Soluble guanylate cyclase

How do heme-protein sensors exclude oxygen? Lessons learned from cytochrome c', Nostoc puntiforme heme nitric oxide/oxygen-binding domain, and soluble guanylyl cyclase

SIGNIFICANCE

Ligand selectivity for dioxygen (O(2)), carbon monoxide (CO), and nitric oxide (NO) is critical for signal transduction and is tailored specifically for each heme-protein sensor. Key NO sensors, such as soluble guanylyl cyclase (sGC), specifically recognized low levels of NO and achieve a total O(2) exclusion. Several mechanisms have been proposed to explain the O(2) insensitivity, including lack of a hydrogen bond donor and negative electrostatic fields to selectively destabilize bound O(2), distal steric hindrance of all bound ligands to the heme iron, and restriction of in-plane movements of the iron atom.

RECENT ADVANCES

Crystallographic structures of the gas sensors, Thermoanaerobacter tengcongensis heme-nitric oxide/oxygen-binding domain (Tt H-NOX(1)) or Nostoc puntiforme (Ns) H-NOX, and measurements of O(2) binding to site-specific mutants of Tt H-NOX and the truncated β subunit of sGC suggest the need for a H-bonding donor to facilitate O(2) binding.

CRITICAL ISSUES

However, the O(2) insensitivity of full length sGC with a site-specific replacement of isoleucine by a tyrosine on residue 145 and the very slow autooxidation of Ns H-NOX and cytochrome c' suggest that more complex mechanisms have evolved to exclude O(2) but retain high affinity NO binding. A combined graphical analysis of ligand binding data for libraries of heme sensors, globins, and model heme shows that the NO sensors dramatically inhibit the formation of six-coordinated NO, CO, and O(2) complexes by direct distal steric hindrance (cyt c'), proximal constraints of in-plane iron movement (sGC), or combinations of both following a sliding scale rule. High affinity NO binding in H-NOX proteins is achieved by multiple NO binding steps that produce a high affinity five-coordinate NO complex, a mechanism that also prevents NO dioxygenation.

FUTURE DIRECTIONS

Knowledge advanced by further extensive test of this "sliding scale rule" hypothesis should be valuable in guiding novel designs for heme based sensors.

Conformationally distinct five-coordinate heme-NO complexes of soluble guanylate cyclase elucidated by multifrequency electron paramagnetic resonance (EPR)

Soluble guanylate cyclase (sGC) is a heme-containing enzyme that senses nitric oxide (NO). Formation of a heme Fe-NO complex is essential to sGC activation, and several spectroscopic techniques, including electron paramagnetic resonance (EPR) spectroscopy, have been aimed at elucidating the active enzyme conformation. Of these, only EPR spectra (X-band ~9.6 GHz) have shown differences between low- and high-activity Fe-NO states, and these states are modeled in two different heme domain truncations of sGC, β1(1-194) and β2(1-217), respectively (Derbyshire et al., Biochemistry 2008, 47, 3892-3899). The EPR signal of the low-activity sGC Fe-NO complex exhibits a broad lineshape that has been interpreted as resulting from site-to-site inhomogeneity, and simulated using g strain, a continuous distribution about the principal values of a given g tensor. This approach, however, fails to account for visible features in the X-band EPR spectra as well as the g anisotropy observed at higher microwave frequencies. Herein we analyze X-, Q-, and D-band EPR spectra and show that both the broad lineshape and the spectral structure of the sGC EPR signal at multiple microwave frequencies can be simulated successfully with a superposition of only two distinct g tensors. These tensors represent different populations that likely differ in Fe-NO bond angle, hydrogen bonding, or the geometry of the amino acid residues. One of these conformations can be linked to a form of the enzyme with higher activity.

Immunohistochemical and functional characterization of nitric oxide signaling pathway in isolated aorta from Crotalus durissus terrificus

We characterized the nitric oxide (NO)-cyclic GMP-phosphodiesterase-5 (PDE5) pathway in Crotalus durissus terrificus aorta. Concentration responses curves to acetylcholine (ACh), sodium nitroprusside (SNP), BAY41-2272 (soluble guanylyl cyclase [sGC] stimulator), BAY60-2770 (sGC activator) and tadalafil (PDE5 inhibitor) were constructed in phenylephrine (10 μM)-precontracted tissues with intact (E(+)) or denuded (E(-)) endothelium. ACh (0.0001-10 μM) and SNP (0.0001-10 μM) relaxed aorta, which were reduced by the NO synthase (L-NAME,100 μM) or the sGC inhibitors (ODQ, 10 μM). Tadalafil (0.0001-10 μM) relaxed E(+) rings with potency (pEC(50)) and maximal response (E(max)) values of 7.34±0.02 and 105±8%, respectively. E(-) or ODQ treatment significantly (P<0.05) reduced tadalafil relaxations (66±18% and 71±7%, respectively). BAY41-2272 (0.0001-300 nM) produced concentration-dependent relaxations in E(+) rings, which were reduced by addition of either ODQ or L-NAME (16.0- and 5.2-fold rightward shifts, respectively). The relaxation of BAY60-2770 was markedly potentiated by ODQ and L-NAME (41.0- and 9.7-fold leftward shifts, respectively), whereas in E(-) the pEC(50) values were shifted by 7-fold to the right. Immunohistochemistry, followed validation by transcriptomic analysis, revealed the presence of eNOS in endothelium, whereas nNOS was observed only in perivascular nerves. sGC and PDE5 were expressed in smooth muscle. Thus, NO-sGC-PDE5 pathway is evolutionarily present in Crotalus sp. vessels, and has a remarkable degree of functional similarity to mammalian vessels.

