Fig. 9.

Monomeric and dimeric forms of NOS. (Panel A) NOS monomers can transfer electrons from NADPH to the flavins in the reductase domain where limited amounts of ${\mathrm{O}}_2^{·-}$ can be formed. NOS monomers cannot bind the cofactor (BH₄) or the substrate (arginine) and cannot generate NO. (Panel B) Formation of a NOS dimer requires heme, which allows for the transfer of electrons from the reductase domain of one monomer to the oxidase domain of the adjacent monomer. When sufficient arginine and BH₄ are present, NOS dimers couple their heme, allowing for O₂ reduction and the synthesis of NO. From Forstermann and Sessa [396] where the structure and catalytic function of NOS are discussed in greater detail.