Discovery of a Ferredoxin : NAD(+)-Oxidoreductase (Rnf) in acetobacterium woodii - A novel potential coupling site in acetogens

Acetogens use the Wood-Ljungdahl pathway for reduction of carbon dioxide to acetate. This pathway not only allows reoxidation of reducing equivalents during heterotrophic growth but also supports chemolithoautotrophic growth on H-2 + CO2. The latter argues for this pathway being a source for net energy conservation, but the mechanism involved remains unknown. In addition to CO2, acetogens can use alternative electron acceptors, such as nitrate or caffeate. Caffeate respiration in the model acetogen Acetobacterium woodii is coupled to energy conservation via a chemiosmotic mechanism, with Na+ as coupling ion. The pathway and its bioenergetics were solved in some detail very recently. This review focuses on the regulation of caffeate respiration, describes the enyzmes involved, summarizes the evidence for a potential Na+-translocating ferredoxin:NAD(+)-oxidoreductase (Rnf complex) as a new coupling site, and hypothesizes on the role of this Rnf complex in the Wood-Ljungdahl pathway.