Menaquinone‐dependent succinate dehydrogenase of bacteria catalyzes reversed electron transport driven by the proton potential

Succinate dehydrogenases from bacteria and archaea using menaquinone (MK) as an electron acceptor (succinate/menaquinone oxidoreductases) contain, or are predicted to contain, two heme‐B groups in the membrane‐anchoring protein(s), located close to opposite sides of the membrane. All succinate/ubiquinone oxidoreductases, however, contain only one heme‐B molecule. In Bacillus subtilis and other bacteria that use MK as the respiratory quinone, the succinate oxidase activity (succinate←O2), and the succinate/menaquinone oxidoreductase activity were specifically inhibited by uncoupler (CCCP, carbonyl cyanide m‐chlorophenylhydrazone) or by agents dissipating the membrane potential (valinomycin). Other parts of the respiratory chains were not affected by the agents. Succinate oxidase or succinate/ubiquinone oxidoreductase from bacteria using ubiquinone as an acceptor were not inhibited. We propose that the endergonic electron transport from succinate (E+′ = +30 mV) to MK (E+′ ≅ −80 mV) in succinate/menaquinone oxidoreductase includes a reversed electron transport across the cytoplasmic membrane from the inner (negative) to the outer (positive) side via the two heme‐B groups. The reversed electron transport is driven by the proton or electrical potential, which provides the driving force for MK reduction.