Cytochrome b6f complex

The cytochrome b6f complex (plastoquinol&mdash;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 (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:

 

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.