Kinetics and equilibria of soluble guanylate cyclase ligation by CO: effect of YC-1

Per-ARNT-Sim Domains in Nitric Oxide Signaling by Soluble Guanylyl Cyclase

Nitric oxide (NO) regulates large swaths of animal physiology including wound healing, vasodilation, memory formation, odor detection, sexual function, and response to infectious disease. The primary NO receptor is soluble guanyly/guanylate cyclase (sGC), a dimeric protein of ∼150 kDa that detects NO through a ferrous heme, leading to a large change in conformation and enhanced production of cGMP from GTP. In humans, loss of sGC function contributes to multiple disease states, including cardiovascular disease and cancer, and is the target of a new class of drugs, sGC stimulators, now in clinical use. sGC evolved through the fusion of four ancient domains, a heme nitric oxide / oxygen (H-NOX) domain, a Per-ARNT-Sim (PAS) domain, a coiled coil, and a cyclase domain, with catalysis occurring at the interface of the two cyclase domains. In animals, the predominant dimer is the α1β1 heterodimer, with the α1 subunit formed through gene duplication of the β1 subunit. The PAS domain provides an extensive dimer interface that remains unchanged during sGC activation, acting as a core anchor. A large cleft formed at the PAS-PAS dimer interface tightly binds the N-terminal end of the coiled coil, keeping this region intact and unchanged while the rest of the coiled coil repacks, and the other domains reposition. This interface buries ∼3000 Å2 of monomer surface and includes highly conserved apolar and hydrogen bonding residues. Herein, we discuss the evolutionary history of sGC, describe the role of PAS domains in sGC function, and explore the regulatory factors affecting sGC function.

Cellular Factors That Shape the Activity or Function of Nitric Oxide-Stimulated Soluble Guanylyl Cyclase

NO-stimulated guanylyl cyclase (SGC) is a hemoprotein that plays key roles in various physiological functions. SGC is a typical enzyme-linked receptor that combines the functions of a sensor for NO gas and cGMP generator. SGC possesses exclusive selectivity for NO and exhibits a very fast binding of NO, which allows it to function as a sensitive NO receptor. This review describes the effect of various cellular factors, such as additional NO, cell thiols, cell-derived small molecules and proteins on the function of SGC as cellular NO receptor. Due to its vital physiological function SGC is an important drug target. An increasing number of synthetic compounds that affect SGC activity via different mechanisms are discovered and brought to clinical trials and clinics. Cellular factors modifying the activity of SGC constitute an opportunity for improving the effectiveness of existing SGC-directed drugs and/or the creation of new therapeutic strategies.

Soluble guanylyl cyclase: Molecular basis for ligand selectivity and action in vitro and in vivo

Nitric oxide (NO), carbon monoxide (CO), oxygen (O₂), hydrogen sulfide (H₂S) are gaseous molecules that play important roles in the physiology and pathophysiology of eukaryotes. Tissue concentrations of these physiologically relevant gases vary remarkable from nM range for NO to high μM range of O₂. Various hemoproteins play a significant role in sensing and transducing cellular signals encoded by gaseous molecules or in transporting them. Soluble guanylyl cyclase (sGC) is a hemoprotein that plays vital roles in a wide range of physiological functions and combines the functions of gaseous sensor and signal transducer. sGC uniquely evolved to sense low non-toxic levels of NO and respond to elevated NO levels by increasing its catalytic ability to generate the secondary signaling messenger cyclic guanosine monophosphate (cGMP). This review discusses sGC's gaseous ligand selectivity and the molecular basis for sGC function as high-affinity and selectivity NO receptor. The effects of other gaseous molecules and small molecules of cellular origin on sGC's function are also discussed.

Mapping of the sGC Stimulator BAY 41-2272 Binding Site on H-NOX Domain and Its Regulation by the Redox State of the Heme