Mechanism of binding of NO to soluble guanylyl cyclase: implication for the second NO binding to the heme proximal site

Soluble guanylyl cyclase (sGC), the key enzyme for the formation of second messenger cyclic GMP, is an authentic sensor for nitric oxide (NO). Binding of NO to sGC leads to strong activation of the enzyme activity. Multiple molecules and steps of binding of NO to sGC have been implicated, but the target of the second NO and the detailed binding mechanism remain controversial. In this study, we used (15)NO and (14)NO and anaerobic sequential mixing-freeze-quench electron paramagnetic resonance to unambiguously confirm that the heme Fe is the target of the second NO. The linear dependence on NO concentration up to 600 s(-1) for the observed rate of the second step of NO binding not only indicates that the binding site of the second NO is different from that in the first step, i.e., the proximal site of the heme, but also supports a concerted mechanism in which the dissociation of the His105 proximal ligand occurs simultaneously with the binding of the second NO molecule. Computer modeling successfully predicts the kinetics of formation of a set of five-coordinate NO complexes with the ligand on either the distal or proximal site and supports the selective release of NO from the distal side of the transient bis-NO-sGC complex. Thus, as has been demonstrated with cytochrome c', a five-coordinate NO-sGC complex containing a proximal NO is formed after the binding of the second NO.

Photolability of NO in designed metal nitrosyls with carboxamido-N donors: a theoretical attempt to unravel the mechanism

During the past few years, photoactive metal nitrosyls (NO complexes of metals) have drawn attention as potential drugs for delivery of nitric oxide (NO) to biological targets under the control of light. Major success in this area has been achieved with designed metal nitrosyls derived from ligands that contain carboxamide group(s). A number of iron, manganese and ruthenium {MNO}(6) nitrosyls of such kind exhibit excellent NO photolability under low-power visible and near-IR light. The results of theoretical studies on these NO-donors have provided insight into (a) the electronic transitions that lead to photorelease of NO and (b) the structural features of the ligands that dictate the sensitivity of the nitrosyls to light of specific wavelengths. In addition, the results have afforded clear understanding of the electronic configurations of the various nitrosyls. This article highlights these results in a coherent manner. Good matches between the predicted and observed spectral features and NO photolability strongly suggest that theoretical studies should be an integral part of the smart design of such NO-donors in the future research.

The diversity of microbial responses to nitric oxide and agents of nitrosative stress close cousins but not identical twins

Nitric oxide and related nitrogen species (reactive nitrogen species) now occupy a central position in contemporary medicine, physiology, biochemistry, and microbiology. In particular, NO plays important antimicrobial defenses in innate immunity but microbes have evolved intricate NO-sensing and defense mechanisms that are the subjects of a vast literature. Unfortunately, the burgeoning NO literature has not always been accompanied by an understanding of the intricacies and complexities of this radical and other reactive nitrogen species so that there exists confusion and vagueness about which one or more species exert the reported biological effects. The biological chemistry of NO and derived/related molecules is complex, due to multiple species that can be generated from NO in biological milieu and numerous possible reaction targets. Moreover, the fate and disposition of NO is always a function of its biological environment, which can vary significantly even within a single cell. In this review, we consider newer aspects of the literature but, most importantly, consider the underlying chemistry and draw attention to the distinctiveness of NO and its chemical cousins, nitrosonium (NO(+)), nitroxyl (NO(-), HNO), peroxynitrite (ONOO(-)), nitrite (NO(2)(-)), and nitrogen dioxide (NO(2)). All these species are reported to be generated in biological systems from initial formation of NO (from nitrite, NO synthases, or other sources) or its provision in biological experiments (typically from NO gas, S-nitrosothiols, or NO donor compounds). The major targets of NO and nitrosative damage (metal centers, thiols, and others) are reviewed and emphasis is given to newer "-omic" methods of unraveling the complex repercussions of NO and nitrogen oxide assaults. Microbial defense mechanisms, many of which are critical for pathogenicity, include the activities of hemoglobins that enzymically detoxify NO (to nitrate) and NO reductases and repair mechanisms (e.g., those that reverse S-nitrosothiol formation). Microbial resistance to these stresses is generally inducible and many diverse transcriptional regulators are involved-some that are secondary sensors (such as Fnr) and those that are "dedicated" (such as NorR, NsrR, NssR) in that their physiological function appears to be detecting primarily NO and then regulating expression of genes that encode enzymes with NO as a substrate. Although generally harmful, evidence is accumulating that NO may have beneficial effects, as in the case of the squid-Vibrio light-organ symbiosis, where NO serves as a signal, antioxidant, and specificity determinant. Progress in this area will require a thorough understanding not only of the biology but also of the underlying chemical principles.

