Allosteric modulation by S-nitrosation in the low-O₂ affinity myoglobin from rainbow trout

NO and Heme Proteins: Cross-Talk between Heme and Cysteine Residues

Heme proteins are a diverse group that includes several unrelated families. Their biological function is mainly associated with the reactivity of the heme group, which-among several other reactions-can bind to and react with nitric oxide (NO) and other nitrogen compounds for their production, scavenging, and transport. The S-nitrosylation of cysteine residues, which also results from the reaction with NO and other nitrogen compounds, is a post-translational modification regulating protein activity, with direct effects on a variety of signaling pathways. Heme proteins are unique in exhibiting this dual reactivity toward NO, with reported examples of cross-reactivity between the heme and cysteine residues within the same protein. In this work, we review the literature on this interplay, with particular emphasis on heme proteins in which heme-dependent nitrosylation has been reported and those for which both heme nitrosylation and S-nitrosylation have been associated with biological functions.

Nitric Oxide Production and Regulation in the Teleost Cardiovascular System

Nitric Oxide (NO) is a free radical with numerous critical signaling roles in vertebrate physiology. Similar to mammals, in the teleost system the generation of sufficient amounts of NO is critical for the physiological function of the cardiovascular system. At the same time, NO amounts are strictly controlled and kept within basal levels to protect cells from NO toxicity. Changes in oxygen tension highly influence NO bioavailability and can modulate the mechanisms involved in maintaining the NO balance. While NO production and signaling appears to have general similarities with mammalian systems, the wide range of environmental adaptations made by fish, particularly with regards to differing oxygen availabilities in aquatic habitats, creates a foundation for a variety of in vivo models characterized by different implications of NO production and signaling. In this review, we present the biology of NO in the teleost cardiovascular system and summarize the mechanisms of NO production and signaling with a special emphasis on the role of globin proteins in NO metabolism.

Expression and characterization of rainbow trout Oncorhynchus mykiss recombinant myoglobin

Recombinant expression system was established for rainbow trout myoglobin (Mb) considering its unique primary structure of having one unusual deletion and two cysteine residues in contrast to the other fish Mbs. The obtained recombinant Mb without His-tag showed non-cooperative thermal denaturation profile. The presence of free cysteine residue(s) in rainbow trout Mb was demonstrated by reacting with a sulfhydryl agent, 4, 4´-dithiodipyridine, which ultimately resulted in the oxidation of Mb with characteristic changes in visible absorption spectra. Besides, the recombinant Mb displayed steady peroxidase reactivity indicating in vivo roles of Mb as a reactive oxygen species scavenger. The findings of the present study indicate that the solitary rainbow trout Mb, which ultimately manifest typical secondary structure pattern and corroborate characteristic functionality, can be over expressed in recombinant system devoid of fusion tag.

Thermal denaturation and autoxidation profiles of carangid fish myoglobins

Although myoglobin (Mb) has been considered to be one of the well-characterized proteins, screening of post-genomic era databases revealed the lack of adequate information on teleost Mbs. The present study was aimed to investigate stability and functional features of Mbs from three teleosts of the same family. To unfold how primary structure influences the stability and function of proteins, Mbs were purified from the dark muscles of three carangids, namely, yellowtail, greater amberjack, and silver trevally. Thermostabilities measured by circular dichroism (CD) spectrometry revealed species-specific thermal denaturation pattern, i.e., silver trevally > yellowtail > greater amberjack Mbs. On the other hand, autoxidation rate constants of the ferrous forms of those three carangid Mbs showed positive correlation between the ferrous state of the heme iron and rising temperature. The order of autoxidation rate was in the order of greater amberjack > yellowtail > silver trevally Mbs. The finding of the present study denotes that the thermal stability is not necessarily correlated with the functional stability of carangid Mbs even though their primary structures shared high homology (84-94%).

