On the principle of ion selectivity in Na ⁺ /H ⁺ -coupled membrane proteins: Experimental and theoretical studies of an ATP synthase rotor

Abstract

Numerous membrane transporters and enzymes couple their mechanisms to the permeation of Na + or H + , thereby harnessing the energy stored in the form of transmembrane electrochemical potential gradients to sustain their activities. The molecular and environmental factors that control and modulate the ion specificity of most of these systems are, however, poorly understood. Here, we use isothermal titration calorimetry to determine the Na + /H + selectivity of the ion-driven membrane rotor of an F-type ATP synthase. Consistent with earlier theoretical predictions, we find that this rotor is significantly H + selective, although not sufficiently to be functionally coupled to H + , owing to the large excess of Na + in physiological settings. The functional Na + specificity of this ATP synthase thus results from two opposing factors, namely its inherent chemical selectivity and the relative availability of the coupling ion. Further theoretical studies of this membrane rotor, and of two others with a much stronger and a slightly weaker H + selectivity, indicate that, although the inherent selectivity of their ion-binding sites is largely set by the balance of polar and hydrophobic groups flanking a conserved carboxylic side chain, subtle variations in their structure and conformational dynamics, for a similar chemical makeup, can also have a significant contribution. We propose that the principle of ion selectivity outlined here may provide a rationale for the differentiation of Na + -and H + -coupled systems in other families of membrane transporters and enzymes. molecular motor | ion-coupled transport | binding thermodynamics | energy transduction | membrane bioenergetics G radients in the electrochemical potential of H + or Na + across biological membranes sustain a wide range of essential cellular process. The resulting proton or sodium motive forces (pmf, smf) are the predominant energy source for secondary-active membrane transporters, which mediate the uptake of many substances required by the cell (1-3), and also enable pathogenic bacteria to protect themselves from human-made antibiotics and other toxic compounds (4-6). Downhill membrane permeation of Na + and H + across the membrane also powers the ATP synthase (7), which produces most of cellular ATP, and energizes the rotation of bacterial flagella (8). Thus, the importance of this mode of energy transduction in cells cannot be overstated. Nevertheless, little is known about the factors that control and modulate the specificity for Na + or H + in most of these processes.It seems clear, although, that there is no correlation between function type and ion specificity; that is, the same process in different species can be coupled to either Na + or H + (2, 5-7, 9-11). Organism-specific environmental factors, such as temperature or pH, also do not provide a consistent rationale; for example, ATP synthases from thermoalkaliphilic bacteria use a H + gradient despite the scarcity of H + and the potentially greater degree of H + leakage across the membrane at hig...

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