Cytochrome b6f complex
Mastigocladus laminosus (cyanobacteria)
The cytochrome b6f complex is a membrane-spanning protein complex embedded in a thylakoide membrane of photosynthetic organisms. Molecular weight of the complex is 220,000 as a dimmer with 26 transmembrane helices. The photosynthetic unit of oxygenic photosynthesis is organized as two large multimolecular membrane complexes, photosystem II that extracts low-energy electrons from water and photosystem I that raises the energy level of those electrons using light energy to produce a strong reductant, NADPH. The cytochrome b6f complex connects the two photosystems electronically by oxidizing lipophilic plastoquinol (PQH2) and reducing plastocyanin (PC) or cytochorm c6 (cyt c6). This electron transfer process is coupled to proton translocation across the membrane that generates a transmembrane electrochemical protein gradient used to synthesize ATP.
According to the x-ray structure, the monomeric unit of the 220 kD dimeric b6f complex consists of four large subunits, cytochrome f, cytochrome b6, the Rieske iron-sulfur protein, and subunit IV, and four small subunits, PetG, PetL, PetM, and PetN. In addition, it contains several natural prosthetic groups: one c-type heme, two b-type hemes, one novel heme, one [2Fe-2S] cluster, one chlorophyll a, and one beta-craotene. The cytochrome b6f accepts one electron from PQH2 through a high-potential electron transfer chain that consists of the Rieske iron-sulfur protein and cytochrome f, and the electron transfer protein, PC or cyt c6. This results in the release of two protons to the aqueous lumenal phase of the membrane. The second electron from PQH2 is transferred across the complex through two b-type hemes or as an anionic plastosemiquinone, and the resulting protein uptake from the stromal side.
Protein Data Bank (PDB)
Kurisu, G. Zhang, H. Smith, J.L. Cramer, W.A.; "Structure of the Cytochrome B6F Complex of Oxygenic Photosynthesis: Tuning the Cavity"; Science; (2003) 302:1009-1014 PubMed:14526088.
author: Genji Kurisu