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

Fo F1 -ATP synthase (Fo F1 ) 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 Fo F1 , whose c /β is 10/3. Proteoliposomes reconstituted with the Fo F1 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.