title: Nitric oxide activation of soluble guanylyl cyclase reveals high and low affinity sites that mediate allosteric inhibition by calcium.
abstract: Cyclic GMP (cGMP) and Ca(2+) regulate opposing mechanisms in (patho)physiological processes reflected in the reciprocal regulation of their intracellular concentrations. Although mechanisms by which cGMP regulates [Ca(2+)](i) have been described, those by which Ca(2+) regulates [cGMP](i) are less well understood. In the present study, Ca(2+) inhibited purified sGC activated by sodium nitroprusside (SNP), a precursor of nitric oxide (NO), employing Mg-GTP as substrate in a concentration-dependent fashion, but was without effect on basal enzyme activity. Ca(2+) inhibited sGC stimulated by protoporphyrin IX or YC-1 suggesting that inhibition was not NO-dependent. In contrast, Ca(2+) was without effect on sGC activated by SNP employing Mn-GTP as substrate, demonstrating that inhibition did not reflect displacement of heme from sGC. Ligand activation of sGC unmasked negative allosteric sites of high (K(i) similar 10(-7) M) and low (K(i) approximately 10(-5) M) affinity for Ca(2+) that mediated noncompetitive and uncompetitive inhibition, respectively. Free Mg(2+) in excess of substrate did not alter the concentration-response relationship of Ca(2+) inhibition at high affinity sites, but produced a rightward shift in that relationship at low affinity sites. Similarly, Ca(2+) inhibition at high affinity sites was noncompetitive, whereas inhibition at low affinity sites was competitive, with respect to free Mg(2+). Purified sGC specifically bound (45)Ca(2+) in the presence of a 1000-fold excess of Mg(2+) and in the absence of activating ligands. These data suggest that sGC is a constitutive Ca(2+) binding protein whose allosteric function is conditionally dependent upon ligand activation.
authors.affiliation: Division of Clinical Pharmacology, Department of Medicine and Biochemistry, Thomas Jefferson University, Philadelphia, PA 19107, USA.
authors.forename: Shiva
authors.initials: S
authors.lastname: Kazerounian
authors.forename: Giovanni M
authors.initials: GM
authors.lastname: Pitari
authors.forename: Inez
authors.initials: I
authors.lastname: Ruiz-Stewart
authors.forename: Stephanie
authors.initials: S
authors.lastname: Schulz
authors.forename: Scott A
authors.initials: SA
authors.lastname: Waldman
mesh_terms: D000494:Allosteric Regulation; D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D002118:Calcium / Q000502:physiology*; D002417:Cattle; D004789:Enzyme Activation; D006162:Guanylate Cyclase / Q000378:metabolism*; D007700:Kinetics; D008168:Lung / Q000201:enzymology; D009569:Nitric Oxide / Q000502:physiology*; D020030:Nitric Oxide Donors / Q000494:pharmacology; D009599:Nitroprusside / Q000494:pharmacology
chemical_list: D020030:Nitric Oxide Donors; D009599:Nitroprusside; D009569:Nitric Oxide; D006162:Guanylate Cyclase; D002118:Calcium