Cytochrome bf complex

Rao, S. B. K., A. M. Tyryshkin et al. (2000). Inhibitory copper binding site on the spinach cytochrome bf complex Implications for Q0 site catalysis. Biochemistry 39 3285-3296. 

In photosystem I, absorption of a photon leads to an excited state that functions as a reducing agent. The electrons are passed from one species to another with several intermediate species that include ferrodoxin (a protein containing iron and sulfur) before finally reducing C02. In photosystem II, electrons are transferred to a series of intermediates, of which a cytochrome bf complex is one entity. Ultimately, the transfer of electrons leads to the reaction... 

The cytochrome bf complex is a proton pump and, during electron transport, pumps H+ ions from the stroma into the thylakoid space, creating an H+ gradient. H+ ions are also released into the thylakoid space when photosystem II oxidizes water to produce oxygen whilst the H+ ions used to reduce NADP+ to NADPH are taken up from the stroma. Both effects contribute to the H+ gradient. 

When little NADP+ is available to accept electrons, an alternative electron transport pathway is used. The high-energy electron donated by photosystem I passes to ferredoxin, then the cytochrome bf complex, then plastocyanin and back to the P700 of photosystem I. The resulting proton gradient generated by the cytochrome bf complex drives ATP synthesis (cyclic photophosphorylation) but no NADPH is made and no 02 is produced. 

The electrons are now passed from PQH2 via the cytochrome bf complex (also called cytochrome b6f complex) to plastocyanin (PC). PC is a copper-containing protein that accepts electrons by the copper cycling between Cu2+ and Cu+ states ... 

Kapazoglou A, Mould RM, Gray JC (2000) Assembly of the Rieske iron-sulphur protein into the cytochrome bf complex in thylakoid membranes of isolated pea chloroplasts. Eur J Biochem 267 352-360... 

PSI and PSII. PSII contains the site of water cleavage, and utilizes the electrons extracted from water to reduce plastoquinone to plastoquinol. The latter diffuses through the membrane until it is reoxidized by another membrane protein, the cytochrome bf complex, which transfers the electrons to a water-soluble electron carrier (plastocyanin or cytochrome c6). This carrier in turn is oxidized by PSI, which delivers the electrons via ferredoxin to the enzymes that produce NADPH (Figure 11.6) [12],... 

Figure 11.10 Second part of electron transfer pathway in photosystem II (PSII) (cytochrome bf complex) involving metal compounds (Fe)...

The two protons from plastoquinol are released into the thylakoid lumen. This reaction is reminiscent of that catalyzed by ubiquinol cytochrome c oxidoreductase in oxidative phosphorylation. Indeed, most components of the enzyme complex that catalyzes the reaction, the cytochrome bf complex, are homologous to those of ubiquinol cytochrome c oxidoreductase. The cytochrome hf complex includes four subunits a 23-kd cytochrome with two Z>-type hemes, a 20-kd Rieske-type Fe-S protein, a 33-kd cytochrome/with a c-type cytochrome, and a 17-kd chain. 

Figure 19.18. Cytochrome BF Contribution to Proton Gradient. The cytochrome bf complex oxidizes QH2 to Q...

The principles by which ATP synthesis takes place in chloroplasts are nearly identical with those for oxidative phosphorylation. We have seen how light induces electron transfer through photosystems II and I and the cytochrome bf complex. At various stages in this process, protons are released into the thylakoid lumen or taken up from the stroma, generating a proton gradient. Such a gradient can be maintained because the thylakoid membrane is essentially... 

We can now estimate the overall stoichiometry for the light reactions. The absorption of 4 photons hy photosystem II generates 1 molecule of O2 and releases 4 protons into the thylakoid lumen. The 2 molecules of plastoquinol are oxidized hy the Q cycle of the cytochrome bf complex to release 8 protons into the lumen. Finally, the electrons from 4 molecules of reduced plastocyanin are driven to ferredoxin by the absorption of 4 additional photons. The 4 molecules of reduced ferredoxin generate 2 molecules of NADPH. Thus, the overall reaction is ... 

Figure 19.26. Cyclic Photophosphorylation. In this pathway, electrons from reduced ferredoxin are transferred to the cytochrome bf complex rather than to ferredoxin-NADP+ reductase. The flow of electrons through cytochrome bf pumps...

DM Kramer and AR Crofts (1994) Re-examination of the properties of the b cytochromes of the thylakoid cytochrome bf complex. Biochim Biophys Acta 1184 193-201... 

PR Rich, P Heathcote, MCW Evans and DS Bendall (1980) An EPR analysis of the partially purified cytochrome bf complex of higher-plant chloroplasts. FEBS Lett 116 51-56... 

Electrons flow from photosystem I to photosystem II through the cytochrome bf complex. This complex catalyzes the transfer of electrons from plastoquinoi (QH2) to plastocyanin (Pc), a small, soluble copper protein in the thylakoid lumen. 

Cytochrome bf complex, a proton transporter of chloroplast thylakold... 

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