The Cytochromeb₆Complex of Oxygenic Photosynthesis

Abstract

The cytochrome b 6 f complex is the central complex in the electron transport chain of oxygenic photosynthesis. It provides the electronic connection between the two photosynthetic reaction centers and generates most of the transmembrane electrochemical proton gradient that is used for ATP synthesis. It is a hetero‐oligomeric 220‐kDa homo‐dimeric complex consisting of 8 polypeptide subunits, 13 transmembrane helical segments, and 7 tightly bound prosthetic groups per monomer. It is one of relatively few hetero‐oligomeric membrane proteins whose crystal structure has been solved to a resolution ≤3.0 Å. Among the seven prosthetic groups per monomer, five have a redox function: two b ‐type hemes, b p and b n , on the electrochemically positive and negative sides of the complex, the p‐side heme of cytochrome f , a unique c ‐type cytochrome, and a p‐side [2Fe–2S] center, and a unique heme c n covalently bound to the cytochrome b subunit, which interacts electronically with hem b n . These groups are arranged to accomplish the oxidation and reduction of plastoquinol/plastoquinone (PQH 2 /PQ) on the p‐ and n‐sides of the membrane, and thereby accomplish a net transfer of as many as two protons across the membrane per electron transferred through the complex, thus providing the major input to the transmembrane proton electrochemical gradient. The redox reactions of PQ/PQH 2 occur within the two quinone‐exchange cavities formed by the two monomers of the complex. The p‐side redox reactions are similar to those of the cytochrome bc 1 complex. However, redox reactions on the n‐side are different, partly because of the presence of heme c n .

Mechanism of Enhanced Superoxide Production in the Cytochrome b6f Complex of Oxygenic Photosynthesis

“…Cyanobacterial thylakoid membranes and the b 6 f complex were prepared and purified, respectively, as described previously, as were spinach chloroplast thylakoid membranes and the cytochrome b 6 f complex. , Membranes were resuspended (chlorophyll a concentration of 2 mg/mL) in TNE [30 mM Tris-HCl (pH 7.5), 50 mM NaCl, 1 mM EDTA, and 0.3 M sucrose, with protease inhibitors benzamidine (2 mM) and ε-aminocaproic acid (2 mM)].…”

“…Plastocyanin from Nostoc and spinach was purified as described previously, and for the latter, a modified procedure included size exclusion and anion-exchange chromatography.…”

Metalloproteins Containing Cytochrome, Iron–Sulfur, or Copper Redox Centers

“…282,1473−1476 It is located in between the PSII and PSI reaction centers in oxygenic photosynthesis (Figure 68). The b 6 f complex is analogous to the bc 1 complex of the mitochondrial electron transport chain.…”

“…Cyt b 6 f (plastoquinol–plastocyanin or cyt c 6 oxidoreductase) is a protein complex belonging to a “Rieske–cytochrome b ” family of energy-transducing protein complexes found in the thylakoid membrane in the chloroplasts of green algae, cyanobacteria, and plants and catalyzes ET from plastoquinol to plastocyanin or cyt c 6 (PSII to PSI) coupled with the proton translocation across the membrane for ATP generation. ,− It is located in between the PSII and PSI reaction centers in oxygenic photosynthesis (Figure ). The b 6 f complex is analogous to the bc 1 complex of the mitochondrial electron transport chain.…”

Methods for Studying Interactions of Detergents and Lipids with α‐Helical and β‐Barrel Integral Membrane Proteins

“…For additional details, see (Zhang et al, 2003). The native lipid can then be replaced by a lipid of choice during crystallization (Kurisu et al, 2003b;Yan et al, 2006;Yamashita et al, 2007;Baniulis et al, 2009), as described in detail elsewhere (Cramer et al, 2011).…”