ABSTRACTTwo putativeMethanococcus jannaschiiisocitrate dehydrogenase genes, MJ1596 and MJ0720, were cloned and overexpressed inEscherichia coli, and their gene products were tested for the ability to catalyze the NAD- and NADP-dependent oxidative decarboxylation ofdl-threo-3-isopropylmalic acid,threo-isocitrate,erythro-isocitrate, and homologs ofthreo-isocitrate. Neither enzyme was found to use any of the isomers of isocitrate as a substrate. The protein product of the MJ1596 gene, designated AksF, catalyzed the NAD-dependent decarboxylation of intermediates in the biosynthesis of 7-mercaptoheptanoic acid, a moiety of methanoarchaeal coenzyme B (7-mercaptoheptanylthreonine phosphate). These intermediates included (−)-threo-isohomocitrate [(−)-threo-1-hydroxy-1,2,4-butanetricarboxylic acid], (−)-threo-iso(homo)2citrate [(−)-threo-1-hydroxy-1,2,5-pentanetricarboxylic acid], and (−)-threo-iso(homo)3citrate [(−)-threo-1-hydroxy-1,2,6-hexanetricarboxylic acid]. The protein product of MJ0720 was found to be α-isopropylmalate dehydrogenase (LeuB) and was found to catalyze the NAD-dependent decarboxylation of one isomer ofdl-threo-isopropylmalate to 2-ketoisocaproate; thus, it is involved in the biosynthesis of leucine. The AksF enzyme proved to be thermostable, losing only 10% of its enzymatic activity after heating at 100°C for 10 min, whereas the LeuB enzyme lost 50% of its enzymatic activity after heating at 80°C for 10 min.
摘要在大肠杆菌中克隆并过表达了两个推定的甲球菌异柠檬酸脱氢酶基因 MJ1596 和 MJ0720,并检测了其基因产物催化 NAD 和 NADP 依赖性氧化脱羧dl-苏-3-异丙基苹果酸、苏-异柠檬酸、赤-异柠檬酸和苏-异柠檬酸同系物的能力。两种酶都不使用异柠檬酸异构体作为底物。MJ1596 基因的蛋白产物被命名为 AksF,它能催化 7-巯基庚酸(甲羟戊酸辅酶 B(7-巯基庚酰苏氨酸磷酸酯)的一个分子)生物合成过程中的中间产物的 NAD 依赖性脱羧反应。这些中间体包括(-)-异柠檬酸苏氨酸[(-)-苏-1-羟基-1,2,4-丁烷三羧酸]、(-)-异柠檬酸苏氨酸[(-)-苏-1-羟基-1、2,5-戊烷三羧酸] 和 (-)-threo-iso(homo)3citrate [(-)-threo-1-hydroxy-1,2,6-hexanetricarboxylic acid]。研究发现,MJ0720 的蛋白产物是 α-异丙基丙二酸脱氢酶(LeuB),它能催化 NAD 依赖性脱羧作用,将 dl-硫代异丙基丙二酸的一种异构体转化为 2-酮异己酸;因此,它参与了亮氨酸的生物合成。事实证明,AksF 酶具有恒温性,在 100°C 下加热 10 分钟后,其酶活性仅损失 10%,而 LeuB 酶在 80°C 下加热 10 分钟后,其酶活性损失 50%。