Rat and human short/branched chain acyl-CoA dehydrogenases exhibit key differences in substrate specificity despite an overall amino acid identity of 85% between them. Rat short/branched chain acyl-CoA dehydrogenases (SBCAD) are more active toward substrates with longer carbon side chains than human SBCAD, whereas the human enzyme utilizes substrates with longer primary carbon chains. The mechanism underlying this difference in substrate specificity was investigated with a novel surface plasmon resonance assay combined with absorbance and circular dichroism spectroscopy, and kinetics analysis of wild type SBCADs and mutants with altered amino acid residues in the substrate binding pocket. Results show that a relatively few amino acid residues are critical for determining the difference in substrate specificity seen between the human and rat enzymes and that alteration of these residues influences different portions of the enzyme mechanism. Molecular modeling of the SBCAD structure suggests that position 104 at the bottom of the substrate binding pocket is important in determining the length of the primary carbon chain that can be accommodated. Conformational changes caused by alteration of residues at positions 105 and 177 directly affect the rate of electron transfer in the dehydrogenation reactions, and are likely transmitted from the bottom of the substrate binding pocket to -sheet 3. Differences between the rat and human enzyme at positions 383, 222, and 220 alter substrate specificity without affecting substrate binding. Modeling predicts that these residues combine to determine the distance between the flavin ring of FAD and the catalytic base, without changing the opening of the substrate binding pocket.The acyl-CoA dehydrogenases (ACDs) 1 are a family of related enzymes that catalyze the ␣, -dehydrogenation of acylCoA esters, transferring electrons to electron transferring flavoprotein (1-4). Deficiencies of these enzymes are important causes of human disease (3-7). Biochemical and immunological studies have identified at least 9 distinct members of this enzyme family, each having a characteristic substrate utilization pattern (8 -13). Very long, long, medium, and short chain acyl-CoA dehydrogenases (VLCAD, LCAD, MCAD, and SCAD, respectively) catalyze the first step in the mitochondrial -oxidation of fatty acids with substrate optima of 16, 16, 8, and 4 carbon chains, respectively (3, 14 -15). A new ACD has been identified recently (ACAD9) that is also active against long chain acyl-CoA substrates (16). Isovaleryl-CoA dehydrogenase (IVD), short/branched chain acyl-CoA dehydrogenase (SBCAD, also known as 2-methyl branched chain acyl-CoA dehydrogenase), and isobutyryl-CoA dehydrogenase (IBD) catalyze the third step in leucine, isoleucine, and valine metabolism, respectively (2-4, 11, 12, 17, 18), whereas glutaryl-CoA dehydrogenase is active in lysine metabolism (19).We have shown previously that SCAD, SBCAD, and IBD can all utilize butyryl-CoA as substrate (albeit with different efficiencies), whereas ...