Proton efflux through the chloroplast ATP synthase (CF0 · CF1) in the presence of sulfhydryl-modifying agents

Abstract

The rate of photosynthetic electron transport measured in the absence of ADP and Pi is stimulated by low levels of Hg2+ or Ag+ (50% stimulation ≈ 3 Hg2+ or 6 Ag+/100 chlorophyll) to a plateau equal to the transport rate under normal phosphorylating conditions (i.e. +ADP, +Pi). Chloroplasts pretreated in the light under energizing conditions with N-ethylmaleimide show a similar stimulation of non-phosphorylating electron transport. The stimulations of non-phosphorylating electron transport by Hg2+, Ag+ and N-ethylmaleimide are reversed by the CF1 inhibitor phlorizin, the CF0 inhibitor triphenyltin chloride, and can be further stimulated by uncouplers such as methylamine. The Hg2+ and N-ethylmaleimide stimulations, but not the Ag+ stimulation, are completely reversed by low levels of ADP (2 μM), ATP (2 μM), and Pi (400 μM). Ag+, which is a potent inhibitor of ATP synthesis, has little or no effect upon phosphorylating electron transport (+ADP, +Pi). Concomitant with the stimulations of non-phosphorylating electron transport by Hg2+, Ag+ and ADP + Pi, there is a decrease in the level of membrane energization (as measured by atebrin fluorescence quenching) which is reversed when the CF0 channel is blocked by triphenyltin. These results suggest that modification of critical CF1 sulfhydryl residues by Hg2+, Ag+ or N-ethylmaleimide leads to the loss of intra-enzyme coupling between the transmembrane protontransferring and the ATP synthesis activities of the CF0-CF1 ATP synthase complex.