FT-IR Spectroscopic Studies on the Molecular Mechanism for Substrate Specificity/Activation of Medium-Chain Acyl-CoA Dehydrogenase

The interactions of acyl-CoA with medium-chain acyl-CoA dehydrogenases (MCADs) reconstituted with artificial FADs--i.e. 8-CN-, 7,8-Cl₂-, 8-Cl-, 8-OCH₃- and 8-NH₂-FAD--were investigated by UV-visible absorption and FT-IR measurements. Although 8-NH₂-FAD-MCAD did not oxidize acyl-CoA the wavelength of the absorption maximum of the flavin was altered by acyl-CoAs binding. Thus, 8-NH₂-FAD-MCAD is one of the attractive materials for investigation of enzyme-substrate (ES) interaction in ES complex (the complex of oxidized MCAD with acyl-CoA). FT-IR difference spectra between non-labelled and [1-¹³C]-labelled acyl-CoA free in solution and bound to oxidized 8-NH₂-FAD-MCAD were obtained. The broad 1668-cm⁻¹ band of free octanoyl-CoA assigned to the C(1) = O stretching vibration appeared as a sharp signal at 1626 cm⁻¹ in the case of the complex. The downward shift indicates a large polarization of C(1) = O, and the sharpness suggests that the orientation of the C(1) = O in the active-site cavity is fairly limited. The hydrogen-bond enthalpy change responsible for the polarization on the transfer of the substrate from aqueous solution to the active site of MCAD was estimated to be ~15 kcal/mol. The 1626-cm⁻¹ band is noticeably weakened in the case of acyl-CoA with acyl chains longer than C12 which are poor substrates for MCAD, suggesting that C(1) = O is likely to exist in multiple orientations in the active-site cavity, whence the band becomes obscured. A band identical to that of bound C8-CoA was observed in the case of C4-CoA which is a poor substrate, indicating the strong hydrogen bond at C(1) = O.