Biomolecular Interactions Governing Fresh Meat Color in Post-mortem Skeletal Muscle: A Review

Appearance is an important sensory property that significantly influences consumers' perceptions of fresh meat quality. Failure to meet consumer expectations can lead to rejection of meat products, concomitant loss in value, and potential production of organic waste. Immediately after animal harvest, skeletal muscle metabolism changes from aerobic to anaerobic. However, anoxic post-mortem muscle is biochemically active, and biomolecular interaction between myoglobin, mitochondria, metabolites, and lipid oxidation determines meat color. This review examines how metabolites and mitochondrial activity can influence myoglobin oxygenation and metmyoglobin reducing activity. Further, the review highlights recent research that has examined myoglobin redox dynamics, sarcoplasmic metabolite changes, and/or post-mortem biochemistry.

Genetic and functional diversity of the multiple lungfish myoglobins

It is known that the West African lungfish (Protopterus annectens) harbours multiple myoglobin (Mb) genes that are differentially expressed in various tissues and that the Mbs differ in their abilities to confer tolerance towards hypoxia. Here, we show that other lungfish species (Protopterus dolloi, Protopterus aethiopicus and Lepidosiren paradoxa) display a similar diversity of Mb genes and have orthologous Mb genes. To investigate the functional diversification of these genes, we studied the structures, O₂ binding properties and nitrite reductase enzymatic activities of recombinantly expressed P. annectens Mbs (PanMbs). CD spectroscopy and small-angle X-ray scattering revealed the typical globin-fold in all investigated recombinant Mbs, indicating a conserved structure. The highest O₂ affinity was measured for PanMb2 (P₅₀  = 0.88 Torr at 20 °C), which is mainly expressed in the brain, whereas the muscle-specific PanMb1 has the lowest O₂ affinity (P₅₀  = 3.78 Torr at 20 °C), suggesting that tissue-specific O₂ requirements have resulted in the emergence of distinct Mb types. Two of the mainly neuronally expressed Mbs (PanMb3 and PanMb4b) have the highest nitrite reductase rates. These data show different O₂ binding and enzymatic properties of lungfish Mbs, reflecting multiple subfunctionalisation and neofunctionalisation events that occurred early in the evolution of lungfish. Some Mbs may have also taken over the functions of neuroglobin and cytoglobin, which are widely expressed in vertebrates but appear to be missing in lungfish.

Expression levels of myoglobin in muscle and non-muscle tissues of rainbow trout Oncorhynchus mykiss, a hypoxia intolerant species

Myoglobin (Mb) is one of the most intensively studied intracellular respiratory muscle proteins. Since the discovery of the fascinating fact that Mb is not confined only to oxidative muscle tissues but also is co-localized in different non-muscle tissues of cyprinids, hypoxia tolerant cyprinids have been established as the model teleost. Mb both at mRNA and protein levels have been reported in this study for the first time from a number of muscle and non-muscle tissues of rainbow trout Oncorhynchus mykiss, a hypoxia intolerant species. Mb transcript levels were high in the heart and slow skeletal muscle, and were comparatively high in the gonad and gill among the non-muscle tissues. Western-blotting by using anti-rainbow trout Mb peptide rabbit antibody detected Mb protein in the muscles and several non-muscle tissues. By both RNA in situ hybridization and immunofluorescence, Mb was localized in the cardiomyocytes and oxidative muscle fibers. On the other hand, Mb both at mRNA and protein levels was restricted to the lamellar epithelial cells of the gill, epithelial layers of hepato-biliary duct, neurons and endothelial cells of brain, ooplasm of gonad, kidney tubules, endothelial cells, and epithelial layer of intestine. Neuroglobin isoform 1 and 2 mRNAs along with Mb mRNA were localized in the granular layer of cerebellum. Considering the previous data reported for cyprinids, the expression sites of Mb in the muscle and non-muscle tissues of teleost could be universal, where Mb concerted with the other globins might play meaningful physiological roles.

