Vinyard David J, Askerka Mikhail, Debus Richard J, Batista Victor S, Brudvig Gary W
Department of Chemistry, Yale University , New Haven, Connecticut 06520-8107, United States.
Department of Biochemistry, University of California , Riverside, California 92521, United States.
Biochemistry. 2016 Aug 9;55(31):4432-6. doi: 10.1021/acs.biochem.6b00543. Epub 2016 Jul 28.
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.
氨与光系统II（PSII）的放氧复合体（OEC）中的两个位点结合。第一个位点是作为S2状态下Mn的末端配体，第二个位点在OEC外部，与氯离子存在竞争关系。氨在这后一个二级位点的结合导致S2状态下S = （5）/2自旋异构体比S = （1）/2自旋异构体更受青睐。利用电子顺磁共振光谱，我们发现氨与野生型集胞藻PCC 6803 PSII中的二级位点结合，但不与不结合氯离子的D2-K317A突变型PSII中的该位点结合。通过将这些结果与量子力学/分子力学计算相结合，我们提出氨在二级位点的结合是与D1-D61竞争，作为OEC末端水配体W1的氢键受体。文中还讨论了氨通过其一级位点直接与OEC中的Mn4结合的机制的相关影响。
