The late steps of anaerobic heme biosynthesis in E. coli: role for quinones in protoporphyrinogen oxidation.

The anaerobic oxidation of protoporphyrinogen with fumarate as electron acceptor in cell-free extracts of $\text{E. coli}$ is inhibited by ultra-violet irradiation. The activity of irradiated extracts is restored by addition of menadione and the restored activity is blocked by the electron-transport inhibitor 2-heptyl-4-hydroxy quinoline-N-oxide. These observations suggest that quinones are required as electron transport carriers at this late step in the pathway of anaerobic heme biosynthesis. These findings have important implications both for the mechanism of anaerobic heme synthesis and for the physiology of cytochrome biosynthesis in anaerobic microorganisms.