Functional roles of globin proteins in hypoxia-tolerant ectothermic vertebrates

Globins are heme-containing proteins ubiquitously expressed in vertebrates, where they serve a broad range of biological functions, directly or indirectly related to the tight control of oxygen levels and its toxic products in vivo. Perhaps the most investigated of all proteins, hemoglobin and myoglobin are primarily involved in oxygen transport and storage, but also in facilitating arterial vasodilation, suppressing mitochondrial respiration, and preventing tissue oxidative damage via accessory redox enzymatic activities during hypoxia. By contrast, the more recently discovered neuroglobin and cytoglobin do not seem to function as reversible oxygen carriers and are instead involved in redox activities, although their exact biological roles remain to be clarified. In this context, hypoxia-tolerant ectotherms, such as freshwater turtles and members of the carp family that survive winter in extreme hypoxia, have proven as excellent models to appreciate the diversity of biological functions of globin proteins. Unraveling physiological roles of globin proteins in these extreme animals will clarify an important part of the adaptive mechanisms for surviving extreme fluctuations of oxygen availability that are prohibitive to mammals.

Molecular evolution of globin genes in Gymnotiform electric fishes: relation to hypoxia tolerance

BACKGROUND

Nocturnally active gymnotiform weakly electric fish generate electric signals for communication and navigation, which can be energetically taxing. These fish mainly inhabit the Amazon basin, where some species prefer well-oxygenated waters and others live in oxygen-poor, stagnant habitats. The latter species show morphological, physiological, and behavioral adaptations for hypoxia-tolerance. However, there have been no studies of hypoxia tolerance on the molecular level. Globins are classic respiratory proteins. They function principally in oxygen-binding and -delivery in various tissues and organs. Here, we investigate the molecular evolution of alpha and beta hemoglobins, myoglobin, and neuroglobin in 12 gymnotiforms compared with other teleost fish.

RESULTS

The present study identified positively selected sites (PSS) on hemoglobin (Hb) and myoglobin (Mb) genes using different maximum likelihood (ML) methods; some PSS fall in structurally important protein regions. This evidence for the positive selection of globin genes suggests that the adaptive evolution of these genes has helped to enhance the capacity for oxygen storage and transport. Interestingly, a substitution of a Cys at a key site in the obligate air-breathing electric eel (Electrophorus electricus) is predicted to enhance oxygen storage of Mb and contribute to NO delivery during hypoxia. A parallel Cys substitution was also noted in an air-breathing African electric fish (Gymnarchus niloticus). Moreover, the expected pattern under normoxic conditions of high expression of myoglobin in heart and neuroglobin in the brain in two hypoxia-tolerant species suggests that the main effect of selection on these globin genes is on their sequence rather than their basal expression patterns.

CONCLUSION

Results indicate a clear signature of positive selection in the globin genes of most hypoxia-tolerant gymnotiform fishes, which are obligate or facultative air breathers. These findings highlight the critical role of globin genes in hypoxia tolerance evolution of Gymnotiform electric fishes.

Hypoxia tolerance, nitric oxide, and nitrite: lessons from extreme animals

Among vertebrates able to tolerate periods of oxygen deprivation, the painted and red-eared slider turtles (Chrysemys picta and Trachemys scripta) and the crucian carp (Carassius carassius) are the most extreme and can survive even months of total lack of oxygen during winter. The key to hypoxia survival resides in concerted physiological responses, including strong metabolic depression, protection against oxidative damage and-in air-breathing animals-redistribution of blood flow. Each of these responses is known to be tightly regulated by nitric oxide (NO) and during hypoxia by its metabolite nitrite. The aim of this review is to highlight recent work illustrating the widespread roles of NO and nitrite in the tolerance to extreme oxygen deprivation, in particular in the red-eared slider turtle and crucian carp, but also in diving marine mammals. The emerging picture underscores the importance of NO and nitrite signaling in the adaptive response to hypoxia in vertebrate animals.

