Electrochemical Evidence for Neuroglobin Activity on NO at Physiological Concentrations

Hydrogen peroxide induces heme degradation and protein aggregation in human neuroglobin: roles of the disulfide bridge and hydrogen-bonding in the distal heme cavity

In the present study, human neuroglobin (hNgb) was found to undergo H₂ O₂ -induced breakdown of the heme center at a much slower rate than other globins, namely in the timescale of hours against minutes. We investigated how the rate of the process is affected by the Cys46/Cys55 disulfide bond and the network of non-covalent interactions in the distal heme side involving Tyr44, Lys67, the His64 heme iron axial ligand and the heme propionate-7. The rate is increased by the Tyr44 to Ala and Phe mutations; however the rate is lowered by Lys67 to Ala swapping. The absence of the disulfide bridge slows down the reaction further. Therefore, the disulfide bond-controlled accessibility of the heme site and the residues at position 44 and 67 affect the activation barrier of the reaction. Wild-type and mutated species form β-amyloid aggregates in the presence of H₂ O₂ producing globular structures. Furthermore, the C46A/C55A, Y44A, Y44F and Y44F/C46A/C55A variants yield potentially harmful fibrils. Finally, the nucleation and growth kinetics for the aggregation of the amyloid structures can be successfully described by the Finke-Watzky model.

GLB-3: A resilient, cysteine-rich, membrane-tethered globin expressed in the reproductive and nervous system of Caenorhabditis elegans

The popular genetic model organism Caenorhabditis elegans (C. elegans) encodes 34 globins, whereby the few that are well-characterized show divergent properties besides the typical oxygen carrier function. Here, we present a biophysical characterization and expression analysis of C. elegans globin-3 (GLB-3). GLB-3 is predicted to exist in two isoforms and is expressed in the reproductive and nervous system. Knockout of this globin causes a 99% reduction in fertility and reduced motility. Spectroscopic analysis reveals that GLB-3 exists as a bis-histidyl-ligated low-spin form in both the ferrous and ferric heme form. A function in binding of diatomic gases is excluded on the basis of the slow CO-binding kinetics. Unlike other globins, GLB-3 is also not capable of reacting with H₂O₂, H₂S, and nitrite. Intriguingly, not only does GLB-3 contain a high number of cysteine residues, it is also highly stable under harsh conditions (pH = 2 and high concentrations of H₂O₂). The resilience diminishes when the N- and C-terminal extensions are removed. Redox potentiometric measurements reveal a slightly positive redox potential (+8 ± 19 mV vs. SHE), suggesting that the heme iron may be able to oxidize cysteines. Electron paramagnetic resonance shows that formation of an intramolecular disulphide bridge, involving Cys70, affects the heme-pocket region. The results suggest an involvement of the globin in (cysteine) redox chemistry.

Carbon Monoxide Signaling: Examining Its Engagement with Various Molecular Targets in the Context of Binding Affinity, Concentration, and Biologic Response

Carbon monoxide (CO) has been firmly established as an endogenous signaling molecule with a variety of pathophysiological and pharmacological functions, including immunomodulation, organ protection, and circadian clock regulation, among many others. In terms of its molecular mechanism(s) of action, CO is known to bind to a large number of hemoproteins with at least 25 identified targets, including hemoglobin, myoglobin, neuroglobin, cytochrome c oxidase, cytochrome P450, soluble guanylyl cyclase, myeloperoxidase, and some ion channels with dissociation constant values spanning the range of sub-nM to high μM. Although CO's binding affinity with a large number of targets has been extensively studied and firmly established, there is a pressing need to incorporate such binding information into the analysis of CO's biologic response in the context of affinity and dosage. Especially important is to understand the reservoir role of hemoglobin in CO storage, transport, distribution, and transfer. We critically review the literature and inject a sense of quantitative assessment into our analyses of the various relationships among binding affinity, CO concentration, target occupancy level, and anticipated pharmacological actions. We hope that this review presents a picture of the overall landscape of CO's engagement with various targets, stimulates additional research, and helps to move the CO field in the direction of examining individual targets in the context of all of the targets and the concentration of available CO. We believe that such work will help the further understanding of the relationship of CO concentration and its pathophysiological functions and the eventual development of CO-based therapeutics. SIGNIFICANCE STATEMENT: The further development of carbon monoxide (CO) as a therapeutic agent will significantly rely on the understanding of CO's engagement with therapeutically relevant targets of varying affinity. This review critically examines the literature by quantitatively analyzing the intricate relationships among targets, target affinity for CO, CO level, and the affinity state of carboxyhemoglobin and provide a holistic approach to examining the molecular mechanism(s) of action for CO.

