Kinetic Effects of the Electrochemical Proton Gradient on Plastoquinone Reduction at the Qi Site of the Cytochrome b 6 f Complex

Abstract

We have investigated the effects of the light-induced thylakoid transmembrane potential on the turnover of the b 6 f complex in cells of the unicellular green alga Chlamydomonas reinhardtii. The reduction of the potential by either decreasing the light intensity or by adding increasing concentrations of the ionophore carbonylcyanide p-(trifluoromethoxy)phenylhydrazone (FCCP) revealed a marked inhibition of the cytochrome b 6 oxidation rate (10-fold) without substantial modifications of cytochrome f oxidation kinetics. Partial recovery of this inhibition could be obtained in the presence of ionophores provided that the membrane potential was re-established by illumination with a train of actinic flashes fired at a frequency higher than its decay. Measurements of isotopic effects on the kinetics of cytochrome b 6 oxidation revealed a synergy between the effects of ionophores and the H 2 O-D 2 O exchange. We propose therefore, that protonation events influence the kinetics of cytochrome b 6 oxidation at the Qi site and that these reactions are strongly influenced by the lightdependent generation of a transmembrane potential.The cytochrome b 6 f complex is a central component of the photosynthetic chain, transferring electrons from the photosystem II (PSII) 1 to the photosystem I (PSI). It comprises four major subunits (1, 2): the Rieske protein, which binds a Fe 2 S 2 cluster, with an E m of ϩ290 mV (3), a c-type cytochrome, cytochrome f, with an E m of ϩ330 mV (2), a b-type cytochrome, cytochrome b 6 , which binds the high and low potential hemes, b h and b l , with E m values of Ϫ84 mV and Ϫ158 mV, respectively (2), and the subunit IV. Four additional small subunits PetG, PetL, PetM and PetN (2, 4 -5) have also been evidenced. Sequence comparisons have shown that cytochrome b 6 and subunit IV are, respectively, homologous to the N-and C-terminal parts of mitochondrial and bacterial cytochromes b (6).As a member of the bc-type proteins, cytochrome b 6 f complex couples proton translocation across the membrane to electron transfer from a lipophilic two-electron donor (plastoquinol) to a hydrophilic one-electron acceptor protein (plastocyanin or a c-type cytochrome). This electron transfer operates through a high potential chain (also called the linear path) formed by the Rieske protein and cytochrome f. The most widely accepted hypothesis to explain the function mechanism of bc complexes is the "Q cycle" hypothesis of Mitchell (7). It predicts that oxidation of quinols not only involves the linear path but also a cyclic path composed of the b hemes, b l and b h , and termed the low potential chain. This mechanism, as modified by Crofts et al. (8), postulates both an oxidation and a reduction of plastoquinol at two distinct sites of the protein, the Qo and Qi sites, on opposite sides of the membrane. The oxidation of plastoquinol at the Qo site is associated with the reduction of both cytochrome f and b l (7-8) and the release of two protons into the lumen.The recent elucidation of the structure of the respirator...

Cytochrome b6 Arginine 214 of Synechococcus sp. PCC 7002, a Key Residue for Quinone-reductase Site Function and Turnover of the Cytochrome bf Complex

“…With the wild type, we obtained results closely resembling those previously reported for C. reinhardtii (37). Both the reduction and oxidation rates of the cytochrome b 6 hemes decreased substantially in D 2 O-enriched medium (compare the solid and open squares in Fig.…”

“…6). This implies that steps in both plastoquinol deprotonation at the Q o site and plastoquinone protonation at the Q i site affect the overall reactions at these sites as shown previously for chloroplasts (37,54). The R214H mutant showed a small isotope effect on b 6 heme reduction but virtually none on b 6 heme oxidation, indicating that an electron transfer step has been slowed at least as much as any of the protonation steps.…”

“…At intervals following an actinic flash, the transmission of weak, monochromatic light passing through the sample was measured relative to a reference sample shielded from actinic light. Data were expressed as the difference (⌬I) in light transmission between the sample and reference cuvettes, normalized to light transmission (I) by the reference cuvette as in (36,37). Cells were collected in the exponential phase of growth and resuspended in 20 mM Hepes buffer, pH 7.2, containing 20% Ficoll (w/v) to prevent cell sedimentation.…”

“…In particular, this effect mostly concerns the reactions that follow plastoquinol oxidation at the Q o site (41) and plastoquinone reduction (or cytochrome b 6 oxidation) at the Q i site (37). The interpretation for this isotopic effect is that deprotonation and protonation of the quinones kinetically limits the reactions occurring in both the Q o and Q i sites.…”

Thylakoid potassium channel is required for efficient photosynthesis in cyanobacteria

“…Beside enhancement of the activation of ATP synthase, the higher ΔΨ might have also additional effects. For example, it has been reported to restrain electron transport at the level of the cytochrome b 6 f complex (22). Because electron transfer is more sensitive to ΔpH than ΔΨ, at high acidic luminal pH, the electron transfer would slow down because of the photosynthetic control, thus limiting the total pmf.…”