EPR & Optical Studies of Photosynthetic Water Oxidation

项目描述（申请人提供）：项目总体目标是描述光合作用水氧化的分子机制。催化位点位于光系统II （PSII）中，包含一个（Mn）4-Ca簇，该簇与氧化还原活性酪氨酸残基Yz相互作用。Yz自由基从（Mn）4-Ca簇中提取电子和质子，导致水氧化并释放02作为副产物。该项目的具体目标是表征（Mn）4簇在其暗稳定的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发现的基础上：(1)(Mn)4簇在S1氧化态表现出平行极化多线EPR信号，(2)从YD到P680+的电子转移速率比之前认为的要快5个数量级（YD是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）。拟议的研究将利用S1态多线EPR信号作为（Mn）4团簇S1氧化状态的光谱探针，就像过去20年利用S2态多线EPR信号作为（Mn）4团簇S2氧化状态的探针一样。我们观察到，从YD到P680+的电子转移发生迅速，这具有重要的机制意义，并提出了关于控制酪氨酸Yz反应性的因素的各种假设，以及可能控制P680+阳离子在构成P680的叶绿素分子中的位置的因素。拟议中的研究将检验这些假设。了解光合作用水氧化的机制将有助于深入了解两类酶的机制，这两类酶目前是生物医学研究的重点：金属自由基酶和机制涉及质子耦合电子转移反应的酶。光系统II在研究这两类酶的催化起始方面具有独特的优势，因为PSII中的催化可以在一闪而过的情况下启动，从而促进了高时间分辨率的反应循环中间体动力学研究。

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.