Soluble guanylate cyclase: effect of YC-1 on ligation kinetics with carbon monoxide

Picosecond to second dynamics reveals a structural transition in Clostridium botulinum NO-sensor triggered by the activator BAY-41-2272

Soluble guanylate cyclase (sGC) is the mammalian endogenous nitric oxide (NO) receptor that synthesizes cGMP upon NO activation. In synergy with the artificial allosteric effector BAY 41-2272 (a lead compound for drug design in cardiovascular treatment), sGC can also be activated by carbon monoxide (CO), but the structural basis for this synergistic effect are unknown. We recorded in the unusually broad time range from 1 ps to 1 s the dynamics of the interaction of CO binding to full length sGC, to the isolated sGC heme domain β(1)(200) and to the homologous bacterial NO-sensor from Clostridium botulinum. By identifying all phases of CO binding in this full time range and characterizing how these phases are modified by BAY 41-2272, we show that this activator induces the same structural changes in both proteins. This result demonstrates that the BAY 41-2272 binding site resides in the β(1)(200) sGC heme domain and is the same in sGC and in the NO-sensor from Clostridium botulinum.

Structure and regulation of soluble guanylate cyclase

Nitric oxide (NO) is an essential signaling molecule in biological systems. In mammals, the diatomic gas is critical to the cyclic guanosine monophosphate (cGMP) pathway as it functions as the primary activator of soluble guanylate cyclase (sGC). NO is synthesized from l-arginine and oxygen (O(2)) by the enzyme nitric oxide synthase (NOS). Once produced, NO rapidly diffuses across cell membranes and binds to the heme cofactor of sGC. sGC forms a stable complex with NO and carbon monoxide (CO), but not with O(2). The binding of NO to sGC leads to significant increases in cGMP levels. The second messenger then directly modulates phosphodiesterases (PDEs), ion-gated channels, or cGMP-dependent protein kinases to regulate physiological functions, including vasodilation, platelet aggregation, and neurotransmission. Many studies are focused on elucidating the molecular mechanism of sGC activation and deactivation with a goal of therapeutic intervention in diseases involving the NO/cGMP-signaling pathway. This review summarizes the current understanding of sGC structure and regulation as well as recent developments in NO signaling.

NO-independent, haem-dependent soluble guanylate cyclase stimulators

The nitric oxide (NO) signalling pathway is altered in cardiovascular diseases, including systemic and pulmonary hypertension, stroke, and atherosclerosis. The vasodilatory properties of NO have been exploited for over a century in cardiovascular disease, but NO donor drugs and inhaled NO are associated with significant shortcomings, including resistance to NO in some disease states, the development of tolerance during long-term treatment, and non-specific effects such as post-translational modification of proteins. The development of pharmacological agents capable of directly stimulating the NO receptor, soluble guanylate cyclase (sGC), is therefore highly desirable. The benzylindazole compound YC-1 was the first sGC stimulator to be identified; this compound formed a lead structure for the development of optimized sGC stimulators with improved potency and specificity for sGC, including CFM-1571, BAY 41-2272, BAY 41-8543, and BAY 63-2521. In contrast to the NO- and haem-independent sGC activators such as BAY 58-2667, these compounds stimulate sGC activity independent of NO and also act in synergy with NO to produce anti-aggregatory, anti-proliferative, and vasodilatory effects. Recently, aryl-acrylamide compounds were identified independent of YC-1 as sGC stimulators; although structurally dissimilar to YC-1, they have a similar mode of action and promote smooth muscle relaxation. Pharmacological stimulators of sGC may be beneficial in the treatment of a range of diseases, including systemic and pulmonary hypertension, heart failure, atherosclerosis, erectile dysfunction, and renal fibrosis. An sGC stimulator, BAY 63-2521, is currently in clinical development as an oral therapy for patients with pulmonary hypertension. It has demonstrated efficacy in a proof-of-concept study, reducing pulmonary vascular resistance and increasing cardiac output from baseline. A full, phase 2 trial of BAY 63-2521 in pulmonary hypertension is underway.

