Photosynthetic electron transport from water to NADP+ driven by photosystem II in inside-out chloroplast vesicles

AUTOR(ES)

Albertsson, Per-Åke

RESUMO

It is now widely held that the light-induced noncyclic (linear) electron transport from water to NADP+ requires the collaboration in series of the two photosystems that operate in oxygen-evolving cells: photosystem II (PSII) photooxidizes water and transfers electrons to photosystem I (PSI); PSI photoreduces ferredoxin, which in turn reduces NADP+ (the Z scheme). However, a recently described alternative scheme envisions that PSII drives the noncyclic electron transport from water to ferredoxin and NADP+ without the collaboration of PSI, whose role is limited to cyclic electron transport [Arnon, D. I., Tsujimoto, H. Y. & Tang, G. M.-S. (1981) Proc. Natl. Acad. Sci. USA 78, 2942-2946]. Reported here are findings at variance with the Z scheme and consistent with the alternative scheme. Thylakoid membrane vesicles were isolated from spinach chloroplasts by the two-phase aqueous polymer partition method. Vesicles, originating mainly from appressed chloroplast membranes that are greatly enriched in PSII, were turned inside-out with respect to the original sidedness of the membrane. With added plastocyanin, ferredoxin, and ferredoxin-NADP+ reductase, the inside-out vesicles enriched in PSII gave a significant photoreduction of NADP+ with water as electron donor, under experimental conditions that appear to exclude the participation of PSI.

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