Photoreduction of ferredoxin with various electron donors: Support for the Z scheme of photosynthetic electron transport.

The currently accepted scheme for photosynthetic electron flow from water to ferredoxin (and subsequently to NADP), known as the Z scheme, envisions a linear electron flow that requires two photosystems joined by several electron carriers. The observation that the extent of photoreduction of ferredoxin depends on whether electrons are provided by water (a donor to photosystem II) or by artificial electron donors to photosystem I led Arnon et al. [Arnon, D. I., Tsujimoto, H. Y. & Tang, G. M.-S. (1980) Proc. Natl. Acad. Sei. USA 77, 2676-2680] to question the validity of the Z scheme for photosynthetic electron transport. Our results show that this difference is not due to any inherent difference in electron transport but to the fact that when electron donors are added to chloroplasts they oxidize (in their oxidized state) reduced ferredoxin. Different electron donors oxidize reduced ferredoxin to a different extent; dehydroascorbate is a more potent oxidant than dithiothreitol. Dichlorophenol-indophenol is also a potent oxidant of reduced ferredoxin. The rate of NADP reduction and the K(m) for NADP with various electron donors reflect the oxidative capacities of the electron donors and mediators. These results also explain the fact that NADP reduction with electron donors to photosystem I is less than that with water, despite the fact that electron flow from donor to artificial dyes can proceed at high rates.