3.D.3.5.5
The Ubiquinol-cytochrome oxidase supercomplex with 8 subunits. In the mycobacterial electron-transport chain, respiratory complex III passes electrons from menaquinol to complex IV, which in turn reduces oxygen, the terminal acceptor. Electron transfer is coupled to transmembrane proton translocation. Wiseman et al. 2018 isolated, biochemically characterized, and determined the structure of the obligate III2IV2 supercomplex from Mycobacterium smegmatis. The supercomplex has quinol:O₂ oxidoreductase activity without exogenous cytochrome c and includes a superoxide dismutase subunit that may detoxify reactive oxygen species produced during respiration. Menaquinone is bound in both the Qo and Qi sites of complex III. The complex III-intrinsic diheme cytochrome cc subunit, which functionally replaces both cytochrome c1 and soluble cytochrome c in canonical electron-transport chains, displays two conformations: one in which it provides a direct electronic link to complex IV and another in which it serves as an electrical switch interrupting the connection (Wiseman et al. 2018).