Athina Zouni

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 Q_A and Q_B 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.

The multiline signal of the S 2 state in Photosystem II was measured both in frozen-solution and single-crystal preparations from the cyanobacterium Thermosynechococcus elongatus . The frozen-solution EPR spectrum shows a gaussian-like line shape without any resolution of Mn hyperfine couplings. This line shape can be understood on the basis of the single-crystal spectra, where a strong orientation dependence of partially resolved hyperfine structures appears. Simulation of the frozen-solution spectrum on the basis of Mn hyperfine couplings taken from published pulse-ENDOR data yields a fully rhombic g -matrix for the multiline signal with principal components 1.997, 1.970, and 1.965. The resulting isotropic g -value g iso = 1.977 is surprisingly small compared to other manganese complexes containing manganese ions in the formal oxidation states three and four. Keywords Photosystem II Oxygen evolving complex Manganese cluster EPR Multiline signal