粪肠球菌基因组包含两个烯酰-ACP还原酶基因,FabK和FabI,编码具有非常不同结构的蛋白质。烯酰-ACP还原酶催化II型脂肪酸合成途径的延伸循环的最后一步。fabK基因位于大的脂肪酸合成操纵子内,而fabI与不饱和脂肪酸合成所需的两个基因fabN和fabO一起定位。先前的工作表明,由于翻译起始不良,FabK弱表达,因此几乎所有细胞烯酰ACP还原酶活性都是由fabI编码的。由于FabK是一种功能齐全的酶,问题是为什么FabI是一种必需的酶。为什么不增加FabK活性?我们报告说,FabK的过度生产是致命的,而FabI的过度生产只会减缓生长,而不是致命的。在这两种情况下,通过添加油酸可以恢复正常生长,不饱和脂肪酸,表明烯酰ACP还原酶的过量生产破坏了不饱和脂肪酸的合成。我们报告说这是由于与FabO的竞争,通过FabN推定的3-酮脂酰-ACP合酶I,提供证明烯酰-ACP还原酶必须与不饱和脂肪酸合成基因匹配的脱水酶/异构酶。FabO被认为与大肠杆菌FabB具有相同的活性,我们在体外报告了这种情况的证据,而FabN是一种脱水酶/异构酶,具有大肠杆菌FabA的活性。然而,FabN比FabA大得多,它是六聚体而不是像FabA那样的二聚体。
The Enterococcus faecalis genome contains two enoyl-ACP reductases genes,
fabK and fabI, which encode proteins having very different structures. Enoyl-ACP reductase catalyzes the last step of the elongation cycle of type II fatty acid synthesis pathway. The
fabK gene is located within the large fatty acid synthesis operon whereas fabI is located together with two genes fabN and fabO required for unsaturated fatty acid synthesis. Prior work showed that FabK is weakly expressed due to poor translational initiation and hence virtually all the cellular enoyl ACP reductase activity is that encoded by fabI. Since
FabK is a fully functional enzyme, the question is why FabI is an essential enzyme. Why not increase
FabK activity? We report that overproduction of
FabK is lethal whereas FabI overproduction only slows the growth and is not lethal. In both cases, normal growth is restored by the addition of oleic acid, an unsaturated fatty acid, to the medium indicating that enoyl ACP reductase overproduction disrupts unsaturated fatty acid synthesis. We report that this is due to competition with FabO, a putative 3-ketoacyl-ACP synthase I via FabN, a dehydratase/isomerase providing evidence that the enoyl-ACP reductase must be matched to the unsaturated fatty acid synthetic genes. FabO has been ascribed the same activity as E. coli FabB and we report in vitro evidence that this is the case, whereas FabN is a dehydratase/isomerase, having the activity of E. coli FabA. However, FabN is much larger than FabA, it is a hexamer rather than a dimer like FabA.