Cytochrome b6f complex

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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 (b_L and b_H); 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 c_i (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.
• 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. 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. 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. PMID 14647374.