Energy charge, phosphorylation potential and proton motive force in chloroplasts

Abstract

Adenylate concentrations were measured in intact chloroplasts under a variety of conditions. Energy charge was significant in the dark and increased in the light, but remained far below values expected from observed phosphorylation potentials in broken chloroplasts, which were 80 000 M-1 or more in the light. With nitrite as electron acceptor, phosphorylation potentials in intact chloroplasts were about 80 M-1 in the dark and only 300 M-1 in the light. Similar phosphorylation potentials were observed, when oxaloacetate, phosphoglycerate or bicarbonate were used as substrates. delta G'ATP was -42 kJ/mol in darkened intact chloroplasts, -46 kJ/mol in illuminated intact chloroplasts and -60 kJ/mol in illuminated broken chloroplasts. Uncoupling by NH4Cl, which stimulated electron transport to nitrite or oxaloacetate and decreased the proton gradient, failed to decrease the phosphorylation potential of intact chloroplasts. Also, it did not increase the quantum requirement of CO2 reduction. It is concluded that the proton motive force as conventionally measured and phosphorylation potentials are far from equilibrium in intact chloroplasts. The insensitivity of CO2 reduction and of the phosphorylation potential to a decrease in the proton motive force suggests that intact chloroplasts are over-energized even under low intensity illumination. However, such a conclusion is at variance with available data on the magnitude of the proton motive force.