Expression, purification, and crystallization of neuro- and cytoglobin

The effect of pH and nitrite on the haem pocket of GLB-33, a globin-coupled neuronal transmembrane receptor of Caenorhabditis elegans

Out of the 34 globins in Caenorhabditis elegans, GLB-33 is a putative globin-coupled transmembrane receptor with a yet unknown function. The globin domain (GD) contains a particularly hydrophobic haem pocket, that rapidly oxidizes to a low-spin hydroxide-ligated haem state at physiological pH. Moreover, the GD has one of the fastest nitrite reductase activity ever reported for globins. Here, we use a combination of electronic circular dichroism, resonance Raman and electron paramagnetic resonance (EPR) spectroscopy with mass spectrometry to study the pH dependence of the ferric form of the recombinantly over-expressed GD in the presence and absence of nitrite. The competitive binding of nitrite and hydroxide is examined as well as nitrite-induced haem modifications at acidic pH. Comparison of the spectroscopic results with data from other haem proteins allows to deduce the important effect of Arg at position E10 in stabilization of exogenous ligands. Furthermore, continuous-wave and pulsed EPR indicate that ligation of nitrite occurs in a nitrito mode at pH 5.0 and above. At pH 4.0, an additional formation of a nitro-bound haem form is observed along with fast formation of a nitri-globin.

Expression of Recombinant Cold-Adapted (Hemo)Globins from Marine Bacteria

The production of recombinant proteins in bacteria made possible to obtain large quantities of proteins essential for basic and applied research. Escherichia coli remains one of the organisms of choice for recombinant proteins because of its ability to grow at high density and availability of a vast catalog of cloning vectors and mutant host strains. Here, we describe the protocols for the expression of cold-adapted (hemo)globins in Escherichia coli.

Structural modeling of a novel membrane-bound globin-coupled sensor in Geobacter sulfurreducens

Globin-coupled sensors (GCS) usually consist of three domains: a sensor/globin, a linker, and a transmitter domain. The globin domain (GD), activated by ligand binding and/or redox change, induces an intramolecular signal transduction resulting in a response of the transmitter domain. Depending on the nature of the transmitter domain, GCSs can have different activities and functions, including adenylate and di-guanylate cyclase, histidine kinase activity, aerotaxis and/or oxygen sensing function. The gram-negative delta-proteobacterium Geobacter sulfurreducens expresses a protein with a GD covalently linked to a four transmembrane domain, classified, by sequence similarity, as GCS (GsGCS). While its GD is fully characterized, not so its transmembrane domain, which is rarely found in the globin superfamily. In the present work, GsGCS was characterized spectroscopically and by native ion mobility-mass spectrometry in combination with cryo-electron microscopy. Although lacking high resolution, the oligomeric state and the electron density map were valuable for further rational modeling of the full-length GsGCS structure. This model demonstrates that GsGCS forms a transmembrane domain-driven tetramer with minimal contact between the GDs and with the heme groups oriented outward. This organization makes an intramolecular signal transduction less likely. Our results, including the auto-oxidation rate and redox potential, suggest a potential role for GsGCS as redox sensor or in a membrane-bound e-/H+ transfer. As such, GsGCS might act as a player in connecting energy production to the oxidation of organic compounds and metal reduction. Database searches indicate that GDs linked to a four or seven helices transmembrane domain occur more frequently than expected.

Native mass spectrometry for the design and selection of protein bioreceptors for perfluorinated compounds

Biosensing platforms are answering the increasing demand for analytical tools for environmental monitoring of small molecules, such as per- and polyfluoroalkyl substances (PFAS). By transferring toxicological findings in bioreceptor design we can develop innovative pathways for biosensor design. Indeed, toxicological studies provide fundamental information about PFAS-biomolecule complexes that can help evaluate the applicability of the latter as bioreceptors. The toolbox of native mass spectrometry (MS) can support this evaluation, as shown by the two case studies reported in this work. The analysis of model proteins' (i.e. albumin, haemoglobin, cytochrome c and neuroglobin) interactions with well-known PFAS, such as perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), demonstrated the potential of this native MS screening approach. In the first case study, untreated albumin and delipidated albumin were compared in the presence and absence of PFOA confirming that the delipidation step increases albumin affinity for PFOA without affecting protein stability. In the second case study, the applicability of our methodology to identify potential bioreceptors for PFOS/PFOA was extended to other proteins. Structurally related haemoglobin and neuroglobin revealed a 1 : 1 complex, whereas no binding was observed for cytochrome c. These studies have value as a proof-of-concept for a general application of native MS to identify bioreceptors for toxic compounds.

