NAD+ and NAD+ analogues in horse liver alcohol dehydrogenase. Relationship between reactivity and conformation simulated with molecular mechanics.

In the present study we show that the enzymatic activity of the coenzyme nicotinamide adenine dinucleotide (NAD+) and its analogues (C(O)NH2 replaced by C(S)NH2, C(O)CH3, C(O)H and CN) with horse liver alcohol dehydrogenase (LADH) (alcohol:NAD+ oxidoreductase, EC 1.1.1.1) can be rationalized by their conformation in the active site determined with molecular mechanics (AMBER, assisted model building with energy refinement). In order to establish the relation between the hydride transfer rate and the conformation of the NAD+ and its analogues, kinetic experiments with the poor substrate isopropanol were carried out. It appears that the enzymatic activity can be readily explained by the geometry of the pyridinium ring, in particular the magnitude of the 'out-of-plane' rotation of the carboxamide side chain (or analogues). The latter is nicely illustrated in the case of 3-cyanopyridine adenine dinucleotide which lacks any 'out-of-plane' rotation and concomitantly exhibits no significant enzymatic activity.