Ordered Motions in the Nitric-Oxide Dioxygenase Mechanism of Flavohemoglobin and Assorted Globins with Tightly Coupled Reductases

Nitric-oxide dioxygenases (NODs) activate and combine O₂ with NO to form nitrate. A variety of oxygen-binding hemoglobins with associated partner reductases or electron donors function as enzymatic NODs. Kinetic and structural investigations of the archetypal two-domain microbial flavohemoglobin-NOD have illuminated an allosteric mechanism that employs selective tunnels for O₂ and NO, gates for NO and nitrate, transient O₂ association with ferric heme, and an O₂ and NO-triggered, ferric heme spin crossover-driven, motion-controlled, and dipole-regulated electron-transfer switch. The proposed mechanism facilitates radical-radical coupling of ferric-superoxide with NO to form nitrate while preventing suicidal ferrous-NO formation. Diverse globins display the structural and functional motifs necessary for a similar allosteric NOD mechanism. In silico docking simulations reveal monomeric erythrocyte hemoglobin alpha-chain and beta-chain intrinsically matched and tightly coupled with NADH-cytochrome b₅ oxidoreductase and NADPH-cytochrome P450 oxidoreductase, respectively, forming membrane-bound flavohemoglobin-like mammalian NODs. The neuroprotective neuroglobin manifests a potential NOD role in a close-fitting ternary complex with membrane-bound NADH-cytochrome b₅ oxidoreductase and cytochrome b₅. Cytoglobin interfaces weakly with cytochrome b₅ for O₂ and NO-regulated electron-transfer and coupled NOD activity. The mechanistic model also provides insight into the evolution of O₂ binding cooperativity in hemoglobin and a basis for the discovery of allosteric NOD inhibitors.

Structural and (Pseudo-)Enzymatic Properties of Neuroglobin: Its Possible Role in Neuroprotection

Neuroglobin (Ngb), the third member of the globin family, was discovered in human and murine brains in 2000. This monomeric globin is structurally similar to myoglobin (Mb) and hemoglobin (Hb) α and β subunits, but it hosts a bis-histidyl six-coordinated heme-Fe atom. Therefore, the heme-based reactivity of Ngb is modulated by the dissociation of the distal HisE7-heme-Fe bond, which reflects in turn the redox state of the cell. The high Ngb levels (~100–200 μM) present in the retinal ganglion cell layer and in the optic nerve facilitate the O₂ buffer and delivery. In contrast, the very low levels of Ngb (~1 μM) in most tissues and organs support (pseudo-)enzymatic properties including NO/O₂ metabolism, peroxynitrite and free radical scavenging, nitrite, hydroxylamine, hydrogen sulfide reduction, and the nitration of aromatic compounds. Here, structural and (pseudo-)enzymatic properties of Ngb, which are at the root of tissue and organ protection, are reviewed, envisaging a possible role in the protection from neuronal degeneration of the retina and the optic nerve.

Utility of fluorescent heme analogue ZnPPIX to monitor conformational heterogeneity in vertebrate hexa-coordinated globins

Here, we report the preparation and photo-physical characterization of hexa-coordinated vertebrate globins, human neuroglobin (hNgb) and cytoglobin (hCygb), with the native iron protoporphyrin IX (FePPIX) cofactor replaced by a fluorescent isostructural analogue, zinc protoporphyrin IX (ZnPPIX). To facilitate insertion of ZnPPIX into hexa-coordinated globins, apoproteins prepared via butanone extraction were unfolded by the addition of GuHCl and subsequently slowly refolded in the presence of ZnPPIX. The absorption/emission spectra of ZnPPIX reconstituted hCygb are similar to those observed for ZnPPIX reconstituted myoglobin whereas the absorption and emission spectra of ZnPPIX reconstituted hNgb are blue shifted by ∼2 nm. Different steady state absorption and emission properties of ZnPPIX incorporated in hCygb and hNgb are consistent with distinct hydrogen bonding interactions between ZnPPIX and the globin matrix. The fluorescence lifetime of ZnPPIX in hexa-coordinated globins is bimodal pointing towards increased heterogeneity of the heme binding cavity in hCygb and hNgb. ZnPPIX reconstituted Ngb binds to cytochrome c with the same affinity as reported for the native protein, suggesting that fluorescent analogues of Cygb and Ngb can be readily employed to monitor interactions between vertebrate hexa-coordinated globins and other proteins.

