The ferredoxin-reductase (FNR) like C-terminal domain of the nitric oxide synthase (NOS) fuses with a heme-containing N-terminal oxidase domain. The reductase portion is similar in structure to NADPH dependent cytochrome-450 reductase (CYPOR), having an inserted connecting sub-domain within the FAD binding portion of FNR. NOS differs from CYPOR in a requirement for the cofactor tetrahydrobiopterin and unlike most CYPOR is dimeric. Nitric oxide synthase produces nitric oxide in the conversion of L-arginine to L-citruline. NOS has been implicated in a variety of processes including cytotoxicity, anti-inflamation, neurotransmission, and vascular smooth muscle relaxation.