Role of the soluble guanylyl cyclase alpha1/alpha2 subunits in the relaxant effect of CO and CORM-2 in murine gastric fundus

Carbon monoxide (CO) has been shown to cause enteric smooth muscle relaxation by activating soluble guanylyl cyclase (sGC). In gastric fundus, the sGCalpha1beta1 heterodimer is believed to be the most important isoform. The aim of our study was to investigate the role of the sGCalpha1/alpha2 subunits in the relaxant effect of CO and CORM-2 in murine gastric fundus using wild-type (WT) and sGCalpha1 knock-out (KO) mice. In WT mice, CO (bolus)-induced relaxations were abolished by the sGC inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ), while CORM-2- and CO (infusion)-induced relaxations were only partially inhibited by ODQ. In sGCalpha1 KO mice, relaxant responses to CO and CORM-2 were significantly reduced when compared with WT mice, but ODQ still had an inhibitory effect. The sGC sensitizer 1-benzyl-3-(5'-hydroxymethyl-2'-furyl-)-indazol (YC-1) was able to potentiate CO- and CORM-2-induced relaxations in WT mice but lost this potentiating effect in sGCalpha1 KO mice. Both in WT and sGCalpha1 KO mice, CO-evoked relaxations were associated with a significant cGMP increase; however, basal and CO-elicited cGMP levels were markedly lower in sGCalpha1 KO mice. These data indicate that besides the predominant sGCalpha1beta1 isoform, also the less abundantly expressed sGCalpha2beta1 isoform plays an important role in the relaxant effect of CO in murine gastric fundus; however, the sGC stimulator YC-1 loses its potentiating effect towards CO in sGCalpha1 KO mice. Prolonged administration of CO-either by the addition of CORM-2 or by continuous infusion of CO-mediates gastric fundus relaxation in both a sGC-dependent and sGC-independent manner.

Allostery in recombinant soluble guanylyl cyclase from Manduca sexta

Soluble guanylyl/guanylate cyclase (sGC), the primary biological receptor for nitric oxide, is required for proper development and health in all animals. We have expressed heterodimeric full-length and N-terminal fragments of Manduca sexta sGC in Escherichia coli, the first time this has been accomplished for any sGC, and have performed the first functional analyses of an insect sGC. Manduca sGC behaves much like its mammalian counterparts, displaying a 170-fold stimulation by NO and sensitivity to compound YC-1. YC-1 reduces the NO and CO off-rates for the approximately 100-kDa N-terminal heterodimeric fragment and increases the CO affinity by approximately 50-fold to 1.7 microm. Binding of NO leads to a transient six-coordinate intermediate, followed by release of the proximal histidine to yield a five-coordinate nitrosyl complex (k(6-5) = 12.8 s(-1)). The conversion rate is insensitive to nucleotides, YC-1, and changes in NO concentration up to approximately 30 microm. NO release is biphasic in the absence of YC-1 (k(off1) = 0.10 s(-1) and k(off2) = 0.0015 s(-1)); binding of YC-1 eliminates the fast phase but has little effect on the slower phase. Our data are consistent with a model for allosteric activation in which sGC undergoes a simple switch between two conformations, with an open or a closed heme pocket, integrating the influence of numerous effectors to give the final catalytic rate. Importantly, YC-1 binding occurs in the N-terminal two-thirds of the protein. Homology modeling and mutagenesis experiments suggest the presence of an H-NOX domain in the alpha subunit with importance for heme binding.

