Cytochrome B6f Complex

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 b6f 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 cytochromef (for the Latinfrons, "leaf"). Electrons flow through the cytochrome b6f complex from PQbH2 to cytochrome f, then to plastocyanin, and finally to P700, thereby reducing it.

^Heme f

^Heme f

Cytochrome B6f Cycle
Stroma (n side)
Rieske Iron Protein

Rieske iron-sulfur protein

Thylakoid lumen (p side)

Rieske iron-sulfur protein

Thylakoid lumen (p side)

Cytochrome B6f Cycle

/Q cycle

Subunit IV

Stroma (N side)

/Q cycle

Subunit IV

Stroma (N side)

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 cofac-tors 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.

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Essentials of Human Physiology

Essentials of Human Physiology

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