Electron transfer between the two photosystems. I. Flash excitation under oxidizing conditions - Joliot, P.; Joliot, A. (Elsevier)
The redox changes of cytochrome b-563 (cytochrome b), cytochrome f, plastocyanin and P-700 were measured on dark-adapted chloroplasts after illumination by a series of flashes in oxidizing conditions (0.1 mM ferricyanide). In these conditions, the plastoquinone pool is fully oxidized and the only available plastoquinol are those formed by Photosystem (PS) II reaction. According to the two-electron gate mechanism proposed by Bouges-Bocquet (Bouges-Bocquet, B. (1973) Biochim. Biophys. Acta 314, 250-256), plastoquinol is mainly formed after the second and the fourth flashes. After the second flash, the reoxidation of plastoquinol occurs by a concerted reaction which reduces most of the cytochrome b present and a fraction of PS I donors. Most of these electrons are stored on P-700, which implies a large equilibrium constant between the secondary PS I donors and P-700. One electron is stored on cytochrome b during a time (t"1"2 ~ 1 s) much longer than the dark interval between flashes. After the fourth flash, a new plastoquinol molecule is formed, which induces the reduction of PS I donors with no corresponding further reduction of cytochrome b. The number of electrons transferred after the fourth flash is larger than that transferred after the second flash although the rate of transfer is lower. To interpret these data, we assume that the plastoquinol formed after the fourth flash is reoxidized by a second concerted reaction: one electron is directly transferred to PS I donors while the other cooperates with the electron stored on cytochrome b to reduce a plastoquinone molecule localized on a site close to the outer face of the membrane. This newly formed plastoquinol crosses the membrane and transfers a second electron to PS I donors. This interpretation resembles a model proposed by Velthuys (Velthuys, B.R. (1979) Proc. Natl. Acad. Sci. U.S.A. 76, 2765-2769) and which belongs to the modified Q-cycle class of models.
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Keywords: W; V; WW-YZ; U