Ammonia Binding in the Second Coordination Sphere of the Oxygen-Evolving Complex of Photosystem II

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.