Impaired iNOS-sGC-cGMP signalling contributes to chronic hypoxic and hypercapnic pulmonary hypertension in rat

Nitric oxide (NO) is an important vascular modulator in the development of pulmonary hypertension. NO exerts its regulatory effect mainly by activating soluble guanylate cyclase (sGC) to synthesize cyclic guanosine monophosphate (cGMP). Exposure to hypoxia causes pulmonary hypertension. But in lung disease, hypoxia is commonly accompanied by hypercapnia. The aim of this study was to examine the changes of sGC enzyme activity and cGMP content in lung tissue, as well as the expression of inducible nitric oxide synthase (iNOS) and sGC in rat pulmonary artery after exposure to hypoxia and hypercapnia, and assess the role of iNOS-sGC-cGMP signal pathway in the development of hypoxic and hypercapnic pulmonary hypertension. Male Sprague-Dawley rats were exposed to hypoxia and hypercapnia for 4 weeks to establish model of chronic pulmonary hypertension. Weight-matched rats exposed to normoxia served as control. After exposure to hypoxia and hypercapnia, mean pulmonary artery pressure, the ratio of right ventricle/left ventricle+septum, and the ratio of right ventricle/body weight were significantly increased. iNOS mRNA and protein levels were significantly increased, but sGC α(1) mRNA and protein levels were significantly decreased in small pulmonary arteries of hypoxic and hypercapnic exposed rat. In addition, basal and stimulated sGC enzyme activity and cGMP content in lung tissue were significantly lower after exposure to hypoxia and hypercapnia. These results demonstrate that hypoxia and hypercapnia lead to the upregulation of iNOS expression, downregulation of sGC expression and activity, which then contribute to the development of pulmonary hypertension.

Modulation of cellular respiration by endogenously produced nitric oxide in rat hippocampal slices

Nitric oxide (•NO) is a ubiquitous signaling molecule that participates in neuromolecular phenomena associated with memory formation as well as in excitotoxicity. In the hippocampus, neuronal •NO production is coupled to the activation of the NMDA-type of glutamate receptor. More recently, Cytochrome c oxidase has emerged as a novel target for •NO, which competes with O 2 for binding to this mitochondrial complex. This reaction establishes •NO not only as a regulator of cellular metabolism but possibly also as a regulator of mitochondrial production of reactive oxygen species which participate in cellular signaling. A major gap in the understanding of •NO bioactivity, namely, in the hippocampus, has been the lack of knowledge of its concentration dynamics. Here, we present a detailed description of the simultaneous recording of •NO and O2 concentration dynamics in rat hippocampal slices. Carbon fi ber microelectrodes are fabricated and applied for real-time measurements of both gases in a system close to in vivo models. This approach allows for a better understanding of the current paradigm by which an intricate interplay between •NO and O 2 regulates cellular respiration.

Thrombospondin-1 and angiotensin II inhibit soluble guanylyl cyclase through an increase in intracellular calcium concentration

Nitric oxide (NO) regulates cardiovascular hemostasis by binding to soluble guanylyl cyclase (sGC), leading to cGMP production, reduced cytosolic calcium concentration ([Ca(2+)](i)), and vasorelaxation. Thrombospondin-1 (TSP-1), a secreted matricellular protein, was recently discovered to inhibit NO signaling and sGC activity. Inhibition of sGC requires binding to cell-surface receptor CD47. Here, we show that a TSP-1 C-terminal fragment (E3CaG1) readily inhibits sGC in Jurkat T cells and that inhibition requires an increase in [Ca(2+)](i). Using flow cytometry, we show that E3CaG1 binds directly to CD47 on the surface of Jurkat T cells. Using digital imaging microscopy on live cells, we further show that E3CaG1 binding results in a substantial increase in [Ca(2+)](i), up to 300 nM. Addition of angiotensin II, a potent vasoconstrictor known to increase [Ca(2+)](i), also strongly inhibits sGC activity. sGC isolated from calcium-treated cells or from cell-free lysates supplemented with Ca(2+) remains inhibited, while addition of kinase inhibitor staurosporine prevents inhibition, indicating inhibition is likely due to phosphorylation. Inhibition is through an increase in K(m) for GTP, which rises to 834 μM for the NO-stimulated protein, a 13-fold increase over the uninhibited protein. Compounds YC-1 and BAY 41-2272, allosteric stimulators of sGC that are of interest for treating hypertension, overcome E3CaG1-mediated inhibition of NO-ligated sGC. Taken together, these data suggest that sGC not only lowers [Ca(2+)](i) in response to NO, inducing vasodilation, but also is inhibited by high [Ca(2+)](i), providing a fine balance between signals for vasodilation and vasoconstriction.

Oxidation and loss of heme in soluble guanylyl cyclase from Manduca sexta

Oxidation and loss of heme in soluble guanylyl/guanylate cyclase (sGC), the nitric oxide receptor, is thought to be a major contributor to cardiovascular disease and is the target of compounds BAY 58-2667 and HMR1766. Using spectroelectrochemical titration, we found a truncated sGC to be highly stable in the ferrous state (234 mV) and to bind ferrous heme tightly even in the presence of NO, despite the NO-induced release of the proximal histidine. In contrast, oxidized sGC readily loses ferric heme to myoglobin (0.47 ± 0.02 h(-1)). Peroxynitrite, the presumed cellular oxidant, readily oxidizes sGC in 5 mM glutathione.