Soluble guanylate cyclase (sGC) is the main receptor of nitric oxide (NO) and by converting GTP to cGMP regulates numerous biological processes. The β1 subunit of the most abundant, α1β1 heterodimer, harbors an N-terminal domain called H-NOX, responsible for heme and NO binding and thus sGC activation. Dysfunction of the NO/sGC/cGMP axis is causally associated with pathological states such as heart failure and pulmonary hypertension. Enhancement of sGC enzymatic function can be effected by a class of drugs called sGC “stimulators,” which depend on reduced heme and synergize with low NO concentrations. Until recently, our knowledge about the binding mode of stimulators relied on low resolution cryo-EM structures of human sGC in complex with known stimulators, while information about the mode of synergy with NO is still limited. Herein, we couple NMR spectroscopy using the H-NOX domain of the Nostoc sp. cyanobacterium with cGMP determinations in aortic smooth muscle cells (A7r5) to study the impact of the redox state of the heme on the binding of the sGC stimulator BAY 41-2272 to the Ns H-NOX domain and on the catalytic function of the sGC. BAY 41-2272 binds on the surface of H-NOX with low affinity and this binding is enhanced by low NO concentrations. Subsequent titration of the heme oxidant ODQ, fails to modify the conformation of H-NOX or elicit loss of the heme, despite its oxidation. Treatment of A7r5 cells with ODQ following the addition of BAY 41-2272 and an NO donor can still inhibit cGMP synthesis. Overall, we describe an analysis in real time of the interaction of the sGC stimulator, BAY 41-2272, with the Ns H-NOX, map the amino acids that mediate this interaction and provide evidence to explain the characteristic synergy of BAY 41-2272 with NO. We also propose that ODQ can still oxidize the heme in the H-NOX/NO complex and inhibit sGC activity, even though the heme remains associated with H-NOX. These data provide a more-in-depth understanding of the molecular mode of action of sGC stimulators and can lead to an optimized design and development of novel sGC agonists.

Docking Investigation on Bis (Nitro Indazolyl) Methanes; Synthesis and Antimicrobial Activity Towards Breast Cancer Applications

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Bismuth (III) nitrate pentahydrate (BN) was found to be a mild and efficient catalyst for the electrophilic condensation of 5-nitroindazole with a wide range of aldehydes to obtain Bis (5-nitro indazolyl) methanes 3 (a-h) at ambient temperature. This was structurally confirmed from FTIR, NMR, and HR-MS technique. Molecular docking studies of all compounds was carried out using breast cancer-causing human estrogen receptor (ER) from Molegro Virtual Docker software. Hydroxy Bis (nitro indazolyl) methanes (3b) were shown better binding affinities and the score obtained was -150.146 Kcal/mol compared with Tamoxifen drug. The major H-bond interactions were observed with the compound 3f and the value was -5.679. The antimicrobial activity results revealed that compounds 3b and 3d showed promising activity against bacteria Escherichia coli, Klebsiella pneumonia, Staphylococcus aureus, and Pseudomonas aeruginosa and maximum inhibition against Aspergillus niger and Aspergillus flavus. Methoxy derivatives of Bis (nitro indazolyl methanes) (3e) have shown better antioxidant activity and low MIC (6.25 µg/ml) observed for the compounds 3a and 3b. The synthesized compounds have a very promising starting point for the development and improvement of anti-breast cancer drugs.

Carbon monoxide signaling and soluble guanylyl cyclase: Facts, myths, and intriguing possibilities

The endogenous signaling roles of carbon monoxide (CO) have been firmly established at the pathway level. For CO's molecular mechanism(s) of actions, hemoproteins are generally considered as possible targets. Importantly, soluble guanylyl cyclase (sGC) is among the most widely referenced molecular targets. However, the affinity of CO for sGC (K_d: 240 μM) is much lower than for other highly abundant hemoproteins in the body, such as myoglobin (K_d: 29 nM) and hemoglobin (K_d: 0.7 nM-4.5 μM), which serve as CO reservoirs. Further, most of the mechanistic studies involving sGC activation by CO were based on in-vitro or ex-vivo studies using CO concentrations not readily attenable in vivo and in the absence of hemoglobin as a competitor in binding. As such, whether such in-vitro/ex-vivo results can be directly extrapolated to in-vivo studies is not clear because of the need for CO to be transferred from a high-affinity binder (e.g., hemoglobin) to a low-affinity target if sGC is to be activated in vivo. In this review, we discuss literature findings of sGC activation by CO and the experimental conditions; examine the myths in the disconnect between the low affinity of sGC for CO and the reported activation of sGC by CO; and finally present several possibilities that may lead to additional studies to improve our understanding of this direct CO-sGC axis, which is yet to be convincingly established as playing generally critical roles in CO signaling in vivo.

Activation mechanism of human soluble guanylate cyclase by stimulators and activators

Soluble guanylate cyclase (sGC) is the receptor for nitric oxide (NO) in human. It is an important validated drug target for cardiovascular diseases. sGC can be pharmacologically activated by stimulators and activators. However, the detailed structural mechanisms, through which sGC is recognized and positively modulated by these drugs at high spacial resolution, are poorly understood. Here, we present cryo-electron microscopy structures of human sGC in complex with NO and sGC stimulators, YC-1 and riociguat, and also in complex with the activator cinaciguat. These structures uncover the molecular details of how stimulators interact with residues from both β H-NOX and CC domains, to stabilize sGC in the extended active conformation. In contrast, cinaciguat occupies the haem pocket in the β H-NOX domain and sGC shows both inactive and active conformations. These structures suggest a converged mechanism of sGC activation by pharmacological compounds.

