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| - Effect of a Charge-Transfer Interaction on the Catalytic Activity of Acyl-CoADehydrogenase: A Theoretical Study of the Role of Oxidized Flavin
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| - To examine the role of charge transfer (CT) interactions in the catalytic activity of acyl-CoA dehydrogenase,model systems for the reactant complex of tricyclic or bicyclic models of FAD (Flox) with the thioenolate portion of acyl-CoA were studied. Singlet excitation energies have been estimated at the TD-B3LYP/6-31G(d)//B3LYP/6-31+G(d,p) level of theory. Location and intensity of the charge-transfer band in theUV-absorption spectrum of the thioenolate/Flox complex depends on the distance between model substrateand isoalloxazine ring and becomes prominent at approximately the van der Waals contact distance. Theprereaction complex for hydride transfer from the thioenolate to flavin [fully optimized at B3LYP/6-31+G(d,p)]possesses a sandwich-like structure with a contact distance of about 3.7 Å between participants and has about0.7 electrons transferred from the enolate to the flavin. We suggest that the formation of a CT complex in theenzyme has a significant impact on the barrier to α-proton abstraction; it reduces the α-proton affinity of thethioester, thereby lowering the activation energy for the first step in this enzymatic process.
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