Calcium ion downregulates soluble guanylyl cyclase activity: Evidence for a two-metal ion catalytic mechanism

The inhibition of soluble guanylyl cyclase by Ca2+ has been kinetically characterized and the results support a two-metal-ion catalytic mechanism for formation of cGMP. Ca2+ reversibly inhibits both the basal and NO-stimulated forms of bovine lung soluble guanylyl cyclase, Inhibition is independent of the activator identity and concentration, revealing that Ca2+ interacts with a site independent of the heme regulatory site. Inhibition by Ca2+ is competitive with respect to Mg2+ in excess of substrate, with K-is values of 29 +/- 4 and 6.6 +/- 0.6 muM for the basal and activated states, respectively. Ca2+ inhibits noncompetitively with respect to the substrate MgGTP in both activity states. The qualitatively similar inhibition pattern and quantitatively different K-i values between the basal and NO-stimulated states suggest that the Ca2+ binding site undergoes some structural modification upon activation of the enzyme. The competitive nature of Ca2+ inhibition with respect to excess Mg2+ is consistent with a two-metal-ion mechanism for cyclization. (C) 2001 Academic Press.