Kinetics of Proton‐Transport‐Coupled ATP Synthesis in Chloroplasts. Activation of the ATPase by an Artificially Generated ΔpH and Δψ

Abstract

AbstractThe kinetics of proton transport coupled ATP synthesis at the chloroplast membrane was investigated upon energization of the membrane by an artificially generated ΔpH and an electric potential difference, Δψ. Using a rapid mixing system, rates of ATP synthesis were studied at short reaction times (<150 ms) where all relevant parameters (ΔpH, Δψ, substrate and product concentrations) remain practically constant at their initial values. Under these conditions it was found: The maximal rate of ATP synthesis obtained under these artificial conditions is the same as that obtained by light‐induced ATP synthesis. The turnover number of the ATPase is 410 s−1. The rate of ATP synthesis depends in a sigmoidal way on the transmembrane electrochemical potential difference of protons, Δψ, regardless of the relative contributions by ΔpH and Δψ. The functional dependence of the rate of ATP synthesis on Δμ does not reflect the kinetics of the catalytic events at the ATPase but the transformation of the ATPase from an inactive to a catalytically active state. The binding of two protons to the ATPase from the inside of the membrane is necessary for activation.