Modes of action of uncouplers in thylakoids

The pH dependence of electron transport in isolated lettuce chloroplasts can be simulated by a model involving four ionizable groups: two facing the inside and two facing the outside of the thylakoids. The pH dependence can be resolved into separate pH profiles for the outside and the inside of the thylakoids, with optima at 7.1 and 5.6, respectively. A ΔpH of 3 units across the thylakoid membrane allows only partial overlapping of the two profiles and relatively low rates of electron transport. Uncouplers stimulate electron transport by reducing the ΔApH, thereby improving the overlapping of the two pH profiles.