Oxidation of protoporphyrinogen IX in Escherichia coli is mediated by the aerobic coproporphyrinogen oxidase

Protoporphyrinogen oxidase, the penultimate enzyme involved in the biosynth etic pathway for heme, catalyzes the removal of six electrons from protopor phyrinogen IX to generate protoporphyrin IX. In Escherichia coli, this enzy me is encoded by the hemG gene. In this study we examined possible alternat e pathways for the oxidation of protoporphyrinogen IX to protoporphyrin IX, by isolating and investigating E. coli mutants that can still grow normall y when the hemG gene is disrupted. One of these mutants was characterized i n 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 eleva ted by the mutation. Overexpression of a wild-type hemF gene cloned in a mu lticopy plasmid also restored the growth of Delta hemG strain, Extracts fro m cells that overexpress hemF exhibited an increased ability to oxidize pro toporphyrinogen IX to protoporphyrin IX. These findings suggest that the E. coli aerobic coproporphyrinogen oxidase has an intrinsic capacity to oxidi ze not only coproporphyrinogen III but also protoporphyrinogen IX.