Structural, functional, and genetic characterization of Gastrophilus hemoglobin

The Globin Gene Family in Arthropods: Evolution and Functional Diversity

Globins are small heme-proteins that reversibly bind oxygen. Their most prominent roles in vertebrates are the transport and storage of O₂ for oxidative energy metabolism, but recent research has suggested alternative, non-respiratory globin functions. In the species-rich and ecologically highly diverse taxon of arthropods, the copper-containing hemocyanin is considered the main respiratory protein. However, recent studies have suggested the presence of globin genes and their proteins in arthropod taxa, including model species like Drosophila. To systematically assess the taxonomic distribution, evolution and diversity of globins in arthropods, we systematically searched transcriptome and genome sequence data and found a conserved, widespread occurrence of three globin classes in arthropods: hemoglobin-like (HbL), globin X (GbX), and globin X-like (GbXL) protein lineages. These globin types were previously identified in protostome and deuterostome animals including vertebrates, suggesting their early ancestry in Metazoa. The HbL genes show multiple, lineage-specific gene duplications in all major arthropod clades. Some HbL genes (e.g., Glob2 and 3 of Drosophila) display particularly fast substitution rates, possibly indicating the evolution of novel functions, e.g., in spermatogenesis. In contrast, arthropod GbX and GbXL globin genes show high evolutionary stability: GbXL is represented by a single-copy gene in all arthropod groups except Brachycera, and representatives of the GbX clade are present in all examined taxa except holometabolan insects. GbX and GbXL both show a brain-specific expression. Most arthropod GbX and GbXL proteins, but also some HbL variants, include sequence motifs indicative of potential N-terminal acylation (i.e., N-myristoylation, 3C-palmitoylation). All arthropods except for the brachyceran Diptera harbor at least one such potentially acylated globin copy, confirming the hypothesis of an essential, conserved globin function associated with the cell membrane. In contrast to other animals, the fourth ancient globin lineage, represented by neuroglobin, appears to be absent in arthropods, and the putative arthropod orthologs of the fifth metazoan globin lineage, androglobin, lack a recognizable globin domain. Thus, the remarkable evolutionary stability of some globin variants is contrasted by occasional dynamic gene multiplication or even loss of otherwise strongly conserved globin lineages in arthropod phylogeny.

Transcriptomes reveal expression of hemoglobins throughout insects and other Hexapoda

Insects have long been thought to largely not require hemoglobins, with some notable exceptions like the red hemolymph of chironomid larvae. The tubular, branching network of tracheae in hexapods is traditionally considered sufficient for their respiration. Where hemoglobins do occur sporadically in plants and animals, they are believed to be either convergent, or because they are ancient in origin and their expression is lost in many clades. Our comprehensive analysis of 845 Hexapod transcriptomes, totaling over 38 Gbases, revealed the expression of hemoglobins in all 32 orders of hexapods, including the 29 recognized orders of insects. Discovery and identification of 1333 putative hemoglobins were achieved with target-gene BLAST searches of the NCBI TSA database, verifying functional residues, secondary- and tertiary-structure predictions, and localization predictions based on machine learning. While the majority of these hemoglobins are intracellular, extracellular ones were recovered in 38 species. Gene trees were constructed via multiple-sequence alignments and phylogenetic analyses. These indicate duplication events within insects and a monophyletic grouping of hemoglobins outside other globin clades, for which we propose the term insectahemoglobins. These hemoglobins are phylogenetically adjacent and appear structurally convergent with the clade of chordate myoglobins, cytoglobins, and hemoglobins. Their derivation and co-option from early neuroglobins may explain the widespread nature of hemoglobins in various kingdoms and phyla. These results will guide future work involving genome comparisons to transcriptome results, experimental investigations of gene expression, cell and tissue localization, and gas binding properties, all of which are needed to further illuminate the complex respiratory adaptations in insects.

Surprising differences in the respiratory protein of insects: A spectroscopic study of haemoglobin from the European honeybee and the malaria mosquito

Only recently it was discovered that haemoglobin (Hb) belongs to the standard gene repertoire of insects, although their tracheal system is used for respiration. A classical oxygen-carrying function of Hb is only obvious for hexapods living in hypoxic environments. In other insect species, including the common fruit fly Drosophila melanogaster, the physiological role of Hb is yet unclear. Here, we study recombinant haemoglobin from the European honeybee Apis mellifera (Ame) and the malaria mosquito Anopheles gambiae (Aga). Spectroscopic evidence shows that both proteins can be classified as hexacoordinate Hbs with a strong affinity for the distal histidine. AgaHb1 is proposed to play a role in oxygen transport or sensing based on its multimeric state, slow autoxidation, and small but significant amount of five-coordinated haem in the deoxy ferrous form. AmeHb appears to behave more like vertebrate neuroglobin with a complex function given its diversified distribution in the genome.

