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

Abstract

It is now widely held that the light-induced noncyclic (linear) electron transport from water to NADP+ requires the collaboration in series of the 2 photosystems that operate in O2-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. 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 2 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.