In the electron transport chain, complex I (CI) catalyzes the reduction of ubiquinone (UQ) to ubiquinol (UQH) by the transfer of two electrons from reduced nicotinamide adenine dinucleotide (NADH) which translocates four protons from the mitochondrial matrix to the IMS:
Complex III (CIII) catalyzes the Q-cycle. The first step involving the transfer of two electrons from the UQH reduced by CI to two molecules of oxidized cytochrome c at the Q site. In the second step, two more electrons reduce UQ to UQH at the Q site. The total reaction is:
Complex IV (CIV) catalyzes the transfer of two electrons from the cytochrome c reduced by CIII to one half of a full oxygen. Utilizing one full oxygen in oxidative phosphorylation requires the transfer of four electrons. The oxygen will then consume four protons from the matrix to form water while another four protons are pumped into the IMS, to give a total reaction