The Function of the b Cytochromes in the Electron Transport from Formate to Fumarate of Vibrio succinogenes

Abstract

The reduction of fumarate by formate of the anaerobic Vibrio succinogenes is catalyzed by an electron transport chain which involves menaquinone and an iron‐sulfur protein as redox mediators between the formate dehydrogenase and the fumarate reductase. The electron transport from formate to menaquinone is specifically inhibited by 2‐(n‐nonyl)‐4‐hydroxyquinoline‐N‐oxide and that from menaquinone to fumarate by 4‐chloromercuriphenyl sulfonate which reacts specifically with the iron‐sulfur protein (Kröger and Innerholer (1976), accompanying paper). The function of the cytochromes in this electron transport chain is investigated. V. succinogenes contains b and c cytochromes, whereas cytochromes of the a, d and o type are missing. The molar absorption coefficients of the b and c cytochromes are determined using the pyridine hemochromogens of heme IX and heme C respectively. By redox titration it is shown that the b and c cytochromes consist of two species each. The midpoint potentials of the b cytochromes are −200 and −20 mV and those of the c cytochromes are −160 and +70 mV. The latter values hold also for the c cytochromes present in the soluble fraction of the cell. The oxidation of cytochrome b (−200 mV) by fumarate is inhibited by the extraction of men aquinone, whereas the reduction by formate is unaffected. In contrast, the reduction of cytochrome b (−20 mV) by formate, but not the oxidation by fumarate is dependent on the presence of menaquinone. The electron transport inhibitor 2‐(n‐nonyl)‐4‐hydroxyquinoline‐N‐oxide affects the redox reactions of the two b cytochromes in the same way as the extraction menaquinone. The oxidation by fumarate of both the b and the c cytochromes is blocked by 4‐chloromercuriphenyl sulfonate, whereas the reduction of the cytochromes by formate is unaffected. Cytochrome b (−20 mV) is reduced by succinate via the fumarate reductase. This reaction is independent of the presence of menaquinone and is inhibited by 4‐chloromercuriphenyl sulfonate. It is concluded that the two b cytochromes participate in the reduction of fumarate by formate. Cytochrome b (−200 mV) interacts between the formate dehydrogenase and menaquinone and cytochrome b (−20 mV) between menaquinone and the iron‐sulfur protein.