Millisecond kinetics of ATP synthesis driven by externally imposed electrochemical potentials in chloroplasts.
We have used rapid mixing and quenching techniques to measure the initial ATP synthesis rates and the duration of the ATP synthetic capacity derived from artificially imposed proton gradients and valinomycin-mediated K+ diffusion potentials in chloroplasts. The initial rate of ATP synthesis driven by a K+ diffusion potential was 10-fold slower than that driven by an acid-base transition of equivalent electrochemical potential. Total yields of ATP resulting from a K+ concentration shift were only slightly affected by the absence of Cl-, indicating that Cl- permeability does not significantly reduce the K+ diffusion potential. The ATP synthetic capacity decayed with a half-life of 0.2 s in the case of a K+ diffusion potential and a half-life of 1.0 s in the case of an acid-base shift. In both cases, ATP, added at the time of the pH or [KCl] shift, slowed the decay of the ATP synthesis rates, indicating that the coupling factor controls a channel for proton efflux, as proposed earlier (Portis, A.R., and McCarty, R.E. (1974) J. Biol. Chem. 249, 6250-6254). Because the proton gradient has a longer half-life than the K+ diffusion potential, when combinations of the two are employed to drive ATP synthesis, the proton gradient will make a larger contribution to the initial rate and total yield than that predicted from a strictly linear proportionality of the initial magnitudes of the two gradients.