DFT analysis of axial and equatorial effects on heme-CO vibrational modes: applications to CooA and H-NOX heme sensor proteins

Determinants of the Fe-CO and C-O stretching frequencies in (imidazole)heme-CO adducts have been investigated via density functional theory (DFT) analysis, in connection with puzzling characteristics of the heme sensor protein CooA and of the H-NOX (Heme-Nitric Oxide and/or OXygen binding) family of proteins, including soluble guanylate cyclase (sGC). The computations show that two mechanisms of Fe-histidine bond weakening have opposite effects on the nuFeC/nuCO pattern. Mechanical tension is expected to raise nuFeC with little change in nuCO whereas the weakening of H-bond donation from the imidazole ligand has the opposite effect. Data on CooA indicate imidazole H-bond weakening associated with heme displacement, as part of the activation mechanism. The computations also reveal that protein-induced distortion of the porphyrin ring, a prominent structural feature of the H-NOX protein TtTar4H (Thermoanaerobacter tengcongensis Tar4 protein heme domain), has surprisingly little effect on nuFeC or nuCO. However, another structural feature, strong H-bonding to the propionates, is suggested to account for the weakened back bonding that is evident in sGC. TtTar4H-CO itself has an elevated nuFeC, which is successfully modeled as a compression effect, resulting from steric crowding in the distal pocket. nuFeC/nuCO data, in conjunction with modeling, can provide valuable insight into mechanisms for heme-protein modulation.

Resonance Raman and Infrared Spectroscopic Studies of High-Output Forms of Human Soluble Guanylyl Cyclase

The allosteric regulator BAY-41-2272 converts the CO adduct of soluble guanylyl cyclase (CO-sGC) enzyme from a low- to high-output form, with respect to production of cGMP. Resonance Raman (RR) and Fourier Transform Infrared (FTIR) spectroscopic techniques are used to show that the CO-sGC exists as major and minor conformers, both having nu(Fe-CO) and nu(C-O) modes characteristic of 6-coordinate species. It is further shown that addition of BAY-41-2272 to the CO adduct induces the transition of some fraction of the initial CO-heme adducts into two new CO-heme complexes, the fractional conversion being dependent on the temperature. One new complex displays vibrational modes characteristic of pentacoordinated CO-adduct, and its formation is not affected by temperature. The second complex, although slightly different from the original CO-adducts, is hexacoordinated, and its formation is facilitated by temperature. The production of substantial amounts of the 5-coordinate CO adduct upon addition of BAY-41-2272, reveals the fact that several out-of-plane heme deformation modes are simultaneously activated, an observation similar to that realized upon NO activation. While the precise nature of these modes will require elucidation by isotopic labeling experiments, by analogy with earlier studies of other heme proteins, several bands associated with modes attributable to peripheral substituent deformations and methine carbon movements are implicated. The documented formation of two new forms upon addition of Bay-41-2272 (a 5-coordinate and a new 6-coordinate form) is discussed with respect to the implications for enzyme activation.

Interactions of soluble guanylate cyclase with diatomics as probed by resonance Raman spectroscopy

Soluble guanylate cyclase (sGC, EC 4.6.1.2) acts as a sensor for nitric oxide (NO), but is also activated by carbon monoxide in the presence of an allosteric modulator. Resonance Raman studies on the structure-function relations of sGC are reviewed with a focus on the CO-adduct in the presence and absence of allosteric modulator, YC-1, and substrate analogues. It is demonstrated that the sGC isolated from bovine lung contains one species with a five-coordinate (5c) ferrous high-spin heme with the Fe-His stretching mode at 204 cm(-1), but its CO adduct yields two species with different conformations about the heme pocket with the Fe-CO stretching (nuFe-CO) mode at 473 and 489 cm(-1), both of which are His- and CO-coordinated 6c ferrous adducts. Addition of YC-1 to it changes their population and further addition of GTP yields one kind of 6c (nuFe-CO=489 cm(-1)) in addition to 5c CO-adduct (nuFe-CO=521 cm(-1)). Under this condition the enzymatic activity becomes nearly the same level as that of NO adduct. Addition of gamma-S-GTP yields the same effect as GTP does but cGMP and GDP gives much less effects. Unexpectedly, ATP cancels the effects of GTP. The structural meaning of these spectroscopic observations is discussed in detail.

