The Rnf Complex Is an Energy-Coupled Transhydrogenase Essential To Reversibly Link Cellular NADH and Ferredoxin Pools in the Acetogen Acetobacterium woodii

Abstract

Ferredoxin and NAD + are key electron carriers in anaerobic bacteria, but energetically, they are not equivalent, since the redox potential of ferredoxin is lower than that of the NADH/NAD + couple. We describe by mutant studies in Acetobacterium woodii that the main function of Rnf is to energetically link cellular pools of ferredoxin and NAD + . When ferredoxin is greater than NADH, exergonic electron flow from ferredoxin to NAD + generates a chemiosmotic potential. This is essential for energy conservation during autotrophic growth. When NADH is greater than ferredoxin, Rnf works in reverse. This reaction is essential for growth on low-energy substrates to provide reduced ferredoxin, indispensable for biosynthesis and CO 2 reduction. Our studies put a new perspective on the cellular function of the membrane-bound ion-translocating Rnf complex widespread in bacteria.