Cyclic electron transfer in photosystem II in the marine diatom Phaeodactylum tricornutum

International audienceIn Phaeodactylum tricornutum Photosystem II is unusually resistant to damage by exposure to high light intensities. Not only is the capacity to dissipate excess excitations in the antenna much larger and induced more rapidly than in other organisms, but in addition an electron transfer cycle in the reaction center appears to prevent oxidative damage when secondary electron transport cannot keep up with the rate of charge separations. Such cyclic electron transfer had been inferred from oxygen measurements suggesting that some of its intermediates can be reduced in the dark and can subsequently compete with water as an electron donor to Photosystem II upon illumination. Here, the proposed activation of cyclic electron transfer by illumination is confirmed and shown to require only a second. On the other hand the dark reduction of its intermediates, specifically of tyrosine Y D , the only Photosystem II component known to compete with water oxidation, is ruled out. It appears that the cyclic electron transfer pathway can be fully opened by reduction of the plastoquinone pool in the dark. Oxygen evolution reappears after partial oxidation of the pool by Photosystem I, but the pool itself is not involved in cyclic electron transfer

Spin conversion of cytochrome b559 in photosystem II induced by exogenous high potential quinone

The spin-state of cytochrome b559 (Cyt b559) was studied in photosystem II (PSII) membrane fragments by low-temperature EPR spectroscopy. Treatment of the membranes with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) converts the native low-spin (LS) form of Cyt b559 to the high-spin (HS) form characterized with the g= 6.19 and g= 5.95 split signal. The HS Cyt b559 was pH dependent with the amplitude increasing toward more acidic pH values (pH 5.5-8.5). The HS state was not photochemically active upon 77 and 200 K continuous illumination under our conditions and was characterized by a low reduction potential (=<0 V). It was also demonstrated that DDQ treatment damages the oxygen evolving complex, leading to inhibition of oxygen evolution, decrease of the S2-state EPR multiline signal and release of Mn2+. In parallel, studies of model systems containing iron(III) protoporphyrin IX chloride (FeIIIPor), which is a good model compound for the Cyt b559 prosthetic group, were performed by using optical and EPR spectroscopy. The interaction of FeIIIPor with imidazole (Im) in weakly polar solvent results in formation of bis-imidazole coordinated heme iron (FeIIIPor Im2) which mimic the bis-histidine axial ligation of Cyt b559. The reaction of DDQ with the LS FeIIIPor Im2 complex leads to its transformation into the HS state (g@?=5.95, g@?=2.00). It was shown that the spin conversion occurs due to the donor-acceptor interaction of coordinated imidazole with this high-potential quinone causing the displacement of imidazole from the axial position. The similar mechanism of DDQ-induced spin change is assumed to be valid for the native membrane Cyt b559 in PSII centers