NADP+ Reduction with Reduced Ferredoxin and NADP+ Reduction with NADH Are Coupled via an Electron-Bifurcating Enzyme Complex in Clostridium kluyveri

ABSTRACT It was recently found that the cytoplasmic butyryl-coenzyme A (butyryl-CoA) dehydrogenase-EtfAB complex from Clostridium kluyveri couples the exergonic reduction of crotonyl-CoA to butyryl-CoA with NADH and the endergonic reduction of ferredoxin with NADH via flavin-based electron bifurcation. We report here on a second cytoplasmic enzyme complex in C. kluyveri capable of energetic coupling via this novel mechanism. It was found that the purified iron-sulfur flavoprotein complex NfnAB couples the exergonic reduction of NADP+ with reduced ferredoxin (Fdred) and the endergonic reduction of NADP+ with NADH in a reversible reaction: Fdred 2− + NADH + 2 NADP+ + H+ = Fdox + NAD+ + 2 NADPH. The role of this energy-converting enzyme complex in the ethanol-acetate fermentation of C. kluyveri is discussed.