Soluble guanylate cyclase (sGC) is a validated drug target for cardiovascular diseases. Here, the authors report structures of human sGC in complex with NO and sGC stimulators or activator, providing insight into the mechanism of sGC activation by pharmacological compounds.

Mechanistic Insights into the Activation of Soluble Guanylate Cyclase by Carbon Monoxide: A Multistep Mechanism Proposed for the BAY 41-2272 Induced Formation of 5-Coordinate CO-Heme

Soluble guanylate cyclase (sGC) is a heme-containing enzyme that catalyzes cGMP production upon sensing NO. While the CO adduct, sGC-CO, is much less active, the allosteric regulator BAY 41-2272 stimulates the cGMP productivity to the same extent as that of sGC-NO. The stimulatory effect has been thought to be likely associated with Fe-His bond cleavage leading to 5-coordinate CO-heme, but the detailed mechanism remains unresolved. In this study, we examined the mechanism under the condition including BAY 41-2272, 2'-deoxy-3'-GMP and foscarnet. The addition of these effectors caused the original 6-coordinate CO-heme to convert to an end product that was an equimolar mixture of a 5- and a new 6-coordinate CO-heme, as assessed by IR spectral measurements. The two types of CO-hemes in the end product were further confirmed by CO dissociation kinetics. Stopped-flow measurements under the condition indicated that the ferrous sGC bound CO as two reversible steps, where the primary step was assigned to the full conversion of the ferrous enzyme to the 6-coordinate CO-heme, and subsequently followed by the slower second step leading a partial conversion of the 6-coordinate CO-heme to the 5-coordinate CO-heme. The observed rates for both steps linearly depended on CO concentrations. The unexpected CO dependence of the rates in the second step supports a multistep mechanism, in which the 5-coordinate CO-heme is led by CO release from a putative bis-carbonyl intermediate that is likely provided by the binding of a second CO to the 6-coordinate CO-heme. This mechanism provides a new aspect on the activation of sGC by CO.

Structure and Activation of Soluble Guanylyl Cyclase, the Nitric Oxide Sensor

SIGNIFICANCE

Soluble guanylyl/guanylate cyclase (sGC) is the primary receptor for nitric oxide (NO) and is central to the physiology of blood pressure regulation, wound healing, memory formation, and other key physiological activities. sGC is increasingly implicated in disease and is targeted by novel therapeutic compounds. The protein displays a rich evolutionary history and a fascinating signal transduction mechanism, with NO binding to an N-terminal heme-containing domain, which activates the C-terminal cyclase domains. Recent Advances: Crystal structures of individual sGC domains or their bacterial homologues coupled with small-angle x-ray scattering, electron microscopy, chemical cross-linking, and Förster resonance energy transfer measurements are yielding insight into the overall structure for sGC, which is elongated and likely quite dynamic. Transient kinetic measurements reveal a role for individual domains in lowering NO affinity for heme. New sGC stimulatory drugs are now in the clinic and appear to function through binding near or directly to the sGC heme domain, relieving inhibitory contacts with other domains. New sGC-activating drugs show promise for recovering oxidized sGC in diseases with high inflammation by replacing lost heme.

CRITICAL ISSUES

Despite the many recent advances, sGC regulation, NO activation, and mechanisms of drug binding remain unclear. Here, we describe the molecular evolution of sGC, new molecular models, and the linked equilibria between sGC NO binding, drug binding, and catalytic activity.

FUTURE DIRECTIONS

Recent results and ongoing studies lay the foundation for a complete understanding of structure and mechanism, and they open the door for new drug discovery targeting sGC. Antioxid. Redox Signal. 26, 107-121.

Thiol-Based Redox Modulation of Soluble Guanylyl Cyclase, the Nitric Oxide Receptor

SIGNIFICANCE

Soluble guanylyl cyclase (sGC), which produces the second messenger cyclic guanosine 3', 5'-monophosphate (cGMP), is at the crossroads of nitric oxide (NO) signaling: sGC catalytic activity is both stimulated by NO binding to the heme and inhibited by NO modification of its cysteine (Cys) thiols (S-nitrosation). Modulation of sGC activity by thiol oxidation makes sGC a therapeutic target for pathologies originating from oxidative or nitrosative stress. sGC has an unusually high percentage of Cys for a cytosolic protein, the majority solvent exposed and therefore accessible modulatory targets for biological and pathophysiological signaling. Recent Advances: Thiol oxidation of sGC contributes to the development of cardiovascular diseases by decreasing NO-dependent cGMP production and thereby vascular reactivity. This thiol-based resistance to NO (e.g., increase in peripheral resistance) is observed in hypertension and hyperaldosteronism.

CRITICAL ISSUES

Some roles of specific Cys thiols have been identified in vitro. So far, it has not been possible to pinpoint the roles of specific Cys of sGC in vivo and to investigate the molecular mechanisms in an animal model.

