MODEL STUDIES OF ACETYL COENZYME A SYNTHASE

提出的研究旨在阐明含镍乙酰辅酶A合成酶（ACS）催化的基本步骤的机理参数，使用明确的低分子量体系。这些步骤包括将一个甲基从类椰蛋白（C/Fe-SP）非酶转移到a簇，CO插入到CH3-A簇键和将硫代盐添加到CH3C(O)-A簇。为此，首先有必要制备含有催化位点（A簇）关键特征的合成类似物。与所有光谱数据一致的结构模型（X射线结构不可用）包括（N/O）2S2供体环境中的单个镍离子通过共价桥X连接到[Fe3S4]2+簇。化学反应性研究将需要探索使用合成类似物的化学计量转换。个别反应将系统地询问使用机械无机化学协议，包括产品分析，动力学测量，立体化学和自由基时钟探针调查。该项目的长期目标是对生物异质金属簇的结构、电子和化学性质如何优化以实现预期的催化转化进行详细的机制理解。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，包括与消化性溃疡疾病、胃癌和胃淋巴瘤有关的幽门螺杆菌的生殖力，以及通过产生降解DNA的氧化物质致癌。此外，产丙酮和产甲烷细菌，这些含有ACS的生物，可能对人类消化功能和功能障碍很重要，因为它们占据了结肠的很大体积。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合的基本理解，例如[NiFe]氢化酶、亚硫酸盐还原酶、细胞色素c氧化酶、[CuZn]超氧化物歧化酶和氮酶辅助因子中的催化位点。

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.