A new catalytic site in the cytochrome b6f complex of Chlamydomonas reinhardtii

Abstract

The Q-cycle model for the operation of the cytochrome b6 f complex is based on the "oxidant-induced-reduction" phenomenon and proposes that the obligatory sequence of events that, after a flash, leads to the reduction of cytochrome b is: 1) oxidation of cytochrome f, 2) oxidation of the FeS center by cytochrome f (i.e. re-reduction of cytochrome f), 3) oxidation of PQH2 to semiquinone by the FeS center, and 4) reduction of the cytochrome b chain by semiquinone. Therefore, cytochrome b (s) should be reduced no faster than cytochrome f. C. reinhardtii mutants of cytochrome f show inhibited rates of cytochrome f re-reduction (a 100 fold range; t1/2 for wt is ~ 3 ms). Instead, the photoreduction of cytochrome b in those mutants is slightly or not inhibited (t1/2 for wt is ~ 3 ms). Thus, the reduction of cytochrome b can precede the re-reduction of cytochrome f, violating the Q-cycle tenets. Besides, the re-reductions of P700 and of plastocyanin that are sensitive to Qo-site inhibitors are not correspondingly inhibited in the mutants and are much faster than the re-reduction of cytochrome f (t1/2= 700 m s for P700); this is also true in the wt. Therefore, although cytochrome f shows redox activity, it is not an obligatory pathway for electrons flowing through the cytochrome b6 f complex from PQH2 to Photosystem I. Steady-state photosynthetic electron transfer activities support this conclusion. We propose that the oxidizing equivalents from Photosystem I can be delivered to the high potential chain of the cytochrome b6 f complex both at the level of cytochrome f and, independently, at another site that is connected to the Qo site, possibly the FeS center. (1) J.Biol.Chem. (2001), accepted.