FUTURE DIRECTIONS

The role of Cys as redox sensors, intermediates of activation, and mediators of change in sGC conformation, activity, and dimerization remains largely unexplored. To understand modulation of sGC activity, it is critical to investigate the roles of specific oxidative thiol modifications that are formed during these processes. Where the redox state of sGC thiols contribute to pathologies (vascular resistance and sGC desensitization by NO donors), it becomes crucial to design therapeutic strategies to restore sGC to its normal, physiological thiol redox state. Antioxid. Redox Signal. 26, 137-149.

YC-1 binding to the β subunit of soluble guanylyl cyclase overcomes allosteric inhibition by the α subunit

Soluble guanylate cyclase (sGC) is a heterodimeric heme protein and the primary nitric oxide receptor. NO binding stimulates cyclase activity, leading to regulation of cardiovascular physiology and making sGC an attractive target for drug discovery. YC-1 and related compounds stimulate sGC both independently and synergistically with NO and CO binding; however, where the compounds bind and how they work remain unknown. Using linked equilibrium binding measurements, surface plasmon resonance, and domain truncations in Manduca sexta and bovine sGC, we demonstrate that YC-1 binds near or directly to the heme-containing domain of the β subunit. In the absence of CO, YC-1 binds with a Kd of 9-21 μM, depending on the construct. In the presence of CO, these values decrease to 0.6-1.1 μM. Pfizer compound 25 bound ∼10-fold weaker than YC-1 in the absence of CO, whereas compound BAY 41-2272 bound particularly tightly in the presence of CO (Kd = 30-90 nM). Additionally, we found that CO binds much more weakly to heterodimeric sGC proteins (Kd = 50-100 μM) than to the isolated heme domain (Kd = 0.2 μM for Manduca β H-NOX/PAS). YC-1 greatly enhanced binding of CO to heterodimeric sGC, as expected (Kd ∼ 1 μM). These data indicate the α subunit induces a heme pocket conformation with a lower affinity for CO and NO. YC-1 family compounds bind near the heme domain, overcoming the α subunit effect and inducing a heme pocket conformation with high affinity. We propose this high-affinity conformation is required for the full-length protein to achieve high catalytic activity.

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.

Probing soluble guanylate cyclase activation by CO and YC-1 using resonance Raman spectroscopy

Soluble guanylate cyclase (sGC) is weakly activated by carbon monoxide (CO) but is significantly activated by the binding of YC-1 to the sGC-CO complex. In this report, resonance Raman (RR) spectroscopy was used to study selected sGC variants. Addition of YC-1 to the sGC-CO complex alters the intensity pattern of RR bands assigned to the vinyl and propionate heme substituents, suggesting changes in the tilting of the pyrrole rings to which they are attached. YC-1 also shifts the RR intensity of the nu(FeC) and nu(CO) bands from 473 and 1985 cm(-1) to 487 and 1969 cm(-1), respectively, and induces an additional nu(FeC) band, at 521 cm(-1), assigned to five-coordinate heme-CO. Site-directed variants in the proximal heme pocket (P118A) or in the distal heme pocket (V5Y and I149Y) reduce the extent of YC-1 activation, along with the 473 cm(-1) band intensity. These lower-activity sGC variants display another nu(FeC) band at 493 cm(-1) which is insensitive to YC-1 addition and is attributed to protein that cannot be activated by the allosteric activator. The results are consistent with a model in which YC-1 binding to the sGC-CO complex results in a conformational change that activates the protein. Specifically, YC-1 binding alters the heme geometry via peripheral nonbonded contacts and also relieves an intrinsic electronic effect that weakens FeCO backbonding in the native, YC-1 responsive protein. This electronic effect might involve neutralization of the heme propionates via H-bond contacts or negative polarization by a distal cysteine residue. YC-1 binding also strains the Fe-histidine bond, leading to a population of the five-coordinate sGC-CO complex in addition to a conformationally distinct population of the six-coordinate sGC-CO complex. The loss of YC-1 activation in the sGC variants might involve a weakening of the heme-protein contacts that are thought to be critical to a YC-1-induced conformational change.

Thrombospondin-1 is an inhibitor of pharmacological activation of soluble guanylate cyclase

BACKGROUND AND PURPOSE

Soluble guanylate cyclase (sGC) is the signal transduction enzyme most responsible for mediating the effects of nitric oxide (NO). Recently, NO-independent small molecule activators of sGC have been developed that have promising clinical activities. We have shown that the secreted matrix protein thrombospondin-1 (TSP-1) binds to CD47 and potently inhibits NO stimulation of sGC in endothelial and vascular smooth muscle cells (VSMCs) and platelets. Here we show that TSP-1 signalling via CD47 inhibits sGC activation by NO-independent sGC activating small molecules.