Enhancement of protein stability by an additional disulfide bond designed in human neuroglobin

Human neuroglobin (Ngb) forms an intramolecular disulfide bond between Cys46 and Cys55, with a third Cys120 near the protein surface, which is a promising protein model for heme protein design. In order to protect the free Cys120 and to enhance the protein stability, we herein developed a strategy by designing an additional disulfide bond between Cys120 and Cys15 via A15C mutation. The design was supported by molecular modeling, and the formation of Cys15–Cys120 disulfide bond was confirmed experimentally by ESI-MS analysis. Molecular modeling, UV-Vis and CD spectroscopy showed that the additional disulfide bond caused minimal structural alterations of Ngb. Meanwhile, the disulfide bond of Cys15–Cys120 was found to enhance both Gdn·HCl-induced unfolding stability (increased by ∼0.64 M) and pH-induced unfolding stability (decreased by ∼0.69 pH unit), as compared to those of WT Ngb with a single native disulfide bond of Cys46–Cys55. Moreover, the half denaturation temperature (T_m) of A15C Ngb was determined to be higher than 100 °C. In addition, the disulfide bond of Cys15–Cys120 has slight effects on protein function, such as an increase in the rate of O₂ release by ∼1.4-fold. This study not only suggests a crucial role of the artificial disulfide in protein stabilization, but also lays the groundwork for further investigation of the structure and function of Ngb, as well as for the design of other functional heme proteins, based on the scaffold of A15C Ngb with an enhanced stability.

A disulfide bond of Cys120 and Cys15 was rationally designed in human neuroglobin (Ngb) by A15C mutation, which caused minimal structural alterations, whereas enhanced both chemical and pH stability, with a thermal stability higher than 100 °C.

The NO-heme signaling hypothesis

While the biological role of nitric oxide (NO) synthase (NOS) is appreciated, several fundamental aspects of the NOS/NO-related signaling pathway(s) remain incompletely understood. Canonically, the NOS-derived NO diffuses through the (inter)cellular milieu to bind the prosthetic ferro(Fe²⁺)-heme group of the soluble guanylyl cyclase (sGC). The formation of ternary NO-ferroheme-sGC complex results in the enzyme activation and accelerated production of the second messenger, cyclic GMP. This paper argues that cells dynamically generate mobile/exchangeable NO-ferroheme species, which activate sGC and regulate the function of some other biomolecules. In contrast to free NO, the mobile NO-ferroheme may ensure safe, efficient and coordinated delivery of the signal within and between cells. The NO-heme signaling may contribute to a number of NOS/NO-related phenomena (e.g. nitrite bioactivity, selective protein S-(N-)nitrosation, endothelium and erythrocyte-dependent vasodilation, some neural and immune NOS functions) and predicts new NO-related discoveries, diagnostics and therapeutics.

Topography of human cytochrome b5/cytochrome b5 reductase interacting domain and redox alterations upon complex formation

Cytochrome b₅ is the main electron acceptor of cytochrome b₅ reductase. The interacting domain between both human proteins has been unidentified up to date and very little is known about its redox properties modulation upon complex formation. In this article, we characterized the protein/protein interacting interface by solution NMR and molecular docking. In addition, upon complex formation, we measured an increase of cytochrome b₅ reductase flavin autofluorescence that was dependent upon the presence of cytochrome b_5. Data analysis of these results allowed us to calculate a dissociation constant value between proteins of 0.5±0.1μM and a 1:1 stoichiometry for the complex formation. In addition, a 30mV negative shift of cytochrome b₅ reductase redox potential in presence of cytochrome b₅ was also measured. These experiments suggest that the FAD group of cytochrome b₅ reductase increase its solvent exposition upon complex formation promoting an efficient electron transfer between the proteins.

