NOS catalyzes the oxidation of L-arginine to produce NO and L-citrulline. This reaction occurs in two steps: the first involves the conversion of L-arginine to N-hydroxyarginine, and the second transforms N-hydroxyarginine to NO and L-citrulline. The enzyme requires several cofactors such as NADPH, FAD, FMN, and tetrahydrobiopterin (BH4) for its activity.