title: Ammonia Binding in the Second Coordination Sphere of the Oxygen-Evolving Complex of Photosystem II.
abstract: 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.
authors.affiliation: Department of Chemistry, Yale University , New Haven, Connecticut 06520-8107, United States.
authors.forename: David J
authors.initials: DJ
authors.lastname: Vinyard
authors.affiliation: Department of Chemistry, Yale University , New Haven, Connecticut 06520-8107, United States.
authors.forename: Mikhail
authors.initials: M
authors.lastname: Askerka
authors.affiliation: Department of Biochemistry, University of California , Riverside, California 92521, United States.
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Debus
authors.affiliation: Department of Chemistry, Yale University , New Haven, Connecticut 06520-8107, United States.
authors.forename: Victor S
authors.initials: VS
authors.lastname: Batista
authors.affiliation: Department of Chemistry, Yale University , New Haven, Connecticut 06520-8107, United States.
authors.forename: Gary W
authors.initials: GW
authors.lastname: Brudvig
mesh_terms: D019943:Amino Acid Substitution; D000641:Ammonia / Q000378:metabolism*; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D002712:Chlorides / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D006860:Hydrogen Bonding; D008958:Models, Molecular; D056004:Molecular Dynamics Simulation; D016297:Mutagenesis, Site-Directed; D010100:Oxygen / Q000378:metabolism; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D046939:Synechocystis / Q000235:genetics / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D002712:Chlorides; D045332:Photosystem II Protein Complex; D000641:Ammonia; D010100:Oxygen