Efficient Reduction of Vertebrate Cytoglobins by the Cytochrome b5/Cytochrome b5 Reductase/NADH System

Cytoglobin is a heme-containing protein ubiquitous in mammalian tissues. Unlike the evolutionarily related proteins hemoglobin and myoglobin, cytoglobin shows a six-coordinated heme binding, with the heme iron coordinated by two histidine side chains. Cytoglobin is involved in cytoprotection pathways through yet undefined mechanisms, and it has recently been demonstrated that cytoglobin has redox signaling properties via nitric oxide (NO) and nitrite metabolism. The reduced, ferrous cytoglobin can bind oxygen and will react with NO in a dioxygenation reaction to form nitrate, which dampens NO signaling. When deoxygenated, cytoglobin can bind nitrite and reduce it to NO. This oxidoreductase activity could be catalytic if an effective reduction system exists to regenerate the reduced heme species. The nature of the physiological cytoglobin reducing system is unknown, although it has been proposed that ascorbate and cytochrome b₅ could fulfill this role. Here we describe that physiological concentrations of cytochrome b₅ and cytochrome b₅ reductase can reduce human and fish cytoglobins at rates up to 250-fold higher than those reported for their known physiological substrates, hemoglobin and myoglobin, and up to 100-fold faster than 5 mM ascorbate. These data suggest that the cytochrome b₅/cytochrome b₅ reductase system is a viable reductant for cytoglobin in vivo, allowing for catalytic oxidoreductase activity.

Determinants of neuroglobin plasticity highlighted by joint coarse-grained simulations and high pressure crystallography

Investigating the effect of pressure sheds light on the dynamics and plasticity of proteins, intrinsically correlated to functional efficiency. Here we detail the structural response to pressure of neuroglobin (Ngb), a hexacoordinate globin likely to be involved in neuroprotection. In murine Ngb, reversible coordination is achieved by repositioning the heme more deeply into a large internal cavity, the “heme sliding mechanism”. Combining high pressure crystallography and coarse-grain simulations on wild type Ngb as well as two mutants, one (V101F) with unaffected and another (F106W) with decreased affinity for CO, we show that Ngb hinges around a rigid mechanical nucleus of five hydrophobic residues (V68, I72, V109, L113, Y137) during its conformational transition induced by gaseous ligand, that the intrinsic flexibility of the F-G loop appears essential to drive the heme sliding mechanism, and that residue Val 101 may act as a sensor of the interaction disruption between the heme and the distal histidine.

Neuroglobin Can Prevent or Reverse Glaucomatous Progression in DBA/2J Mice

Mitochondrial dysfunction is responsible for hereditary optic neuropathies. We wished to determine whether preserving mitochondrial bioenergetics could prevent optic neuropathy in a reliable model of glaucoma. DBA/2J mice exhibit elevated intraocular pressure, progressive degeneration of their retinal ganglion cells, and optic neuropathy that resembles glaucoma. We established that glaucoma in these mice is directly associated with mitochondrial dysfunction: respiratory chain activity was compromised in optic nerves 5 months before neuronal loss began, and the amounts of some mitochondrial proteins were reduced in retinas of glaucomatous mice. One of these proteins is neuroglobin, which has a neuroprotective function. Therefore, we investigated whether gene therapy aimed at restoring neuroglobin levels in the retina via ocular administration of an adeno-associated viral vector could reduce neuronal degeneration. The approach of treating 2-month-old mice impeded glaucoma development: few neurons died and respiratory chain activity and visual cortex activity were comparable to those in young, asymptomatic mice. When the treatment was performed in 8-month-old mice, the surviving neurons acquired new morphologic and functional properties, leading to the preservation of visual cortex activity and respiratory chain activity. The beneficial effects of neuroglobin in DBA/2J retinas confirm this protein to be a promising candidate for treating glaucoma.

Neuroglobin, a Factor Playing for Nerve Cell Survival

Cell death represents the final outcome of several pathological conditions of the central nervous system and available evidence suggests that in both acute injuries and neurodegenerative diseases it is often associated with mitochondrial dysfunction. Thus, the possibility to prevent mitochondrial events involved in cell death might represent efficient tools to limit neuronal damage. In recent years, increased attention has been paid to the endogenous protein neuroglobin, since accumulating evidence showed that its high expression was associated with preserved mitochondrial function and to an increased survival of nerve cells in vitro and in vivo in a variety of experimental models of cell insult. The biological and structural features of neuroglobin and the mitochondria-related mechanisms of neuroglobin-induced neuroprotection will be here briefly discussed. In this respect, the inhibition of the intrinsic pathway of apoptosis emerges as a key neuroprotective effect induced by the protein. These findings could open the possibility to develop efficient neuroglobin-mediated therapeutic strategies aimed at minimizing the neuronal cell death occurring in impacting neurological pathologies like stroke and neurodegenerative diseases.