2.B.124.  The Na⁺ Ionopohore, ETH2120 (ETH2120) Family 

Imkamp and Müller 2002 prepared cell suspensions of Acetobacterium woodii from cultures grown on fructose plus caffeate. These cultures catalyzed caffeate reduction with electrons derived from molecular hydrogen. Hydrogen-dependent caffeate reduction was strictly Na⁺-dependent with a K(m) for Na⁺ of 0.38 mM; Li⁺ could substitute for Na⁺. The sodium ionophore ETH2120, but not protonophores, stimulated hydrogen-dependent caffeate reduction by 280%, indicating that caffeate reduction is coupled to the buildup of a membrane potential generated by Na⁺ extrusion. Caffeate reduction was coupled to the synthesis of ATP, and ATP synthesis coupled to hydrogen-dependent caffeate reduction was strictly Na⁺ dependent and abolished by ETH2120, but not by protonophores, indicating the involvement of a transmembrane Na⁺ gradient in ATP synthesis. The ATPase inhibitor N,N'-dicyclohexylcarbodiimide (DCCD) abolished ATP synthesis, and at the same time, hydrogen-dependent caffeate reduction was inhibited. This inhibition could be relieved by ETH2120. These experiments are compatible with a chemiosmotic mechanism of ATP synthesis with Na⁺ as the coupling ion during hydrogen-dependent caffeate reduction by A. woodii (Imkamp and Müller 2002).