Mechanisms of regulation by calmodulin of nitric oxide synthase

: The Ca(2+)-calmodulin system controls the neuronal and endothelial isoforms of NOS, whereas the inducible isoform is calcium independent apparently because CaM is a tightly bound subunit of iNOS. The canonical CaM-binding site is located between the oxygenase and reductase NOS domains. CaM controls eNOS dimerization rather then iNOS one. The proteins with the so-called "IQ" motif bind calmodulin in a Ca(2+)-independent manner. This group of proteins does not include iNOS, which has the canonical CaM-binding motif. In the experiments with synthetic peptides was demonstrated that the interaction between the calmodulin and CaM-binding site of iNOS does not depend on the Ca2+ concentration. On the other hand, in the experiments with fusion, mutant and truncated NOSs was shown that these features of CaM-binding region of iNOS is not enough for the enzyme to bind calmodulin Ca(2+)-independently; this interaction requires the additional binding sites both in reductase and oxygenase domains of iNOS. In the experiments with fusion calmodulins the mechanism of calmodulin regulation of electron transfer in NOS was elaborated. The concept of autoinhibitory control element in the FMN-binding site of constitutive NOS is discussed.