Acyl-CoA dehydrogenase 9 (ACAD 9) is the ninth member of ACADs involved in mitochondrial fatty acid oxidation and possibly complex I assembly. Sequence alignment suggested that Glu389 of rat ACAD 9 was highly conserved and located near the active center and might act as an important base for the dehydrogenation reaction. The role of Glu389 in the catalytic reaction was investigated by site-directed mutagenesis. Both wild-type and mutant ACAD 9 proteins were purified and their catalytic characterization was studied. When Glu389 was replaced by other residues, the enzyme activity could be lost to a large extent. Those results suggested that Glu389 could function as the catalytic base that abstracted the a-proton of the acyl-CoA substrate in a proposed catalytic mechanism.