Structural and Functional Characterization of the Globin-Coupled Sensors of Azotobacter vinelandii and Bordetella pertussis

Aims: Structural and functional characterization of the globin-coupled sensors (GCSs) from Azotobacter vinelandii (AvGReg) and Bordetella pertussis (BpeGReg). Results: Ultraviolet/visible and resonance Raman spectroscopies confirm the presence in AvGReg and BpeGReg of a globin domain capable of reversible gaseous ligand binding. In AvGReg, an influence of the transmitter domain on the heme proximal region of the globin domain can be seen, and k'_CO is higher than for other GCSs. The O₂ binding kinetics suggests the presence of an open and a closed conformation. As for BpeGReg, the fully oxygenated AvGReg show a very high diguanylate cyclase activity. The carbon monoxide rebinding to BpeGReg indicates that intra- and intermolecular interactions influence the ligand binding. The globin domains of both proteins (AvGReg globin domain and BpeGRegGb with cysteines (Cys16, 45, 114, 154) mutated to serines [BpeGReg-Gb*]) share the same GCS fold, a similar proximal but a different distal side structure. They homodimerize through a G-H helical bundle as in other GCSs. However, BpeGReg-Gb* shows also a second dimerization mode. Innovation: This article extends our knowledge on the GCS proteins and contributes to a better understanding of the GCSs role in the formation of bacterial biofilms. Conclusions:AvGReg and BpeGReg conform to the GCS family, share a similar overall structure, but they have different properties in terms of the ligand binding. In particular, AvGReg shows an open and a closed conformation that in the latter form will very tightly bind oxygen. BpeGReg has only one closed conformation. In both proteins, it is the fully oxygenated GCS form that catalyzes the production of the second messenger.

The effect of reactive oxygen and nitrogen species on the structure of cytoglobin: A potential tumor suppressor

Many current anti-cancer therapies rely on increasing the intracellular reactive oxygen and nitrogen species (RONS) contents with the aim to induce irreparable damage, which subsequently results in tumor cell death. A novel tool in cancer therapy is the use of cold atmospheric plasma (CAP), which has been found to be very effective in the treatment of many different cancer cell types in vitro as well as in vivo, mainly through the vast generation of RONS. One of the key determinants of the cell's fate will be the interaction of RONS, generated by CAP, with important proteins, i.e. redox-regulatory proteins. One such protein is cytoglobin (CYGB), a recently discovered globin proposed to be involved in the protection of the cell against oxidative stress. In this study, the effect of plasma-produced RONS on CYGB was investigated through the treatment of CYGB with CAP for different treatment times. Spectroscopic analysis of CYGB showed that although chemical modifications occur, its secondary structure remains intact. Mass spectrometry experiments identified these modifications as oxidations of mainly sulfur-containing and aromatic amino acids. With longer treatment time, the treatment was also found to induce nitration of the heme. Furthermore, the two surface-exposed cysteine residues of CYGB were oxidized upon treatment, leading to the formation of intermolecular disulfide bridges, and potentially also intramolecular disulfide bridges. In addition, molecular dynamics and docking simulations confirmed, and further show, that the formation of an intramolecular disulfide bond, due to oxidative conditions, affects the CYGB 3D structure, thereby opening the access to the heme group, through gate functioning of His₁₁₇. Altogether, the results obtained in this study (1) show that plasma-produced RONS can extensively oxidize proteins and (2) that the oxidation status of two redox-active cysteines lead to different conformations of CYGB.