Potentiation of tumor necrosis factor-alpha expression by YC-1 in alveolar macrophages through a cyclic GMP-independent pathway

Using cultured rat alveolar NR 8383 macrophages, this study investigated the effect of YC-1 [3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole], a soluble guanylyl cyclase (sGC) activator, on the production of tumor necrosis factor-alpha (TNF alpha). YC-1 enhanced lipopolysaccharide and interferon-gamma (LPS/IFN gamma)-induced TNF alpha formation in a concentration- and time-dependent fashion. YC-1 also caused an increasing effect on the TNF alpha mRNA level, suggesting that the transcriptional process was involved. However, further studies suggested that cyclic GMP did not mediate the potentiation of YC-1 on TNF alpha release, because (a) the sGC inhibitor and the protein kinase G inhibitor failed to block the effect; and (b) the cyclic GMP analogues, on the contrary, concentration-dependently diminished LPS/IFN gamma-induced TNF alpha synthesis. In agreement with this finding, YC-1 produced changes in cell function but no changes in cyclic GMP and cyclic AMP levels or sGC activity. Pretreatment of the cells with cyclooxygenase inhibitors, a p38 mitogen-activated protein kinase inhibitor, a mitogen-activated protein kinase kinase (MEK) inhibitor, and a tyrosine kinase inhibitor did not attenuate the potentiation of TNF alpha release by YC-1. Cycloheximide prevented the YC-1-enhanced TNF alpha formation, implying that new protein synthesis was required. Interestingly, protein kinase C inhibitors enhanced the potentiation of YC-1 to a greater extent. Nevertheless, a protein kinase C activator, phorbol 12-myristate 13-acetate, failed to suppress the potentiation of TNFalpha production by YC-1. In summary, potentiation of TNF alpha release by YC-1 in LPS/IFN gamma-activated alveolar macrophages is an additional mode of action of this compound that is independent of the elevation of cyclic GMP. Thus, caution needs to be used in attributing the YC-1-mediated response to the activation of sGC.

YC-1 facilitates release of the proximal His residue in the NO and CO complexes of soluble guanylate cyclase

The benzylindazole compound YC-1 has been shown to activate soluble guanylate cyclase by increasing the sensitivity toward NO and CO. Here we report the action of YC-1 on the coordination of CO- and NO-hemes in the enzyme and correlate the events with the activation of enzyme catalysis. A single YC-1-binding site on the heterodimeric enzyme was identified by equilibrium dialysis. To explore the affect of YC-1 on the NO-heme coordination, the six-coordinate NO complex of the enzyme was stabilized by dibromodeuteroheme substitution. Using the dibromodeuteroheme enzyme, YC-1 converted the six-coordinate NO-heme to a five-coordinate NO-heme with a characteristic EPR signal that differed from that in the absence of YC-1. These results revealed that YC-1 facilitated cleavage of the proximal His-iron bond and caused geometrical distortion of the five-coordinate NO-heme. Resonance Raman studies demonstrated the presence of two iron-CO stretch modes at 488 and 521 cm(-1) specific to the YC-1-bound CO complex of the native enzyme. Together with the infrared C-O stretching measurements, we assigned the 488-cm(-1) band to the iron-CO stretch of a six-coordinate CO-heme and the 521-cm(-1) band to the iron-CO stretch of a five-coordinate CO-heme. These results indicate that YC-1 stimulates enzyme activity by weakening or cleaving the proximal His-iron bond in the CO complex as well as the NO complex.

