In Complex III (cytochrome bc complex or CoQH-cytochrome c reductase; ), the Q-cycle contributes to the proton gradient by an asymmetric absorption/release of protons. Two electrons are removed from QH at the Q site and sequentially transferred to two molecules of cytochrome c, a water-soluble electron carrier located within the intermembrane space. The two other electrons sequentially pass across the protein to the Q site where the quinone part of ubiquinone is reduced to quinol. A proton gradient is formed by one quinol () oxidations at the Q site to form one quinone () at the Q site. (In total, four protons are translocated: two protons reduce quinone to quinol and two protons are released from two ubiquinol molecules.)
When electron transfer is reduced (by a high membrane potential or respiratory inhibitors such as antimycin A), Complex III may leak electrons to molecular oxygen, resulting in superoxide formation.
This complex is inhibited by dimercaprol (British Antilewisite, BAL), naphthoquinone and antimycin.