Calmodulin-dependent regulation of mammalian nitric oxide synthase.
The nitric oxide synthases are large, modular, dimeric enzymes composed of a reductase domain, which is related to cytochrome P450 reductase, and a structurally unique oxygenase domain containing a Cys-ligated haem. Both the neuronal and endothelial isoforms are activated by the reversible binding of calmodulin (CaM) at elevated intracellular Ca(2+) levels to produce NO as part of a number of cell signalling pathways. CaM binds to the linker region between the two domains and activates the enzyme by inducing intramolecular electron transfer. Protein-engineering experiments have shown that a series of unusual autoinhibitory inserts found only in the CaM-dependent NOS isoforms control both CaM binding and the structural rearrangement it induces. These lie in the reductase domain of the enzyme and include a 40-amino-acid autoinhibitory loop in the FMN-binding module, a 30-amino-acid extension to the C-terminus and the CaM-binding site itself. The substrate (NADPH) also plays an important role in defining the CaM-dependence of the reductase domain by inducing a tight conformational lock in the absence of CaM. Both the substrate and the conformational lock appear to be released on CaM binding; the resultant domain mobility leads to activation.
Daff
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Department of Chemistry, University of Edinburgh, The King's Buildings, West Mains Road, Edinburgh EH9 3JJ, UK. Simon.Daff@ed.ac.uk
D000818:Animals; D002147:Calmodulin / Q000502:physiology*; D006706:Homeostasis; D007700:Kinetics; D008322:Mammals; D009569:Nitric Oxide / Q000096:biosynthesis; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D052248:Nitric Oxide Synthase Type I
D002147:Calmodulin; D009569:Nitric Oxide; D019001:Nitric Oxide Synthase; D052248:Nitric Oxide Synthase Type I