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 ubiquinol 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. The cytochrome b subunit contains two ubiquinol reactive sites: the oxidation (QP) site and the reduction (QN) site. 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. Inhibited by the anti-tuberculous drug Q203, an optimized imidazopyridine amide (IPA) compound. Q203 triggers a rapid ATP depletion in M.tuberculosis grown under an aerobic or anaerobic atmosphere (PubMed:23913123). Also inhibited by LPZS (lansoprazole sulfide), a metabolite of LPZ (lansoprazole) generated by intracellular sulfoxide reduction into the host cytoplasm; LPZS is a potent anti-mycobacterial agent that inhibits growth of intracellular M.tuberculosis with good activity against drug-resistant isolates. The inhibition of QcrB by LPZS leads to disruption of the mycobacterial respiratory chain and massive and rapid ATP depletion (PubMed:26158909). Both compounds target the same active site in QcrB (the ubiquinol oxidation QP site), but they have distinct drug-binding mechanisms (PubMed:26158909). The cytochrome bc1 complex is composed of a cytochrome b (QcrB), the Rieske iron-sulfur protein (QcrA) and a diheme cytochrome c (QcrC) subunit. Was identified as a high-confidence drug target. Belongs to the cytochrome b family.