Dihydrolipoamide dehydrogenase (LipDH) transfers two electrons from dihydrolipoamide (DHL) to NAD+ mediated by FAD. Since this reaction is the final step of a series of catalytic reaction of pyruvate dehydrogenase multi-enzyme complex (PDC), LipDH is a key enzyme to maintain the fluent metabolic flow. We reported here the conformational change near the redox center of LipDH induced by NAD+ promoting the access of the DHL to FAD. The increase in the affinity of DHL to redox center was evidenced by the decrease in K M responding to the increase in the concentration of NAD+ in Lineweaver-Burk plots. The fluorescence intensity of FAD transiently reduced by the addition of DHL was not recovered but rather reduced by the binding of NAD+ with LipDH. The fluorescence decay lifetimes of FAD and Trp were prolonged in the presence of NAD+ to show that FAD would be free from the electron transfer from the neighboring Tyrs and the resonance energy transfer efficiency between Trp and FAD lowered. These results consistently reveal that the conformation near the FAD and the surroundings would be so rearranged by NAD+ to allow the easier access of DHL to the redox center of LipDH.
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