A molecular basis for NO selectivity in soluble guanylate cyclase

Soluble guanylate cyclases (sGCs) function as heme sensors that selectively bind nitric oxide (NO), triggering reactions essential to animal physiology. Recent discoveries place sGCs in the H-NOX family (heme nitric oxide/oxygen-binding domain), which includes bacterial proteins from aerobic and anaerobic organisms. Some H-NOX proteins tightly bind oxygen (O-2), whereas others show no measurable affinity for O-2, providing the basis for selective NO signaling in aerobic cells. Using a series of wild-type and mutant H-NOXs, we established a molecular basis for ligand discrimination. A distal pocket tyrosine is requisite for O-2 binding in the H-NOX family. These data suggest that sGC uses a kinetic selection against O-2; we propose that the O-2 dissociation rate in the absence of this tyrosine is fast and that a stable O-2 complex does not form.