Nitric oxide (NO) is short-lived intracellular signal molecule in mammals, and its known physiological roles include the regulation of blood pressure via its vasodilatory action on the vasculature, the release of insulin, angiogenesis and peristalsis. In mammals, NO synthase is a family of three closely related enzymes that catalyse the conversion of L-arginine to NO, via the intermediary N-hydroxy-L-arginine (NOHA). The reaction required NADPH and O2 as co-substrates and tetrahydrobiopterin as a cofactor, and also leads to the formation of L-citruline (figure 1.). Nitric oxide is a haem-containing flavoprotein, with the flavin element transferring electrons from NADPH to haem, which then enables the binding molecular oxygen to the haem in both steps during the synthesis of NO. This transfer of electrons is regulated by the binding of calmodulin to the enzyme, which allows cellular NO synthesis to be regulated by intracellular calcium concentrations.
Figure 1. The synthesis of nitric oxide from L-arginine in mammals.
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