Perfect chemomechanical coupling of F_oF₁-ATP synthase.

Abstract

F_oF₁-ATP synthase (F_oF₁) couples H⁺ flow in F_o 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 <i>c</i> subunits as <i>c</i>/β, the actual H⁺/ATP ratio can vary depending on coupling efficiency. Here, we report actual H⁺/ATP ratio of thermophilic <i>Bacillus</i> F_oF₁, whose <i>c</i>/β is 10/3. Proteoliposomes reconstituted with the F_oF₁ 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 <i>c</i>/β ratio. Thus, chemomechanical coupling between F_o and F₁ is perfect.