title: Millisecond kinetics of ATP synthesis driven by externally imposed electrochemical potentials in chloroplasts.
abstract: 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.
authors.forename: R D
authors.initials: RD
authors.lastname: Horner
authors.forename: E N
authors.initials: EN
authors.lastname: Moudrianakis
mesh_terms: D000136:Acid-Base Equilibrium; D000255:Adenosine Triphosphate / Q000096:biosynthesis*; D002712:Chlorides / Q000378:metabolism; D002736:Chloroplasts / Q000378:metabolism*; D004563:Electrochemistry; D007700:Kinetics; D008564:Membrane Potentials; D011188:Potassium / Q000378:metabolism; D011522:Protons
chemical_list: D002712:Chlorides; D011522:Protons; D000255:Adenosine Triphosphate; D011188:Potassium