Structural and energetic basis for H+ versus Na+ binding selectivity in ATP synthase Fo rotors

The functional mechanism of the F1Fo ATP synthase, like many membrane transporters and pumps, entails a conformational cycle that is coupled to the movement of H+ or Na+ ions across its transmembrane domain, down an electrochemical gradient. This coupling is an efficient means of energy transduction and regulation, provided that ion binding to the membrane domain, known as Fo, is appropriately selective. In this study we set out to establish the structural and energetic basis for the ion-binding selectivity of the membrane-embedded Fo rotors of two representative ATP synthases. First, we use a biochemical approach to demonstrate the inherent binding selectivity of these rotors, that is, independently from the rest of the enzyme. We then use atomically detailed computer simulations of wild-type and mutagenized rotors to calculate and rationalize their selectivity, on the basis of the structure, dynamics and coordination chemistry of the binding sites. We conclude that H+ selectivity is most likely a robust property of all Fo rotors, arising from the prominent presence of a conserved carboxylic acid and its intrinsic chemical propensity for protonation, as well as from the structural plasticity of the binding sites. In H+-coupled rotors, the incorporation of hydrophobic side chains to the binding sites enhances this inherent H+ selectivity. Size restriction may also favor H+ over Na+, but increasing size alone does not confer Na+ selectivity. Rather, the degree to which Fo rotors may exhibit Na+ coupling relies on the presence of a sufficient number of suitable coordinating side chains and/or structural water molecules. These ligands accomplish a shift in the relative binding energetics, which under some physiological conditions may be sufficient to provide Na+ dependence.