| Two types of model systems were developed in order to clarify the components and mechanisms involved in electron transfer between Photosystems I and II. The first type involved Photosystem I-driven cyclic phosphorylation catalyzed by various low potential mediators, and the second type involved phosphorylation driven by artificial transmembrane redox reactions.;Conditions were devised which allow ferredoxin catalyzed cyclic phosphorylation to be studied without interference from noncyclic phosphorylation. Cyclic phosphorylation catalyzed by ferredoxin was inhibited by both dibromothymoquinone (DBMIB) and antimycin A. The DBMIB inhibition, but not the antimycin A inhibition, could be reversed by the addition of low concentrations of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD). This was interpreted in terms of TMPD forming a bypass of electron transfer on the internal side of the thylakoid vesicle.;Low potential quinones could also catalyze cyclic phosphorylation if they were prereduced to a proper redox potential. Both lipophilic and hydrophilic quinones catalyzed this reaction, and in all cases, cyclic phosphorylation was inhibited by DBMIB, but not by antimycin A. This suggests that the plastoquinone pool, but not cytochrome b(,563), is involved in quinone mediated cyclic phosphorylation.;Cyclic phosphorylation mediated by certain quinols was also very sensitive to inhibition by the dinitrophenyl ether of iodonitrothymol (DNPINT), a compound which inhibits electron transfer between the bound plastoquinone, "B," and the plastoquinone pool. Cyclic phosphorylation mediated by quinols that were appreciably deprotonated at the assay pH was much less sensitive to inhibition by DNPINT than cyclic phosphorylation mediated by protonated quinols. When assay conditions were varied to increase the concentration of the deprotonated quinol, the DNPINT sensitivity of cyclic phosphorylation was decreased. These data were interpreted as showing that the protonated form of a quinol must obligately donate electrons to the bound plastoquinone "B," whereas the deprotonated monohydroquinol anion can donate electrons directly to the plastoquinone pool.;Quinones could also act as mediators in an artificial transmembrane redox reaction, transferring both protons and electrons from a reductant located outside the thylakoid vesicle to an oxidant trapped within the vesicle. The protons released within the vesicle drive phosphorylation. This reaction was inhibited by DBMIB only when hydrophilic quinones acted as the mediator. Redox-induced phosphorylation mediated by hydrophilic quinones is therefore thought to utilize the chloroplasts' endogenous plastoquinone pool as a proton/electron shuttle.;As a step towards developing other model systems which catalyze ATP synthesis in thylakoid vesicles, techniques were devised which allow the entrapment of proteins within thylakoid vesicles. |
