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 phot osynthetic electron transfer, denitrification, and nitrogen fixation i n a phylogenetically diverse collection of bacteria. In all of these o rganisms, the cytochrome bc(1) complex transfers electrons from ubiqui nol to cytochrome c and links this electron transfer to translocation of protons across the membrane in which it resides, thus converting th e available free energy of the oxidation-reduction reaction into an el ectrochemical proton gradient. The mechanism by which the cytochrome b c(1) complex achieves this energy transduction is the protonmotive Q c ycle. The Q cycle mechanism has been documented by extensive experimen tation, and recent investigations have focused on structural features of the three redox subunits of the bc(1) complex essential to the prot onmotive and electrogenic activities of this membranous enzyme.