Transfer of electrons through the succinate dehydrogenase complex Authored: Jassal, B, 2005-06-10 10:46:07 EC Number: 1.3.5.1 GENE ONTOLOGYGO:0006121 Pubmed8682198 Reactome Database ID Release 43163213 Reactome, http://www.reactome.org ReactomeREACT_6360 This event is deduced on the basis of bovine experimental data.<br>Complex II (succinate dehydrogenase) transfers electrons from the TCA cycle to ubiquinone. The 6th step in the TCA cycle is where succinate is dehydrogenated to fumarate with subsequent reduction of FAD to FADH₂. FADH₂ provides the electrons for the transport chain. Succinate dehydrogenase belongs to subclass 1 of the SQR family (succinate:quinone reductase) (classified by Hagerhall, C and Hederstedt, L [1996]).<br>It consists of 4 subunits (referred to as A, B, C and D), all nuclear-encoded and is located on the matrix side of the inner mitochondrial membrane. Subunits A and B are hydrophilic whereas subunits C and D are integral proteins of the inner membrane. SQRs usually contain 3 Fe-S clusters bound by the B subunit. Succinate dehydrogenase contains one [2Fe-2S] cluster, one [4Fe-4S] cluster and one [3Fe-4S] cluster. Additionally, the A subunit has a covalently-bound FAD group. Reduced complex II has this FAD converted to FADH₂. The electrons from complex II are transferred to ubiquinone (also called Q, Coenzyme Q or CoQ), a small mobile carrier of electrons located within the inner membrane. Ubiquinone is reduced to ubiquinol during this process. Succinate dehydrogenase complex (oxidised) Succinate dehydrogenase complex (reduced)