EPR & Optical Studies of Photosynthetic Water Oxidation

描述（由申请人提供）：该项目的总体目标是描述光合水氧化的分子机制。该催化位点位于光系统II（PSII）中，并且包含与称为Yz的氧化还原活性酪氨酸残基相互作用的（Mn）4-Ca簇。Yz自由基从（Mn）4-Ca簇中提取电子和质子，导致水的氧化和作为副产物的O2的释放。该项目的具体目标是表征（Mn）4簇在其暗稳定S1氧化态下的底物、抑制剂和辅因子结合特性，并进一步表征控制酪氨酸Yz反应性的环境因素。这些目标建立在我们最近的发现上，即（1）处于S1氧化态的（Mn）4簇表现出平行极化多线EPR信号，以及（2）从YD到P680+的电子转移速率比以前认为的快五个数量级（YD，PSII中的第二氧化还原活性酪氨酸，不参与水氧化的初级电子转移反应）。所提出的研究将利用S1状态的多线EPR信号作为光谱探针的（Mn）4簇在其S1氧化态的相同的方式，S2状态的多线EPR信号已被利用在过去的二十年作为探针的（Mn）4簇在其S2氧化态。我们的观察，电子转移从YD到P680+发生迅速具有重要的机械影响，并提出了各种假说的因素，管理酪氨酸Yz的反应性，并可能，管理的P680+阳离子的位置之间的叶绿素分子，构成P680的因素。拟议的研究将检验这些假设。了解光合作用水氧化的机制，应提供深入了解的两个重要类别的酶的机制，是目前密集的生物医学研究的主题：金属自由基酶和酶的机制涉及质子耦合电子转移反应。光系统II具有独特的优势，研究这两类酶的催化起始，因为在PSII催化可以启动闪光灯，从而促进动力学研究的反应周期中间体具有高的时间分辨率。

DESCRIPTION (provided by applicant): The project's overall goal is to describe the molecular mechanism of photosynthetic water oxidation. The catalytic site is located in Photosystem II (PSII) and contains a (Mn)4-Ca cluster that interacts with a redox-active tyrosine residue known as Yz. The Yz radical extracts electrons and protons from the (Mn)4-Ca cluster, leading to the oxidation of water and the release of 02 as a by-product. The project's specific goals are to characterize the substrate, inhibitor, and cofactor binding properties of the (Mn)4 cluster in its dark-stable S1 oxidation state and to further characterize the environmental factors that control the reactivity of tyrosine Yz. These objectives build on our recent discoveries that (1) the (Mn)4 cluster in its S1 oxidation state exhibits a parallel polarization multiline EPR signal, and (2) the rate of electron transfer from YD to P680+ is five orders of magnitude faster than previously believed (YD, a second redox-active tyrosine in PSII, does not participate in the primary electron transfer reactions of water oxidation). The proposed studies will exploit the S1 state multiline EPR signal as a spectroscopic probe of the (Mn)4 cluster in its S1 oxidation stale in the same manner that the S2 state multiline EPR signal has been exploited over the last two decades as a probe of the (Mn)4 cluster in its S2 oxidation state. Our observation that electron transfer from YD to P680+ takes place rapidly has important mechanistic implications and suggests a variety of hypotheses regarding the factors that govern the reactivity of tyrosine Yz and, possibly, the factors that govern location of the P680+ cation among the chlorophyll molecules that constitute P680. The proposed studies will test these hypotheses. Understanding the mechanism of photosynthetic water oxidation should provide insight into the mechanisms of two important classes of enzymes that are currently the subjects of intensive bio-medical research: metallo-radical enzymes and enzymes whose mechanisms involve proton-coupled electron transfer reactions. Photosystem II possesses unique advantages for studying the initiation of catalysis in these two classes of enzymes because catalysis in PSII can be initiated with a flash of light, thereby facilitating kinetic studies of reaction cycle intermediates with high time resolution.