Photoreduction of NADP+ by isolated reaction centers of photosystem II: requirement for plastocyanin

The carrier of photosynthetically generated reducing power is the iron-sulfur protein ferredoxin, which provides directly, or via NADP+, reducing equivalents needed for CO2assimilation and other metabolic reactions in the cell. It is now widely held that, in oxygenic photosynthesis, the generation of reduced ferredoxin-NADP+requires the collaboration in series of two photosystems: photosystem II (PSII), which energizes electrons to an intermediate reducing potential and transfers them to photosystem I (PSI), which in turn is solely competent to energize electrons to the strong reducing potential required for the reduction of ferredoxin-NADP+(the Z scheme). This investigation tested the premise of an alternative scheme, which envisions that PSII, without the involvement of PSI, is also capable of photoreducing ferredoxin-NADP+. We report here unexpected findings consistent with the alternative scheme. Isolated PSII reaction centers (completely free of PSI components), when supplemented with ferredoxin, ferredoxin-NADP+oxidoreductase, and a PSII electron donor, 1,5-diphenylcarbazide, gave a significant photoreduction of NADP+. A striking feature of this electron transfer from a PSII donor to the perceived terminal acceptor of PSI was its total dependence on catalytic quantities of plastocyanin, a copper-containing electron-transport protein hitherto known only as an electron donor to PSI.