Unusual folded conformation of nicotinamide adenine dinucleotide bound to flavin reductase P

The 2.1 Angstrom resolution crystal structure of flavin reductase P with th e inhibitor nicotinamide adenine dinucleotide (NAD) bound in the active sit e has been determined. NAD adopts a novel, folded conformation in which the nicotinamide and adenine rings stack in parallel with an inter-ring distan ce of 3.6 Angstrom. The pyrophosphate binds next to the flavin cofactor iso alloxazine, while the stacked nicotinamide/adenine moiety faces away from t he flavin. The observed NAD conformation is quite different from the extend ed conformations observed in other enzyme/NAD(P) structures; however, it re sembles the conformation proposed for NAD in solution. The flavin reductase P/NAD structure provides new information about the conformational diversit y of NAD, which is important for understanding catalysis. This structure of fers the first crystallographic evidence of a folded NAD with ring stacking , and it is the first enzyme structure containing an FMN cofactor interacti ng with NAD(P). Analysis of the structure suggests a possible dynamic mecha nism underlying NADPH substrate specificity and product release that involv es unfolding and folding of NADP(H).