ENERGY TRANSDUCTION BY COUPLING OF PROTON TRANSLOCATION TO ELECTRON TRANSFER BY THE CYTOCHROME bc, COMPLEX*

The cytochrome bcl complex is an oligomeric membrane protein complex which is a component of the mitochondrial respiratory chain and of the electron transfer chains of numerous bacteria which use oxygen, nitrogen, and sulfur compounds as terminal electron acceptors. The cytochrome bcl complex also participates in the cyclic transfer of electrons to and from the photosynthetic reaction centers in anoxygenic photosynthetic bacteria. In all of these species the cytochrome bcl complex transfers electrons from ubiquinol to cytochrome c and links this electron transfer to translocation of protons across the membrane in which the bcl complex resides. The mechanism by which the cytochrome bcl complex links electron transfer to proton translocation is the protonmotive Q cycle (1). This protonmotive electron transfer is one of the most important mechanisms of cellular energy transduction, found in a phylogenetically diverse range of organisms (2). The purpose of this review is to explain the protonmotive Q cycle.