Opposite modulation by uncoupling and electron transport limitation of the [formula omitted] of ADP for photophosphorylation

Abstract

The K m (app) of ADP for photophosphorylation in lettuce chloroplasts was measured both at various light intensities and in the presence of various uncoupler (nigericin + K +) concentrations. Lowering the light intensity results in both, a decrease in the rate of phosphorylation and a several fold decrease in the K m (app) of ADP for the reaction. However, when increasing concentrations of the uncoupler nigericin + K + are employed, the rate of photophosphorylation is decreased but a several-fold increase in the K m (app) of ADP for the reaction is observed. The results are discussed in terms of the chemiosmotic hypothesis. It is suggested that these effects might indicate the existence of a mechanism controlling the rate of ATP formation which is different than the formation of the electrochemical gradient.