The ins and outs of Na bioenergetics in Acetobacterium woodii

The acetogenic bacterium Acetobacterium woodii uses a transmembrane electrochemical sodium ion potential for bioenergetic reactions. A primary sodium ion potential is established during carbonate (acetogenesis) as well as caffeate respiration. The electrogenic Na⁺ pump connected to the Wood-Ljungdahl pathway (acetogenesis) still remains to be identified. The pathway of caffeate reduction with hydrogen as electron donor was investigated and the only membrane-bound activity was found to be a ferredoxin-dependent NAD⁺ reduction. This exergonic electron transfer reaction may be catalyzed by the membrane-bound Rnf complex that was discovered recently and is suggested to couple exergonic electron transfer from ferredoxin to NAD⁺ to the vectorial transport of Na⁺ across the cytoplasmic membrane. Rnf may also be involved in acetogenesis. The electrochemical sodium ion potential thus generated is used to drive endergonic reactions such as flagellar rotation and ATP synthesis. The ATP synthase is a member of the F₁F_O class of enzymes but has an unusual and exceptional feature. Its membrane-embedded rotor is a hybrid made of F_O and V_O-like subunits in a stoichiometry of 9:1. This stoichiometry is apparently not variable with the growth conditions. The structure and function of the Rnf complex and the Na⁺ F₁F_O ATP synthase as key elements of the Na⁺ cycle in A. woodii are discussed.