Mechanism of YC-1-induced activation of soluble guanylyl cyclase.

The signaling molecule nitric oxide (NO) mediates many of its effects by the stimulation of soluble guanylyl cyclase (sGC). The activation process is initiated by high-affinity binding of NO to the enzyme's prosthetic heme group. Despite its poor sGC-activating properties, carbon monoxide (CO) has also been suggested as a physiological activator of sGC. Recently, we have shown that the substance YC-1, a benzyl indazole derivative, stimulates sGC by 10-fold (independently of NO) potentiates the stimulatory effect of NO, and turns CO into a potent activator of sGC. In the present study, we show that activation of sGC by protoporphyrin IX, a ligand-independent activator, was potentiated by YC-1, yet a shift of the concentration-response curve as seen with NO and CO was not observed. YC-1 slowed down the dissociation rates for NO and CO from the activated enzyme as monitored by cGMP accumulation after addition of the NO and CO scavenger oxyhemoglobin. A direct interaction of YC-1 with the heme group can be ruled out because YC-1 did not change the Soret absorption of basal or stimulated sGC and, in addition, still bound to the heme-depleted enzyme. Together, our results indicate that YC-1 increases the maximal catalytic rate and sensitizes the enzyme toward its gaseous activators by binding to an allosteric site on sGC molecules, thereby reducing the ligand dissociation rate from the heme group.