An unusual conformation of NAD+ bound to sorbitol dehydrogenase? A time-dependent transferred nuclear Overhauser effect study.

The conformation of NAD+ in the sheep liver sorbitol dehydrogenase-NAD+ binary complex has been investigated using time-dependent proton-proton transferred nuclear Overhauser enhancement measurements to determine interproton distance ratios and distances between bound NAD+ protons. The conformation about both the adenosine and nicotinamide riboside glycosidic bonds is anti, the conformations of the adenosine and nicotinamide ribose rings are C3'-endo and C1'-exo, respectively, and the conformations about the adenosine and nicotinamide riboside C4'-C5' bonds are g+ and t, respectively, similar to those found in complexes of NAD+ with other A type dehydrogenases. In addition, however, the distance data are indicative of an unusual overall conformation of NAD+ in the sorbitol dehydrogenase-NAD+ binary complex, with the planes of the nicotinamide and adenine rings separated by 6 to 8 A and at approximately 120 degrees to each other. This overall conformation differs from the concensus extended conformation found in the NAD+-dehydrogenase complexes crystallized to date, where the planes of the nicotinamide and adenine rings are 12 to 14 A apart and nearly perpendicular to each other.