On the Role of Quinones in Bacterial Electron Transport

Abstract

The individual roles of ubiquinone and menaquinone in the electron transport of Proteus rettgeri have been investigated and their functional positions with respect to the cytochromes and dehydrogenases elucidated.1. A method has been developed for the parallel determination of both ubiquinone and menaquinone in the nanomole range in the same extract.2. Ubiquinone and menaquinone are not separated by various methods of cell disintegration and remain associated with the cytochromes.3. The relative contents of ubiquinone and menaquinone are independent of the hydrogen source and depend on the redox acceptor offered in the growth medium. As in Escherichia coli, ubiquinone is preferentially formed in cutures with maximal oxygen supply and menaquinone in anaerobic cultures. The activity offurnarate reduction by NADH and by formate parallels the content of menaquinone. 4.On extraction of the quinones the electron transport is inhibited. Ubiquinone reactivates the respiration with succinate, NADH and formate and menaquinone restores the fumarate reduction. I n anaerobically grown P . rettgeri the respiration with NADH and formate is partly restored by menaquinone, too.5. In the anaerobic states, ubiquinone is reduced with succinate, NADH and formate, and menaquinone with NADH and formate only. Ubiquinone is partly oxidized in the steady state of respiration and completely reduced in the steady state of fumarate reduction, where menaquinone is partly oxidized.6. 2-n-Heptyl-4-hydroxyquinoline-N-oxide inhibits the respiration of succinate, NADH and formate a t a common site located on the oxygen side of ubiquinone.7. Two b-type cytochromes (besides cytochrome 0) are differentialed: (1) Cytochrome b B is situated on the substrate side of 2-n-heptyl-4-hydroxyquinoline-N-oxide action and is reduced by formate only. (2) Cytochrome b1 is located on the oxygen side of this action and is reducible by all substrates.I n a previous communication [l] the role of the naphthoquinone derivative, menaquinone, was studied in a gram-positive organism (Bacillus megaterium) where it is the only occurring quinone. A basically similar role in the electron transport was attributed to menaquinone (MK) as to ubiquinone (Q) in mitochondria. A more penetrating insight into the particular role of both quinones and their differences is expected to be gained from the study of an organism where both quinones occur together, such as in the entero-bacteria, Escherichia and Proteus [2,3]. Different roles for Q and MK can be expected from the considerable differences or their redox potentials (see Discussion).Unusual Abbreviations. Ubiquinone, Q ; oxidized ubiquinone, Qox ; reduced ubiquinone, Qred ; menaquinone (vitamin K2), MK; oxidized menaquinone, MK,,; reduced menaquinone, =red.It is generally assumed that both quinones interact in the electron transport system of these organisms [4-91 although no redox reactions of MK have been demonstrated so far. Furthermore, the individual roles as reported in the literature have not been understood so far [lo-12...

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