Cytochrome b subunit of the cytochrome bc1 complex, an essential component of the respiratory electron transport chain required for ATP synthesis. The bc1 complex catalyzes the oxidation of menaquinol and the reduction of cytochrome c in the respiratory chain. The bc1 complex operates through a Q-cycle mechanism that couples electron transfer to generation of the proton gradient that drives ATP synthesis. a quinol + 2 Fe(III)-[cytochrome c](out) = a quinone + 2 Fe(II)-[cytochrome c](out) + 2 H(+)(out) Binds 2 heme groups non-covalently per subunit. The cytochrome bc1 complex is composed of a cytochrome b (QcrB), the Rieske protein iron-sulfur (QcrA) and a diheme cytochrome c (QcrC) subunit. Belongs to the cytochrome b family.