The ubiquinol cytochrome c reductase (cytochrome bc1) complex is a respiratory chain that generates an electrochemical potential coupled to ATP synthesis. The bc1 complex contains 11 subunits, 3 respiratory subunits (cytochrome B, cytochrome C1, Rieske protein), 2 core proteins and 6 low-molecular weight proteins. Each subunit of the cytochrome bc1 complex provides a single helix (this domain) to make up the transmembrane region of the complex.