Electrochemical Evidence for Neuroglobin Activity on NO at Physiological Concentrations

The true function of neuroglobin (Ngb) and, particularly, human Ngb (NGB) has been under debate since its discovery 15 years ago. It has been expected to play a role in oxygen binding/supply, but a variety of other functions have been put forward, including NO dioxygenase activity. However, in vitro studies that could unravel these potential roles have been hampered by the lack of an Ngb-specific reductase. In this work, we used electrochemical measurements to investigate the role of an intermittent internal disulfide bridge in determining NO oxidation kinetics at physiological NO concentrations. The use of a polarized electrode to efficiently interconvert the ferric (Fe(3+)) and ferrous (Fe(2+)) forms of an immobilized NGB showed that the disulfide bridge both defines the kinetics of NO dioxygenase activity and regulates appearance of the free ferrous deoxy-NGB, which is the redox active form of the protein in contrast to oxy-NGB. Our studies further identified a role for the distal histidine, interacting with the hexacoordinated iron atom of the heme, in oxidation kinetics. These findings may be relevant in vivo, for example, in blocking apoptosis by reduction of ferric cytochrome c, and gentle tuning of NO concentration in the tissues.

Characterization of zebrafish neuroglobin and cytoglobins 1 and 2: Zebrafish cytoglobins provide insights into the transition from six-coordinate to five-coordinate globins

Neuroglobin (Ngb) and cytoglobin (Cygb) are two six-coordinate heme proteins of unknown physiological function. Although studies on the mammalian proteins have elucidated aspects of Ngb and Cygb biophysics and indicated potential functions, the properties of non-mammalian Ngbs and Cygbs are largely uncharacterized. We have expressed the recombinant zebrafish proteins Ngb, Cygb1, and Cygb2 in Escherichia coli and characterized their nitrite reduction rates, spectral properties, autoxidation rate constants, redox potentials and lipid binding properties. The three zebrafish proteins can catalyze the reduction of nitrite to nitric oxide with a broad range of reaction rate constants. (Ngb, 0.68 ± 0.04 M(-1) s(-1); Cygb1, 28.6 ± 3.1 M(-1) s(-1); Cygb2, 0.94 ± 0.18 M(-1) s(-1)). We observe that zebrafish Ngb and Cygb2 have comparable spectral features to those of human Ngb and Cygb, consistent with a six-coordinate heme, whereas unexpectedly Cygb1 has a five-coordinate heme, a slower autoxidation and in general has properties more akin to oxygen transport proteins. In agreement with a possible oxygen carrier and nitrite reductase role, we detect mRNA transcript for Cygb1 but not Cygb2 or Ngb in zebrafish blood. Unlike human Cygb, neither of the zebrafish globins binds oleic acid with high affinity. This finding suggests that lipid binding may be a trait acquired later during evolution and not an ancestral property of cytoglobins. Altogether, our results uncover unexpected properties of zebrafish globins and reveal the pivotal role of cytoglobins in the transition of heme globins from six-coordinate to five-coordinate oxygen carriers and nitrite reductases.

Kinetic properties and heme pocket structure of two domains of the polymeric hemoglobin of Artemia in comparison with the native molecule

In this project, we studied some physicochemical properties of two different globin domains of the polymeric hemoglobin of the brine shrimp Artemia salina and compared them with those of the native molecule. Two domains (AsHbC1D1 and AsHbC1D5) were cloned and expressed in BL21(DE3)pLysS strain of Escherichia coli. The recombinant proteins as well as the native hemoglobin (AfHb) were purified from bacteria and frozen Artemia, respectively by standard chromatographic methods and assessed by SDS-PAGE. The heme environment of these proteins was studied by optical spectroscopy and ligand-binding kinetics (e.g. CO association and O2 binding affinity) were measured for the two recombinant proteins and the native hemoglobin. This indicates that the CO association rate for AsHbC1D1 is higher than that of AsHbC1D5 and AfHb, while the calculated P50 value for AsHbC1D1 is lower than that of AsHbC1D5 and AfHb. The geminate and bimolecular rebinding parameters indicate a significant difference between both domains. Moreover, EPR results showed that the heme pocket in AfHb is in a more closed conformation than the heme pocket in myoglobin. Finally, the reduction potential of -0.13V versus the standard hydrogen electrode was determined for AfHb by direct electrochemical measurements. It is about 0.06V higher than the potential of the single domain AsHbC1D5. This work shows that each domain in the hemoglobin of Artemia has different characteristics of ligand binding.