EXPERIMENTAL APPROACH

Vascular smooth muscle cells and washed human platelets were pretreated with TSP-1 (2.2 nM) in the presence of haeme-dependent sGC activators (YC-1, BAY 41-2272), and a haeme-independent activator (meso-porphyrin IX), and cGMP levels were measured. The effect of sGC activators on platelet aggregation and contraction of VSMC embedded in collagen gels was also assayed in the presence and absence of TSP-1.

KEY RESULTS

Thrombospondin-1 inhibited sGC activator-dependent increase in cGMP in VSMC and platelets. TSP-1 pretreatment also inhibited the ability of these agents to delay thrombin-induced platelet aggregation. TSP-1 pretreatment reduced the ability of sGC activating agents to abrogate VSMC contraction in vitro.

CONCLUSIONS AND IMPLICATIONS

This work demonstrates that TSP-1 is a universal inhibitor of sGC, blocking both haeme-dependent and haeme-independent activation. These data coupled with the reported increases in TSP-1 with age, diabetes, ischaemia/reperfusion, and atherosclerosis implies that the therapeutic potential of all drugs that activate sGC could be compromised in disease states where TSP-1/CD47 signalling is elevated.

Biochemistry of soluble guanylate cyclase

Nitric oxide (NO) functions in biology as both a critical cytotoxic agent and an essential signaling molecule. The toxicity of the diatomic gas has long been accepted; however, it was not known to be a signaling molecule until it was identified as the endothelium-derived relaxing factor (EDRF). Since this discovery, the physiological signaling pathways that are regulated by NO have been the focus of numerous studies. Many of the cellular responses that NO modulates are mediated by the heme protein soluble guanylate cyclase (sGC). NO binds to sGC at a diffusion controlled rate, and leads to a several 100-fold increase in the synthesis of the second messenger cGMP from GTP. Other diatomic gases either do not bind (dioxygen), or do not significantly activate (carbon monoxide) sGC. This provides selectivity and efficiency for NO even in an aerobic environment, which is critical due to the high reactivity of NO. Several biochemical studies have focused on elucidating the mechanism of NO activation and O(2) discrimination. Significant advances in our understanding of these topics have occurred with the identification and characterization of the sGC-like homologues termed Heme-Nitric oxide and OXygen binding (H-NOX) proteins.

Reactivity of nitric oxide with the [4Fe-4S] cluster of dihydroxyacid dehydratase from Escherichia coli

Although the NO (nitric oxide)-mediated modification of iron-sulfur proteins has been well-documented in bacteria and mammalian cells, specific reactivity of NO with iron-sulfur proteins still remains elusive. In the present study, we report the first kinetic characterization of the reaction between NO and iron-sulfur clusters in protein using the Escherichia coli IlvD (dihydroxyacid dehydratase) [4Fe-4S] cluster as an example. Combining a sensitive NO electrode with EPR (electron paramagnetic resonance) spectroscopy and an enzyme activity assay, we demonstrate that NO is rapidly consumed by the IlvD [4Fe-4S] cluster with the concomitant formation of the IlvD-bound DNIC (dinitrosyl-iron complex) and inactivation of the enzyme activity under anaerobic conditions. The rate constant for the initial reaction between NO and the IlvD [4Fe-4S] cluster is estimated to be (7.0+/-2.0)x10(6) M(-2) x s(-1) at 25 degrees C, which is approx. 2-3 times faster than that of the NO autoxidation by O2 in aqueous solution. Addition of GSH failed to prevent the NO-mediated modification of the IlvD [4Fe-4S] cluster regardless of the presence of O2 in the medium, further suggesting that NO is more reactive with the IlvD [4Fe-4S] cluster than with GSH or O2. Purified aconitase B [4Fe-4S] cluster from E. coli has an almost identical NO reactivity as the IlvD [4Fe-4S] cluster. However, the reaction between NO and the endonuclease III [4Fe-4S] cluster is relatively slow, apparently because the [4Fe-4S] cluster in endonuclease III is less accessible to solvent than those in IlvD and aconitase B. When E. coli cells containing recombinant IlvD, aconitase B or endonuclease III are exposed to NO using the Silastic tubing NO delivery system under aerobic and anaerobic conditions, the [4Fe-4S] clusters in IlvD and aconitase B, but not in endonuclease III, are efficiently modified forming the protein-bound DNICs, confirming that NO has a higher reactivity with the [4Fe-4S] clusters in IlvD and aconitase B than with O2 or GSH. The results suggest that the iron-sulfur clusters in proteins such as IlvD and aconitase B may constitute the primary targets of the NO cytotoxicity under both aerobic and anaerobic conditions.

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.

