The effects of chloroplast coupling factor reduction on the energetics of activation and on the energetics and efficiency of ATP formation.

There are two separate aspects to the energetic control over the initiation of ATP formation by chloroplast thylakoid membranes: (i) the free energy requirement for the phosphorylation of ADP and (ii) the energy-dependent transformation of the coupling factor into a catalytically active state. Using flash excitation, we have been able to distinguish between limitations imposed on ATP formation by the energetics of coupling factor activation from limitations imposed by the thermodynamics of ADP phosphorylation. We have placed particular emphasis on the effects of coupling factor reduction. ATP hydrolysis, ATP synthesis, and the release of tightly bound [14C]ADP were initiated by fewer flashes in thylakoid membranes in which the coupling factors were prereduced than in membrane samples in which the coupling factor remained oxidized. Only under conditions in which ATP synthesis occurred against a large chemical potential (i.e. delta GATP) did the onset of net ATP synthesis require a larger electrochemical potential than did the activation of the coupling factor. Thus, only at delta GATP values greater than about 51 kJ.mol-1 for thylakoid membranes with oxidized coupling factors and about 45 kJ.mol-1 when the coupling factors are reduced was the onset of ATP synthesis dictated by the thermodynamic equilibrium between ATP and ADP. At lower delta GATP values, the electrochemical potential became energetically adequate to phosphorylate ADP prior to becoming adequate to activate the coupling factor. These data indicate that reduction of the coupling factor lowers the delta pH necessary for activation from approximately 2.9 to approximately 2.6 units. Accompanying the lower energetic requirement for the activation of the reduced coupling factor was an increase in the efficiency of ATP synthesis under conditions of limiting energization.