The mitochondrial inner membrane constitutes a series of five multi-subunit enzyme complexes responsible for transport of electrons from high-energy carriers, NADH, and FADH2, in an energetically downhill sequence, to a low-energy electron acceptor- oxygen. The first complex-NADH-Q oxidoreductase, is the largest enzyme complex in the series, transferring electrons from NADH to coenzyme Q. This L-shaped complex includes 45 different subunits, of which the mitochondrial genome encodes seven. Its major catalytic components are the NADH-binding site, the primary electron acceptor- FMN, and multiple iron-sulfur clusters. The second complex is part of both the citric acid cycle and the electron transport chain. It transports electrons from succinate to FADH2 and finally to coenzyme Q via iron-sulphur clusters. This complex is therefore known as the succinate-Q reductase. It is a nuclear-encoded tetramer with two hydrophilic subunits – A and B. Subunit-A is a flavoprotein with FAD cofactor and a succinate binding site. Subunit-B is an iron-sulfur protein with three iron-sulfur clusters. The other two subunits – C and D are hydrophobic integral-membrane proteins that contain a Q-binding site.