A structural model for human dihydrolipoamide dehydrogenase.

The hypothesis that dihydrolipoamide dehydrogenases (E3s) have tertiary structures very similar to that of human glutathione reductase (GR) was tested in detail by three separate criteria: (1) by analyzing each putative secondary structural element for conservation of appropriate polar/nonpolar regions, (2) by detailed comparison of putative active site residues in E3s with their authentic counterparts in human GR, and (3) by comparison of residues at the putative dimeric interface of the E3s with the authentic residues in GR. All three criteria are satisfied in a convincing way for the 7 E3s that were considered, supporting the conclusion that the structural scaffolding and the overall tertiary structure (which determines the location of functional sites and residues) are remarkably similar for the E3s and for GR. These analyses together with the crystal structures of human erythrocyte GR formed the basis for construction of a molecular model for human E3. The cofactor FAD and the substrates NAD and lipoic acid were also included in the model. Unexpectedly, the surface residues in the cleft that holds the lipoamide were found to be highly charged and predominantly acidic, allowing us to predict that the region around the lipoamide in the subunit should be basic in nature. The molecular model can be tested by site-directed mutagenesis of residues predicted to be in the dihydrolipoamide acetyltransferase subunit binding cleft.