Editing Electron transport chain (section)

====Complex III====
In [[Complex III]] ([[cytochrome bc1 complex|cytochrome ''bc<sub>1</sub>'' complex]] or CoQH{{sub|2}}-cytochrome ''c'' reductase; {{EC number|1.10.2.2}}), the [[Q cycle|Q-cycle]] contributes to the proton gradient by an asymmetric absorption/release of protons. Two electrons are removed from QH{{sub|2}} at the Q<sub>O</sub> site and sequentially transferred to two molecules of [[cytochrome c|cytochrome ''c'']], a water-soluble electron carrier located within the intermembrane space. The two other electrons sequentially pass across the protein to the Q<sub>i</sub> site where the quinone part of ubiquinone is reduced to quinol. A proton gradient is formed by one quinol (<chem>2H+2e-</chem>) oxidations at the Q<sub>o</sub> site to form one quinone (<chem>2H+2e-</chem>) at the Q<sub>i</sub> site. (In total, four protons are translocated: two protons reduce quinone to quinol and two protons are released from two ubiquinol molecules.){{cn|date=January 2025}}
: <chem>  QH2 + 2</chem><math> \text{ cytochrome }c</math><chem>(Fe^{III}) + 2 H</chem><math>^+_\text{in}</math><chem> -> Q + 2</chem><math> \text{ cytochrome }c</math><chem>(Fe^{II}) + 4 H</chem><math>^+_\text{out}</math>

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 Anti-Lewisite, BAL), [[naphthoquinone]] and antimycin.