Antagonism of soluble guanylyl cyclase attenuates cutaneous vasodilation during whole body heat stress and local warming in humans

We hypothesized that nitric oxide activation of soluble guanylyl cyclase (sGC) participates in cutaneous vasodilation during whole body heat stress and local skin warming. We examined the effects of the sGC inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), on reflex skin blood flow responses to whole body heat stress and on nonreflex responses to increased local skin temperature. Blood flow was monitored by laser-Doppler flowmetry, and blood pressure by Finapres to calculate cutaneous vascular conductance (CVC). Intradermal microdialysis was used to treat one site with 1 mM ODQ in 2% DMSO and Ringer, a second site with 2% DMSO in Ringer, and a third site received Ringer. In protocol 1, after a period of normothermia, whole body heat stress was induced. In protocol 2, local heating units warmed local skin temperature from 34 to 41°C to cause local vasodilation. In protocol 1, in normothermia, CVC did not differ among sites [ODQ, 15 ± 3% maximum CVC (CVC(max)); DMSO, 14 ± 3% CVC(max); Ringer, 17 ± 6% CVC(max); P > 0.05]. During heat stress, ODQ attenuated CVC increases (ODQ, 54 ± 4% CVC(max); DMSO, 64 ± 4% CVC(max); Ringer, 63 ± 4% CVC(max); P < 0.05, ODQ vs. DMSO or Ringer). In protocol 2, at 34°C local temperature, CVC did not differ among sites (ODQ, 17 ± 2% CVC(max); DMSO, 18 ± 4% CVC(max); Ringer, 18 ± 3% CVC(max); P > 0.05). ODQ attenuated CVC increases at 41°C local temperature (ODQ, 54 ± 5% CVC(max); DMSO, 86 ± 4% CVC(max); Ringer, 90 ± 2% CVC(max); P < 0.05 ODQ vs. DMSO or Ringer). sGC participates in neurogenic active vasodilation during heat stress and in the local response to direct skin warming.

Acyloxy nitroso compounds as nitroxyl (HNO) donors: kinetics, reactions with thiols, and vasodilation properties

Acyloxy nitroso compounds hydrolyze to nitroxyl (HNO), a nitrogen monoxide with distinct chemistry and biology. Ultraviolet-visible spectroscopy and mass spectrometry show hydrolysis rate depends on pH and ester group structure with the observed rate being trifluoroacetate (3) > acetate (1) > pivalate (2). Under all conditions, 3 rapidly hydrolyzes to HNO. A combination of spectroscopic, kinetic, and product studies show that addition of thiols increases the decomposition rate of 1 and 2, leading to hydrolysis and HNO. Under conditions that favor thiolates, the thiolate directly reacts with the nitroso group, yielding oximes without HNO formation. Biologically, 3 behaves like Angeli's salt, demonstrating thiol-sensitive nitric oxide-mediated soluble guanylate cyclase-dependent vasorelaxation, suggesting HNO-mediated vasorelaxation. The slow HNO-donor 1 demonstrates weak thiol-insensitive vasorelaxation, indicating HNO release kinetics determine HNO bioavailability and activity. These results show that acyloxy nitroso compounds represent new HNO donors capable of vasorelaxation depending on HNO release kinetics.

Interactions of multiple gas-transducing systems: hallmarks and uncertainties of CO, NO, and H2S gas biology

The diverse physiological actions of the "biologic gases," O2, CO, NO, and H2S, have attracted much interest. Initially viewed as toxic substances, CO, NO, and H2S play important roles as signaling molecules. The multiplicity of gas actions and gas targets and the difficulty in measuring local gas concentrations obscures detailed mechanisms whereby gases exert their actions, and many questions remain unanswered. It is now readily apparent, however, that heme-based proteins play central roles in gas-generation/reception mechanisms and provide a point where multiple gases can interact. In this review, we consider a number of key issues related to "gas biology," including the effective tissue concentrations of these gases and the importance and significance of the physical proximity of gas-producing and gas-receptor/sensors. We also take an integrated approach to the interaction of gases by considering the physiological significance of CO, NO, and H2S on mitochondrial cytochrome c oxidase, a key target and central mediator of mitochondrial respiration. Additionally, we consider the effects of biologic gases on mitochondrial biogenesis and "suspended animation." By evaluating gas-mediated control functions from both in vitro and in vivo perspectives, we hope to elaborate on the complex multiple interactions of O2, NO, CO, and H2S.

Dynamic and interacting profiles of *NO and O2 in rat hippocampal slices

Nitric oxide (*NO) is a ubiquitous signaling molecule that participates in the neuromolecular phenomena associated with memory formation. In the hippocampus, neuronal *NO production is coupled to the activation of the NMDA-type of glutamate receptor. Although *NO-mediated signaling has been associated with soluble guanylate cyclase activation, cytochrome oxidase is also a target for this gaseous free radical, for which *NO competes with O(2). Here we show, for the first time in a model preserving tissue cytoarchitecture (rat hippocampal slices) and at a physiological O(2) concentration, that endogenous NMDA-evoked *NO production inhibits tissue O(2) consumption for submicromolar concentrations. The simultaneous real-time recordings reveal a direct correlation between the profiles of *NO and O(2) in the CA1 subregion of the hippocampal slice. These results, obtained in a system close to in vivo models, strongly support the current paradigm for O(2) and *NO interplay in the regulation of cellular respiration.