Exploring the mechanisms of the reductase activity of neuroglobin by site-directed mutagenesis of the heme distal pocket

Neuroglobin (Ngb) is a six-coordinate globin that can catalyze the reduction of nitrite to nitric oxide. Although this reaction is common to heme proteins, the molecular interactions in the heme pocket that regulate this reaction are largely unknown. We have shown that the H64L Ngb mutation increases the rate of nitrite reduction by 2000-fold compared to that of wild-type Ngb [Tiso, M., et al. (2011) J. Biol. Chem. 286, 18277–18289]. Here we explore the effect of distal heme pocket mutations on nitrite reduction. For this purpose, we have generated mutations of Ngb residues Phe28(B10), His64(E7), and Val68(E11). Our results indicate a dichotomy in the reactivity of deoxy five- and six-coordinate globins toward nitrite. In hemoglobin and myoglobin, there is a correlation between faster rates and more negative potentials. However, in Ngb, reaction rates are apparently related to the distal pocket volume, and redox potential shows a poor relationship with the rate constants. This suggests a relationship between the nitrite reduction rate and heme accessibility in Ngb, particularly marked for His64(E7) mutants. In five-coordinate globins, His(E7) facilitates nitrite reduction, likely through proton donation. Conversely, in Ngb, the reduction mechanism does not rely on the delivery of a proton from the histidine side chain, as His64 mutants show the fastest reduction rates. In fact, the rate observed for H64A Ngb (1120 M^–1 s^–1) is to the best of our knowledge the fastest reported for a heme nitrite reductase. These differences may be related to a differential stabilization of the iron–nitrite complexes in five- and six-coordinate globins.

Functional and structural roles of the N-terminal extension in Methanosarcina acetivorans protoglobin

Functional and structural properties of protoglobin from Methanosarcina acetivorans, whose Cys(101)E20 residue was mutated to Ser (MaPgb*), and of mutants missing either the first 20 N-terminal amino acids (MaPgb*-ΔN20 mutant), or the first 33 N-terminal amino acids [N-terminal loop of 20 amino acids and a 13-residue Z-helix, preceding the globin fold A-helix; (MaPgb*-ΔN20Z mutant)] have been investigated. In keeping with the MaPgb*-ΔN20 mutant crystal structure, here reported at 2.0Å resolution, which shows an increased exposure of the haem propionates to the solvent, the analysis of ligand binding kinetics highlights high accessibility of ligands to the haem pocket in ferric MaPgb*-ΔN20. CO binding to ferrous MaPgb*-ΔN20 displays a marked biphasic behavior, with a fast binding process close to that observed in MaPgb* and a slow carbonylation process, characterized by a rate-limiting step. Conversely, removal of the first 33 residues induces a substantial perturbation of the overall MaPgb* structure, with loss of α-helical content and potential partial collapse of the protein chain. As such, ligand binding kinetics are characterized by very slow rates that are independent of ligand concentration, this being indicative of a high energy barrier for ligand access to the haem, possibly due to localized misfolding. This article is part of a Special Issue entitled: Oxygen Binding and Sensing Proteins.

Is the heme pocket region modulated by disulfide-bridge formation in fish and amphibian neuroglobins as in humans?

Neuroglobin, a globin characterized by a bis-histidine ligation of the heme iron, has been identified in mammalian and non-mammalian vertebrates, including fish, amphibians and reptiles. In human neuroglobin, the presence of an internal disulfide bond in the CD loop (CD7-D5) is found to modulate the ligand binding through a change in the heme pocket structure. Although the neuroglobin sequences mostly display conserved Cys at positions CD7, D5 and G18/19, a number of exceptions are known. In this study, neuroglobins from amphibian (Xenopus tropicalis) and fish (Chaenocephalus aceratus, Dissostichus mawsoni and Danio rerio) are investigated using electron paramagnetic resonance and optical absorption spectroscopy. All these neuroglobins differ from human neuroglobin in their Cys-positions. It is demonstrated that if disulfide bonds are formed in fish and amphibian neuroglobins, the reduction of these bonds does not result in alteration of the heme pocket in these globins. Furthermore, it is shown that mutagenesis of the Cys residues of X. tropicalis neuroglobin influences the protein structure. The amphibian neuroglobin is also found to be more resistant to H2O2-induced denaturation than the other neuroglobins under study, although all show an overall large stability in high concentrations of this oxidant. This article is part of a Special Issue entitled: Oxygen Binding and Sensing Proteins.

