Mandal Manoj, Askerka Mikhail, Banerjee Gourab, Amin Muhammed, Brudvig Gary W, Batista Victor S, Gunner M R
Department of Physics, City College of New York, C.U.N.Y. New York 10031, USA.
Dalton Trans. 2017 Nov 28;46(46):16089-16095. doi: 10.1039/c7dt03901h.
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 of ammonium ion is 9.25, it is calculated to only bind as NH, 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 S state (OEC Mn oxidation state 3444) while ammonia only binds in the g = 4.1 form of the S 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 NHK317D61, 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.
通过连续静电/蒙特卡罗（MCCE）、量子力学/分子力学（QM/MM）和密度泛函理论（DFT）计算相结合的方法，对光系统II（PSII）放氧复合体（OEC）附近的第二壳层氨结合位点进行了表征，并与新的和早期的实验测量结果进行了比较。MCCE结果表明，氨在6个位置具有显著亲和力，但只有两个位置对OEC有显著影响。尽管铵离子的pK值为9.25，但计算结果表明其仅以NH形式结合，这与其在低pH下的低亲和力一致。这些计算结果也有助于解释实验观察到的氨与氯的竞争性结合。氨和氯竞争一个位点。静电相互作用导致氯在另外两个位点影响氨。氯稳定了S态（OEC锰氧化态为3444）的多线g = 2.0形式，而由于锰1和锰4之间正电荷的移动，氨仅以S态（氧化态为4443）的g = 4.1形式结合。一个氨在锰4附近结合，并与D2-K317共享一个质子，形成离子对NHK317D61，使得氨的结合对K317A突变敏感。氨的亲和力还取决于水2（锰4的主要配体）的质子化状态。
