ELECTRON SUPPLY AND CATALYTIC OXIDATION OF NITROGEN BY CYTOCHROME P450 AND NITRIC OXIDE SYNTHASE

Cytochrome P450 and nitric oxide synthase (NOS) oxidize nitrogen atoms, although the substrates and transformations are highly restricted for NOS. The first reaction catalyzed by NOS is mediated by a P450-like ferryl species, although it is generated by a distinct process in which a tetrahydrobiopterin molecule in NOS serves as a transient electron donor. The second NOS reaction appears to be mediated by an iron dioxygen precursor of the ferryl species. The transient tetrahydrobiopterin radical formed in these reactions is quenched by electron transfer from the NOS flavin domain. Electron transfer from the flavins is controlled by the binding of calmodulin, the presence of peptide inserts in the flavin domain, the substrate structure, and phosphorylation of the enzyme.