Fig. 3.

A previously uncharacterized model for sGC activation and deactivation. When ATP and GTP are present (the most likely physiological scenario), the ferrous-nitrosyl species is extremely stable at low NO concentrations and has low activity. Higher NO concentrations fully activate sGC, albeit transiently. When ATP levels drop (such as under conditions of cellular stress), GTP and NO at the heme are sufficient for full enzyme activity, which is also transient. Although ATP and GTP are depicted binding to the same or overlapping sites, it remains possible that the nucleotides bind to separate sites on the enzyme.