The protonmotive Q cycle in mitochondria and bacteria

Crit Rev Biochem Mol Biol. 1994;29(3):165-97. doi: 10.3109/10409239409086800.


The cytochrome bc1 complex is an oligomeric electron transfer enzyme located in the inner membrane of mitochondria and the plasma membrane of bacteria. The cytochrome bc1 complex participates in respiration in eukaryotic cells and also participates in respiration, cyclic photosynthetic electron transfer, denitrification, and nitrogen fixation in a phylogenetically diverse collection of bacteria. In all of these organisms, the cytochrome bc1 complex transfers electrons from ubiquinol to cytochrome c and links this electron transfer to translocation of protons across the membrane in which it resides, thus converting the available free energy of the oxidation-reduction reaction into an electrochemical proton gradient. The mechanism by which the cytochrome bc1 complex achieves this energy transduction is the protonmotive Q cycle. The Q cycle mechanism has been documented by extensive experimentation, and recent investigations have focused on structural features of the three redox subunits of the bc1 complex essential to the protonmotive and electrogenic activities of this membranous enzyme.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.
  • Review

MeSH terms

  • Amino Acid Sequence
  • Bacteria / metabolism*
  • Electron Transport Complex III / metabolism*
  • Electron Transport*
  • Iron-Sulfur Proteins / metabolism
  • Mitochondria / metabolism*
  • Molecular Sequence Data
  • Protons
  • Ubiquinone / metabolism


  • Iron-Sulfur Proteins
  • Protons
  • Rieske iron-sulfur protein
  • Ubiquinone
  • Electron Transport Complex III