Perfect chemomechanical coupling of FF-ATP synthase.

FF-ATP synthase (FF) couples H flow in F domain and ATP synthesis/hydrolysis in F domain through rotation of the central rotor shaft, and the H/ATP ratio is crucial to understand the coupling mechanism and energy yield in cells. Although H/ATP ratio of the perfectly coupling enzyme can be predicted from the copy number of catalytic β subunits and that of H binding subunits as /β, the actual H/ATP ratio can vary depending on coupling efficiency. Here, we report actual H/ATP ratio of thermophilic FF, whose /β is 10/3. Proteoliposomes reconstituted with the FF were energized with ΔpH and Δψ by the acid-base transition and by valinomycin-mediated diffusion potential of K under various [ATP]/([ADP]⋅[Pi]) conditions, and the initial rate of ATP synthesis/hydrolysis was measured. Analyses of thermodynamically equilibrated states, where net ATP synthesis/hydrolysis is zero, show linear correlation between the chemical potential of ATP synthesis/hydrolysis and the proton motive force, giving the slope of the linear function, that is, H/ATP ratio, 3.3 ± 0.1. This value agrees well with the /β ratio. Thus, chemomechanical coupling between F and F is perfect.