Hemoglobin βCys93 is essential for cardiovascular function and integrated response to hypoxia

Oxygen delivery by Hb is essential for vertebrate life. Three amino acids in Hb are strictly conserved in all mammals and birds, but only two of those, a His and a Phe that stabilize the heme moiety, are needed to carry O2. The third conserved residue is a Cys within the β-chain (βCys93) that has been assigned a role in S-nitrosothiol (SNO)-based hypoxic vasodilation by RBCs. Under this model, the delivery of SNO-based NO bioactivity by Hb redefines the respiratory cycle as a triune system (NO/O2/CO2). However, the physiological ramifications of RBC-mediated vasodilation are unknown, and the apparently essential nature of βCys93 remains unclear. Here we report that mice with a βCys93Ala mutation are deficient in hypoxic vasodilation that governs blood flow autoregulation, the classic physiological mechanism that controls tissue oxygenation but whose molecular basis has been a longstanding mystery. Peripheral blood flow and tissue oxygenation are decreased at baseline in mutant animals and decline excessively during hypoxia. In addition, βCys93Ala mutation results in myocardial ischemia under basal normoxic conditions and in acute cardiac decompensation and enhanced mortality during transient hypoxia. Fetal viability is diminished also. Thus, βCys93-derived SNO bioactivity is essential for tissue oxygenation by RBCs within the respiratory cycle that is required for both normal cardiovascular function and circulatory adaptation to hypoxia.

A globin domain in a neuronal transmembrane receptor of Caenorhabditis elegans and Ascaris suum: molecular modeling and functional properties

We report the structural and biochemical characterization of GLB-33, a putative neuropeptide receptor that is exclusively expressed in the nervous system of the nematode Caenorhabditis elegans. This unique chimeric protein is composed of a 7-transmembrane domain (7TM), GLB-33 7TM, typical of a G-protein-coupled receptor, and of a globin domain (GD), GLB-33 GD. Comprehensive sequence similarity searches in the genome of the parasitic nematode, Ascaris suum, revealed a chimeric protein that is similar to a Phe-Met-Arg-Phe-amide neuropeptide receptor. The three-dimensional structures of the separate domains of both species and of the full-length proteins were modeled. The 7TM domains of both proteins appeared very similar, but the globin domain of the A. suum receptor surprisingly seemed to lack several helices, suggesting a novel truncated globin fold. The globin domain of C. elegans GLB-33, however, was very similar to a genuine myoglobin-type molecule. Spectroscopic analysis of the recombinant GLB-33 GD showed that the heme is pentacoordinate when ferrous and in the hydroxide-ligated form when ferric, even at neutral pH. Flash-photolysis experiments showed overall fast biphasic CO rebinding kinetics. In its ferrous deoxy form, GLB-33 GD is capable of reversibly binding O2 with a very high affinity and of reducing nitrite to nitric oxide faster than other globins. Collectively, these properties suggest that the globin domain of GLB-33 may serve as a highly sensitive oxygen sensor and/or as a nitrite reductase. Both properties are potentially able to modulate the neuropeptide sensitivity of the neuronal transmembrane receptor.

Hydrogen sulfide and nitric oxide metabolites in the blood of free-ranging brown bears and their potential roles in hibernation

During winter hibernation, brown bears (Ursus arctos) lie in dens for half a year without eating while their basal metabolism is largely suppressed. To understand the underlying mechanisms of metabolic depression in hibernation, we measured type and content of blood metabolites of two ubiquitous inhibitors of mitochondrial respiration, hydrogen sulfide (H2S) and nitric oxide (NO), in winter-hibernating and summer-active free-ranging Scandinavian brown bears. We found that levels of sulfide metabolites were overall similar in summer-active and hibernating bears but their composition in the plasma differed significantly, with a decrease in bound sulfane sulfur in hibernation. High levels of unbound free sulfide correlated with high levels of cysteine (Cys) and with low levels of bound sulfane sulfur, indicating that during hibernation H2S, in addition to being formed enzymatically from the substrate Cys, may also be regenerated from its oxidation products, including thiosulfate and polysulfides. In the absence of any dietary intake, this shift in the mode of H2S synthesis would help preserve free Cys for synthesis of glutathione (GSH), a major antioxidant found at high levels in the red blood cells of hibernating bears. In contrast, circulating nitrite and erythrocytic S-nitrosation of glyceraldehyde-3-phosphate dehydrogenase, taken as markers of NO metabolism, did not change appreciably. Our findings reveal that remodeling of H2S metabolism and enhanced intracellular GSH levels are hallmarks of the aerobic metabolic suppression of hibernating bears.

