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

Respiratory complex I couples NADH:quinone oxidoreduction to ion translocation across the membrane, contributing to the buildup of the transmembrane difference of electrochemical potential. H⁺ is well recognized to be the coupling ion of this system but some studies suggested that this role could be also performed by Na⁺. We have previously observed NADH-driven Na⁺ transport opposite to H⁺ translocation by menaquinone-reducing complexes I, which indicated a Na⁺/H⁺ antiporter activity in these systems. Such activity was also observed for the ubiquinone-reducing mitochondrial complex I in its deactive form. The relation of Na⁺ with complex I may not be surprising since the enzyme has three subunits structurally homologous to bona fide Na⁺/H⁺ antiporters and translocation of H⁺ and Na⁺ ions has been described for members of most types of ion pumps and transporters. Moreover, no clearly distinguishable motifs for the binding of H⁺ or Na⁺ have been recognized yet. 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.