Alternative Electron Acceptors for Photosystem II

Photosystem II (PSII) is conserved in all oxygenic photosynthetic organisms and is important for its unique ability to use energy from light to split water, generate molecular oxygen in the Earth’s atmosphere and drive electrons into the photosynthetic electron transport chain by reducing the plastoquinone (PQ) pool in the thylakoid membrane. The focus of this chapter is on alternative electron-transfer pathways on the acceptor side of PSII. Upon close examination of the literature there is evidence of exogenous electron acceptors that are reduced directly by the primary PQ electron acceptor (QA), bypassing the canonical terminal PQ-reduction (QB) site. These herbicide-insensitive electron-acceptor molecules include but are not limited to ferricyanide, synthetic cobalt coordination complexes, and cytochrome c. We also discuss experimental treatments to PSII such as cation exchange and herbicide treatment that have been shown to alter the redox midpoint potential (Em) of QA and impact electron transfer from QA to QB. The results described in this chapter provide a platform for understanding how electrons generated in PSII by photochemical water oxidation can be extracted from the electron-acceptor side of PSII for energy applications.