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H/D exchange mass spectrometry reveals calmodulin‐controlled regulatory interactions in neuronal nitric oxide synthase

Neuronal nitric oxide synthase (nNOS) produces nitric oxide (NO), a unique signaling gas molecule involved in retrograde neuronal signaling. Homodimeric nNOS is composed of a reductase and oxidase subdomain, connected by a mobile FMN subdomain. The reductase domain delivers reducing equivalents from NADPH through FAD and FMN to the heme of the oxidase domain, where NO is generated from arginine. Delivery of reducing equivalents is tightly regulated by Ca2+ via the intermediary Ca2+‐sensor calmodulin (CaM). We examined conformational changes induced by CaM‐binding that encourage the inter‐domain delivery using hydrogen‐deuterium exchange mass spectrometry (HDX‐MS).