Questions about Structure Function at the Outset

Among the mechanistic questions implicit in Figure 7.1a are: (i) the site(s) of entry/exit of reduced plastoquinol/oxidized plastoquinone (PQH2/PQ) from/into the membrane lipid bilayer into/from the b6 fcomplex; (ii) the pathway of the movement of the snake-like PQH2/PQ, a six carbon dimethyl-substituted benzene ring

Figure 7.3 Modified "Q-cycle" model of electron transfer in the cytochrome b6f complex of oxygenic photosynthesis. The modifications, relative to the standard model (Table 7.1A) used to describe the function of the cytochrome bc1 complex in the mitochondrial respiratory chain and

photosynthetic bacteria, involve (a) the presence of heme cn in addition to the heme bn that is present in both bc, and b6f complexes, and the possibility of injection of one electron from ferredoxin that is reduced through the PSI- I inked cyclic electron transport pathway.

that drags a 45 carbon tail made of nine isoprenoid units between its site of oxidation and reduction on the lumenal p-side and stromal n-side of the complex. The problem of diffusion of such quinones in the membrane bilayer has been discussed extensively [18], but not that of movement between redox sites through the interstices of a lipophilic oligomeric membrane protein. (iii) The pathway(s) for PSI-and ferredoxin-dependent cyclic electron transfer, as well as the regulation of cyclic relative to linear electron transfer, present major questions. A pathway involving electron feedback into the bfcomplex is shown (Figure 7.1a) in which electrons would be transferred via ferredoxin from the stromal side of PSI to the stromal n-side (heme bn) of the bf complex [19], and thereby to bound and mobile PQ, allowing additional H+ transfer and ATP synthesis without net reduction of NADP+. It has been argued that heme bn of the bf complex is not an intermediate in the cyclic pathway because of contrasting effects of antimycin A on inhibition of cyclic electron transfer and an absence of inhibition of flash-induced reduction of heme bn [20]. However, realization of the short distance and tight coupling between hemes bn and cn [7-9, 17], and structure details of modes of action of quinone analog inhibitors, mean that the effect of antimycin A on the transient net reduction of heme bn will depend on the detailed mode of action of this inhibitor.

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