Perfect chemomechanical coupling of F(o)F(1)-ATP synthase.

F(o)F(1)-ATP synthase (F(o)F(1)) couples H(+) flow in F(o) domain and ATP synthesis/hydrolysis in F(1) 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 F(o)F(1), whose c/β is 10/3. Proteoliposomes reconstituted with the F(o)F(1) 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 F(o) and F(1) is perfect.