The relationship between proton fluxes and the regulation of electron transport in chloroplasts

Effect of low growth temperature on coupling between electron transport and proton flux in Vicia faba thylakoids

Coupling between electron transport and proton flux has been compared in chloroplasts from Vicia faba (cv. Windsor) plants grown at 20 and 5°C. Proton uptake by warm-grown thylakoids was sensitive to external pH and stimulated by micromolar adenine nucleotide above pH 7.0. Electron transport was modulated by pH, adenine nucleotide and energy transfer inhibitors (triphenyltin and Hg(2+) ). By contrast, proton uptake by cold-grown thylakoids was generally lower and was insensitive to micromolar ATP. The rate of non-phosphorylating electron flow in cold-grown thylakoids was relatively insensitive to pH and Hg(2+) and was not modulated by adenine nucleotides or triphenyltin. Stimulation of electron transport by phosphorylating conditions in cold-grown thylakoids was generally lower and insensitive to pH. It is concluded that the control of proton efflux through CF(0) -CF(1) differs in thylakoids of V. faba grown at warm and cold temperatures.

Physical mechanism of ATP formation in biomembranes

Membrane-bound and isolated H+ ATPases of various origin are able to synthesize ATP from ADP and Pi after a jump-like pH increase. In the course of this increase the pH of solution (or suspension) must cross a value corresponding to pK of certain acid groups in the catalytic component of ATPase. In the case of isolated soluble enzymes it is possible to obtain up to 10 ATP molecules per one pH jump per one enzyme molecule. A physical mechanism of this phenomenon as well as of oxidative and photosynthetic phosphorylation is suggested.

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

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 approximately or equal to 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-ethylmalemide stimulations, but not the Ag+ stimulation, are completely reversed by low levels of ADP (2 microM), ATP (2 microM), AND Pi (400 microM). 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-ethylmalemide leads to the loss of intra-enzyme coupling between the transmembrane proton-transferring and the ATP synthesis activities of the CF0-CF1 ATP synthase complex.