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Nitric oxide synthase (NOS) in mammals is a family of multidomain proteins in which interdomain electron transfer (IET) is controlled by domain-domain interactions. Calmodulin (CaM) binds to the canonical CaM-binding site in the linker region between the FMN and heme domains of NOS and allows tethered FMN domain motions, enabling an intersubunit FMN-heme IET in the output state for NO production. Our previous cross-linking mass spectrometric (XL MS) results demonstrated site-specific protein dynamics in the CaM-responsive regions of rat neuronal NOS (nNOS) reductase construct, a monomeric protein [Jiang et al., Biochemistry , 2023, 62, 2232-2237]. In this work, we have extended our combined approach of XL MS structural mapping and AlphaFold structural prediction to examine the homodimeric nNOS oxygenase/FMN (oxyFMN) construct, an established model of the NOS output state. We employed parallel reaction monitoring (PRM) based quantitative XL MS (qXL MS) to assess the CaM-induced changes in interdomain dynamics and interactions. Intersubunit cross-links were identified by mapping the cross-links onto top AlphaFold structural models, which was complemented by comparing their relative abundances in the cross-linked dimeric and monomeric bands. Furthermore, contrasting the CaM-free and CaM-bound nNOS samples shows that CaM enables the formation of the intersubunit FMN-heme docking complex and that CaM binding induces extensive, allosteric conformational changes across the NOS regions. Moreover, the observed cross-links sites specifically respond to changes in ionic strength. This indicates that interdomain salt bridges are responsible for stabilizing and orienting the output state for efficient FMN-heme IET. Taken together, our targeted qXL MS results have revealed that CaM and ionic strength modulate specific dynamic changes in the CaM/FMN/heme complexes, particularly in the context of intersubunit interdomain FMN-heme interactions.

Nitric oxide (NO) signaling regulates key processes in cardiovascular physiology, specifically vasodilation, platelet aggregation, and leukocyte rolling. Soluble guanylate cyclase (sGC), the mammalian NO sensor, transduces an NO signal into the classical second messenger cyclic GMP (cGMP). NO binds to the ferrous (Fe(2+)) oxidation state of the sGC heme cofactor and stimulates formation of cGMP several hundred-fold. Oxidation of the sGC heme to the ferric (Fe(3+)) state desensitizes the enzyme to NO. The heme-oxidized state of sGC has emerged as a potential therapeutic target in the treatment of cardiovascular disease. Here, we investigate the molecular mechanism of NO desensitization and find that sGC undergoes a reductive nitrosylation reaction that is coupled to the S-nitrosation of sGC cysteines. We further characterize the kinetics of NO desensitization and find that heme-assisted nitrosothiol formation of β1Cys-78 and β1Cys-122 causes the NO desensitization of ferric sGC. Finally, we provide evidence that the mechanism of reductive nitrosylation is gated by a conformational change of the protein. These results yield insights into the function and dysfunction of sGC in cardiovascular disease.