Differential calmodulin-modulatory and electron transfer properties of neuronal nitric oxide synthase mu compared to the alpha variant

•nNOSμ contains 34 extra residues, compared to nNOSα, in a regulatory element.•nNOSμ exhibits slower electron transfer and heme nitrosyl formation, and lower CaM-stimulation.•nNOSμ activity is adapted specifically for the skeletal muscle environment. Neuronal nitric oxide synthase μ (nNOSμ) contains 34 additional residues in an autoregulatory element compared to nNOSα. Cytochrome c and flavin reductions in the absence of calmodulin (CaM) were faster in nNOSμ than nNOSα, while rates in the presence of CaM were smaller. The magnitude of stimulation by CaM is thus notably lower in nNOSμ. No difference in NO production was observed, while electron transfer between the FMN and heme moieties and formation of an inhibitory ferrous-nitrosyl complex were slower in nNOSμ. Thus, the insert affects electron transfer rates, modulation of electron flow by CaM, and heme–nitrosyl complex formation.