Conserved supersecondary structural motif in NAD-dependent dehydrogenases.

L- and D-specific nicotinamide adenine dinucleotide (NAD)-dependent dehydrogenases map to the same structural protein superfamily as defined by the Structural Classification of Proteins (SCOP) and are based on the Rossmann fold type domains. A detailed classification of these domains is proposed using a novel diagnostic parameter of the rms per aligned pair. The catalytic domain in D-specific dehydrogenases shows a strong structural homology to the coenzyme binding domain. A topologically conserved part within the dehydrogenase superfamily reveals a supersecondary structural motif comprising the 5-stranded left-handedly twisted parallel beta-sheet with one complete and one partial Rossmann fold units and two alpha-helices, the long helix, adjacent to and running roughly parallel with the beta-sheet plane and the helix connecting two Rossmann folds.