Oxygen-linked S-nitrosation in fish myoglobins: a cysteine-specific tertiary allosteric effect

The discovery that cysteine (Cys) S-nitrosation of trout myoglobin (Mb) increases heme O₂ affinity has revealed a novel allosteric effect that may promote hypoxia-induced nitric oxide (NO) delivery in the trout heart and improve myocardial efficiency. To better understand this allosteric effect, we investigated the functional effects and structural origin of S-nitrosation in selected fish Mbs differing by content and position of reactive cysteine (Cys) residues. The Mbs from the Atlantic salmon and the yellowfin tuna, containing two and one reactive Cys, respectively, were S-nitrosated in vitro by reaction with Cys-NO to generate Mb-SNO to a similar yield (∼0.50 SH/heme), suggesting reaction at a specific Cys residue. As found for trout, salmon Mb showed a low O₂ affinity (P₅₀ = 2.7 torr) that was increased by S-nitrosation (P₅₀ = 1.7 torr), whereas in tuna Mb, O₂ affinity (P₅₀ = 0.9 torr) was independent of S-nitrosation. O₂ dissociation rates (k_off) of trout and salmon Mbs were not altered when Cys were in the SNO or N-ethylmaleimide (NEM) forms, suggesting that S-nitrosation should affect O₂ affinity by raising the O₂ association rate (k_on). Taken together, these results indicate that O₂-linked S-nitrosation may occur specifically at Cys107, present in salmon and trout Mb but not in tuna Mb, and that it may relieve protein constraints that limit O₂ entry to the heme pocket of the unmodified Mb by a yet unknown mechanism. UV-Vis and resonance Raman spectra of the NEM-derivative of trout Mb (functionally equivalent to Mb-SNO and not photolabile) were identical to those of the unmodified Mb, indicating that S-nitrosation does not affect the extent or nature of heme-ligand stabilization of the fully ligated protein. The importance of S-nitrosation of Mb in vivo is confirmed by the observation that Mb-SNO is present in trout hearts and that its level can be significantly reduced by anoxic conditions.

Hemoglobin isoform differentiation and allosteric regulation of oxygen binding in the turtle, Trachemys scripta

When freshwater turtles acclimatize to winter hibernation, there is a gradual transition from aerobic to anaerobic metabolism, which may require adjustments of blood O2 transport before turtles become anoxic. Here, we report the effects of protons, anionic cofactors, and temperature on the O2-binding properties of isolated hemoglobin (Hb) isoforms, HbA and HbD, in the turtle Trachemys scripta. We determined the primary structures of the constituent subunits of the two Hb isoforms, and we related the measured functional properties to differences in O2 affinity between untreated hemolysates from turtles that were acclimated to normoxia and anoxia. Our data show that HbD has a consistently higher O2 affinity compared with HbA, whereas Bohr and temperature effects, as well as thiol reactivity, are similar. Although sequence data show amino acid substitutions at two known β-chain ATP-binding site positions, we find high ATP affinities for both Hb isoforms, suggesting an alternative and stronger binding site for ATP. The high ATP affinities indicate that, although ATP levels decrease in red blood cells of turtles acclimating to anoxia, the O2 affinity would remain largely unchanged, as confirmed by O2-binding measurements of untreated hemolysates from normoxic and anoxic turtles. Thus, the increase in blood-O2 affinity that accompanies winter acclimation is mainly attributable to a decrease in temperature rather than in concentrations of organic phosphates. This is the first extensive study on freshwater turtle Hb isoforms, providing molecular evidence for adaptive changes in O2 transport associated with acclimation to severe hypoxia.

