NOS3 Regulation

Abstract

Endothelial nitric oxide synthase (NOS3) was first purified and characterized in 1991 as the enzyme responsible for the production of the endothelium-dependent relaxation factor. NOS3 converts l-arginine and molecular oxygen to l-citrulline and NO. Over the last 14 years, regulation of NOS3 enzymatic activity has been shown to be mediated by transcription and translation, substrate and cofactor availability, subcellular localization, protein–protein interactions, and phosphorylation. NOS3 is a homodimer with each monomer possessing a reductase domain (FMN, FAD, and NADPH binding region) and an oxygenase domain (arginine, heme, Fe, and tetrahydrobiopterin binding region) that are “linked” by the calmodulin-binding domain (CBD). NOS3 activity is Ca2+/calmodulin-dependent, and it has been proposed that calmodulin binds to the CBD and relieves the inhibition from the auto-inhibitory loop, thereby enabling the flow of electrons from the reductase domain to the oxygenase domain and the production of NO. “Uncoupled NOS” refers to the condition in which O2 is the final electron acceptor and thus NOS produces O2·−. NOS3 is also known to be myristoylated and palmitoylated at the amino-terminal region, which renders the enzyme membrane-bound and has been described to be active in the plasma membrane and golgi. However, there is evidence that under pathological conditions, NOS3 is upregulated and expressed in the cytosolic compartment.1 NOS3 is known to interact with numerous proteins that modulate enzymatic activity under normal and pathological conditions. The overwhelming majority of studies regarding regulation, and especially phosphorylation, of NOS3 have been described in the vasculature and particularly in cultured aortic endothelial cells. Thus far, 5 serine/threonine phosphorylation sites and, most recently, a tyrosine phosphorylation site (Ser114, Thr495, Ser617, Ser633, Ser1177, and Tyr83) have been elucidated on NOS3 with varying effects on the status of activity (for …