complex II CII Eukaryotic mitochondrial complex II (succinate dehydrogenase) and E. coli succinate:quinone oxidoreductase (SQR) are composed of two hydrophilic subunits, a flavoprotein (SDHA) and iron-sulfur protein (SDHB) subunit, and two hydrophobic membrane anchor subunits, SDHC and SDHD. GO:0045273 GO:0045281 IMR:0001008 SQR molecule_role succinate dehydrogenase succinate-ubiquinone oxidoreductase succinate:quinone oxidoreductase dehydrogenase