Knockdown of Drosophila hemoglobin suggests a role in O2 homeostasis

Almost all insects are equipped with a tracheal system, which appears to be sufficient for O2 supply even in phases of high metabolic activity. Therefore, with the exception of a few species dwelling in hypoxic habitats, specialized respiratory proteins had been considered unnecessary in insects. The recent discovery and apparently universal presence of intracellular hemoglobins in insects has remained functionally unexplained. The fruitfly Drosophila melanogaster harbors three different globin genes (referred to as glob1-3). Glob1 is the most highly expressed globin and essentially occurs in the tracheal system and the fat body. To better understand the functions of insect globins, the levels of glob1 were modulated in Drosophila larvae and adults by RNAi-mediated knockdown and transgenic over-expression. No effects on the development were observed in flies with manipulated glob1 levels. However, the knockdown of glob1 led to a significantly reduced survival rate of adult flies under hypoxia (5% and 1.5% O2). Surprisingly, the glob1 knockdown flies also displayed increased resistance towards the reactive oxygen species-forming agent paraquat, which may be explained by a restricted availability of O2 resulting in decreased formation of harmful O2(-). In summary, our results suggest an important functional role of glob1 in O2 homeostasis, possibly by enhancing O2 supply.

Evolution of Respiratory Proteins across the Pancrustacea

Respiratory proteins enhance the capacity of the blood for oxygen transport and support intracellular storage and delivery of oxygen. Hemocyanin and hemoglobin are the respiratory proteins that occur in the Pancrustacea. The copper-containing hemocyanins evolved from phenoloxidases in the stem lineage of arthropods. For a long time, hemocyanins had only been known from the malacostracan crustaceans but recent studies identified hemocyanin also in Remipedia, Ostracoda, and Branchiura. Hemoglobins are common in the Branchiopoda but have also been sporadically found in other crustacean classes (Malacostraca, Copepoda, Thecostraca). Respiratory proteins had long been considered unnecessary in the hexapods because of the tracheal system. Only chironomids, some backswimmers, and the horse botfly, which all live under hypoxic conditions, were known exceptions and possess hemoglobins. However, recent data suggest that hemocyanins occur in most ametabolous and hemimetabolous insects. Phylogenetic analysis showed the hemocyanins of insects and Remipedia to be similar, suggesting a close relationship of these taxa. Hemocyanin has been lost in dragonflies, mayflies, and Eumetabola (Hemiptera + Holometabola). In cockroaches and grasshoppers, hemocyanin expression is restricted to the developing embryo while in adults oxygen is supplied solely by the tracheal system. This pattern suggests that hemocyanin was the oxygen-transport protein in the hemolymph of the last common ancestor of the pancrustaceans. The loss was probably associated with miniaturization, a period of restricted availability of oxygen, a change in life-style, or morphological changes. Once lost, hemocyanin was not regained. Some pancrustaceans also possess cellular globin genes with uncertain functions, which are expressed at low levels. When a respiratory protein was again required, hemoglobins evolved several times independently from cellular globins.

Tertiary and quaternary effects in the allosteric regulation of animal hemoglobins

In the last decade, protein allostery has experienced a major resurgence, boosted by the extension of the concept to systems of increasing complexity and by its exploitation for the development of drugs. Expansion of the field into new directions has not diminished the key role of hemoglobin as a test molecule for theory and experimental validation of allosteric models. Indeed, the diffusion of hemoglobins in all kingdoms of life and the variety of functions and of quaternary assemblies based on a common tertiary fold indicate that this superfamily of proteins is ideally suited for investigating the physical and molecular basis of allostery and firmly maintains its role as a main player in the field. This review is an attempt to briefly recollect common and different strategies adopted by metazoan hemoglobins, from monomeric molecules to giant complexes, exploiting homotropic and heterotropic allostery to increase their functional dynamic range. This article is part of a Special Issue entitled: Oxygen Binding and Sensing Proteins.

Characterization of the hemoglobin of the backswimmer Anisops deanei (Hemiptera)

While O(2)-binding hemoglobin-like proteins are present in many insects, prominent amounts of hemoglobin have only been found in a few species. Backswimmers of the genera Anisops and Buenoa (Notonectidae) have high concentrations of hemoglobin in the large tracheal cells of the abdomen. Oxygen from the hemoglobin is delivered to a gas bubble and controls the buoyant density, which enables the bugs to maintain their position without swimming and to remain stationary in the mid-water zone where they hunt for prey. We have obtained the cDNA sequences of three Anisops deanei hemoglobin chains by RT-PCR and RACE techniques. The deduced amino acid sequences show an unusual insertion of a single amino acid in the conserved helix E, but this does not affect protein stability or ligand binding kinetics. Recombinant A. deanei hemoglobin has an oxygen affinity of P(50) = 2.4 kPa (18 torr) and reveals the presence of a dimeric fraction or two different conformations. The absorption spectra demonstrate that the Anisops hemoglobin is a typical pentacoordinate globin. Phylogenetic analyses show that the backswimmer hemoglobins evolved within Heteroptera and most likely originated from an intracellular hemoglobin with divergent function.

