The succinate dehydrogenases of aerobic bacteria catalyze the oxidation of succinate by respiratory quinones (‘succinate:quinone reductase’), and the quinols are reoxidized by O2 (‘succinate oxidase’). Depending on the quinone type present in the bacterial membrane, succinate:ubiquinone or succinate:menaquinone reductases are found. Succinate:ubiquinone and succinate:menaquinone reductases share many properties, but show characteristic differences in the quinone reactive subunit(s) (reviewed in [1,2]).
In Gram-positive bacteria like Bacillus subtilis and other prokaryotes containing succinate:menaquinone reductases, the activities of succinate oxidase and of succinate:menaquinone reductase are lost when the proton potential across the membrane is degraded by disruption of the bacteria or by the addition of a protonophore [3,4]. Such inhibition is not observed with bacteria containing succinate:ubiquinone reductases. This difference has been explained by the fact that succinate (E0′ succinate/fumarate = +30 mV) oxidation by ubiquinone (E0′ ubiquinol/ubiquinone =+110 mV) is exergonic, whereas succinate oxidation by menaquinone (E0′ menaquinol/menaquinone = −80 mV) is endergonic. It has been suggested that the endergonic reduction of menaquinone by succinate is driven by the electrochemical proton potential (Δp) across the membrane in a process of reverse electron transport . The protons consumed in the reduction of menaquinone were envisaged to be taken up from the bacterial outside (Eqn 1), while the protons liberated in the oxidation of succinate are released on the cytoplasmic side (Eqn 2).
The two reactions are linked by the transfer of two electrons.
If the endergonic succinate oxidation by menaquinone is driven at the expense of Δp, the reverse reaction (fumarate reduction by menaquinol) catalyzed by the same enzyme should generate a Δp across the membrane. This hypothesis will be tested in this communication. To this end the question was addressed whether the succinate dehydrogenase in the membrane of B. subtilis can operate as a fumarate reductase. Furthermore it was tested whether electrons can be transported from NADH to fumarate via NADH:menaquinone reductase and menaquinone, and whether this process is coupled to the generation of a Δp.