Mitochondrial complex III - supernumerary subunits form part of complex III that catalyzes the electron transfer reaction from the lipophilic substrate ubiquinol (QH2) to cytochrome C (cyt c), coupled with proton translocation across the membrane. As a result, for every QH2 molecule oxidized, four protons are transferred from the mitochondrial matrix to the intermembrane space and two molecules of cyt c are reduced. The subunit composition of complex III varies between species, though all complexes contain three redox subunits that from the catalytic core. All additional subunits, referred to as supernumerary subunits, have no well-established cellular function, except for the so-called core-1 and core-2 subunits which are metalloproteases, and thus are believed to contribute to the increased stability of these complexes. (Adapted from PMID:23201476 and PMID: 23871937.)