Activation of soluble guanylate cyclase by carbon monoxide and nitric oxide: A mechanistic model

Soluble guanylate cyclase (GC) from bovine lung is activated 4-fold by carb on monoxide (CO) and 400-fold by nitric oxide (NO). Spectroscopic and kinet ic data for ligation of CO and NO with GC are summarized and compared with similar data for myoglobin (Mb), hemoglobin (Hb), and heme model compounds. Kinetic, thermodynamic, and structural data form a basis on which to const ruct a model for the manner in which the two ligands affect protein structu re near the heme for heme proteins in general and for GC in particular. The most significant datum is that although association rates of ligands with GC are similar to those with Mb and Hb, their dissociation rates are dramat ically faster. This suggests a delicate balance between five- and six-coord inate heme iron in both NO and CO complexes. Based on these and other data, a model for GC activation is proposed: The first step is formation of a si x-coordinate species concomitant with tertiary and quaternary structural ch anges in protein structure and about a 4-fold increase in enzyme activity. In the second step, applicable to NO, the bond from iron to the proximal hi stidine ruptures, leading to additional relaxation in the quaternary and te rtiary structure and a further 100-fold increase in activity. This is the m ain event in activation, available to NO and possibly other activators or c ombinations of activators. It is proposed, finally, that the proximal base freed in step 2, or some other protein base suitably positioned as a result of structural changes following ligation, may provide a center for nucleop hilic substitution catalyzing the reaction GTP --> cGMP. An example is prov ided for a similar reaction in a derivatized protoheme model compound. The reaction mechanism attempts to rationalize the relative enzymatic activitie s of GC, heme-deficient GC, GC-CO, and G -NO on a common basis and makes pr edictions for new activators that may be discovered in the future. (C) 1999 Academic Press.