Perfect chemomechanical coupling of FₒF₁-ATP synthase

FₒF₁-ATP synthase (FₒF₁) couples H⁺ flow in Fₒ domain and ATP synthesis/hydrolysis in F₁ 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 c subunits as c/β, the actual H⁺/ATP ratio can vary depending on coupling efficiency. Here, we report actual H⁺/ATP ratio of thermophilic Bacillus FₒF₁, whose c/β is 10/3. Proteoliposomes reconstituted with the FₒF₁ 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 c/β ratio. Thus, chemomechanical coupling between Fₒ and F₁ is perfect.