Hedstrom L, Wang C C
Department of Pharmaceutical Chemistry, School of Pharmacy, University of California, San Francisco 94143.
Biochemistry. 1990 Jan 30;29(4):849-54. doi: 10.1021/bi00456a001.
Inosine 5'-monophosphate dehydrogenase (IMPDH) catalyzes the oxidation of inosine 5'-monophosphate (IMP) to xanthosine 5'-monophosphate (XMP) with the conversion of NAD to NADH. An ordered sequential mechanism where IMP is the first substrate bound and XMP is the last product released was proposed for Tritrichomonas foetus IMPDH on the basis of product inhibition studies. Thiazole adenine dinucleotide (TAD) is an uncompetitive inhibitor versus IMP and a noncompetitive inhibitor versus NAD, which suggests that TAD binds to both E-IMP and E-XMP. Mycophenolic acid is also an uncompetitive inhibitor versus IMP and noncompetitive versus NAD. Multiple-inhibitor experiments show that TAD and mycophenolic acid are mutually exclusive with each other and with NADH. Therefore, mycophenolic acid most probably binds to the dinucleotide site of T. foetus IMPDH. The mycophenolic acid binding site was further localized to the nicotinamide subsite within the dinucleotide site: mycophenolic acid was mutually exclusive with tiazofurin, but could form ternary enzyme complexes with ADP or adenosine diphosphate ribose. NAD inhibits the IMPDH reaction at concentrations greater than 3 mM. NAD substrate inhibition is uncompetitive versus IMP, which suggests that NAD inhibits by binding to E-XMP. TAD is mutually exclusive with both NAD and NADH in multiple-inhibitor experiments, which suggests that there is one dinucleotide binding site. The ordered mechanism predicts that multiple-inhibitor experiments with XMP and TAD, mycophenolic acid, or NAD should have an interaction constant (alpha) between 0 and 1. However, alpha was greater than 1 in all cases.(ABSTRACT TRUNCATED AT 250 WORDS)
肌苷5'-单磷酸脱氢酶（IMPDH）催化肌苷5'-单磷酸（IMP）氧化为黄苷5'-单磷酸（XMP），同时将NAD转化为NADH。基于产物抑制研究，有人提出胎儿三毛滴虫IMPDH的反应机制为有序序列机制，其中IMP是第一个结合的底物，XMP是最后一个释放的产物。噻唑腺嘌呤二核苷酸（TAD）对IMP是反竞争性抑制剂，对NAD是非竞争性抑制剂，这表明TAD同时结合E-IMP和E-XMP。霉酚酸对IMP也是反竞争性抑制剂，对NAD是非竞争性抑制剂。多抑制剂实验表明，TAD和霉酚酸相互排斥，且与NADH也相互排斥。因此，霉酚酸很可能结合在胎儿三毛滴虫IMPDH的二核苷酸位点上。霉酚酸结合位点进一步定位在二核苷酸位点内的烟酰胺亚位点：霉酚酸与噻唑呋林相互排斥，但能与ADP或二磷酸腺苷核糖形成三元酶复合物。当浓度大于3 mM时，NAD会抑制IMPDH反应。NAD的底物抑制对IMP是反竞争性的，这表明NAD通过结合E-XMP来发挥抑制作用。在多抑制剂实验中，TAD与NAD和NADH都相互排斥，这表明存在一个二核苷酸结合位点。有序机制预测，用XMP和TAD、霉酚酸或NAD进行的多抑制剂实验的相互作用常数（α）应在0到1之间。然而，在所有情况下α都大于1。（摘要截短于250字）