Alcohol does not modulate the augmented acetylcholine-induced vasodilatory response in hemorrhaged rodents

Our previous studies have shown that acute alcohol intoxication (AAI) decreases blood pressure, exacerbates hypotension after hemorrhagic shock, impairs the pressor response to fluid resuscitation, and blunts neuroendocrine activation. We hypothesized that impaired hemodynamic compensation during and after hemorrhagic shock in the acute alcohol-intoxicated host is the result of blunted neuroendocrine activation or, alternatively, of an impaired vascular responsiveness to vasoactive agents. The aim of this study was to examine the effects of AAI, AAI and hemorrhagic shock, and AAI and hemorrhagic shock and resuscitation on reactivity of isolated blood vessel rings to phenylephrine and acetylcholine. Chronically instrumented, conscious male Sprague-Dawley rats (300-350 g) received a primed continuous 15-h intragastric alcohol infusion (2.5 g x kg(-1) + 300 mg x kg(-1) x h(-1)), and time-matched controls received an isocaloric-isovolumic dextrose infusion. At completion of infusions, animals were randomized to sham, 60-min fixed-pressure hemorrhage, or hemorrhagic shock followed by resuscitation with lactated Ringer's solution. At the completion of the experimental protocols, animals were killed, and thoracic aorta and mesenteric artery ring segments (1-2 mm) were prepared and studied in myograph baths. Acute alcohol intoxication did not produce significant alterations in either pressor or dilator responses in aortic or mesenteric rings. These findings suggest that impaired hemodynamic counterregulation during hemorrhagic shock in AAI is not due to decreased vasopressor responsiveness. However, our results suggest a role for accentuated vasodilatory responses that may be central in progression to decompensatory shock.

The effects of nitric oxide on the immune system during Trypanosoma cruzi infection

Trypanosoma cruzi infection triggers substantial production of nitric oxide (NO), which has been shown to have protective and toxic effects on the host's immune system. Sensing of trypomastigotes by phagocytes activates the inducible NO-synthase (NOS2) pathway, which produces NO and is largely responsible for macrophage-mediated killing of T. cruzi. NO is also responsible for modulating virtually all steps of innate and adaptive immunity. However, NO can also cause oxidative stress, which is especially damaging to the host due to increased tissue damage. The cytokines IFN-gamma and TNF-alpha, as well as chemokines, are strong inducers of NOS2 and are produced in large amounts during T. cruzi acute infection. Conversely, TGF-beta and IL-10 negatively regulate NO production. Here we discuss the recent evidence describing the mechanisms by which NO is able to exert its antimicrobial and immune regulatory effects, the mechanisms involved in the oxidative stress response during infection and the implications of NO for the development of therapeutic strategies against T. cruzi.

A structural basis for H-NOX signaling in Shewanella oneidensis by trapping a histidine kinase inhibitory conformation

Heme nitric oxide/oxygen (H-NOX) proteins are found in eukaryotes where they are typically part of a larger protein such as soluble guanylate cyclase and in prokaryotes where they are often found in operons with a histidine kinase, suggesting that H-NOX proteins serve as sensors for NO and O(2) in signaling pathways. The Fe(II)-NO complex of the H-NOX protein from Shewanella oneidensis inhibits the autophosphorylation of the operon-associated histidine kinase, whereas the ligand-free H-NOX has no effect on the kinase. NMR spectroscopy was used to determine the structures of the Fe(II)-CO complex of the S. oneidensis H-NOX and the Fe(II)-CO complex of the H103G H-NOX mutant as a mimic of the ligand-free and kinase-inhibitory Fe(II)-NO H-NOX, respectively. The results provide a molecular glimpse into the ligand-induced conformational changes that may underlie kinase inhibition and the subsequent control of downstream signaling.

Nitric oxide/cGMP signalling induces Escherichia coli K1 receptor expression and modulates the permeability in human brain endothelial cell monolayers during invasion

Escherichia coli K1 invasion of human brain microvascular endothelial cells (HBMEC) mediated by outer membrane protein A (OmpA) results in the leakage of HBMEC monolayers. Despite the influence of nitric oxide (NO) in endothelial cell tight junction integrity, its role in E. coli-induced HBMEC monolayer permeability is poorly defined. Here, we demonstrate that E. coli invasion of HBMEC stimulates NO production by increasing the inducible nitric oxide synthase (iNOS) expression. Exposure to NO-producing agents enhanced the invasion of OmpA(+)E. coli and thereby increased the permeability of HBMEC. OmpA(+)E. coli-induced NO production lead to increased generation of cGMP and triggered the expression of OmpA receptor, Ec-gp96 in HBMEC. Pre-treatment of HBMEC with iNOS inhibitors or by introducing siRNA to iNOS, but not to eNOS or cGMP inhibitors abrogated the E. coli-induced expression of Ec-gp96. Overexpression of the C-terminal truncated Ec-gp96 in HBMEC prevented NO production and its downstream effector, cGMP generation and consequently, the invasion of OmpA(+)E. coli. NO/cGMP production also activates PKC-alpha, which is previously shown to be involved in HBMEC monolayer leakage. These results indicate that NO/cGMP signalling pathway plays a novel role in OmpA(+)E. coli invasion of HBMEC by enhancing the surface expression of Ec-gp96.

