Proton Translocation and ATP Formation Coupled to Electron Transport from H2O to the Primary Acceptor of Photosystem 2

Abstract

The rate of electron transport from H2O to silicomolybdate in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) (which involves the oxygen-evolving enzyme, the photochemistry of photosystem 2 and the primary electron acceptor of photosystem 2) is controlled by internal pH. This is based on the shift of the pH profile of the rate of electron transport upon addition of uncouplers, or by using EDTA-treated chloroplasts [lettuce]. Both stimulation and inhibition of electron transport by addition of uncouplers (depending on external pH) was observed. These effects are obtained in the diuron-insensitive photoreductions of either silicomolybdate or ferricyanide. These experiments provide strong evidence that a proton translocating site exists in the sequence of the electron transport H2O .fwdarw. Q (the primary acceptor of photosystem 2). The photoreduction of silicomolybdate in the presence of diuron causes the formation of .DELTA.pH. The value of .DELTA.pH depends on the external pH and its maximal value was 2.4. The calculated internal pH at different external pH values was rather constant, namely between 5.1-5.2. Electron transport from H2O to silicomolybdate (in the presence of diuron) does not support ATP formation. Probably this is due to the fact that the .DELTA.pH formed is below the threshold .DELTA.pH required for the synthesis of ATP. By adding an additional source of energy in the form of a dark diffusion potential created in the presence of K+ and valinomycin, significant amounts of ATP are formed in this system.