NADP-DEPENDENT ENZYMES .1. CONSERVED STEREOCHEMISTRY OF COFACTOR BINDING

The ubiquitous redox cofactors nicotinamide adenine dinucleotides [NAD and NADP] are very similar molecules, despite their participation in substantially different biochemical processes. NADP differs from NAD i n only the presence of an additional phosphate group esterified to the 2'-hydroxyl group of the ribose at the adenine end and yet NADP is co nfined with few exceptions to the reactions of reductive biosynthesis, whereas NAD is used almost exclusively in oxidative degradations, The discrimination between NAD and NADP is therefore an impressive exampl e of the power of molecular recognition by proteins, The many known te rtiary structures of NADP complexes affords the possibility for an ana lysis of their discrimination. A systematic analysis of several crysta l structures of NAD (P)-protein complexes show that: 1) the NADP coenz ymes are more flexible in conformation than those of NAD; 2) although the protein-cofactor interactions are largely conserved in the NAD com plexes, they are quite variable in those of NADP; and 3) in both cases the pocket around the nicotinamide moiety is substrate dependent, The conserved and variable interactions between protein and cofactors in the respective binding pockets are reported in detail, Discrimination between NAD and NADP is essentially a consequence of the overall pocke t and not of a few residues. A clear finger-print in NAD complexes is a carboxylate side chain that chelates the diol group at the ribose ne ar the adenine, whereas in NADP complexes an arginine side chain faces the adenine plane and interacts with the phosphomonoester. The latter type of interaction might be a general feature of recognition of nucl eotides by proteins. Other features such as strand-like hydrogen bondi ng between the NADP diphosphate moeties and the protein are also signi ficant. The NADP binding pocket properties should prove useful in prot ein engineering and design. (C) 1997 Wiley Liss, Inc.