NITRIC-OXIDE SYNTHASES REVEAL A ROLE FOR CALMODULIN IN CONTROLLING ELECTRON-TRANSFER

Nitric oxide (NO) is synthesized within the immune, vascular, and nerv ous systems, where it acts as a wide-ranging mediator of mammalian phy siology. The NO synthases (EC 1.14.13.39) isolated from neurons or end othelium are calmodulin dependent. Calmodulin binds reversibly to neur onal NO synthase in response to elevated Ca2+, triggering its NO produ ction by an unknown mechanism. Here we show that calmodulin binding al lows NADPH-derived electrons to pass onto the heme group of neuronal N O synthase. Calmodulin-triggered electron transfer to heme was indepen dent of substrate binding, caused rapid enzymatic oxidation of NADPH i n the presence of O2, and was required for NO synthesis. An NO synthas e isolated from cytokine-induced macrophages that contains tightly bou nd calmodulin catalyzed spontaneous electron transfer to its heme, con sistent with bound calmodulin also enabling electron transfer within t his isoform. Together, these results provide a basis for how calmoduli n may regulate NO synthesis. The ability of calmodulin to trigger elec tron transfer within an enzyme is unexpected and represents an additio nal function for calcium-binding proteins in biology.