Succinate - coenzyme Q reductase

Succinate - coenzyme Q reductase also called succinate dehydrogenase is an enzyme complex found in the matrix part of the inner mitochondrial membrane. The enzyme complex has two main parts where one is part of both the citric acid cycle with the task of converting succinate into fumarate and the other is complex II of the electron transport chain, which uses electrons freed from succinate, to reduce ubiquinone to ubiquinol , ubiquinol is the input of the next complex in the electron transport chain, complex III, coenzyme Q - cytochrome c reductase.

In the citric acid cycle succinate is oxidized by the succinate dehydrogenase part of the complex and 2e^- + 2H⁺ is transferred from succinate to FAD, a coenzyme which is covalently attached to a histidine residue in the enzyme. FADH₂, the reduced form of FAD, will then transfer the two electrons to iron-sulfur clusters inside the enzyme and release the 2H⁺ back into the micochondrial matrix. The iron-sulfur clusters will reduce ubiquinone, Q, to ubiquinol, QH₂.