CO rebinding kinetics and molecular dynamics simulations highlight dynamic regulation of internal cavities in human cytoglobin

Cytoglobin (Cygb) was recently discovered in the human genome and localized in different tissues. It was suggested to play tissue-specific protective roles, spanning from scavenging of reactive oxygen species in neurons to supplying oxygen to enzymes in fibroblasts. To shed light on the functioning of such versatile machinery, we have studied the processes supporting transport of gaseous heme ligands in Cygb. Carbon monoxide rebinding shows a complex kinetic pattern with several distinct reaction intermediates, reflecting rebinding from temporary docking sites, second order recombination, and formation (and dissociation) of a bis-histidyl heme hexacoordinated reaction intermediate. Ligand exit to the solvent occurs through distinct pathways, some of which exploit temporary docking sites. The remarkable change in energetic barriers, linked to heme bis-histidyl hexacoordination by HisE7, may be responsible for active regulation of the flux of reactants and products to and from the reaction site on the distal side of the heme. A substantial change in both protein dynamics and inner cavities is observed upon transition from the CO-liganded to the pentacoordinated and bis-histidyl hexacoordinated species, which could be exploited as a signalling state. These findings are consistent with the expected versatility of the molecular activity of this protein.

Biophysical characterisation of neuroglobin of the icefish, a natural knockout for hemoglobin and myoglobin. Comparison with human neuroglobin

The Antarctic icefish Chaenocephalus aceratus lacks the globins common to most vertebrates, hemoglobin and myoglobin, but has retained neuroglobin in the brain. This conserved globin has been cloned, over-expressed and purified. To highlight similarities and differences, the structural features of the neuroglobin of this colourless-blooded fish were compared with those of the well characterised human neuroglobin as well as with the neuroglobin from the retina of the red blooded, hemoglobin and myoglobin-containing, closely related Antarctic notothenioid Dissostichus mawsoni. A detailed structural and functional analysis of the two Antarctic fish neuroglobins was carried out by UV-visible and Resonance Raman spectroscopies, molecular dynamics simulations and laser-flash photolysis. Similar to the human protein, Antarctic fish neuroglobins can reversibly bind oxygen and CO in the Fe(2+) form, and show six-coordination by distal His in the absence of exogenous ligands. A very large and structured internal cavity, with discrete docking sites, was identified in the modelled three-dimensional structures of the Antarctic neuroglobins. Estimate of the free-energy barriers from laser-flash photolysis and Implicit Ligand Sampling showed that the cavities are accessible from the solvent in both proteins.Comparison of structural and functional properties suggests that the two Antarctic fish neuroglobins most likely preserved and possibly improved the function recently proposed for human neuroglobin in ligand multichemistry. Despite subtle differences, the adaptation of Antarctic fish neuroglobins does not seem to parallel the dramatic adaptation of the oxygen carrying globins, hemoglobin and myoglobin, in the same organisms.

The anti-apoptotic role of neuroglobin

The small heme-protein neuroglobin is expressed at high concentrations in certain brain neurons and in the rod cells of the retina. This paper reviews the many studies which have recently identified a protective role for neuroglobin, in a wide range of situations involving apoptotic cell death. The origins of this protective mechanism are discussed in terms of both experimental results and computational modeling of the intrinsic pathway of apoptosis, which shows that neuroglobin can intervene in this process by a reaction with released mitochondrial cytochrome c. An integrated model, based on the various molecular actions of both neuroglobin and cytochrome c, is developed, which accounts for the cellular distribution of neuroglobin.

