The cytochrome bc complexes of the mitochondrial respiratory and photosynthetic electron transport chains are hetero-oligomeric integral membrane proteins. These proteins are responsible for most of the energy transduction and transport activities across biological membranes. Such complexes cannot be cloned and expressed. Only about two dozen such complexes have been solved by x-ray crystallographic techniques to a resolution of 3.0 A, compared to 40,000 independent structures of soluble proteins in the protein data bank. The cytochrome b6f complex of oxygenic photosynthesis, which is the central electron transfer and proton translocating complex of oxygenic photosynthesis, has been solved to a resolution of 3.0 A. It is a dimeric structure of 220,000 molecular weight, containing 8 polypeptide subunits, with 7 tightly bound electron transfer and pigment groups per monomer. These groups, along with the lipophilic proton and electron carrier, plastoquinone, provide the pathways of electron and proton transfer though the complex. This charge transfer generates the trans-membrane proton electrochemical gradient that is utilized for synthesis of the chemical high energy intermediate, ATP. Among the 7 prosthetic groups, three, a chlorophyll, a beta-carotene, and a unique c-type cytochrome, are unique in terms of their structure or placement in the complex. Recent data on the location of quinone analogue inhibitors define its pathway of transfer through the internal labyrinthine structure of the b6f complex [Ann. Rev. Biochem., 75, 769-790, 2006].
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