Binding of YC-1 or BAY 41-2272 to soluble guanylyl cyclase induces a geminate phase in CO photolysis

Soluble guanylyl/guanylate cyclase (sGC), a heme-containing heterodimeric protein of approximately 150 kDa, is the primary receptor for nitric oxide, an endogenous molecule of immense physiological importance to animals. Recent studies have identified compounds such as YC-1 and BAY 41-2272 that stimulate sGC independently of NO binding, properties of importance for the treatment of endothelial dysfunction and other diseases linked to malfunctioning NO signaling pathways. We have developed a novel expression system for sGC from Manduca sexta (the tobacco hornworm) that retains the N-terminal two-thirds of both subunits, including heme, but is missing the catalytic domain. Here, we show that binding of compounds YC-1 or BAY 41-2272 to the truncated protein leads to a change in the heme pocket such that photolyzed CO cannot readily escape from the protein matrix. Geminate recombination of the trapped CO molecules with heme takes place with a measured rate of 6 x 10(7) s(-1). These findings provide strong support for an allosteric regulatory model in which YC-1 and related compounds can alter the sGC heme pocket conformation to retain diatomic ligands and thus activate the enzyme alone or in synergy with either NO or CO.

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.

An enzyme-linked receptor mechanism for nitric oxide-activated guanylyl cyclase

Nitric oxide (NO) exerts physiological effects by activating specialized receptors that are coupled to guanylyl cyclase activity, resulting in cGMP synthesis from GTP. Despite its widespread importance as a signal transduction pathway, the way it operates is still understood only in descriptive terms. The present work aimed to elucidate a formal mechanism for NO receptor activation and its modulation by GTP, ATP, and allosteric agents, such as YC-1 and BAY 41-2272. The model comprised a module in which NO, the nucleotides, and allosteric agents bind and the protein undergoes a conformational change, dovetailing with a catalytic module where GTP is converted to cGMP and pyrophosphate. Experiments on NO-activated guanylyl cyclase purified from bovine lung allowed values for all of the binding and isomerization constants to be derived. The catalytic module was a modified version of one describing the kinetics of adenylyl cyclase. The resulting enzyme-linked receptor mechanism faithfully reproduces all of the main functional properties of NO-activated guanylyl cyclase reported to date and provides a thermodynamically sound interpretation of those properties. With appropriate modification, it also replicates activation by carbon monoxide and the remarkable enhancement of that activity brought about by the allosteric agents. In addition, the mechanism enhances understanding of the behavior of the receptor in a cellular setting.

Butyl isocyanide as a probe of the activation mechanism of soluble guanylate cyclase. Investigating the role of non-heme nitric oxide

Nitric oxide (NO) is a physiologically relevant activator of the hemoprotein soluble guanylate cyclase (sGC). In the presence of NO, sGC is activated several hundredfold above the basal level by a mechanism that remains to be elucidated. The heme ligand n-butyl isocyanide (BIC) was used to probe the mechanism of NO activation of sGC. Electronic absorption spectroscopy was used to show that BIC binds to the sGC heme, forming a 6-coordinate complex with an absorbance maximum at 429 nm. BIC activates sGC 2-5-fold, and synergizes with the allosteric activator YC-1, to activate the enzyme 15-25-fold. YC-1 activates the sGC-BIC complex, and leads to an increase in both the V(max) and K(m). BIC was also used to probe the mechanism of NO activation. The activity of the sGC-BIC complex increases 15-fold in the presence of NO, without displacing BIC at the heme, which is consistent with previous reports that proposed the involvement of a non-heme NO binding site in the activation process.

Effect of strain in the proximal ligand on the binding of nitric oxide and carbon monoxide to chelated protoheme complexes

The binding of NO and CO to chelated protoheme-l-histidine methyl ester (HM-H), protoheme-glycyl-l-histidine methyl ester (HM-GH), and free protoheme (HM) has been studied in methanol-DMSO solution. In all cases, the NO adducts are five-coordinated, indicating that binding of NO occurs with displacement of the axial base, and confirms the strong negative trans effect exerted by NO in heme complexes, though it is found that the presence of strain in the iron-histidine bond of HM-H has a positive influence on NO binding, making it thermodynamically more favorable than for HM-GH. The equilibrium constants thus decrease in the series: HM > HM-H > HM-GH. In contrast to NO, CO has a positive trans effect, and therefore, an opposite trend is observed in the binding of this ligand to the heme complexes.

Microvascular and systemic effects following top load administration of saturated carbon monoxide-saline solution

OBJECTIVE

To determine how top loads with different doses of carbon monoxide (CO)-saturated saline solutions (CO-saline) affect microvascular and systemic hemodynamics and to delineate the corresponding biochemical mechanisms.

DESIGN

Prospective study.

SETTING

University research laboratory.

SUBJECTS

Male Golden Syrian hamsters.

INTERVENTIONS

Hamsters implemented with a dorsal window chamber were given different volumes (characterized as percent of blood volume, BV) by intravenous injection of CO-saturated saline.