Heme orientation modulates histidine dissociation and ligand binding kinetics in the hexacoordinated human neuroglobin

Neuroglobin (Ngb) is a globin present in the brain and retina of mammals. This hexacoordinated hemoprotein binds small diatomic molecules, albeit with lower affinity compared with other globins. Another distinctive feature of most mammalian Ngb is their ability to form an internal disulfide bridge that increases ligand affinity. As often seen for prosthetic heme b containing proteins, human Ngb exhibits heme heterogeneity with two alternative heme orientations within the heme pocket. To date, no details are available on the impact of heme orientation on the binding properties of human Ngb and its interplay with the cysteine oxidation state. In this work, we used ¹H NMR spectroscopy to probe the cyanide binding properties of different Ngb species in solution, including wild-type Ngb and the single (C120S) and triple (C46G/C55S/C120S) mutants. We demonstrate that in the disulfide-containing wild-type protein cyanide ligation is fivefold faster for one of the two heme orientations (the A isomer) compared with the other isomer, which is attributed to the lower stability of the distal His64–iron bond and reduced steric hindrance at the bottom of the cavity for heme sliding in the A conformer. We also attribute the slower cyanide reactivity in the absence of a disulfide bridge to the tighter histidine–iron bond. More generally, enhanced internal mobility in the CD loop bearing the disulfide bridge hinders access of the ligand to heme iron by stabilizing the histidine–iron bond. The functional impact of heme disorder and cysteine oxidation state on the properties of the Ngb ligand is discussed.

Ligation tunes protein reactivity in an ancient haemoglobin: kinetic evidence for an allosteric mechanism in Methanosarcina acetivorans protoglobin

Protoglobin from Methanosarcina acetivorans (MaPgb) is a dimeric globin with peculiar structural properties such as a completely buried haem and two orthogonal tunnels connecting the distal cavity to the solvent. CO binding to and dissociation from MaPgb occur through a biphasic kinetics. We show that the heterogenous kinetics arises from binding to (and dissociation from) two tertiary conformations in ligation-dependent equilibrium. Ligation favours the species with high binding rate (and low dissociation rate). The equilibrium is shifted towards the species with low binding (and high dissociation) rates for the unliganded molecules. A quantitative model is proposed to describe the observed carbonylation kinetics.

Functional differentiation of myoglobin isoforms in hypoxia-tolerant carp indicates tissue-specific protective roles

Because of a recent whole genome duplication, the hypoxia-tolerant common carp and goldfish are the only vertebrates known to possess two myoglobin (Mb) paralogs. One of these, Mb1, occurs in oxidative muscle but also in several other tissues, including capillary endothelial cells, whereas the other, Mb2, is a unique isoform specific to brain neurons. To help understand the functional roles of these diverged isoforms in the tolerance to severe hypoxia in the carp, we have compared their O(2) equilibria, carbon monoxide (CO) and O(2) binding kinetics, thiol S-nitrosation, nitrite reductase activities, and peroxidase activities. Mb1 has O(2) affinity and nitrite reductase activity comparable to most vertebrate muscle Mbs, consistent with established roles for Mbs in O(2) storage/delivery and in maintaining nitric oxide (NO) homeostasis during hypoxia. Both Mb1 and Mb2 can be S-nitrosated to similar extent, but without oxygenation-linked allosteric control. When compared with Mb1, Mb2 displays faster O(2) and CO kinetics, a lower O(2) affinity, and is slower at converting nitrite into NO. Mb2 is therefore unlikely to be primarily involved in either O(2) supply to mitochondria or the generation of NO from nitrite during hypoxia. However, Mb2 proved to be significantly faster at eliminating H(2)O(2,) a major in vivo reactive oxygen species (ROS), suggesting that this diverged Mb isoform may have a specific protective role against H(2)O(2) in the carp brain. This property might be of particular significance during reoxygenation following extended periods of hypoxia, when production of H(2)O(2) and other ROS is highest.

Electron transfer function versus oxygen delivery: a comparative study for several hexacoordinated globins across the animal kingdom

Caenorhabditis elegans globin GLB-26 (expressed from gene T22C1.2) has been studied in comparison with human neuroglobin (Ngb) and cytoglobin (Cygb) for its electron transfer properties. GLB-26 exhibits no reversible binding for O(2) and a relatively low CO affinity compared to myoglobin-like globins. These differences arise from its mechanism of gaseous ligand binding since the heme iron of GLB-26 is strongly hexacoordinated in the absence of external ligands; the replacement of this internal ligand, probably the E7 distal histidine, is required before binding of CO or O(2) as for Ngb and Cygb. Interestingly the ferrous bis-histidyl GLB-26 and Ngb, another strongly hexacoordinated globin, can transfer an electron to cytochrome c (Cyt-c) at a high bimolecular rate, comparable to those of inter-protein electron transfer in mitochondria. In addition, GLB-26 displays an unexpectedly rapid oxidation of the ferrous His-Fe-His complex without O(2) actually binding to the iron atom, since the heme is oxidized by O(2) faster than the time for distal histidine dissociation. These efficient mechanisms for electron transfer could indicate a family of hexacoordinated globin which are functionally different from that of pentacoordinated globins.

