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

The cytochrome b₆f complex (plastoquinol—plastocyanin reductase; ) 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, ?-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:

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

External links

• 1Q90 - PDB structure of cytochrome b₆f complex from Chlamydomonas reinhardtii
• 1VF5 - PDB structure of cytochrome b₆f complex from Mastigocladus laminosus
• Cytochrome ''b''₆''f'' page @ Purdue University, USA
• - Calculated positions of b6f and related complexes in membranes

es:Complejo del citocromo b6f

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