MEASUREMENTS AND MAIN RESULTS

Hamsters were observed until 90 mins after infusion of CO-saline solution. In the 20% BV CO-saline infusion group, observation was extended until 180 mins. Systemic variables measured included mean arterial pressure, heart rate, systemic arterial blood gases, and cardiac output and index. Microvascular hemodynamic measurements included vessel diameter, red blood cell velocity, and functional capillary density. Cyclic guanosine monophosphate (cGMP) content in the chamber tissue was measured by enzyme immunoassay. 10% BV of CO-saline increased flow maximally in the microcirculation at 30 mins after infusion (207% in arterioles and 238% in venules, p < .05 vs. baseline). Functional capillary density was significantly increased in both 10% and 15% groups (p < .05 vs. baseline), and cardiac index increased 130% (p < .05 vs. baseline) at 10 mins after 10% CO-saline infusion. There were no changes of microhemodynamic variables and functional capillary density with 2.5%, 5%, and 20% CO-saline infusion during the observation period. Microvascular hemodynamic changes by 10% CO-saline infusion were inhibited completely by L-NAME pretreatment and partially by 1H-[1,2,4]oxadiazole[4,3-a]quinoxqalin-1-one pretreatment. cGMP content in skin fold tissues was related to changes of vessel diameter.

CONCLUSIONS

Intravenous injection of CO-saturated saline caused vasodilation and improved microvascular hemodynamics in the hamster window chamber model in a dose-dependent manner. These changes were related to increased cardiac output and local cGMP content. These results support the possible use of CO-saturated solutions as a vasodilator in critical conditions.

Synthesis and structural characterization of 1- and 2-substituted indazoles: ester and carboxylic acid derivatives

A series of indazoles substituted at the N-1 and N-2 positions with ester-containing side chains -(CH2)(n)CO2R of different lengths (n = 0-6, 9, 10) are described. Nucleophilic substitution reactions on halo esters (X(CH2)(n)CO2R) by 1H-indazole inalkaline solution lead to mixtures of N-1 and N-2 isomers, in which the N-1 isomer predominates. Basic hydrolysis of the ester derivatives allowed the synthesis of the corresponding indazole carboxylic acids. All compounds were fully characterised by multinuclear NMR and IR spectroscopies, MS spectrometry and elemental analysis; the NMR spectroscopic data were used for structural assignment of the N-1 and N-2 isomers. The molecular structure of indazol-2-yl-acetic acid (5b) was determined by X-ray diffraction, which shows a supramolecular architecture involving O2-H...N1 intermolecular hydrogen bonds.

Soluble guanylate cyclase

Soluble guanylate cyclase (sGC) is a mammalian nitric oxide (NO) sensor. When NO binds to the sGC heme, its GTP cyclase activity markedly increases, thus generating cyclic GMP, which serves to regulate several cell signaling functions. A good deal is known about the kinetics and equilibrium of binding of NO to sGC, leading to a proposed multistep mechanism of sGC activation that involves at least two NO-binding sites. The crystal structure of a member of a recently discovered family of prokaryotic sGC homologues has provided important insights into structure-function relationships within the sGC family of proteins.

CO as a cellular signaling molecule

Many biological functions of heme oxygenase (HO), such as cytoprotection against oxidative stress, vasodilation, neurotransmission in the central or peripheral nervous systems, and anti-inflammatory, anti-apoptotic, or anti-proliferative potential, have been attributed to its enzymatic byproduct carbon monoxide (CO), although roles for biliverdin/bilirubin and iron have also been proposed. In addition to these well-characterized effects, recent findings reveal that HO-derived CO may act as an oxygen sensor and circadian modulator of heme biosynthesis. In lymphocytes, CO may participate in regulatory T cell function. A number of the known signaling effects of CO depend on stimulation of soluble guanylate cyclase and/or activation of mitogen-activated protein kinases (MAPK). Furthermore, modulation of caveolin-1 status may serve as an essential component of certain aspects of CO action, such as growth control. In this review, we summarize recent findings of the beneficial or detrimental effects of endogenous CO with an emphasis on the signaling pathways and downstream targets that trigger the action of this gas.

The design and synthesis of YC-1 analogues as probes for soluble guanylate cyclase

Soluble guanylate cyclase (sGC) is highly activated in the presence of both YC-1 (1-benzyl-3-(5'-hydroxymethyl-2'-furyl)-indazole) and CO. In this report, the design, synthesis, and activity (i.e., sGC activation) of photolabile analogues of 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (YC-1) are presented. Initial results with 6-azido-3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole led to the synthesis of a tritium-labeled analogue. When photoactivated, this analogue labeled the alpha-subunit of sGC.

