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.

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 protons 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 proton 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.

Cyanobacteria Use the Cytochrome b6f Complex and Cytochrome c6 in Both Oxidative Phosphorylation and Photophosphorylation

Cyanobacteria can synthesize ATP by oxidative phos-phorylation or by photophosphorylation, although they have neither mitochondria nor chloroplasts. The enzymatic machinery for both processes is in a highly convoluted plasma membrane (see Fig. 1-6). Two protein components function in both processes (Fig. 19-55). The proton-pumping cytochrome b6 f complex carries electrons from plastoquinone to cytochrome c6 in photosynthesis, and also carries electrons from ubiquinone to cytochrome c6 in oxidative phosphorylation—the role played by cytochrome bc1 in mitochondria. Cytochrome c6, homologous to mitochondrial cytochrome c, carries electrons from Complex III to Complex IV in cyanobac-teria; it can also carry electrons from the cytochrome b6 f complex to PSI—a role performed in plants by plastocyanin. We therefore see the functional homology between the cyanobacterial cytochrome b6 f complex and the mitochondrial cytochrome bc1 complex, and between cyanobacterial cytochrome c6 and plant plastocyanin.

Water Is Split by the Oxygen-Evolving Complex

The ultimate source of the electrons passed to NADPH in plant (oxygenic) photosynthesis is water. Having given up an electron to pheophytin, P680+ (of PSII) must acquire an electron to return to its ground state in preparation for capture of another photon. In principle, the required electron might come from any number of organic or inorganic compounds. Photosynthetic bacteria use a variety of electron donors for this purpose— acetate, succinate, malate, or sulfide—depending on what is available in a particular ecological niche. About 3 billion years ago, evolution of primitive photosynthetic bacteria (the progenitors of the modern cyanobacteria) produced a photosystem capable of taking electrons from a donor that is always available—water. Two water molecules are split, yielding four electrons, four protons, and molecular oxygen:


A single photon of visible light does not have enough energy to break the bonds in water; four photons are required in this photolytic cleavage reaction.

The four electrons abstracted from water do not pass directly to P680+, which can accept only one electron at a time. Instead, a remarkable molecular device, the oxygen-evolving complex (also called the watersplitting complex), passes four electrons one at a



Oxidative phosphorylation

FIGURE 19-55 Dual roles of cytochrome b6f and cytochrome c6 in cyanobacteria. Cyanobacteria use cytochrome b6f, cytochrome c6, and plastoquinone for both oxidative phosphorylation and photophosphorylation. (a) In photophosphorylation, electrons flow (top to bottom) from water to NADP+. (b) In oxidative phosphorylation, electrons flow from NADH to O2. Both processes are accompanied by proton movement across the membrane, accomplished by a Q cycle.





Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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