The Coupling of Electron Flow to ATP Synthesis in Pea and Maize Mesophyll Chloroplasts

Abstract

The rate of nouphosphorylating electron transport (in the absence of ADP and inorganic phosphate) in weD-coupled (ATP/2e-= 0.9-1.1) maize mesophyD choroplasts Is not modulated by external pH (6.5-8.5), low levels of ADP or ATP, or energy transfer inhibitors, e.g. triphenyltin andHeg ions. In contrast nonphosphorylating electron flow in pea chloroplasts is sensitive to alterations In medium pH, and to the presence of adenine nucleotides and energy transfer inhdbitors in the assay medium Although ATP is without effect on the rate of basal electron transport in maize chloroplasts, steady-state proton uptake is stimulated 3-to 5-fold by low levels of ATP. These results suggest that differences may exist in the manner in which the coupling factor complex controls proton efflux from the intrathylakoid space in Cs and C4 mesophyll chloroplasts.A number of recent studies have examined the relationship between proton flux through the coupling factor complex (CFo-CF1) and electron flow in chloroplasts (2, 7-11, 24, 25, 27-29). In C3 mesophyll chloroplasts which have developed a large transmembrane proton concentration gradient, there appears to be a pathway for proton efflux through CFo-CF, that is not coupled to ATP synthesis (8,11,17,19,24,27,28 (8,17,24,27,28). Studies of the effects of energy transfer inhibitors, e.g. phlorizin, alkyl tins, Hg2+ ions, etc.on basal electron flow also suggest a role for CFo-CF, in the control of proton leakage from the intrathylakoid space via this pathway (8,11,27,28). Our aim was to compare the effects of adenine nucleotides and energy transfer inhibitors on the rate of nonphosphorylating electron flow and on proton fluxes in mesophyll chloroplasts isolated from both a C3 plant (pea) and a C4 plant (maize).

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

“…4). I interpret these results as indication that non-coupled leakage of protons through CFg-CF, is not operative in cold-grown thylakoids (Cole et al 1981). Both phosphorylating electron flow and response to mercuric ion were also reduced (Tab.…”

“…There is evidence for three parallel pathways of proton efflux from illuminated thylakoids (Cole et al 1981, Gould 1976a, McCarty et al 1971, Portis et al 1975, Schonfeld and Schickler 1984, Underwood and Gould 1980a; (i) a non-specific diffusion of protons through the thylakoid membrane, which is proportional to the permeability of the membrane for protons; (ii) proton translocation through the CF^-CF, ATP-synthetase complex which is coupled to the formation of ATP and (iii) a non-coupled leakage through CFg-CF, in a man-M. ner which does not contribute to phosphorylation. The latter two pathways are sensitive to the pH of the suspending medium, various energy transfer inhibitors, and low concentrations of adenine nucleotide.…”

“…The latter two pathways are sensitive to the pH of the suspending medium, various energy transfer inhibitors, and low concentrations of adenine nucleotide. The relative contribution of each pathway to proton efflux can be assessed by studying the effects of these factors on the steady state proton gradient and the rate of electron flow (Cole et al 1981, McCarty et al 1971, Portis et al 1975, Rumberg et al 1969, Underwood and Gould 1980a.…”

“…Basal (non-phosphorylating) electron flow from water to methyl viologen is modulated by external pH in spinach (McCarty et al 1971, Portis et al 1975) and pea (Cole et al 1981). In thylakoids from V. faba plants grown at 20°C, basal electron flow was stimulated as much as 5-fold at pH 8.5 compared with pH 7.0 ( Fig.…”