NO- and haem-independent activation of soluble guanylyl cyclase: molecular basis and cardiovascular implications of a new pharmacological principle

1. Soluble guanylyl cyclase (sGC) is the only proven receptor for the ubiquitous biological messenger nitric oxide (NO) and is intimately involved in many signal transduction pathways, most notably in regulating vascular tone and platelet function. sGC is a heterodimeric (alpha/ss) protein that converts GTP to cyclic GMP; NO binds to its prosthetic haem group. Here, we report the discovery of a novel sGC activating compound, its interaction with a previously unrecognized regulatory site and its therapeutic implications. 2. Through a high-throughput screen we identified BAY 58-2667, an amino dicarboxylic acid which potently activates sGC in an NO-independent manner. In contrast to NO, YC-1 and BAY 41-2272, the sGC stimulators described recently, BAY 58-2667 activates the enzyme even after it has been oxidized by the sGC inhibitor ODQ or rendered haem deficient. 3. Binding studies with radiolabelled BAY 58-2667 show a high affinity site on the enzyme. 4. Using photoaffinity labelling studies we identified the amino acids 371 (alpha-subunit) and 231 - 310 (ss-subunit) as target regions for BAY 58-2667. 5. sGC activation by BAY 58-2667 results in an antiplatelet activity both in vitro and in vivo and a potent vasorelaxation which is not influenced by nitrate tolerance. 6. BAY 58-2667 shows a potent antihypertensive effect in conscious spontaneously hypertensive rats. In anaesthetized dogs the hemodynamic effects of BAY 58-2667 and GTN are very similar on the arterial and venous system. 7. This novel type of sGC activator is a valuable research tool and may offer a new approach for treating cardiovascular diseases.

YC-1 activation of human soluble guanylyl cyclase has both heme-dependent and heme-independent components

YC-1 [3-(5'-hydroxymethyl-2'furyl)-1-benzyl indazole] is an allosteric activator of soluble guanylyl cyclase (sGC). YC-1 increases the catalytic rate of the enzyme and sensitizes the enzyme toward its gaseous activators nitric oxide or carbon monoxide. In other studies the administration of YC-1 to experimental animals resulted in the inhibition of the platelet-rich thrombosis and a decrease of the mean arterial pressure, which correlated with increased cGMP levels. However, details of YC-1 interaction with sGC and enzyme activation are incomplete. Although evidence in the literature indicates that YC-1 activation of sGC is strictly heme-dependent, this report presents evidence for both heme-dependent and heme-independent activation of sGC by YC-1. The oxidation of the sGC heme by 1H-(1,2,4)oxadiazole(4,3-a)quinoxalin-1-one completely inhibited the response to NO, but only partially attenuated activation by YC-1. We also observed activation by YC-1 of a mutant sGC, which lacks heme. These findings indicate that YC-1 activation of sGC can occur independently of heme, but that activation is substantially increased when the heme moiety is present in the enzyme.

Synthesis of 1-benzyl-3-(5'-hydroxymethyl-2'-furyl)indazole analogues as novel antiplatelet agents

1-Benzyl-3-(5'-hydroxymethyl-2'-furyl)indazole (28, YC-1) was selected as the lead compound for systemic structural modification. After screening for antiplatelet activity, SARs of YC-1 analogues were established. Among these potent active derivatives, compounds 29, 30, 31, 44, and 45 functioned as potent activators of sGC and inhibitors of PDE5 with potency comparable to that of YC-1. In addition, compound 58 was found to be a selective and potent inhibitor of protease-activated receptor type 4 (PAR4)-dependent platelet activation.

Molecular mechanisms involved in the synergistic activation of soluble guanylyl cyclase by YC-1 and nitric oxide in endothelial cells

