Cyclic electron transport in chloroplasts. The Q-cycle and the site of action of antimycin

Cyclic electron transport systems have been set up in broken chloroplasts, with photochemically reduced ferredoxin or 9,10-anthraquinone-2-sulphonate as cofactor. In good agreement with the literature, only the ferredoxin-catalyzed pathway was found to be inhibited by antimycin; but both pathways were found to have a slow electrogenic reaction, both were inhibited by the cytochrome b-563 oxidation inhibitor 2-heptyl-4-hydroxyquinoline N-oxide (the inhibition being strongest at limiting light intensity), and the two pathways had the same proton/electron stoichiometry at limiting light intensity. It is concluded that a Q-cycle can occur in cyclic electron transport with either cofactor; and therefore that the site of action of antimycin in chloroplasts is not within the Q-cycle, as it is believed to be in mitochondria and bacteria. Instead, a ferredoxin-quinone reductase is proposed as the site of action of antimycin in the ferredoxin-catalyzed cyclic pathway. It is also concluded that the data presented here are consistent with the suggestion that the Q-cycle in photosynthetic electron transport is a facultative one, its degree of engagement depending on competition between the Rieske centre and cytochrome b-563 for reducing equivalents from plastosemiquinone.