Structure-Function of the Cytochrome b₆f Complex of Oxygenic Photosynthesis

Book · Thu Mar 20 00:00:00 EDT 2014

DOI:https://doi.org/10.1016/B978-0-12-378630-2.00158-4· OSTI ID:1126221

Cramer, W. A.; Yamashita, E.; Baniulis, D.; Whitelegge, J.; Hasan, S. S. ^[1]

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1. Lithuanian RAF

Structure–function of the major integral membrane cytochrome b₆f complex that functions in cyanobacteria, algae, and green plants to transfer electrons between the two reaction center complexes in the electron transport chain of oxygenic photosynthesis is discussed in the context of recently obtained crystal structures of the complex and soluble domains of cytochrome f and the Rieske iron–sulfur protein. The energy-transducing function of the complex, generation of the proton trans-membrane electrochemical potential gradient, centers on the oxidation/reduction pathways of the plastoquinol/plastoquinone (QH₂/Q), the proton donor/acceptor within the complex. These redox reactions are carried out by five redox prosthetic groups embedded in each monomer, the high potential two iron–two sulfur cluster and the heme of cytochrome f on the electropositive side (p) of the complex, two noncovalently bound b-type hemes that cross the complex and the membrane, and a covalently bound c-type heme (c_n) on the electronegative side (n). These five redox-active groups are organized in high- (cyt f/[2Fe–2S] and low-potential (hemes b_p, b_n, c_n) electron transport pathways that oxidize and reduce the quinol and quinone on the p- and n-sides in a Q-cycle-type mechanism, while translocating as many as 2 H⁺ to the p-side aqueous side for every electron transferred through the high potential chain to the photosystem I reaction center. The presence of heme c_n and the connection of the n-side of the membrane and b₆f complex to the cyclic electron transport chain indicate that the Q cycle in the oxygenic photosynthetic electron transport chain differs from those connected to the bc₁ complex in the mitochondrial respiratory chain and the chain in photosynthetic bacteria. Inferences from the structure and C2 symmetry of the complex for the pathway of QH₂/Q transfer within the complex, problems posed by the presence of lipid in the inter-monomer cavity, and the narrow portal for QH2 passage through the p-side oxidation site proximal to the [2Fe–2S] cluster are discussed.

• Research Organization: Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
• Sponsoring Organization: USDOE Office of Science (SC)
• OSTI ID: 1126221
• Resource Relation: Related Information: Encyclopedia of Biological Chemistry
• Country of Publication: United States
• Language: ENGLISH