The effects of nitroxyl (HNO) on soluble guanylate cyclase activity: interactions at ferrous heme and cysteine thiols

It has been previously proposed that nitric oxide (NO) is the only biologically relevant nitrogen oxide capable of activating the enzyme soluble guanylate cyclase (sGC). However, recent reports implicate HNO as another possible activator of sGC. Herein, we examine the affect of HNO donors on the activity of purified bovine lung sGC and find that, indeed, HNO is capable of activating this enzyme. Like NO, HNO activation appears to occur via interaction with the regulatory ferrous heme on sGC. Somewhat unexpectedly, HNO does not activate the ferric form of the enzyme. Finally, HNO-mediated cysteine thiol modification appears to also affect enzyme activity leading to inhibition. Thus, sGC activity can be regulated by HNO via interactions at both the regulatory heme and cysteine thiols.

ADMA injures the glomerular filtration barrier: role of nitric oxide and superoxide

Chronic kidney disease (CKD) is associated with decreased renal nitric oxide (NO) production and increased plasma levels of methylarginines. The naturally occurring guanidino-methylated arginines N-monomethyl-l-arginine (l-NMMA) and asymmetric dimethyl-l-arginine (ADMA) inhibit NO synthase activity. We hypothesized that ADMA and l-NMMA compromise the integrity of the glomerular filtration barrier via NO depletion. We studied the effect of ADMA on albumin permeability (P(alb)) in isolated glomeruli and examined whether this effect involves NO- and superoxide (O(2)(*-))-dependent mechanisms. ADMA at concentrations found in circulation of patients with CKD decreased cGMP and increased P(alb) in a dose-dependent manner. A similar increase in P(alb) was caused by l-NMMA but at a concentration two orders of magnitude higher than that of ADMA. NO donor DETA-NONOate or cGMP analog abrogated the effect of ADMA on P(alb). The SOD mimetic tempol or the NAD(P)H oxidase inhibitor apocynin also prevented the ADMA-induced increase in P(alb). The NO-independent soluble guanylyl cyclase (sGC) activator BAY 41-2272, at concentrations that increased glomerular cGMP production, attenuated the ADMA-induced increase in P(alb). Furthermore, sGC incapacitation by the heme site-selective inhibitor ODQ increased P(alb). We conclude that ADMA compromises the integrity of the filtration barrier by altering the bioavailability of NO and O(2)(*-) and that NO-independent activation of sGC preserves the integrity of this barrier under conditions of NO depletion. NO-independent activation of sGS may be a useful pharmacotherapeutic approach for preservation of glomerular function in CKD thereby reducing the risk for cardiovascular events.

Iron-based redox switches in biology

By virtue of its unique electrochemical properties, iron makes an ideal redox active cofactor for many biologic processes. In addition to its important role in respiration, central metabolism, nitrogen fixation, and photosynthesis, iron also is used as a sensor of cellular redox status. Iron-based sensors incorporate Fe-S clusters, heme, and mononuclear iron sites to act as switches to control protein activity in response to changes in cellular redox balance. Here we provide an overview of iron-based redox sensor proteins, in both prokaryotes and eukaryotes, that have been characterized at the biochemical level. Although this review emphasizes redox sensors containing Fe-S clusters, proteins that use heme or novel iron sites also are discussed.

Interstitial cells of Cajal contain signalling molecules for transduction of nitrergic stimulation in guinea pig caecum

Nitric oxide (NO) is an inhibitory signalling molecule in the gastrointestinal (GI) tract that is released from neurons and from leucocytes during inflammation. NO stimulates soluble guanylate cyclase (sGC), elevates cyclic guanosine 3',5'-monophospate (cGMP), and subsequently activates cGMP-dependent protein kinase (PKG). Targets for NO in the guinea pig caecum were investigated by characterizing the cellular distribution of sGC, cGMP and PKG. Immunoreactivity for both isoforms of sGC, sGCalpha1 and sGCbeta1, was observed in the interstitial cells of Cajal (ICC) and enteric neurons in the tunica muscularis. Double labelling with anti-Kit and anti-sGC antibodies showed sGCalpha1 and sGCbeta1-like immunoreactivity (LI) in almost all intramuscular (IM) and myenteric ICC. Neuronal processes with neuronal NO synthase were closely apposed to ICC expressing sGC-LI. Cells with sGC-LI possessed ultrastructural features of ICC-IM: caveolae, close association with nerve bundles and contacts with smooth muscle cells (SMC). Sodium nitroprusside, added with the phosphodiesterase inhibitors (3-isobutyl-1-methylxanthine and zaprinast), enhanced cGMP-LI in almost all ICC and in some enteric neurons. Nerve stimulation also increased cGMP-LI in ICC and enteric neurons. In contrast, no resolvable increase in cGMP-LI was observed in any cells when the sGC inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one was present. ICC and SMC also expressed PKG type I-LI. These data show that ICC express the downstream signalling molecules necessary to transduce nitrergic signals and activate inhibitory pathways and thus are primary targets for NO released from neurons and other cells in the GI tract.

