METALLOBIOCHEMISTRY OF METHIONINE SYNTHASE

高水平的同型半胱氨酸在人类已被牵连作为一个因素，有助于心血管疾病。甲硫氨酸合酶是一种甲基钴胺素依赖性酶，可将同型半胱氨酸转化为甲硫氨酸;因此，这种酶是控制人体同型半胱氨酸水平的关键。该建议旨在提供详细的化学先例和酶的研究集中在一个方面的酶，协调组氨酸钴胺素的活性位点。在甲硫氨酸合酶中，甲钴胺中钴的第六配位配体不是附加的5，6-二甲基苯并咪唑碱，而是由酶提供的组氨酸/咪唑残基。针对组氨酸轴向碱基的一个工作假设是，质子化/去质子化循环与不同的Co-N键长相结合，对于增强催化活性很重要。该提议通过结合化学先例和酶促研究来测试该假设，所述酶促研究旨在改变轴向碱基的质子化状态和Co-N键的长度。将在溶液中完成甲基钴胺（甲基钴胺的无碱类似物）在外源性轴向碱存在下的热力学和动力学研究。然后，在溶液中完成有和没有外源性轴向碱的情况下甲基可宾酰胺的动力学研究。然后，在溶液中完成有和没有外源性轴向碱的甲基可宾酰胺的动力学研究。然后动力学研究methylcobinamide有和没有一个外源的轴向基地将完成在酶系统本身，直接探测蛋氨酸合酶的轴向基地的作用。

High levels of homocysteine in humans have been implicated as a factor contributing to cardiovascular disease. Methionine synthase, a methylcobalamin-dependent enzyme, converts homocysteine to methionine; as such, this enzyme is key in controlling the level of homocysteine in humans. This proposal aims to provide detailed chemical precedent and enzymatic studies focusing on one aspect of the enzyme, the coordination of histidine to cobalamin in the active site. In methionine synthase, the sixth coordinating ligand to cobalt in methylcobalamin is not the appended 5,6-dimethylbenzimidazole base, but is a histidine/imidazole residue supplied by the enzyme. A working hypothesis for the purpose of the histidine axial base is that a protonation/deprotonation cycle, in combination with varying Co-N bond lengths, is important in enhancing catalytic activity. This proposal test the hypothesis by combining chemical precedent and enzymatic studies which are designed to vary the protonation state of the axial base and the length of the Co-N bond. Thermodynamic and kinetic studies of methylcobinamide (the base-free analog of methylcobalamin) in the presence of an exogenous axial base will be completed in solution. Then kinetic studies of methylcobinamide with and without an exogenous axial base will be complete with and without an exogenous axial base will be completed in solution. Then kinetic studies of methylcobinamide with and without an exogenous axial base will be completed in solution. Then kinetic studies of methylcobinamide with and without an exogenous axial base will be complete in the enzymatic system itself, directly probing the role of the axial base in methionine synthase.