The Function of Menaquinone, Covalently Bound FAD and Iron‐Sulfur Protein in the Electron Transport from Formate to Fumarate of Vibrio succinogenes

Abstract

The anaerobic bacterium Vibrio succinogenes contains a system of electron transport phosphorylation in which formate functions as the donor and fumarate as the terminal acceptor. The electron transport chain of this system has been investigated. The formate‐fumarate reductase is localized in the membrane fraction of the bacterium, together with acid‐extractable and covalently‐bound FAD, menaquinone, iron‐sulfur protein and b and c cytochromes. The soluble fraction contains FMN, acid‐extractable FAD, iron‐sulfur protein and c cytochromes. The formate‐fumarate reductase is fully inhibited by 2‐(n‐nonyl)‐4‐hydroxyquinoline‐N‐oxide and 4‐chloromercuriphenylsulfonate in amounts which correspond to half the content of the b cytochromes and to the content of iron‐sulfur protein respectively. 2‐(n‐noryl)‐4‐hydroxyquinoline‐N‐oxide specifically inhibits the reduction of menaquinone by formate, 4‐chloromercuriphenylsulfonate the oxidation of reduced menaquinone by fumarate. Formate dehydrogenase, when measured with benzylviologen as acceptor, and fumarate reductase, measured with reduced benzylviologen as the donor, are not affected by the inhibitors. The prosthetic group of fumarate reductase is covalently bound FAD. The specific activity of fumarate reductase is increased to the same extent (about 8‐fold) as the content of the covalently bound FAD when the membrane is fractionated with cholate and ammonium sulfate. The acidextractable FAD is removed by this procedure. The iron‐sulfur protein is the donor for fumarate reductase. This is concluded from the finding that the succinate‐ferricyanide reductase, which is copurified with the fumarate reductase, requires only the covalently bound FAD and the iron‐sulfur protein for activity. The 4‐chloromercuriphenyl‐sulfonate inhibition curve of the succinate‐ferricyanide reductase of the membrane fraction is identical with that of the overall electron transport. Menaquinone is an obligatory redox mediator of formate‐fumarate reductase. The activity is fully inhibited on the extraction of the menaquinone from the membrane fraction, and is reactivated on reincorporation of menaquinone into the membrane. The activities of formate dehydrogenase and succinate‐ferricyanide reductase are independent of the presence of menaquinone. The redox response of membrane‐bound menaquinone confirms participation in electron transport. Copyright © 1976, Wiley Blackwell. All rights reserved