Nitric oxide synthases: Structure, function, and control

Nitric oxide synthases are large modular enzymes that produce NO and citrulline from arginine at the expense of NADPH and O-2. We have modeled the reductase region, which corresponds roughly to the C-terminal half of the molecule, using the available crystal structures of flavodoxin (FMN-binding domain) and ferredoxin NADPH reductase (FAD- and NADPH-binding domains). This has enabled us to identify important sequence regions that interact with cofactors and to show that the N-terminal boundary of the reductase domain extends to within a few residues of the CaM-binding site. A large (40-50 residues) insertion in the FMN-binding domain of cNOS, located similar to 80 residues downstream from the CaM site, is the major sequence difference between iNOS and cNOS. This insertion is directly adjacent to the CaM-binding site on the three-dimensional structure. Several lines of evidence suggest that it functions as a control element.