Insights into BAY 60-2770 activation and S-nitrosylation-dependent desensitization of soluble guanylyl cyclase via crystal structures of homologous nostoc H-NOX domain complexes

The soluble guanylyl cyclase (sGC) is an important receptor for nitric oxide (NO). Nitric oxide activates sGC several hundred fold to generate cGMP from GTP. Because of sGC's salutary roles in cardiovascular physiology, it has received substantial attention as a drug target. The heme domain of sGC is key to its regulation as it not only contains the NO activation site but also harbors sites for NO-independent sGC activators as well an S-nitrosylation site (β1 C122) involved in desensitization. Here we report the crystal structure of the activator BAY 60-2770 bound to the Nostoc H-NOX domain that is homologous to sGC. The structure reveals that BAY 60-2770 has displaced the heme and acts as a heme mimetic via carboxylate-mediated interactions with the conserved YxSxR motif as well as hydrophobic interactions. Comparisons with the previously determined BAY 58-2667 bound structure reveal that BAY 60-2770 is more ordered in its hydrophobic tail region. sGC activity assays demonstrate that BAY 60-2770 has about 10% higher fold maximal stimulation compared to BAY 58-2667. S-Nitrosylation of the BAY 60-2770 substituted Nostoc H-NOX domain causes subtle changes in the vicinity of the S-nitrosylated C122 residue. These shifts could impact the adjacent YxSxR motif and αF helix and as such potentially inhibit either heme incorporation or NO-activation of sGC and thus provide a structural basis for desensitization.

Expression patterns and adaptive functional diversity of vertebrate myoglobins

Recent years have witnessed a new round of research on one of the most studied proteins - myoglobin (Mb), the oxygen (O2) carrier of skeletal and heart muscle. Two major discoveries have stimulated research in this field: 1) that Mb has additional protecting functions, such as the regulation of in vivo levels of the signaling molecule nitric oxide (NO) by scavenging and generating NO during normoxia and hypoxia, respectively; and 2) that Mb in vertebrates (particularly fish) is expressed as tissue-specific isoforms in other tissues than heart and skeletal muscle, such as vessel endothelium, liver and brain, as found in cyprinid fish. Furthermore, Mb has also been found to protect against oxidative stress after hypoxia and reoxygenation and to undergo allosteric, O2-linked S-nitrosation, as in rainbow trout. Overall, the emerging evidence, particularly from fish species, indicates that Mb fulfills a broader array of physiological functions in a wider range of different tissues than hitherto appreciated. This new knowledge helps to better understand how variations in Mb structure and function may correlate with differences in animals' lifestyles and hypoxia-tolerance. This review integrates old and new results on Mb expression patterns and functional properties amongst vertebrates and discusses how these may relate to adaptive variations in different species. This article is part of a special issue entitled: Oxygen Binding and Sensing Proteins.

Integrating nitric oxide, nitrite and hydrogen sulfide signaling in the physiological adaptations to hypoxia: A comparative approach

Hydrogen sulfide (H(2)S), nitric oxide (NO) and nitrite (NO(2)(-)) are formed in vivo and are of crucial importance in the tissue response to hypoxia, particularly in the cardiovascular system, where these signaling molecules are involved in a multitude of processes including the regulation of vascular tone, cellular metabolic function and cytoprotection. This report summarizes current advances on the mechanisms by which these signaling pathways act and may have evolved in animals with different tolerance to hypoxia, as presented and discussed during the scientific sessions of the annual meeting of the Society for Experimental Biology in 2011 in Glasgow. It also highlights the need and potential for a comparative approach of study and collaborative effort to identify potential link(s) between the signaling pathways involving NO, nitrite and H(2)S in the whole-body responses to hypoxia.

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.