Testes-specific hemoglobins in Drosophila evolved by a combination of sub- and neofunctionalization after gene duplication

BACKGROUND

For a long time the presence of respiratory proteins in most insects has been considered unnecessary. However, in recent years it has become evident that globins belong to the standard repertoire of the insect genome. Like most other insect globins, the glob1 gene of Drosophila melanogaster displays a conserved expression pattern in the tracheae, the fat body and the Malpighian tubules.

RESULTS

Here we show that the recently discovered D. melanogaster globin genes glob2 and glob3 both display an unusual male-specific expression in the reproductive tract during spermatogenesis. Both paralogs are transcribed at equivalent mRNA levels and largely overlap in their cellular expression patterns during spermatogenesis. Phylogenetic analyses showed that glob2 and glob3 reflect a gene duplication event that occurred in the ancestor of the Sophophora subgenus at least 40 million years ago. Therefore, flies of the Drosophila subgenus harbor only one glob2/3-like gene.

CONCLUSIONS

Phylogenetic and sequence analyses indicate an evolution of the glob2 and glob3 duplicates by a combination of sub- and neofunctionalization. Considering their restricted, testes-specific expression, an involvement of both globins in alleviating oxidative stress during spermatogenesis is conceivable.

Oxygen binding properties of backswimmer (Notonectidae, Anisops) haemoglobin, determined in vivo

Aquatic backswimmers (Anisops spp.) collect oxygen from the atmosphere in order to breathe underwater, carrying it within a bubble of air on the ventral surface of their body and bound within haemoglobin-filled cells inside their abdomen. These oxygen stores are interconnected via the abdominal spiracles and the tracheal system. Fibre optic oxygen probes were used to measure PO(2) changes within the air bubbles of submerged backswimmers (Anisops deanei) and these measurements were transformed into in vivo haemoglobin-oxygen equilibrium curves (OECs) using a biotonometric approach. The haemoglobin displayed a triphasic, highly sigmoid OEC with a P(50) of 3.90 kPa. Comparisons made with a previous in vitro analysis of Anisops haemoglobin demonstrate that while the apparent cooperativity and oxygen affinity are considerably higher in vivo, both measurements share unusual Hb-O(2) binding characteristics. The affinity and cooperativity of the backswimmers' haemoglobin appears adaptive as it lengthens dives and promotes neutral buoyancy. While there are limitations associated with biotonometry, the in vivo OEC accurately represents the loading and unloading of biologically available oxygen within the backswimmers' haemoglobin cells. Potential errors associated with determining the OEC are small, as evaluated with sensitivity analyses in numerical models.

Unusual flexibility of distal and proximal histidine residues in the haem pocket of Drosophila melanogaster haemoglobin

Several pH-dependent low-spin ferric haem forms are identified in a frozen solution of the ferric ¹²¹Cys→Ser mutant of Drosophila melanogaster haemoglobin (DmHb1*) using electron paramagnetic resonance (EPR) techniques. Different forms with EPR parameters typical of bis-histidine coordinated haem iron centers were observed. Strong pH-dependent changes in the EPR signatures were observed related to changes in the haem pocket. The pulsed EPR data indicate that both the distal and proximal histidine exhibit a large libration around the Fe-N(His) axis. The resonance Raman spectra of the CO-ligated ferrous form of Drosophila melanogaster haemoglobin are typical of an open conformation, with little stabilization of the CO ligand by the surrounding amino-acid residues. The EPR data of the cyanide-ligated ferric DmHb1* indicates a close similarity with cyanide-ligated ferric myoglobin. The structural characteristics of DmHb1* are found to clearly differ from those of other bis-histidine-coordinated globins.

Protein expression profile of Gasterophilus intestinalis larvae causing horse gastric myiasis and characterization of horse immune reaction

Background

Little information is available on the immunological aspect of parasitic Gasterophilus intestinalis (Diptera, Oestridae) larvae causing horse gastric myiasis. The objectives of this research were to analyze the protein content of larval crude extracts of the migrating second and third larvae (L2 and L3) of G. intestinalis in order to characterize the immune response of horses.

