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Cytochrome b6f complex (1q90). Hydrocarbon boundaries of the lipid bilayer are shown by red and blue dots.

The cytochrome b6f complex (plastoquinol—plastocyanin reductase; EC 1.10.99.1) of chloroplasts and cyanobacteria transfers electrons between the two reaction center complexes of oxygenic photosynthetic membranes, photosystem I and photosystem II, and participates in formation of the transmembrane electrochemical proton gradient by also transferring protons from the stromal to the internal lumen compartment. It is minimally composed of four subunits: cytochrome b6, carrying a low- and a high-potential heme groups (bL and bH); cytochrome f with one covalently bound heme c; Rieske iron-sulfur protein (ISP) containing a single [Fe2S2] cluster; and subunit IV (17 kDa protein). In its structure and functions, the cytochrome b6f complex bears extensive analogy to the cytochrome bc1 complex of mitochondria and photosynthetic purple bacteria. However, there are important differences between the two complexes:

  • The single-polypeptide cytochrome b in the cytochrome bc1 complex corresponds to cytochrome b6 and subunit IV in the cytochrome b6f complex
  • Cytochrome f and cytochrome c1 are not homologous
  • The cytochrome b6f complex contains additional chromophores, chlorophyll a, β-carotene and atypical heme ci (heme x), the latter being linked by a single thioether bond to cytochrome b6

The cytochrome b6f complex is responsible for "non-cyclic" (1) and "cyclic" (2) electron transfer between two mobile redox carriers, plastoquinol (QH2) and plastocyanin:

H2O photosystem II QH2 Cyt b6f plastocyanin photosystem I NADP+ (1)
QH2 Cyt b6f plastocyanin photosystem I Q (2)

Electron transfer is coupled with the translocation of protons across the membrane, thus generating proton-motive force in the form of an electrochemical proton potential which can drive ATP synthesis.

The crystal structure of cytochrome b6f complexes from Chlamydomonas reinhardtii and Mastigocladus laminosus have been determined.

References

  • Cramer, W.A., Martinez, S.E., Furbacher, P., Huang, D. and Smith, J.L. (1994). "The cytochrome b6f complex". Curr. Opin. Struct. Biol. 4: 536–544. doi:10.1016/S0959-440X(94)90216-X.  
  • Cramer, W.A., Zhang, H., Yan, J., Kurisu, G. and Smith, J.L. (2004). "Evolution of photosynthesis: time-independent structure of the cytochrome b6f complex". Biochemistry 43: 5921–5929. doi:10.1021/bi049444o. PMID 15147175.  
  • Schoepp, B., Chabaud, E., Breyton, C., Verméglio, A. and Popot, J.-L. (2000). "On the spatial organization of hemes and chlorophyll in cytochrome b6f. A linear and circular dichroism study". J. Biol. Chem. 275: 5275–5283. doi:10.1074/jbc.275.8.5275. PMID 10681499.  
  • Stroebel, D., Choquet, Y., Popot, J.-L. and Picot, D. (2003). "An atypical haem in the cytochrome b6f complex". Nature 426: 413–418. doi:10.1038/nature02155. PMID 14647374.  

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