Hyperthermophilic archaea are close to the origin of life. Some hyperthermophilic anaerobic archaea live under strong energy limitation and have to make a living near thermodynamic equilibrium. Obviously, this requires adaptations of the energy-conserving machinery to harness small energy increments. Their ATP synthases often have an unusual motor subunit c that is predicted to prevent ATP synthesis. We have purified and reconstituted into liposomes such an archaeal ATP synthase found in a mesophilic bacterium. The enzyme indeed synthesized ATP at physiological membrane potentials, despite its unusual c subunit, but the minimal driving force for ATP synthesis was found to be even lower than in ATP synthases with usual c subunits. These data not only reveal an intermediate in the transition from ATP hydrolases to ATP synthases but also give a rationale for a bioenergetic adaptation of microbial growth near the thermodynamic equilibrium.
Keywords: ATP synthesis; acetogenic bacteria; archaea; bioenergetics; driving forces.