FTIR Studies of Photosynthetic Oxygen Evolution

项目描述（申请人提供）：项目总体目标是阐明光合作用释氧的分子机制。这个过程发生在光系统II中，是大气中几乎所有氧气的来源。催化位点包含一个（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，导致水的氧化和氧气的释放。该项目的主要目标是表征在催化循环的各个步骤中伴随（Mn）4-Ca簇氧化的结构变化。这一信息对于理解氧的演化机制是至关重要的，并将补充从x射线晶体学中获得的信息。主要的研究工具将是傅里叶变换红外（FTIR）差分光谱。该项目的具体目标是：(1)确定在催化循环的各个步骤中发生氧化的特定Mn离子；(2)确定催化循环中促进（Mn）4-Ca簇质子偶联氧化的关键碱基氨基酸残基；(3)表征生成氧反应的最终中间体，即可以通过增加环境氧压捕获的中间体；(4)采用改良的细菌反应中心作为模型系统，以表征连接金属离子氧化反应环境中发生的变化；(5)利用近红外激发共振拉曼光谱作为表征（Mn）4-Ca团簇环境的附加工具。除了提供光合作用氧气演化机制的基本见解外，该项目还将提供对金属酶和其机制涉及质子耦合电子转移（PCET）反应的酶的机制的见解。这些酶催化线粒体中的生物能量转导。阐明这些酶的催化机制对于理解线粒体疾病和衰老的分子基础至关重要。光系统II是金属代谢酶的一个很好的例子，也是研究质子耦合电子转移反应的一个独特的实验室。它的优点在于它能够通过单次闪光步进其催化循环，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 elucidate the molecular mechanism of photosynthetic oxygen evolution. This process takes place in Photosystem II and is the source of nearly all atmospheric oxygen. The catalytic site 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 oxygen. The project's primary goal is to characterize the structural changes that accompany the oxidation of the (Mn)4-Ca cluster during the individual steps of the catalytic cycle. This information is crucial to understanding the mechanism of oxygen evolution and will complement the information that is being obtained from X-ray crystallography. The primary investigative tool will be Fourier transform infrared (FTIR) difference spectroscopy. The project's specific aims are: (1) To identify the specific Mn ion(s) that undergo oxidation during the individual steps in the catalytic cycle; (2) To identify the amino acid residues that serve as the critical bases that facilitate the proton-coupled oxidations of the (Mn)4-Ca cluster during the catalytic cycle; (3) To characterize the final intermediate of the oxygen formation reaction, an intermediate that can be trapped by increasing the ambient oxygen pressure; (4) To employ modified bacterial reaction centers as model systems for characterizing changes that occur in the environment of a ligated metal ion in response to its oxidation; (5) To employ near-infrared excitation resonance Raman spectroscopy as an additional tool for characterizing the environment of the (Mn)4-Ca cluster. In addition to providing fundamental insight into the mechanism of photosynthetic oxygen evolution, the project will provide insight into the mechanisms of metalloradical enzymes and enzymes whose mechanisms involve proton-coupled electron transfer (PCET) reactions. Such enzymes catalyze biological energy transduction in mitochondria. Elucidating the catalytic mechanisms of these enzymes is essential for understanding the molecular basis of mitochondrial diseases and aging. Photosystem II is both an excellent example of a metalloradical enzyme and a unique laboratory for studying proton-coupled electron transfer reactions. Its advantages derive from its ability to be stepped through its catalytic cycle with single flashes of light, thereby facilitating kinetic studies of reaction cycle intermediates with high time resolution.