Effects of permeant buffers on the initiation of photosynchronous phosphorylation and postillumination phosphorylation in chloroplasts.

Abstract Under canonical chemiosmotic formulations, the development of a delocalized transmembrane proton gradient should precede and, in the absence of a membrane potential, should account for all the capacity of an energy transducing system to synthesize ATP. Furthermore, any agents, such as permeant proton-absorbing buffers, that slow down the kinetics of the development of this gradient should, consequently, delay ATP synthesis. We have studied the very early (0 through 1000 ms) steps of photosynthetic ATP synthesis utilizing real-time, rapid flow-quench techniques. We have investigated the effect(s) that permeant buffers exert on this process where these buffers show no uncoupling effects, and the transmembrane potential has been collapsed by valinomycin and K+. Experimentally this system was dissected into two ATP synthesizing components, as follows: synthesis of ATP strictly concomitant with light influx and unaffected by the addition of permeant buffers. We refer to this as photosynchronous phosphorylation and synthesis of ATP monitored after the light was extinguished and which was greatly diminished by the addition of proton-absorbing permeant buffers, thus exhibiting the characteristics of conventional postillumination phosphorylation, and we suggest that it represents part of capacitance phosphorylation. The potential for capacitance phosphorylation initiates very rapidly under light and gradually builds up to steady-state level, and it is governed by canonical chemiosmotic principles. We estimate that its contribution to overall ATP yield is minimal during the first few cycles of the system and that it increases gradually towards steady state when it contributes to the majority of ATP synthesized. Neither a delocalized transmembrane proton gradient nor a strictly localized intramembrane proton pathway can account for these observations so we have proposed that a gating mechanism exists which delivers intramembrane protons initially directly to the ATP synthetase complex but subsequently to the lumen as well, and thus, allows the lumen to act as a capacitor during the steady state. This study can reconcile the findings of Ort et al. (Ort, D. R., Dilley, R. A., and Good, N. E. (1976) Biochim. Biophys. Acta 449, 108-124) with the contrasting findings of Vinkler et al. (Vinkler, C., Avron, M., and Boyer, P. D. (1980) J. Biol. Chem. 255, 2263-2266) through the opposite effects which osmotic strength and KCl concentration exert on the two ATP synthetic phases (during and after illumination) of the rapid flash technique used in those studies.(ABSTRACT TRUNCATED AT 400 WORDS)