivalents from Photosystem I are delivered to the high potential chain of the cytochrome b6 f complex both at the cytochrome f level and, independently, at another site connected to the quinol-oxidizing site, possibly the iron-sulfur center

Summary

EXPERIMENTAL PROCEDURES

Genetic Constructs—C. reinhardtii-reconstituted wild type and mutants Y1S and P2V are described in Ref. 20. For P700 photooxidation (percentage of P700 photooxidizable with one flash, as it relates to the total P700 photooxidizable with a 17-Hz train of strong flashes) was measured using permeabilized cells at Eh 420 mV and with 10 ␮M stigmatellin for two different conditions: Condition A, excitation at ␭ Ͼ 695 nm (Schott filter RG695) and monitoring at 543 nm, and condition B, excitation through a blue-green band-pass filter and monitoring at 702–730 nm [20]. The same Km for flash intensity for P700 photooxidation was found for wild type, P2V, and P2V/R156A, and it was 50 Ϯ 5% I and 13 Ϯ 2% I for conditions A and B, respectively. Mathematical Model—The fraction of P700 centers that underwent a given number of turnovers of photooxidation after each flash in a train of flashes was calculated applying the binomial distribution and taking into account the probability of a photochemical event determined by the “flash saturation” (the fraction of P700 photooxidizable with one flash) and the number of flashes fired. K3.A(1 Ϫ k)nϪ3; coeff. c ϭ 0, 0, 0, 1, 4, 10, 20, 35, and 56 for n ϭ 0 – 8, respectively

RESULTS

92 Ϯ 10 a Columns 1– 8 are as follows

DISCUSSION