On an Inhibitor of Ferredoxin Catalyzed Cyclic Photophosphorylation in Thylakoid Membranes*

Abstract

AbstractDibromo‐ and diiodo‐naphthoquinones are shown to be inhibitors of the cytochrome b6/f complex in isolated thylakoid membranes from spinach chloroplasts. Dibromo‐naphthoquinone inhibits ferredoxin catalyzed cyclic photophosphorylation at 0.1 μM concentrations, but non cyclic e‐flow only at 10 μM. It does not inhibit cyclic systems with artifical cofactors, nor non‐cyclic electron flow from duroquinol through photosystem I via the cytochrome b6/f complex. Dibromo‐naphthoquinone does however, lower the stoichiometry for ATP formation in the duroquinol donor system. This inhibitory pattern is quite different from that of DBMIB, but very similar to that of antimycin. This antimycin‐like behaviour of these inhibitors is interpreted to indicate a) the existence of a Qc site in the cytochrome b6/f complex and its obligate function in ferredoxin catalyzed cyclic electron flow and b) a non‐essential role of the Qc site in non‐cyclic electron flow, but which — when operative — pumps an extra proton across the thylakoid membrane increasing the ATP yield.