Amin Muhamed, Vogt Leslie, Szejgis Witold, Vassiliev Serguei, Brudvig Gary W, Bruce Doug, Gunner M R
†Department of Physics, J-419, City College of New York, 138th Street, Convent Avenue, New York, New York 10031, United States.
‡Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, United States.
J Phys Chem B. 2015 Jun 18;119(24):7366-77. doi: 10.1021/jp510948e. Epub 2015 Jan 26.
The oxygen-evolving complex (OEC) of photosystem II (PSII) is a unique Mn4O5Ca cluster that catalyzes water oxidation via four photoactivated electron transfer steps. As the protein influence on the redox and protonation chemistry of the OEC remains an open question, we present a classical valence model of the OEC that allows the redox state of each Mn and the protonation state of bridging μ-oxos and terminal waters to remain in equilibrium with the PSII protein throughout the redox cycle. We find that the last bridging oxygen loses its proton during the transition from S0 to S1. Two possible S2 states are found depending on the OEC geometry: S2 has Mn4(IV) with a proton lost from a terminal water (W1) trapped by the nearby D1-D61 if O5 is closer to Mn4, or Mn1(IV), with partial deprotonation of D1-H337 and D1-E329 if O5 is closer to Mn1. In S3, the OEC is Mn4(IV) with W2 deprotonated. The estimated OEC Em's range from +0.7 to +1.3 V, enabling oxidation by P680(+), the primary electron donor in PSII. In chloride-depleted PSII, the proton release increases during the S1 to S2 transition, leaving the OEC unable to properly advance through the water-splitting cycle.
光系统II（PSII）的析氧复合物（OEC）是一种独特的Mn4O5Ca簇，它通过四个光激活电子转移步骤催化水氧化。由于蛋白质对OEC氧化还原和质子化化学的影响仍是一个悬而未决的问题，我们提出了一种OEC的经典价键模型，该模型允许每个Mn的氧化还原状态以及桥连μ-氧原子和末端水的质子化状态在整个氧化还原循环中与PSII蛋白质保持平衡。我们发现，在从S0到S1的转变过程中，最后一个桥连氧失去了它的质子。根据OEC的几何结构发现了两种可能的S2状态：如果O5更靠近Mn4，则S2具有Mn4(IV)，其质子从被附近D1-D61捕获的末端水（W1）中丢失；如果O5更靠近Mn1，则S2具有Mn1(IV)，伴有D1-H337和D1-E329的部分去质子化。在S3中，OEC为Mn4(IV)，W2去质子化。估计的OEC Em范围为+0.7至+1.3 V，能够被PSII中的初级电子供体P680(+)氧化。在氯化物耗尽的PSII中，质子释放量在S1到S2的转变过程中增加，导致OEC无法在水分解循环中正常推进。
