On the role of quinones in bacterial electron transport. The respiratory system of Bacillus megaterium.

Abstract

The menaquinone of Bacillus megaterium (14581 ATCC) is localized exclusively in the cytoplasmic membrane as are the cytochromes. Menaquinone is contained in a 5 to 10‐fold molar excess over cytochrome b1. The constituent menaquinone is almost completely reduced to menaquinole in the anaerobic state with the substrates respired by the membrane fraction. On the addition of oxygen the menaquinole is reoxidized to menaquinone. Menaquinone and menaquinole are the only forms of MK detected in the steady states of the electron transport. The redox reactions of menaquinone indicate its participation in the electron transport from the substrates α‐glycerophosphate, malate and NADH, to the acceptors oxygen and fumarate. Menaquinone is in a steady state in all the flow systems established with each substrate and both acceptors, and exhibits the steady state response expected for a respiratory component. The cytochrome system of this strain of B. megaterium consists of a b‐type cytochrome with a redox potential of + 8 mV and absorption bands close to those of pure cytochrome b1, two cytochrome oxidases, and α‐type cytochrome and cytochrome o and probably a small amount of c‐type cytochrome. Cytochrome b1 is completely reducible by each of the substrates. 2‐n‐Heptyl‐4‐hydroxyquinoline‐N‐oxide inhibits the electron flow at about the concentration of the constituent menaquinone between menaquinone and the cytochromes. The electron flow from each of the substrates to both acceptors depends completely on the reaction sensitive to the inhibitor. The electron transport from α‐glycerophosphate, malate and NADH to oxygen and fumarate as well as the reduction of cytochrome b1 is dependent on the presence of MK. This is demonstrated by pentane extraction and reincorporation of the MK. The reincorporation leads to the reconstitution of the original pathway, since MK and cytochrome b1 participate in the electron transport and the same sensitivity to 2‐n‐heptyl‐4‐hydroxyquinoline‐N‐oxide is found. The position of menaquinone in the respiratory system of B. megaterium is between the dehydrogenases for α‐glycerophosphate, malate and NADH and cytochrome b1 to which the fumarate reductase and the cytochrome oxidases are linked.