Department of Biology, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario L2S 3A1, Canada.
Biochemistry. 2010 Mar 9;49(9):1873-81. doi: 10.1021/bi901900s.
The CaMn(4) cluster of the oxygen-evolving complex (OEC) of photosynthesis catalyzes the light-driven splitting of water into molecular oxygen, protons, and electrons. The OEC is buried within photosystem II (PSII), a multisubunit integral membrane protein complex, and water must find its way to the CaMn(4) cluster by moving through protein. Channels for water entrance, and proton and oxygen exit, have previously been proposed following the analysis of cavities found within X-ray structures of PSII. However, these analyses do not account for the dynamic motion of proteins and cannot track the movement of water within PSII. To study water dynamics in PSII, we performed molecular dynamics simulations and developed a novel approach for the visualization of water diffusion within protein based on a streamline tracing algorithm used in fluid dynamics and diffusion tensor imaging. We identified a system of branching pathways of water diffusion in PSII leading to the OEC that connect to a number of distinct entrance points on the lumenal surface. We observed transient changes in the connections between channels and entrance points that served to moderate both the flow of water near the OEC and the exchange of water inside and outside of the protein. Water flow was significantly altered in simulations lacking the OEC which were characterized by a simpler and wider channel with only two openings, consistent with the creation of an ion channel that allows entry of Mn(2+), Ca(2+), and Cl(-) as required for construction of the CaMn(4) cluster.
光合作用中放氧复合酶（OEC）的 CaMn(4) 簇催化水在光照下裂解为分子氧、质子和电子。OEC 埋藏在光合系统 II（PSII）中，这是一个多亚基整合膜蛋白复合物，水必须通过蛋白质移动才能到达 CaMn(4)簇。此前，根据 PSII 的 X 射线结构中发现的空腔分析，提出了水进入、质子和氧气逸出的通道。然而，这些分析并未考虑蛋白质的动态运动，也无法追踪 PSII 内水的运动。为了研究 PSII 中的水动力学，我们进行了分子动力学模拟，并开发了一种新方法，基于用于流体动力学和扩散张量成像的流线追踪算法，可视化蛋白质内的水扩散。我们确定了 PSII 中通往 OEC 的水扩散分支途径系统，该系统与腔表面上的多个不同入口点相连。我们观察到通道和入口点之间的连接发生了短暂的变化，这有助于调节 OEC 附近水的流动以及蛋白质内外水的交换。在缺乏 OEC 的模拟中，水的流动发生了显著变化，其特点是通道更简单、更宽，只有两个开口，这与创建一个离子通道一致，该通道允许 Mn(2+)、Ca(2+)和 Cl(-)进入，以满足 CaMn(4)簇的构建要求。
