STAFF PRIORITY TIME XRAY STRUCT ANALYSIS OF INOSINEMONOPHOSPHATE DEHYDROGENASE

肌苷一磷酸脱氢酶（IMPDH）催化 NAD依赖的肌苷一磷酸（IMP）氧化为黄嘌呤 单磷酸（XMP）。 这是嘌呤的限速步骤 生物合成 这种酶的抑制剂已被证明具有 抗肿瘤、免疫抑制和抗寄生虫作用。 抑制 中医胎儿IMPDH在体外阻止生物体的生长， 这种抑制可以通过向培养基中补充GMP来克服。 在SSRL 7-1收集天然、衍生和复杂数据。 良好的硫柳汞、PCMBS和四氯铂酸盐衍生物数据集 并用于MIR定相。 初始低分辨率SIR MIR电子密度图显示了一个扁平的四聚体分子， 与预期的晶体堆积一致， 不对称单元中的氨基酸单体，并且四聚体是 由晶体学四重算符产生。 每个单体 形成非常类似于丙糖的a/B桶的a/B桶 磷酸异构酶（TIM桶）。 后续阶段组合， MIR和改进的部分模型阶段使我们能够拟合大多数 氨基酸主链和侧链。 重原子差映射 清楚地表明5个半胱氨酸残基与PCMBS或 硫代蛋氨酸和3个与氯化铂结合的蛋氨酸残基。 通过活性位点的位置确定活性位点 半胱氨酸与电子密度在差分图中的位置 根据在配体和经修饰物浸泡的 晶体

Inosine monophosphate dehydrogenase (IMPDH) catalyzes the NAD-dependent oxidation of inosine monophosphate (IMP) to xanthine monophosphate (XMP). This is the rate-limiting step in purine biosynthesis. Inhibitors of this enzyme have been shown to have anti-tumor, immunosuppressive, and antiparasitic effects. Inhibition of T. foetus IMPDH in vitro arrests the growth of the organism and this inhibition can be overcome by supplementing the medium with GMP. Native, derivative, and complex data were collected at 7-1 at SSRL. Good thimerosal, PCMBS and tetra-chloro-platinate derivative data sets were obtained and used for MIR phasing. Initial low-resolution SIR and MIR electron density maps indicated a flat, tetrameric molecule, consistent with the expected crystal packing where there is one 503 amino-acid monomer in the asymmetric unit, and the tetramer is generated by the crystallographic four-fold operator. Each monomer forms an a/b barrel resembling very closely the a/b barrel of triose phosphate isomerase (TIM barrel). Subsequent phase combination with the MIR and refined partial model phases has allowed us to fit most of the amino acid backbone and side chains. Heavy-atom difference maps indicate clearly 5 cysteine residues that reacted with either PCMBS or thimerosal, and 3 methionine residues that bound platinum chloride. The active site has been identified by the position of an active-site cysteine and the positions of electron density in difference maps calculated from data collected on ligand and inhibitor-soaked crystals.