Binding of intermediate, product, and substrate analogs to neuronal nitric oxide synthase. Ferriheme is sensitive to ligand-specific effects in the L-arginine binding site

The electron paramagnetic resonance spectra of purified neuronal nitric oxide synthase indicates that the binding of ligands to the arginine site perturbs the environment of the high-spin ferriheme in a highly ligand-specific manner. Four categories of high-spin complex can be distinguished; all are five-coordinate, and all retain the axial thiolate ligand, but they differ in their ligation geometries. These spectroscopic species reveal distinct local conformations which can be stabilized individually by the binding of L-arginine, N-omega-hydroxy-L-arginine, N-omega-methyl-L-arginine, and N-omega-nitro-L-arginine. Other arginine analog inhibitors stabilize one or more of these states, revealing patterns based on the nature of substituents at the terminal amino group.