Two decades of new concepts in nitric oxide signaling: from the discovery of a gas messenger to the mediation of nonenzymatic posttranslational modifications

For the past 20 years, nitric oxide (NO) has established itself as a gaseous free radical with crucial and unpredicted roles in a wide spectrum of biological functions and organisms. We present here a case whereby NO-mediated signaling can be broadly classified into classical (cGMP-mediated) and nonclassical, the latter mainly alluding to posttranslational modifications related to NO and its interaction with reactive groups in proteins.

Heme-copper assembly mediated reductive coupling of nitrogen monoxide (*NO)

A iron-dinitrosyl species ((6)L)Fe(NO)(2) (2), generated from nitrogen monoxide (*NO) binding to its related iron(II)-mononitrosyl complex ((6)L)Fe(NO) (1), efficiently effects reductive coupling of two *NO molecules to release nitrous oxide (N(2)O), when Cu(+) ion and 2 equiv acid are added; the heme/Cu product is [((6)L)Fe(III)...Cu(II)(D)](3+) (D = H(2)O or MeCN). In a control experiment where only ((6)L)Fe(NO)(2) (2) is exposed to 2 equiv acid, no UV-vis change is observed; upon warming, *NO((g)) is released and ((6)L)Fe(NO) is reformed. The copper ion complex within the (6)L ligand framework is required for the *NO coupling chemistry. In a further control experiment Cu(+) ion is added to ((6)L)Fe(NO)(2) without acid present, [((6)L)Fe(NO)...Cu(II)(NO(2)(-))](+) is obtained, with the amount of N(2)O((g)) released fitting with copper(I) ion promoted disproportionation chemistry, 3*NO + ligand-Cu(I) --> N(2)O + ligand-Cu(II)(NO(2)(-)). The chemical system described represents a (stoichiometric) functional model for heme/Cu protein nitric oxide reductase activity.

Activation of aminoimidazole carcinogens by nitrosation: mutagenicity and nucleotide adducts

2-Amino-3-methylimidazo[4,5-f]quinoline (IQ) and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline(MeIQx) are heterocyclic amines (HCAs) derived from high temperature cooking of meat and thought to cause colon cancer in humans. Reactive nitrogen oxygen species, which are mediators of the inflammatory response, can convert these amines to the corresponding N-nitrosamines, N-NO-IQ and N-NO-MeIQx. This study was designed to evaluate whether these N-nitrosamines are genotoxic and could be responsible, in part, for the high incidence of colon cancer in individuals with colitis. Such an association would counsel reduced intake of well-done red meat by colitis patients. Mutagenicity was evaluated by reversion of a lacZ frameshift allele in three different E. coli strains. Strains DJ701 and DJ702 express recombinant(S. typhimurium) aromatic amine N-acetyltransferase (NAT); DJ702 also expresses recombinant human cytochrome P450 1A2 and NADPH-P450 reductase; and DJ2002 served as an N-acetyltransferase negative control. In strain DJ701, N-NO-IQ and N-NO-MeIQx elicited dose-dependent mutagenicity,which was not further increased in DJ702. Neither nitrosamine was mutagenic in strain DJ2002. While both N-nitrosamines are stable for >4 h (pH 7.4, 37 degrees C), they react with DNA or 2'-deoxyguanosine 3'-monophosphate at lower pH (5.5) to form adducts. HOCl, a component of the inflammatory response,increased adduct formation, as measured by 32P-postlabeling. Following treatment with nuclease P1and separation by two-dimensional thin-layer chromatography and then HPLC, N-NO-IQ and N-NOMeIQxwere shown to form the same adducts as those formed by N-OH-MeIQx or N-OH-IQ, namely N-(deoxyguanosin-8-yl) adducts. In summary, these N-nitrosamines are genotoxic and might be alternatives to their hydroxylamine analogues as activated intermediates leading to initiation of colon cancer in individuals with colitis.

Purification and characterization of recombinant human soluble guanylate cyclase produced from baculovirus-infected insect cells

Soluble guanylate cyclase (sGC) has been purified from 100 L cell culture infected by baculovirus using the newer and highly effective titerless infected-cells preservation and scale-up (TIPS) method. Successive passage of the enzyme through DEAE, Ni(2+)-NTA, and POROS Q columns obtained approximately 100mg of protein. The sGC obtained by this procedure was already about 90% pure and suitable for various studies which include high throughput screening (HTS) and hit follow-up. However, in order to obtain enzyme of greater homogeneity and purity for crystallographic and high precision spectroscopic and kinetic studies of sGC with select stimulators, the sGC solution after the POROS Q step was further purified by GTP-agarose affinity chromatography. This additional step led to the generation of 26 mg of enzyme that was about 99% pure. This highly pure and active enzyme exhibited a M(r)=144,933 by static light scattering supportive of a dimeric structure. It migrated as a two-band protein, each of equal intensity, on SDS-PAGE corresponding to the alpha (M(r) approximately 77,000) and beta (M(r) approximately 70,000) sGC subunits. It showed an A(430)/A(280)=1.01, indicating one heme per heterodimer, and a maximum of the Soret band at 430 nm indicative of a penta-coordinated ferrous heme with a histidine as the axial ligand. The Soret band shifted to 398 nm in the presence of an NO donor as expected for the formation of a penta-coordinated nitrosyl-heme complex. Non-stimulated sGC had k(cat)/K(m)=1.7 x 10(-3)s(-1)microM(-1) that increased to 5.8 x 10(-1)s(-1)microM(-1) upon stimulation with an NO donor which represents a 340-fold increase due to stimulation. The novel combination of using the TIPS method for co-expression of a heterodimeric heme-containing enzyme, along with the application of a reproducible ligand affinity purification method, has enabled us to obtain recombinant human sGC of both the quality and quantity needed to study structure-function relationships.

