EPR & Optical Studies of Photosynthetic Water Oxidation

EPR

• 批准号: 6608887
• 负责人: RICHARD J DEBUS
• 金额: $ 17.61万
• 依托单位: UNIVERSITY OF CALIFORNIA RIVERSIDE
• 依托单位国家: 美国
• 财政年份: 2002
• 资助国家: 美国
• 起止时间: 2002-08-01 至 2006-07-31
• 项目状态: 已结题
• 来源: https://reporter.nih.gov/project-details/6608887
• 关键词: affinity chromatography; chlorophyll; electron spin resonance spectroscopy; electron transport; enzyme mechanism; enzyme substrate complex; isozymes; manganese; metalloenzyme; oxidation; oxidation reduction reaction; photosynthesis; photosynthetic reaction centers; photosystem; site directed mutagenesis; spectrometry; tyrosine; water

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.

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。催化位点位于光系统 II (PSII) 中，包含 (Mn)4-Ca 簇，该簇与氧化还原活性酪氨酸残基（称为 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O 2 作为副产物。该项目的具体目标是表征暗稳定 S1 氧化态 (Mn)4 簇的底物、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之上：(1) S1 氧化态的 (Mn)4 簇表现出平行偏振多线 EPR 信号，以及 (2) 从 YD 到 P680+ 的电子转移速率比以前认为（YD，PSII 中的第二个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 簇在其 S1 氧化阶段的光谱探针，就像在过去二十年中利用 S2 态多线 EPR 信号作为探针一样。 (Mn)4 簇处于 S2 氧化态。我们观察到电子从 YD 转移到 P680+ 迅速发生，这具有重要的机制意义，并提出了关于控制酪氨酸 Yz 反应性的因素的多种假设，以及可能控制叶绿素分子中 P680+ 阳离子位置的因素。构成P680。拟议的研究将检验这些假设。了解光合水氧化的机制应该有助于深入了解目前生物医学深入研究的两类重要酶的机制：金属自由基酶和其机制涉及质子耦合电子转移反应的酶。 Photosystem II 在研究这两类酶的催化启动方面具有独特的优势，因为 PSII 中的催化可以通过闪光启动，从而有助于以高时间分辨率进行反应循环中间体的动力学研究。