Figure 1.

The NO-signaling pathway. NOSs convert L-arginine to citrulline and a family of NO-related molecules. Exact N-oxide yields and identities vary as a function of experimental condition. The reaction requires cosubstrates/cofactors NADPH, O₂, and tetrahydrobiopterin (BH₄). Glutathione (GSH) may sustain enzyme activity and/or influence reaction product. Superoxide/hydrogen peroxide (O₂⁻/H₂O₂) are typically generated by NOS, at least to some degree. NOSs may be regulated by Ca²⁺/calmodulin and other associated proteins. NO-elicited signals are mediated by cGMP, nitrosylation, or both. Although a growing list of signaling pathways are influenced by NO, sites of regulation by NO and NO-dependent interactions between signal transduction cascades are as yet poorly characterized. SNO, S-nitrosothiol; NOx, other oxides of nitrogen, in this case perhaps also including low mass metal-nitrosyl compounds.