Complex II or succinate dehydrogenase (quinone) is shared between the TCA cycle and the ETC. It has no proton pumping activity. It is composed of four subunits encoded by nDNA. Two subunits are hydrophillic, SDHA and SDHB, and two subunits are hydrophobic: SDHC and SDHD. Complex II assembly occurs through the independent maturation of these subunits mediated by subunit-specific chaperones.