YC-1 is a direct activator of soluble guanylyl cyclase (sGC) and sensitizes the enzyme for activation by nitric oxide (NO) and CO. Because the potentiating effect of YC-1 on NO-induced cGMP formation in platelets and smooth muscle cells has been shown to be substantially higher than observed with the purified enzyme, the synergism between heme ligands and YC-1 is apparently more pronounced in intact cells than in cell-free systems. Here, we investigated the mechanisms underlying the synergistic activation of sGC by YC-1 and NO in endothelial cells. Stimulation of the cells with YC-1 enhanced cGMP accumulation up to approximately 100-fold. The maximal effect of YC-1 was more pronounced than that of the NO donor DEA/NO (approximately 20-fold increase in cGMP accumulation) and markedly diminished in the presence of L-N(G)-nitroarginine, EGTA, or oxyhemoglobin. Because YC-1 did not activate endothelial NO synthase, the pronounced effect of YC-1 on cGMP accumulation was apparently caused by a synergistic activation of sGC by YC-1 and basal NO. The effect of YC-1 was further enhanced by addition of DEA/NO, resulting in a approximately 160-fold stimulation of cGMP accumulation. Thus, YC-1 increased the NO-induced accumulation of cGMP in intact cells by approximately 8-fold. Addition of endothelial cell homogenate increased the stimulatory effect of YC-1 on NO-activated purified sGC from 1.2- to 3.7-fold. This effect was not observed with heat-denatured homogenates, suggesting that a heat-labile factor present in endothelial cells potentiates the effect of YC-1 on NO-activated sGC.

Human recombinant soluble guanylyl cyclase: expression, purification, and regulation

The alpha1- and beta1-subunits of human soluble guanylate cyclase (sGC) were coexpressed in the Sf9 cells/baculovirus system. In addition to the native enzyme, constructs with hexahistidine tag at the amino and carboxyl termini of each subunit were coexpressed. This permitted the rapid and efficient purification of active recombinant enzyme on a nickel-affinity column. The enzyme has one heme per heterodimer and was readily activated with the NO donor sodium nitroprusside or 3-(5'-hydroxymethyl-2'furyl)-1-benzyl-indazole (YC-1). Sodium nitroprusside and YC-1 treatment potentiated each other in combination and demonstrated a remarkable 2,200-fold stimulation of the human recombinant sGC. The effects were inhibited with 1H-(1,2, 4)oxadiazole(4,3-a)quinoxalin-1one (ODQ). The kinetics of the recombinant enzyme with respect to GTP was examined. The products of the reaction, cGMP and pyrophosphate, inhibited the enzyme. The extent of inhibition by cGMP depended on the activation state of the enzyme, whereas inhibition by pyrophosphate was not affected by the enzyme state. Both reaction products displayed independent binding and cooperativity with respect to enzyme inhibition. The expression of large quantities of active enzyme will facilitate structural characterization of the protein.

Potentiation of carbon monoxide-induced relaxation of rat aorta by YC-1 [3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole]

The hypothesis that endogenous carbon monoxide (CO), produced during the oxidation of heme catalyzed by heme oxygenase (HO), plays a role similar to that of nitric oxide (NO) in the regulation of cardiovascular tone has been criticized because of the low potency of CO compared with NO in relaxing blood vessels and stimulating soluble guanylyl cyclase (sGC). This criticism has been muted by the demonstration that, in the presence of YC-1 [3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole], CO has similar potency to NO in stimulating sGC activity. In this study, we determined that YC-1 potentiated CO-induced relaxation of rat aortic strips (RtAS) by approximately ten-fold. Furthermore, CO-induced relaxation of RtAS was shown to be mediated through stimulation of sGC because vasorelaxation was inhibited by ODQ (1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one), a selective sGC inhibitor, in the absence and presence of YC-1. A gas chromatographic-headspace method was used to measure CO concentration in Krebs' solution following the addition of CO-saturated saline solution to the tissue bath, in order to provide an accurate determination of RtAS exposure to CO. The tissue bath concentration of CO was shown to be approximately one-half of that calculated to be present. We conclude that should an endogenous compound exist with properties similar to that of YC-1, then the potency of CO as a vasorelaxant in the presence of this factor would be increased. As a consequence, CO could play a role in the regulation of cardiovascular tone, comparable to that of NO.Key words: carbon monoxide; YC-1 [3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole]; vasodilation; rat aorta; gas-chromatographic analysis.