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.

Dynamical characterization of the heme NO oxygen binding (HNOX) domain. Insight into soluble guanylate cyclase allosteric transition

Since the discovery of soluble guanylate cyclase (sGC) as the mammalian receptor for nitric oxide (NO), numerous studies have been performed in order to understand how sGC transduces the NO signal. However, the structural basis of sGC activation is still not completely elucidated. Spectroscopic and kinetic studies showed that the key step in the activation mechanism was the NO-induced breaking of the iron proximal histidine bond in the so-called 6c-NO to 5c-NO transition. The main breakthrough in the understanding of sGC activation mechanism came, however, from the elucidation of crystal structures for two different prokaryotic heme NO oxygen (HNOX) domains, which are homologues to the sGC heme domain. In this work we present computer simulation results of Thermoanaerobacter tencogensis HNOX that complement these structural studies, yielding molecular explanations to several poorly understood properties of these proteins. Specifically, our results explain the differential ligand binding patterns of the HNOX domains according to the nature of proximal and distal residues. We also show that the natural dynamics of these proteins is intimately related with the proposed conformational dependent activation process, which involves mainly the alphaFbeta1 loop and the alphaA-alphaC distal subdomain. The results from the sGC models also support this view and suggest a key role for the alphaFbeta1 loop in the iron proximal histidine bond breaking process and, therefore, in the sGC activation mechanism.

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.

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.

Nonequilibrium dynamics simulations of nitric oxide release: comparative study of nitrophorin and myoglobin

Nitrophorin 4 (NP4) is a heme protein that reversibly binds nitric oxide (NO), with release rates modulated by pH change. High-resolution structures of NP4 revealed that pH changes and NO binding induce a large conformational rearrangement in two loops that serve to protect the heme-bound NO molecule from solvent. We used extended (110 ns) molecular dynamics simulations of NP4 at pH 5 and pH 7, modeled by selective deprotonation of acidic groups. Conformational and dynamic changes were observed, consistent with those found in the crystal. Further, major solvent movement and NO escape were observed at pH 7, while the ligand remained in the heme binding pocket at pH 5. As a control, we also performed molecular dynamics (MD) simulations of sperm whale myoglobin, where NO migration into the interior cavities of the protein was observed, consistent with previous reports. We constructed a kinetic model of ligand escape to quantitatively relate the microscopic rate constants to the observed rates, and tested the predictions against the experimental data. The results suggest that release rates of diatomic molecules from heme proteins can be varied by several orders of magnitude through modest adjustments in geminate rebinding and gating behavior.

S-nitrosylation-induced conformational change in blackfin tuna myoglobin

S-nitrosylation is a post-translational protein modification that can alter the function of a variety of proteins. Despite the growing wealth of information that this modification may have important functional consequences, little is known about the structure of the moiety or its effect on protein tertiary structure. Here we report high-resolution x-ray crystal structures of S-nitrosylated and unmodified blackfin tuna myoglobin, which demonstrate that in vitro S-nitrosylation of this protein at the surface-exposed Cys-10 directly causes a reversible conformational change by "wedging" apart a helix and loop. Furthermore, we have demonstrated in solution and in a single crystal that reduction of the S-nitrosylated myoglobin with dithionite results in NO cleavage from the sulfur of Cys-10 and rebinding to the reduced heme iron, showing the reversibility of both the modification and the conformational changes. Finally, we report the 0.95-A structure of ferrous nitrosyl myoglobin, which provides an accurate structural view of the NO coordination geometry in the context of a globin heme pocket.

Structural and thermodynamic consequences of b heme binding for monomeric apoglobins and other apoproteins

The binding of a cofactor to a protein matrix often involves a reorganization of the polypeptide structure. b Hemoproteins provide multiple examples of this behavior. In this minireview, selected monomeric and single b heme proteins endowed with distinct topological properties are inspected for the extent of induced refolding upon heme binding. To complement the data reported in the literature, original results are presented on a two-on-two globin of cyanobacterial origin (Synechococcus sp. PCC 7002 GlbN) and on the heme-containing module of FixL, an oxygen-sensing protein with the mixed alpha/beta topology of PAS domains. GlbN had a stable apoprotein that was further stabilized and locally refolded by heme binding; in contrast, apoFixLH presented features of a molten globule. Sequence analyses (helicity, disorder, and polarity) and solvent accessibility calculations were performed to identify trends in the architecture of b hemoproteins. In several cases, the primary structure appeared biased toward a partially disordered binding pocket in the absence of the cofactor.