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) co ntain, 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 he me-B molecule. In Bacillus subtilis and other bacteria that use MK as the respiratory quinone, the succinate oxidase activity (succinate-->O -2), and the succinate/menaquinone oxidoreductase activity were specif ically inhibited by uncoupler (CCCP, carbonyl cyanide m-chlorophenylhy drazone) or by agents dissipating the membrane potential (valinomycin) . Other parts of the respiratory chains were not affected by the agent s. Succinate oxidase or succinate/ubiquinone oxidoreductase from bacte ria using ubiquinone as an acceptor were not inhibited. We propose tha t the endergonic electron transport from succinate (E degrees' = +30 m V) to MK (E degrees' congruent to -80 mV) in succinate/menaquinone oxi doreductase includes a reversed electron transport across the cytoplas mic membrane from the inner (negative) to the enter (positive) side vi a the two heme-B groups. The reversed electron transport is driven by the proton or electrical potential, which provides the driving force f or MK reduction.