Two pathways of electron transfer in quinol-mediated cyclic phosphorylation in spinach chloroplasts

Low potential quinones are mediators of cyclic phosphorylation in washed spinach thylakoid membranes if they are prereduced to provide the proper redox poise. Cyclic phosphorylation catalyzed by different quinols varies in its sensitivity to the electron transfer inhibitor 2-iodo-6-isopropyl-3-methyl-2',4,4'-trinitrodiphenyl ether (DNPINT), which is thought to inhibit electron flux from the bound plastoquinone (B) to the plastoquinone pool (Trebst, A., Wietoska, H., Draber, W. and Knops, H.J. (1978) Z. Naturforsch. 33c, 919-927). Cyclic phosphorylation catalyzed by uncharged quinols is extremely sensitive to DNPINT, whereas cyclic phosphorylation catalyzed by negatively charged quinols is approximately two orders of magnitude less sensitive. Many quinols have pK1 values in the physiological range (pH 7-9). Increasing the concentration of the deprotonated quinol either by raising the assay pH, increasing the mediator concentration, or increasing the fractional reduction of the quinone results in a decrease in the sensitivity of cyclic phosphorylation to DNPINT. At very high DNPINT concentrations, cyclic phosphorylation catalyzed by all quinols (and ferredoxin) is inhibited, but not phenazine methosulfate catalyzed cyclic phosphorylation. These data suggest that the deprotonated form of the quinol can donate electrons directly to the plastoquinone pool, whereas the uncharged quinol most obligately transfer electrons through the bound plastoquinone 'B'. A second site of DNPINT action after the plastoquinone pool is also observed, which requires much higher DNPINT concentrations for inhibition of phosphorylation.