MECHANISMS OF ENZYMIC AND HYDRIDE TRANSFERS

这个项目的总体目标是了解氢从弱酸性碳和氧中心转移的有效酶催化的结构基础。我们选择了几个有代表性的系统进行研究。对于质子转移催化：磷酸葡萄糖异构酶（PGI），它首先催化质子转移到氧（开环），然后利用酸/碱催化碳之间的质子转移使磷酸糖异构化；d -半乳糖mutarotase (GalM)，它催化糖底物上未磷酸化的氧之间的开环和质子转移。对于氢化物转移：木糖异构酶（xylose isomerase, XyI），它通过金属介导的1,2-氢化物转移在碳中心之间转移氢，也催化氧之间的质子转移，即糖环开环；肌苷5'-单磷酸脱氢酶（IMPDH），在肌苷转化为黄嘌呤的过程中使用NAD+作为氢化物受体。为了测试各种残基在这些酶的机制中的假定作用，我们将使用位点定向诱变和x射线晶体学的组合。除了了解活性位点残基的精确化学和结构作用外，这些研究还将提供反应途径的观点，并对质子和氢化物转移催化的贡献进行定性和半定量评估，例如：通用酸/通用碱催化；亲电催化;短而强的过渡态氢键；带电物质的静电稳定，结合水分子的参与，和底物/辅因子应变。了解这些酶的作用机制对人类健康也有重要影响。PGI在一种严重的遗传性代谢疾病中发生突变。此外，PGI通过一些未知的机制从细胞分泌，在那里它作为一种有效的细胞因子和肿瘤细胞介质。XyI在粮食生产中很重要，也被用于生物技术。Ga1M是半乳糖利用的Leloir通路的一部分，有许多已知的半乳糖代谢紊乱。IMPDH催化GMT生物合成的速率决定步骤，是免疫抑制、抗癌、抗病毒和抗菌药物的靶标。

The overall aim of this project is to understand the structural basis for efficient enzymic catalysis of hydrogen transfer from weakly acidic carbon and oxygen centers. We have selected several representative systems for study. For proton transfer catalysis: phosphoglucose isomerase (PGI), which first catalyzes proton transfer to oxygen (ring-opening) and then uses acid/base catalyzed proton transfer between carbons to isomerize a sugar phosphate, and D-galactose mutarotase (GalM), which catalyses ring-opening and proton transfer between oxygens on a sugar substrate that is not phosphorylated. For hydride transfer: xylose isomerase (XyI), which transfers hydrogen between carbon centers via metal-mediated 1,2-hydride shift and also catalyzes proton transfer between oxygens, i.e., sugar ring opening; and inosine 5'-monophosphate dehydrogenase (IMPDH), which uses NAD+ as a hydride acceptor in the transformation of inosine to xanthosine. To test the putative roles of various residues in the mechanisms of these enzymes, we will use a combination of site-directed mutagenesis and X-ray crystallography. In addition to an understanding of the precise chemical and structural roles of the residues in the active sites, these studies will provide views of the reaction pathways and a qualitative and semiquantitative assessment of the contribution to proton and hydride transfer catalysis of factors such as: general-acid/general-base catalysis; electrophilic catalysis; short, strong hydrogen bonds to the transition state; electrostatic stabilization of charged species, participation of bound water molecules, and substrate/cofactor strain. Understanding the mechanisms of action of these enzymes also has important consequences for human health. PGI is mutated in a severe hereditary metabolic disease. In addition, PGI by some unknown mechanism is secreted from the cell where it moonlights as a potent cytokine and tumor cell meidator. XyI is important in food production and is also being used in biotechnology. Ga1M is part of the Leloir pathway for the utilization of galactose and there are many known galactose metabolic disorders. IMPDH catalyzes the rate-determining step in GMT biosynthesis and is a target for immunosuppressive, anticancer, antiviral and antimicrobial drugs.