 Human SDH shows sequence homology of 27% with the yeast enzyme and 27% with the E. coli enzyme.Additionally, the primary structures are shown to be similar between mammalian SDH and microbial threonine dehydratase, especially in the sequences surrounding the PLP cofactor and the G-residues surrounding the PLP's phosphate group.Thus, in PLP enzymes, there is high conservation of the active site residues during evolution.