The effects of quinone analogues on cytochrome b₆ reduction and oxidation in a reconstituted system

The reconstituted system containing Photosystem I, plastocyanin and the cytochrome b₆-f{hook} complex is used to study the effects of various quinone analogues on the redox behavior of cytochrome b₆. The effects of DBMIB, DNP-INT and HQNO are compared in an attempt to discern the modes of action of these quinone analogues. Both DBMIB and DNP-INT are potent inhibitors of the plastocyanin reductase activity of the isolated cytochrome complex. However, while DBMIB abolished the oxidant-induced reduction of cytochrome b₆, DNP-INT only inhibited about 25% of the net reduction. On the other hand, HQNO does not show any significant inhibition of plastocyanin reductase activity of the isolated cytochrome complex at concentrations up to 20μM. An enhancement of the net amount of cytochrome b₆, reduced is observed in the presence of HQNO. Both DNP-INT and HQNO inhibited the dark oxidation rate of cytochrome b₆. The possible identity of the oxidant for cytochrome b₆ is discussed. Plastoquinone is concluded to be the most likely candidate. DNP-INT is concluded to have at least two sites of inhibition in the cytochrome complex. The implications of these findings on quinone functions in the cytochrome b₆-f{hook} complex are discussed.