Photosynthesis Research

, Volume 92, Issue 1, pp 17–34 | Cite as

Structural basis for the mechanism of electron bifurcation at the quinol oxidation site of the cytochrome bc1 complex



At the heart of the Q cycle hypothesis, the cytochrome bc1 complex (bc1) is required to separate the two electrons from a quinol molecule at the quinol oxidation site. Recent studies have brought to light an intricate mechanism for this bifurcated electron transfer. A survey of the protein data bank shows 30 entries for the structures of bc1 and the homologous b6f complex. These structures provide considerable insights into the structural organization of mitochondrial, bacterial, and plant enzymes. Crystallographic binding studies of bc1 with either quinone reduction (QN) and/or quinol oxidation (QP) site inhibitors offer atomic details on how these compounds interact with residues at their respective sites. Most importantly, the different locations and apparent flexibility observed in crystals for the extrinsic domain of the iron-sulfur protein (ISP) subunit suggest a mechanism for electron bifurcation at the QP site. Analyses of various inhibitor-bound structures revealed two classes of QP site inhibitors: Pm inhibitors that promote ISP mobility and Pf inhibitors that favor the fixation of the ISP conformation. Those analyses also shed light on a possible process by which the ISP motion switch is controlled. The first phase reduction of ISP is shown to be comparable to the reduction of the bL heme by pre-steady state kinetic analysis, whereas the second phase reduction of ISP share similar kinetics with the reduction of the bH heme. The reduction of cyt c1 is measured much slower, indicating that the reduced ISP remains bound at the QP site until the reduced heme bL is oxidized by the heme bH and supporting the existence of a control mechanism for the ISP motion switch.


Crystal structures Cytochrome bc1 complex Inhibitor binding Conformational changes Electron transfer mechanism 



Two-iron-two-sulfur cluster of ISP


Ubiquinol cytochrome c oxidoreductase


High potential b heme


Low potential b heme

cyt b

Cytochrome b subunit

cyt c1

Cytochrome c1 subunit


Electron paramagnetic resonance


Electron transfer


Intermembrane space


Iron-sulfur protein subunit


The extrinsic domain of ISP




2-Nonyl-4-hydroxyquinoline N-oxide


Quinol oxidation site inhibitors that fix ISP-ED conformation


Quinol oxidation site inhibitors that promote ISP-ED movement


Quinol oxidation


Quinone reduction



rms deviation

Root mean square deviation


Rhodobacter sphaeroides bc1






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Copyright information

© Springer Science+Business Media B.V. 2007

Authors and Affiliations

  1. 1.Laboratory of Cell Biology, Center for Cancer ResearchNational Cancer Institute, National Institutes of Health, NIHBethesdaUSA
  2. 2.Department of Biochemistry and Molecular BiologyOklahoma State UniversityStillwaterUSA

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