Generation of a proton potential by succinate dehydrogenase of Bacillus subtilis functioning as a fumarate reductase

The membrane fraction of Bacillus subtilis catalyzes the reduction of fumar ate to succinate by NADH. The activity is inhibited by low concentrations o f 2-(heptyl)-4-hydroxyquinoline-N-oxide (HOQNO), an inhibitor of succinate: quinone reductase. In sdh or aro mutant strains, which lack succinate dehy drogenase or menaquinone, respectively, the activity of fumarate reduction by NADH was missing. In resting cells fumarate reduction required glycerol or glucose as the electron donor, which presumably supply NADH for fumarate reduction. Thus in the bacteria, fumarate reduction by NADH is catalyzed b y an electron transport chain consisting of NADH dehydrogenase (NADH:menaqu inone reductase), menaquinone, and succinate dehydrogenase operating in the reverse direction (menaquinol:fumarate reductase). Poor anaerobic growth o f B. subtilis was observed when fumarate was present. The fumarate reductio n catalyzed by the bacteria in the presence of glycerol or glucose was not inhibited by the protonophore carbonyl cyanide m-chlorophenyl hydrazone (CC CP) or by membrane disruption, in contrast to succinate oxidation by O-2. F umarate reduction caused the uptake by the bacteria of the tetraphenyphosph onium cation (TPP+) which was released after fumarate had been consumed. TP P+ uptake was prevented by the presence of CCCP or HOQNO, but not by N,N'-d icyclohexylcarbodiimide, an inhibitor of ATP synthase. From the TPP+ uptake the electrochemical potential generated by fumarate reduction was calculat ed (Delta psi = -132 mV) which was comparable to that generated by glucose oxidation with O-2 (Delta psi = -120 mV). The Delta psi generated by fumara te reduction is suggested to stem from menaquinol:fumarate reductase functi oning in a redox half-loop.