The inhibitory mechanism of YC-1, a benzyl indazole, on smooth muscle cell proliferation: an in vitro and in vivo study

The pharmacological mechanisms of a synthetic compound 1-benzyl-3-(5'-hydroxymethyl-2'-furyl) indazole (YC-1) in preventing smooth muscle cell proliferation remains to be elucidated. The present study was aimed to explore the effects of YC-1 on certain molecules responsible for cell proliferation, including transforming growth factor (TGF)-beta1, soluble guanylyl cyclase (sGC) and focal adhesion kinase (FAK). The in vivo assay was correlated to the in vitro results of YC-1 on vascular stenosis. YC-1 was applied topically via a pluronic gel onto the balloon-injured rat carotid arteries, which were then harvested two weeks later for histological analysis. Our in vitro results showed that TGF-beta1 was suppressed by YC-1 by 50%. The translational level of sGC was threefold activated by YC-1 while the transcription level of sGC was increased up to 24-fold. FAK, the molecule responsible for cell proliferation and migration, was suppressed by YC-1 on the translational levels for 72%. These in vitro results were in consistent with the in vivo observation that the area ratio of neointima to media was reduced by YC-1. This study provides insights into the pharmacological mechanisms of YC-1 in preventing abnormal smooth muscle cell proliferation and thus supports the use of YC-1 as an adjuvant therapy for balloon injury-induced restenosis.

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.

CO-sensing mechanisms

Carbon monoxide (CO) has long been known to have dramatic physiological effects on organisms ranging from bacteria to humans, but recently there have a number of suggestions that organisms might have specific sensors for CO. This article reviews the current evidence for a variety of proteins with demonstrated or potential CO-sensing ability. Particular emphasis is placed on the molecular description of CooA, a heme-containing CO sensor from Rhodospirillum rubrum, since its biological role as a CO sensor is clear and we have substantial insight into the basis of its sensing ability.

Characterization of a novel type of endogenous activator of soluble guanylyl cyclase

Nitric oxide (NO) remains the only firmly established endogenous modulator of soluble guanylyl cyclase (sGC) activity, but physiological, structural, and biochemical evidence now suggests that in vivo regulation of sGC involves direct interaction with other factors. We searched for such endogenous modulators in human umbilical vein endothelial cells and COS-7 cells. The cytosolic fraction of both cell types stimulated the activity of semipurified sGC severalfold in the absence or presence of a saturating concentration of NO. The cytosolic factor was sensitive to proteinase K and destroyed by boiling, suggesting that it contains a protein component. Size exclusion chromatography revealed peaks of activity between 40 and 70 kDa. The sGC-activating effect was further purified by ion exchange chromatography. In the presence of the benzylindazole YC-1 or NO, the partially purified factor synergistically activated sGC, suggesting that this factor had a mode of activation different from that of YC-1 or NO. Four candidate activators were identified from the final purification step by matrix-assisted laser desorption ionization mass spectrometry analysis. Using an sGC affinity matrix, one of them, the molecular chaperone Hsp70, was shown to directly interact with sGC. This interaction was further confirmed by co-immunoprecipitation in lung tissues and by co-localization in smooth muscle cells. sGC and Hsp70 co-localized at the plasma membrane, supporting the idea that sGC can be translocated to the membrane. Hsp70 co-purifies with the sGC-activating effect, and immunodepletion of Hsp70 from COS-7 cytosol coincided with a marked attenuation of the sGC-activating effect, yet the effect was not rescued by the addition of pure Hsp70. Thus, Hsp70 is a novel sGC-interacting protein that is responsible for the sGC-activating effect, probably in association with other factors or after covalent modification.

Molecular cloning of a novel variant of the rat soluble guanylate cyclase beta2 subunit

Soluble guanylate cyclases (sGCs) are heterodimeric enzymes consisting of alpha and beta subunits and catalyze the formation of cGMP from GTP. The beta1 subunit has been characterized in detail, whereas the function and physiological role of the beta2 subunit are poorly understood. In this study, I isolated two distinct cDNA fragments for the beta2 subunit of sGC (beta2a and beta2b) from a rat brain cDNA library by 3' rapid amplification of cDNA ends using degenerate sense primers based on amino acid sequences conserved among membrane-bound guanylate cyclases. The deduced amino acid sequence of beta2a is identical with the corresponding sequence of the previously described beta2 subunit, whereas that of beta2b is C-terminally shorter by 46 amino acids and thus does not contain a consensus sequence for isoprenylation/carboxymethylation. Reverse transcription-polymerase chain reaction (RT-PCR) analysis demonstrated that both variants are expressed in various tissues, including kidney, liver, and brain. Although the functional significance of the C-terminal region containing the consensus sequence for isoprenylation/carboxymethylation of beta2a remains unclear yet, it is likely that these beta2 subunits play some physiological or pathophysiological role in various tissues.