Respiratory complex I: A dual relation with H+ and Na+?

We noticed that in menaquinone-reducing complexes I, less energy is available for ion translocation, compared to ubiquinone-reducing complexes I. Therefore, we hypothesized that menaquinone-reducing complexes I perform Na+/H+ antiporter activity in order to achieve the stoichiometry of 4H+/2e−. In agreement, the organisms that use ubiquinone, a high potential quinone, would have kept such Na+/H+ antiporter activity, only operative under determined conditions. This would imply a physiological role(s) of complex I besides a simple “coupling” of a redox reaction and ion transport, which could account for the sophistication of this enzyme. This article is part of a Special Issue entitled Respiratory complex I, edited by Volker Zickermann and Ulrich Brandt.