Succinate dehydrogenase (complex II) and epigenetic regulation. SDH is the only membrane-bound enzyme of the Krebs cycle, which is also a functional member (complex II) of the electron transport chain (ETC). SDH forms a complex of four different polypeptides (SDHA, SDHB, SDHC and SDHD) with several other prosthetic groups that include FAD, non-heme iron ubiquinone and heme; TET ten–eleven translocation, KDM lysine (K) demethylase, PHD prolyl hydroxylase