In the electron transport chain, complex I (CI) catalyzes the reduction of ubiquinone (UQ) to ubiquinol (UQH 2) by the transfer of two electrons from reduced nicotinamide adenine dinucleotide (NADH) which translocates four protons from the mitochondrial matrix to the IMS: NADH + H + + UQ + 4 H + ⏟ ⏟ m a t r i x ⟶ ⟶ NAD + + UQH 2 + 4 H + ⏟ ⏟ I M S