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. c1 functions as an electron donor to cytochrome c. The main subunits of complex b-c1 are: cytochrome b, cytochrome c1 and the Rieske protein. Binds 1 heme c group covalently per subunit.