Narita S, Taketani S, Inokuchi H
Department of Biophysics, Faculty of Science, Kyoto University, Japan.
Mol Gen Genet. 1999 Jul;261(6):1012-20. doi: 10.1007/s004380051050.
Protoporphyrinogen oxidase, the penultimate enzyme involved in the biosynthetic pathway for heme, catalyzes the removal of six electrons from protoporphyrinogen IX to generate protoporphyrin IX. In Escherichia coli, this enzyme is encoded by the hemG gene. In this study we examined possible alternate pathways for the oxidation of protoporphyrinogen IX to protoporphyrin IX, by isolating and investigating E. coli mutants that can still grow normally when the hemG gene is disrupted. One of these mutants was characterized in detail and had a mutation in the promoter region of the hemF gene, which encodes aerobic coproporphyrinogen oxidase, the enzyme involved in the step immediately before protoporphyrinogen oxidase. Measurement of the promoter activity of the hemF gene showed that the level of transcription was elevated by the mutation. Overexpression of a wild-type hemF gene cloned in a multicopy plasmid also restored the growth of deltahemG strain. Extracts from cells that overexpress hemF exhibited an increased ability to oxidize protoporphyrinogen IX to protoporphyrin IX. These findings suggest that the E. coli aerobic coproporphyrinogen oxidase has an intrinsic capacity to oxidize not only coproporphyrinogen III but also protoporphyrinogen IX.
原卟啉原氧化酶是血红素生物合成途径中的倒数第二个酶，它催化从原卟啉原IX去除六个电子以生成原卟啉IX。在大肠杆菌中，该酶由hemG基因编码。在本研究中，我们通过分离和研究当hemG基因被破坏时仍能正常生长的大肠杆菌突变体，来检测原卟啉原IX氧化为原卟啉IX的可能替代途径。其中一个突变体被详细表征，其hemF基因的启动子区域发生了突变，hemF基因编码需氧型粪卟啉原氧化酶，该酶参与原卟啉原氧化酶之前的步骤。对hemF基因启动子活性的测量表明，转录水平因该突变而升高。克隆在多拷贝质粒中的野生型hemF基因的过表达也恢复了ΔhemG菌株的生长。过表达hemF的细胞提取物氧化原卟啉原IX为原卟啉IX的能力增强。这些发现表明，大肠杆菌需氧型粪卟啉原氧化酶不仅具有氧化粪卟啉原III的内在能力，还具有氧化原卟啉原IX的内在能力。
