THE PROTONMOTIVE Q-CYCLE IN MITOCHONDRIA AND BACTERIA

The cytochrome bc(1) complex is an oligomeric electron transfer enzyme located in the inner membrane of mitochondria and the plasma membrane of bacteria. The cytochrome bc(1) 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 bc(1) 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 bc(1) 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 bc(1) complex essential to the protonmotive and electrogenic activities of this membranous enzyme.