Complex III (CIII) catalyzes the Q-cycle. The first step involving the transfer of two electrons from the UQH 2 reduced by CI to two molecules of oxidized cytochrome c at the Q o site. In the second step, two more electrons reduce UQ to UQH 2 at the Q i site. The total reaction is: 2 cytochrome c ⏟ ⏟ oxidized + UQH 2 + 2 H + ⏟ ⏟ matrix ⟶ ⟶ 2 cytochrome c ⏟ ⏟ reduced + UQ + 4 H + ⏟ ⏟ IMS