To get a closer insight into the substrate recognition of thermostable 3-isopropylmalate dehydrogenase (IPMDH) derived from Thermus thermophilus HB8, two analogs of the natural substrate, (2R,3S) 3-isopropylmalic acid (IPM) were synthesized according to the chiral transcription methodology which we have recently developed, and the kinetics of the enzyme reaction were analyzed. 2-O-methyl IPM was found to be inactive as a substrate but to function as an uncompetitive inhibitor, which suggests that, although it is the site of oxidation by NAD+, 2-O-methyl IPM was incorporated into one of the active sites of the homodimeric enzyme. The hydroxyl group at C-2 was not essential to the substrate recognition by IPMDH. The 1-carboxamide derivative of IPM was inactive, both as a substrate and as an inhibitor. This clearly implies the prime importance of electrostatic interaction between the C-1 carboxylate of IPM and a cationic function (probably a guanidino group of Arg-104) of the enzyme for the recognition of IPM.
为了更深入地了解嗜热菌HB8中热稳定3-
异丙基丙二酸脱氢酶(
IPMDH)对底物的识别,我们根据最近开发的手性转录方法合成了两种天然底物的类似物(2R,3S)3-
异丙基丙二酸(
IPM),并分析了酶反应的动力学。我们发现2-O-甲基
IPM作为底物不活跃,但可以作为非竞争性
抑制剂发挥作用,这表明尽管它是
NAD+的氧化位点,但2-O-甲基
IPM被整合到了同源二聚酶的一个活性位点中。
IPMDH识别底物时,C-2的羟基不是必需的。
IPM的1-羧酰胺衍
生物作为底物和
抑制剂都不活跃。这清楚地表明,
IPM的C-1
羧酸与酶的阳离子功能(可能是Arg-104的
胍基)之间的静电相互作用对于
IPM的识别至关重要。