© 2016 American Chemical Society. Ammonia binds to two sites in the oxygen-evolving complex (OEC) of Photosystem II (PSII). The first is as a terminal ligand to Mn in the S2 state, and the second is at a site outside the OEC that is competitive with chloride. Binding of ammonia in this latter secondary site results in the S2 state S = 5/2 spin isomer being favored over the S = 1/2 spin isomer. Using electron paramagnetic resonance spectroscopy, we find that ammonia binds to the secondary site in wild-type Synechocystis sp. PCC 6803 PSII, but not in D2-K317A mutated PSII that does not bind chloride. By combining these results with quantum mechanics/molecular mechanics calculations, we propose that ammonia binds in the secondary site in competition with D1-D61 as a hydrogen bond acceptor to the OEC terminal water ligand, W1. Implications for the mechanism of ammonia binding via its primary site directly to Mn4 in the OEC are discussed.
Publication Source (Journal or Book title)
Vinyard, D., Askerka, M., Debus, R., Batista, V., & Brudvig, G. (2016). Ammonia Binding in the Second Coordination Sphere of the Oxygen-Evolving Complex of Photosystem II. Biochemistry, 55 (31), 4432-4436. https://doi.org/10.1021/acs.biochem.6b00543