Calmodulin interaction with nitric-oxide synthases

Nitric oxide is synthesized from arginine in an NADPH-dependent reaction catalyzed by nitric oxide synthase (Fig. 22-31), a dimeric enzyme structurally related to NADPH cytochrome P-450 reductase (see Box 21-1). The reaction is a five-electron oxidation. Each subunit of the enzyme contains one bound molecule of each of four different cofactors FMN, FAD, tetrahydrobiopterin, and Fe heme. NO is an unstable molecule and cannot be stored. Its synthesis is stimulated by interaction of nitric oxide synthase with Ca -calmodulin (see Fig. 12-21). 

Many proteins, particularly those of eukaryotic origin, require an interacting protein partner for correct folding and stability.190-193 Often, these proteins contain intrinsically disordered domains that mediate the protein interaction. Frequently, proteins with unstructured domains cannot be expressed solubly in E. coli. In some cases, coexpression of the interacting protein has improved the stability and solubility of the protein of interest.194-197 As an example, the yield of inducible nitric oxide synthase (iNOS) was 20—25 times greater when coexpressed with its natural partner, calmodulin, than in the absence of calmodulin. Additionally, soluble iNOS could be isolated when expressed without calmodulin but it was deficient in heme and flavins and almost completely inactive.198 Methods for coexpression in both prokaryotic and eukaryotic hosts have been recently reviewed.1... 

Gachhui, R., Abu-Soud, HAl., Ghosha, D.K., Presta, A., Blazing, M.A., Mayer, B., George, S.E., and Stuehr, D.J. (1998). Neuronal nitric-oxide synthase interaction with calmodulin-troponin C chimeras. J. Biol. Chem. 275(10), 5451-5454. 

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