The Central Photochemical Event: Light-Driven Electron Flow:- The Cytochrome b6f Complex Links Photosystems II and I
Electrons temporarily stored in plastoquinol as a result of the excitation of P680 in PSII are carried to P700 of PSI via the cytochrome b6 f complex and the soluble protein plastocyanin (Fig. 19–49, center). Like Complex III of mitochondria, the cytochrome b6 f complex (Fig. 19–54) contains a b-type cytochrome with two heme groups (designated bH and bL), a Rieske iron-sulfur protein (Mr 20,000), and cytochrome f (for the Latin frons, “leaf”). Electrons flow through the cytochrome b6 f com plex from PQBH2 to cytochrome f, then to plastocyanin, and finally to P700, thereby reducing it.
Like Complex III of mitochondria, cytochrome b6 f conveys electrons from a reduced quinone—a mobile, lipid-soluble carrier of two electrons (Q in mitochondria, PQB in chloroplasts)—to a water-soluble protein that carries one electron (cytochrome c in mitochondria, plastocyanin in chloroplasts). As in mitochondria, the function of this complex involves a Q cycle (Fig. 19–12) in which electrons pass, one at a time, from PQBH2 to cytochrome b6. This cycle results in the pumping of pro tons across the membrane; in chloroplasts, the direction of proton movement is from the stromal compartment to the thylakoid lumen, up to four protons moving for each pair of electrons. The result is production of a pro ton gradient across the thylakoid membrane as electrons pass from PSII to PSI. Because the volume of the flattened thylakoid lumen is small, the influx of a small number of protons has a relatively large effect on lumenal pH. The measured difference in pH between the stroma (pH 8) and the thylakoid lumen (pH 5) represents a 1,000-fold difference in proton concentration—a powerful driving force for ATP synthesis.
FIGURE 19–54 Electron and proton flow through the cytochrome b6f complex. (a) The crystal structure of the complex (PDB ID 1UM3) reveals the positions of the cofactors involved in electron transfers. In addition to the hemes of cytochrome b (heme bH and bL; also called heme bN and bP, respectively, because of their proximity to the N and P sides of the bilayer) and that of cytochrome f (heme f), there is a fourth (heme x) near heme bH, and there is a -carotene of unknown function. Two sites bind plasto quinone: the PQH2 site near the P side of the bilayer, and the PQ site near the N side. The Fe-S center of the Rieske protein lies just outside the bilayer on the P side, and the heme f site is on a protein domain that extends well into the thylakoid lumen. (b) The complex is a homodimer arranged to create a cavern connecting the PQH2 and PQ sites (compare with the structure of mitochondrial Complex III in Fig. 19–12). This cavern allows plastoquinone movement between the sites of its oxidation and reduction. (c) Plastoquinol (PQH2) formed in PSII is oxidized by the cytochrome b6f complex in a series of steps like those of the Q cycle in the cytochrome bc1 complex (Complex III) of mitochondria (see Fig. 19–11). One electron from PQH2 passes to the Fe-S center of the Rieske protein (purple), the other to heme bL of cytochrome b6 (green). The net effect is passage of electrons from PQH2 to the soluble protein plastocyanin, which carries them to PSI.