Neuroglobin and prion cellular localization: investigation of a potential interaction

Neuroglobin (Ngb) and the cellular prion protein (PrP(c)), proteins of unknown function in the nervous system, are known to be expressed in the retina and have been observed in different rat retinal cells. The retina is the site of the highest concentration for Ngb, a heme protein of similar size and conformation to myoglobin. In this study, we demonstrated by immunohistochemical analysis of retinal colocalization of Ngb and PrP(c) in the ganglion cell layer. Considering for these two a common protective role in relation to oxidative stress and a possible transient contact during migration of PrP(c) through the eye or upon neuronal degradation, we undertook in vitro studies of the interaction of the purified proteins. Mixing these two proteins leads to rapid aggregation, even at submicromolar concentrations. As observed with the use of dynamic light scattering, particles comprising both proteins evolve to hundreds of nanometers within several seconds, a first report showing that PrP(c) is able to form aggregates without major structural changes. The main effect would then appear to be a protein-protein interaction specific to the surface charge of the Ngb protein with PrP(c) N-terminal sequence. A dominant parameter is the solvent ionic force, which can significantly modify the final state of aggregation. PrP(c), normally anchored to the cell membrane, is toxic in the cytoplasm, where Ngb is present; this could suggest an Ngb function of scavenging proteins capable of forming deleterious aggregates considering a charge complementarity in the complex.

HisE11 and HisF8 provide bis-histidyl heme hexa-coordination in the globin domain of Geobacter sulfurreducens globin-coupled sensor

Among heme-based sensors, recent phylogenomic and sequence analyses have identified 34 globin coupled sensors (GCS), to which an aerotactic or gene-regulating function has been tentatively ascribed. Here, the structural and biochemical characterization of the globin domain of the GCS from Geobacter sulfurreducens (GsGCS(162)) is reported. A combination of X-ray crystallography (crystal structure at 1.5 A resolution), UV-vis and resonance Raman spectroscopy reveals the ferric GsGCS(162) as an example of bis-histidyl hexa-coordinated GCS. In contrast to the known hexa-coordinated globins, the distal heme-coordination in ferric GsGCS(162) is provided by a His residue unexpectedly located at the E11 topological site. Furthermore, UV-vis and resonance Raman spectroscopy indicated that ferrous deoxygenated GsGCS(162) is a penta-/hexa-coordinated mixture, and the heme hexa-to-penta-coordination transition does not represent a rate-limiting step for carbonylation kinetics. Lastly, electron paramagnetic resonance indicates that ferrous nitrosylated GsGCS(162) is a penta-coordinated species, where the proximal HisF8-Fe bond is severed.

Ultrafast heme-residue bond formation in six-coordinate heme proteins: implications for functional ligand exchange

A survey is presented of picosecond kinetics of heme-residue bond formation after photolysis of histidine, methionine, or cysteine, in a broad range of ferrous six-coordinate heme proteins. These include human neuroglobin, a bacterial heme-binding superoxide dismutase (SOD), plant cytochrome b 559, the insect nuclear receptor E75, horse heart cytochrome c and the heme domain of the bacterial sensor protein Dos. We demonstrate that the fastest and dominant phase of binding of amino acid residues to domed heme invariably takes place with a time constant in the narrow range of 5-7 ps. Remarkably, this is also the case in the heme-binding SOD, where the heme is solvent-exposed. We reason that this fast phase corresponds to barrierless formation of the heme-residue bond from a configuration close to the bound state. Only in proteins where functional ligand exchange occurs, additional slower rebinding takes place on the time scale of tens of picoseconds after residue dissociation. We propose that the presence of these slower phases reflects flexibility in the heme environment that allows external ligands (O2, CO, NO, . . .) to functionally replace the internal residue after thermal dissociation of the heme-residue bond.