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 F-o, 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 F-o 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 F-o 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 F-o 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. (C) 2010 Elsevier B.V. All rights reserved.