¹³C-NMR Study on the Interaction of Medium-Chain Acyl-CoA Dehydrogenase with Acetoacetyl-CoA

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The charge-transfer interaction in the complex of pig kidney medium-chain acyl-CoA dehydrogenase (MCAD) with acetoacetyl-CoA was investigated by ¹³C-NMR spectroscopy and molecular orbital treatment. The acyl carbons of acetoacetyl-CoA were separately ¹³C-labeled and ¹³C-NMR spectra of the complexes of MCAD with the ¹³C-labeled acetoace-tyl-CoA were measured. Each ¹³C-carbon atom was observed as a distinct peak and easily distinguished from the protein background. The chemical shift values for free acetoacetyl-CoA were 198.5, 59.9, 208.8, and 32.8ppm for C (1), C (2), C (3), and C (4), respectively, which shifted to 181.3, 103.4, 192.3, and 29.9ppm, respectively, when acetoacetyl-CoA was complexed with MCAD. While C (4) underwent a small upfield shift, the other carbons experienced significant shifts; both the C (1) and C (3) carbonyl carbons shifted upfield by about 17ppm, and the C (2) carbon was observed as a very broad peak at a position shifted downfield by more than 40ppm. These results were compared with ¹³C-NMR spectra of the keto-, enol-, and enolate forms of ethyl acetoacetate labeled with ¹³C at the acyl carbons, and interpreted with reference to the charge-transfer model based on the optimum overlap between the lowest unoccupied molecular orbital (LUMO) of flavin and the highest occupied molecular orbital (HOMO) of the enolate state of the acetoacetyl moiety of acetoacetyl-CoA. The C (2) carbon of acetoacetyl-CoA takes on the sp² configuration in the bound form, indicating that one of the protons at C (2) of acetoacetyl-CoA is abstracted when bound to MCAD. C (1)=O is substantially polarized in the bound form of acetoacetyl-CoA, implying the presence of a machinery that polarizes this carbonyl group at the binding site, which thereby lowers the pK_a value of the α-proton at C (2). This machinery is of fundamental importance in the initial step of MCAD catalysis.