抄録
Medium-chain acyl-CoA dehydrogenase (MCAD) oxidizes acyl-CoA to 2-enoyl-CoA. The acyl-CoAs with C6-C10 chain length are good substrates for MCAD. The hydrogen bonds of C(1)=O of the thioester with ribityl-2'-OH of FAD and with amide N-H of Glu376 are important to activate the thioester. We investigated the molecular mechanism for the substrate specificity of MCAD. MCAD reconstituted with 8-NH2-FAD can bind acyl-CoA, but does not oxidize it, because the oxidizing power is weak. Therefore the complex is a good material to investigate the nature of ES complex (the complex of oxidized MCAD with acyl-CoA). The substrate chain-length dependency on the absorption maximum for 8-NH2-FAD in ES complex is correlated well with the substrate specificity of MCAD. FT-IR study suggests the existence of multiple conformations, i.e., a strong and weak hydrogen bonding forms, in the cases of long-chain (>C12) substrate acyl-CoAs. Isovaleryl-CoA (IV-CoA), which has a branched chain, is a poor substrate for MCAD, but the band of C(1)=O stretch shifted downward by 42 cm−1 in the bound form. The large shift indicates the existence of a strong hydrogen bond(s) in the active site, as in the case of octanoyl-CoA. The low activity of MCAD for IV-CoA is due to deviation from an appropriate orientation of IV-CoA in the active site, rather than the destruction of the hydrogen bond(s) by steric hindrance of isovaleryl group in the active site. [Jpn J Physiol 55 Suppl:S68 (2005)]
