3.D.3.5.4
The three component QcrABC cytochrome bc₁ (bcc) complex.  The bc1 complex catalyzes the oxidation of menaquinol and the reduction of a 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. QcrA is an iron-sulfur (2Fe-2S) protein of 353 aas and 3 central TMSs; QcrB is a cytochrom b protein that contains two quinone binding sites, one for oxidations, and one for reduction of 545 aas and 9 TMSs, while  QcrC is a membrane-bound diheme c-type cytochrome with 269 aas and 2 TMSs, one N-terminal, and one C-terminal.  QcrABC forms a complex with CtaCDEF (TC# 3.D.4.4.5), a cytochrome aa₃ oxidase complex (Falke et al. 2018). This supercomplex is required for spore-specific nitrate reductase 1 activity (Falke et al. 2019).