Calmodulin-induced structural changes in endothelial nitric oxide synthase

► Structures of intact eNOS ± CaM have been derived from cryo-electron micrographs. ► The reductase domains appear to be mobile with respect to the oxygenase domain dimer. ► Densities consistent with oxygenase-docked FMN modules are observed in the presence of CaM. ► Additional densities are observed that are proposed to be bound and docked CaM. ► A novel model is suggested in which docking of CaM promotes docking of the FMN modules.