Nitric oxide synthase

Nitric oxide synthase is an enzyme family that converts L-arginine and oxygen into nitric oxide and L-citrulline. Its three mammalian isoforms regulate signaling, vascular tone, and immune defense.

L-arginine: L-arginine is a protein-building amino acid that also serves as the substrate for nitric oxide synthase. The enzyme oxidizes this substrate to produce nitric oxide and citrulline.
Endothelial nitric oxide synthase: Endothelial nitric oxide synthase is a constitutive isoform that produces nitric oxide in vascular endothelial cells. It is the principal isoform linking endothelial signals to blood-vessel relaxation.
Vasodilation: Vasodilation is the widening of blood vessels through relaxation of their smooth muscle. Endothelial nitric oxide synthase initiates a major pathway that relaxes vascular smooth muscle.
Arginase: Arginase is an enzyme that converts L-arginine into ornithine and urea. It competes with nitric oxide synthase for L-arginine rather than producing nitric oxide.
Tetrahydrobiopterin: Tetrahydrobiopterin is a redox-active cofactor required by several enzymes, including nitric oxide synthase. It supports productive nitric oxide synthesis; its loss can uncouple the enzyme.
Neuronal nitric oxide synthase: Neuronal nitric oxide synthase is an isoform that produces nitric oxide in neurons and some other cells. Its calcium-sensitive activity supports neuronal signaling and synaptic plasticity.
Nitroglycerin: Nitroglycerin is a nitrate drug that releases nitric oxide-related species and treats angina. It bypasses nitric oxide synthase while engaging downstream vascular signaling.
Nitric oxide signaling: Nitric oxide signaling is cellular communication mediated by nitric oxide and its downstream targets. The enzyme family supplies nitric oxide to this broader signaling system.
Calmodulin: Calmodulin is a calcium-binding protein that regulates many cellular enzymes. Calcium-bound calmodulin activates most nitric oxide synthase isoforms by enabling electron transfer.
Inducible nitric oxide synthase: Inducible nitric oxide synthase is an isoform whose expression rises in response to inflammatory signals. Unlike the constitutive isoforms, it can sustain high nitric oxide output during inflammation.

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