Nitric oxide synthase (NOS) produces nitric oxide (NO) by catalyzing a five-electron heme-based oxidation of a guanidine nitrogen of L-arginine to L-citrulline via two successive monooxygenation reactions producing N(omega)-hydroxy-L-arginine (NHA) as an intermediate. In mammals, there are three distinct NOS isozymes: neuronal (nNOS or NOS-1), cytokine-inducible (iNOS or NOS-2) and endothelial (eNOS or NOS-3) . Nitric oxide synthases are homodimers. In eukaryotes, each monomer has an N-terminal oxygenase domain which binds to the substrate L-Arg, zinc, and to the cofactors heme and 18.104.22.168-(6R)-tetrahydrobiopterin (BH4) . Eukaryotic NOSs also have a C-terminal electron supplying reductase region, which is homologous to cytochrome P450 reductase and binds NADH, FAD and FMN. While prokaryotes can produce NO as a byproduct of denitrification, using a completely different set of enzymes than NOS, a few prokaryotes also have a NOS which consists solely of the NOS oxygenase domain. Prokaryotic NOS binds to the substrate L-Arg, zinc, and to the cofactors heme and tetrahydrofolate.