The nonheme iron in photosystem II.

Photosystem II (PSII), the light-driven water:plastoquinone (PQ) oxidoreductase of oxygenic photosynthesis, contains a nonheme iron (NHI) at its electron acceptor side. The NHI is situated between the two PQs QA and QB that serve as one-electron transmitter and substrate of the reductase part of PSII, respectively. Among the ligands of the NHI is a (bi)carbonate originating from CO2, the substrate of the dark reactions of oxygenic photosynthesis. Based on recent advances in the crystallography of PSII, we review the structure of the NHI in PSII and discuss ideas concerning its function and the role of bicarbonate along with a comparison to the reaction center of purple bacteria and other enzymes containing a mononuclear NHI site.