Energy thresholds for ATP synthesis in chloroplasts

Abstract

We have investigated the ATP synthesis associated with acid-base transitions in chloroplast lamellae under conditions which allow simultaneous control of the thermodynamic variables, ΔpH, membrane potential and ΔG ATP. These variables have been directly imposed rather than simply inferred. Since the initiation of labeled P i incorporation seems to measure accurately the initiation of net ATP synthesis, the following conclusions can be drawn: (1) The proton-motive force which is just sufficient for ATP synthesis provides almost exactly the required energy for ΔG ATP if the efflux of three H + is required for each ATP molecule formed. (2) The membrane potential and the ΔpH contribute to the proton-motive force in a precisely additive way. Thus, the threshold can be reached or exceeded by a ΔpH in the absence of a membrane potential, by a membrane potential in the absence of a ΔpH, or by any combination of membrane potential and ΔpH. With a large enough membrane potential, ATP synthesis occurs even against a small inverse ΔpH. In each instance the combined ΔpH and membrane potential necessary for initiation of ATP synthesis represent the same threshold proton-motive force.