Characterization of ammonia binding to the second coordination shell of the oxygen-evolving complex of photosystem II

The second-shell ammonia binding sites near the OEC (oxygen-evolving complex) of PSII are characterized by combined Continuum Electrostatic/Monte Carlo (MCCE), QM/MM and DFT calculations and compared with new and earlier experimental measurements. MCCE shows ammonia has significant affinity at 6 positions but only two significantly influence the OEC. Although the pK_a of ammonium ion is 9.25, it is calculated to only bind as NH₃, in agreement with its low affinity at low pH. The calculations also help explain the experimentally observed competitive binding of ammonia with chloride. Ammonia and Cl^- compete for one site. Electrostatic interactions cause Cl^- to effect ammonia at two other sites. Cl^- stabilizes the multiline g = 2.0 form of the S₂ state (OEC Mn oxidation state 3444) while ammonia only binds in the g = 4.1 form of the S₂ state (oxidation state 4443) due to the movement of the positive charge between Mn1 and Mn4. One ammonia binds near Mn4 and shares a proton with D2-K317, making the ion pair NH₄⁺K317⁰D61^-, making ammonia binding sensitive to the K317A mutation. The affinity of ammonia is also dependent on the protonation state of water 2, a primary ligand to Mn4.