STRUCTURE AND ROLE OF THE CALCIUM COFACTOR IN PHOTOSYNTHETIC OXYGEN EVOLUTION

这个子项目是许多研究子项目中利用 资源由NIH/NCRR资助的中心拨款提供。子项目和 调查员(PI)可能从NIH的另一个来源获得了主要资金， 并因此可以在其他清晰的条目中表示。列出的机构是 该中心不一定是调查人员的机构。 光合作用的水氧化发生在光系统II(PS II)内，它嵌入绿色植物、蓝藻和藻类囊体的膜中。在PSⅡ中，Mn团簇耦合了一电子初级电荷分离(光氧化)和四电子水氧化循环(S态)。S0到S4，连续存储氧化当量，直到达到S4，于是它将两个底物水分子氧化成氧气，并返回到S0状态。钙是一种重要的辅因子，如果没有钙，OEC就不能催化水氧化成氧气、质子和电子。钙已被证明是S2到S3状态转变和随后的放氧所必需的。XANES和EXAFS实验都是在OEC的MnK边进行的，以确定Mn位的氧化态和结构信息。相比之下，对钙辅因子的光谱研究较少。对PSII中的钙的研究大多是生化的，因为钙没有简单的光谱处理。钙辅因子结合位点的结构已经引起了相当大的讨论。为了探索这一点，可以通过在缺钙的锶介质中生长蓝藻来在化学或生物化学上用锶取代OEC中的钙，该介质结合在蛋白质中的钙位置。根据以前的建议，我们小组已经进行了Ca和SrK边EXAFS实验，以解决Ca/Sr与OEC的Mn4-团簇接近的问题。利用天然PSⅡ的Ca EXAFS和锶活化的PS II膜上的Sr EXAFS，我们证实了在OEC的S1暗稳定状态下，3.5A处的Ca/Sr接近于Mn团簇。用极化的锶EXAFS对取向的锶再活化样品，确定了相对于膜法线的平均锶-锰矢量取向在0-23度范围内。目前的挑战是确定MN4-Ca的相互作用。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Photosynthetic water oxidation takes place within Photosystem II (PS II), which is embedded in the thylakoid membranes of green plants, cyanobacteria and algae. Within PS II, the Mn cluster couples the one electron primary charge separation (photo-oxidation) with the four-electron water oxidation cycling through ?S-states? S0 through S4, storing oxidative equivalents serially until it reaches S4, whereupon it oxidizes two substrate water molecules to dioxygen, and returns to the S0 state. Calcium is an essential cofactor and without it the OEC cannot catalyze the oxidation of water into dioxygen, protons, and electrons. Calcium has been shown to be essential for the S2 to S3 state transition and subsequent oxygen evolution. Both XANES and EXAFS experiments have been carried out at the Mn K-edge of the OEC to determine the oxidation states and structural information about the Mn site. In comparison, there have been fewer spectroscopic investigation of the calcium cofactor. Most studies on Ca in PSII have been biochemical, because Ca has no easy spectroscopic handle. The structure of the Ca cofactor binding site has generated considerable discussion. To probe it, Ca in OEC might be substituted by Sr chemically or biochemically by growing cyanobacteria in Ca depleted Sr media, which binds at the Ca site within the protein. Under previous proposals our group has carried out Ca and Sr K-edge EXAFS experiments to resolve the question of proximity of the Ca/Sr to the Mn4-cluster of OEC. By using Ca EXAFS of the native PS II and Sr EXAFS on Sr-reactivated PS II membranes we have confirmed the proximity of the Ca/Sr at 3.5A to the Mn-cluster in the S1 dark stable state of the OEC. Using polarized Sr EXAFS on oriented Sr-reactivated samples the averaged Sr-Mn vector orientation relative to the membrane normal was determined to be within 0-23 degree. The present challenge is to determine the Mn4-Ca interactions.