MODEL STUDIES OF ACETYL COENZYME A SYNTHASE

这项研究旨在使用定义明确的低分子体系来阐明含镍乙酰辅酶A合成酶(ACS)催化基本步骤的机理参数。这些步骤包括从皮质蛋白(C/Fe-SP)到A簇的甲基的非酶转移，CO插入CH3-A簇键和CH3C(O)-A簇的硫酸盐加成。为此，首先必须制备包含催化中心(A簇)关键特征的合成类似物。一个与所有光谱数据一致的结构模型(X射线结构不可用)包括在(N/O)2S2施主环境中通过共价桥X连接到[Fe3S4]2簇的单个镍离子。化学反应性研究将需要使用合成类似物来探索化学计量转换。将使用机械无机化学的方案系统地审问个别反应，包括产品分析、动力学测量、立体化学和自由基时钟探针研究。该项目的长期目标是从机理上详细了解生物异金属簇合物的结构、电子和化学性质如何针对预期的催化转化进行优化。拟议的研究影响了我们对必需微量元素镍的生物学意义的理解，这些元素包括幽门螺杆菌的生殖力，它与消化性溃疡疾病、胃癌和胃淋巴瘤有关，以及通过产生降解DNA的氧化性物种而致癌。此外，产丙酮和产甲烷细菌，即含有ACS的有机体，可能对人类的消化功能和功能障碍很重要，因为它们占据了大量的结肠。更广泛地说，对ACS的深入了解有助于我们对一般类型的生物异金属组装的基本理解，例如[NiFe]氢酶、亚硫酸盐还原酶、细胞色素C氧化酶、[CuZn]超氧化物歧化酶和固氮酶辅助因子的催化部位。

The proposed research is designed to elucidate the mechanistic parameters of the elementary steps catalyzed by the nickel- containing acetyl-coenzyme A synthase (ACS) using well-defined low molecular weight systems. Included among these steps are the nonenzymatic transfer of a methyl group from the corrinoid protein (C/Fe-SP) to the A cluster, CO insertion into the CH3-A cluster bond and thiolate addition to the CH3C(O)-A cluster. To this end, first it will be necessary to prepare synthetic analogs that contain the key features of the catalytic site (A cluster). A structural model consistent with all spectroscopic data (an X- ray structure is not available) includes a single nickel ion in a (N/O)2S2 donor environment linked through a covalent bridge, X, to an [Fe3S4]2+ cluster. Chemical reactivity studies will entail probing stoichiometric transformations using the synthetic analogs. Individual reactions will be systematically interrogated using the protocols of mechanistic inorganic chemistry including product analyses, kinetic measurements, stereochemical and radical clock probe investigations. The long- term goal of this project is to develop a detailed mechanistic understanding of how the structural, electronic and chemical properties of biological heterometallic clusters are optimized for the intended catalytic transformations. The proposed research impacts our understanding of the biological implications of the essential trace element nickel that include the virility of Helicobacter pylori which has been associated with peptic ulcer disease, gastric carcinoma, and gastric lymphoma, and carcinogenesis through production of oxidizing species that degrade DNA. Additionally, acetongenic and methanogenic bacteria, organisms that contain ACS, may be important to human digestive function and dysfunction as they occupy a large volume of the colon. More broadly, a deeper understanding of ACS is valuable to our fundamental understanding of the general class of biological heterometallic assemblies, e.g. the catalytic sites in [NiFe] hydrogenase, sulfite reductase, cytochrome c oxidase, [CuZn] superoxide dismutase and the nitrogenase cofactor.