Component of the ubiquinol-cytochrome c reductase complex (complex III or cytochrome b-c1 complex), which is a respiratory chain that generates an electrochemical potential coupled to ATP synthesis. Binds 2 heme b groups non-covalently. The main subunits of complex b-c1 are: cytochrome b, cytochrome c1 and the Rieske protein. Heme 1 (or BL or b562) is low-potential and absorbs at about 562 nm, and heme 2 (or BH or b566) is high-potential and absorbs at about 566 nm. Belongs to the cytochrome b family.