The Structure of Nitric Oxide Synthase Oxygenase Domain and Inhibitor Complexes

The nitric oxide synthase oxygenase domain (NOS$_{ox}$) oxidizes arginine to synthesize the cellular signal and defensive cytotoxin nitric oxide (NO). Crystal structures determined for cytokine-inducible NOS$_{ox}$ reveal an unusual fold and heme environment for stabilization of activated oxygen intermediates key for catalysis. A winged β sheet engenders a curved α-β domain resembling a baseball catcher's mitt with heme clasped in the palm. The location of exposed hydrophobic residues and the results of mutational analysis place the dimer interface adjacent to the heme-binding pocket. Juxtaposed hydrophobic O$_2$- and polar L-arginine-binding sites occupied by imidazole and aminoguanidine, respectively, provide a template for designing dual-function inhibitors and imply substrate-assisted catalysis.