Alternative hydroxylases for the aerobic and anaerobic biosynthesis of ubiquinone in Escherichia coli

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Abstract
The synthesis of ubiquinone under anaerobic conditions was examined in a variety of strains of E. coli K12. All were shown to synthesize appreciable quantities of ubiquinone 8 when grown anaerobically on glycerol in the presence of fumarate. Under these conditions, ubiquinone 8 was in most cases the principal quinone formed, and levels in the range 50-70% of those obtained aerobically were observed. Studies with mutants blocked in the various reactions of the aerobic pathway for ubiquinone 8 synthesis established that under anaerobic conditions 3 alternative hydroxylation reactions not involving O2 are used to derive the C-4, -5 and -6 O of ubiquinone 8. Thus, mutants blocked in either of the 3 hydroxylation reactions of the aerobic pathway (ubiB, ubiH or ubiF) are each able to synthesize ubiquinone 8 anaerobically, whereas mutants lacking the octaprenyltransferase (ubiA), carboxy-lyase (ubiD) or methyltransferases (ubiE or ubiG) of the aerobic pathways remain blocked anaerobically. The demonstration that E. coli possesses a special mechanism for the anaerobic biosynthesis of ubiquinone suggests that this quinone may play an important role in anaerobic metabolism.