Results

The proteomic profile of L2 and L3, investigated by using one and two dimensional approaches, revealed a migration pattern specific to each larval stage. Furthermore, Western blots were performed with horse sera and with sera of Balb/c mice immunised with the larval crude extracts of L2 or L3, revealing a different immune reaction in naturally infected horses vs. artificially induced immune reaction in mice. The comparisons of the immunoblot profiles demonstrate that the stage L2 is more immunogenic than the stage L3 most likely as an effect of the highest enzymatic production of L2 while migrating through the host tissues. Fifteen proteins were identified by mass spectrometry.

Conclusion

This work provides further information into the understanding of the interaction between G. intestinalis and their host and by contributing a novel scheme of the proteomic profile of the main larval stages.

The hemoglobins of the trematodes Fasciola hepatica and Paramphistomum epiclitum: a molecular biological, physico-chemical, kinetic, and vaccination study

The trematode Fasciola hepatica (Fa.he.) is a common parasite of human and livestock. The hemoglobin (Hb) of Fa.he., a potential immunogen, was chosen for characterization in the search for an effective vaccine. Characterization of trematode Hbs show that they are intracellular single-domain globins with the following remarkable features: (1) Fa.he. expresses two Hb isoforms that differ at two amino acid sites (F1: 119Y/123Q; F2: 119F/123L). Both isoforms are monoacetylated at their N-termini; (2) the genes coding for Fa.he. and Paramphistomum epiclitum (Pa.ep.) Hbs are interrupted by two introns at the conserved positions B12.2 and G7.0.; (3) UV/VIS and resonance Raman spectroscopy identify the recombinant Fa.he. HbF2 as a pentacoordinated high-spin ferrous Hb; (4) electron paramagnetic resonance spectroscopy of cyano-met Fa.he. HbF2 proves that the endogenously bound imidazole has no imidazolate character; (5) the major structural determinants of the globin fold are present, they contain a TyrB10/TyrE7 residue pair on the distal side. Although such distal-site pair is a signature for high oxygen affinity, as shown for Pa.ep. Hb, the oxygen-binding rate parameters for Fa.he. Hb are intermediate between those of myoglobin and those of other trematode Hbs; (6) the three-dimensional structure of recombinant Fa.he. HbF2 from this study closely resembles the three-dimensional structure of Pa.ep. determined earlier. The set of distal-site polar interactions observed in Pa.ep. Hb is matched with small but significant structural adjustments; (7) despite the potential immunogenic character of the fluke Hb, vaccination of calves with recombinant Fa.he. HbF2 failed to promote protection against parasitic infection.

The respiratory proteins of insects

For a long time, respiratory proteins have been considered unnecessary in most insects because the tracheal system was thought to be sufficient for oxygen supply. Only a few species that survive under hypoxic conditions were known exceptions. However, recently it has become evident that (1) intracellular hemoglobins belong to the standard repertoire of insects and (2) that hemocyanin is present in many "lower" insects. Intracellular hemoglobins have been identified in Drosophila, Anopheles, Apis and many other insects. In all investigated species, hemoglobin is mainly expressed in the fat body and the tracheal system. The major Drosophila hemoglobin binds oxygen with high affinity. This hemoglobin type possibly functions as a buffer system for oxygen supply at low partial pressures and/or for the protection from an excess of oxygen. Similar hemoglobins, present in much higher concentrations, store oxygen in specialized tracheal organs of the botfly and some backswimmers. The extracellular hemoglobins in the hemolymph of chironomid midges are evolutionary derivatives of the intracellular insect hemoglobins, which emerged in response to the hypoxic environment of the larvae. In addition, several hemoglobin variants of unknown functions have been discovered in insect genomes. Hemocyanins transport oxygen in the hemolymph of stoneflies, but also in the Entognatha and most hemimetabolan taxa. Apparently, hemocyanin has been lost in Holometabola. At present, no physiological or morphological character is known that could explain the presence or loss of hemocyanins in distinct taxa. Nevertheless, the occurrence of respiratory proteins in insects adds further complexity to our view on insect respiration.

Characterization of two globin genes from the malaria mosquito Anopheles gambiae: divergent origin of nematoceran haemoglobins

The chironomid midges are the only insects that harbour true haemoglobin in their haemolymph. Here we report the identification of haemoglobin genes in two other nematoceran species. Two paralogous haemoglobin genes (glob1 and glob2) from the malaria mosquito Anopheles gambiae were cloned and sequenced. Furthermore, we identified two orthologous haemoglobin genes in the yellow fever mosquito Aedes aegypti. All four haemoglobins were predicted to be intracellular proteins, with the amino acids required for heme- and oxygen-binding being conserved. In situ-hybridization studies showed that glob1 and glob2 expression in An. gambiae is mainly associated with the tracheal system. This pattern resembles that of other insect intracellular globins. We also observed expression of glob2 in visceral muscles. Phylogenetic analyses showed that the globins of the mosquitoes and the Chironomidae are not orthologous. The chironomid haemoglobins share a recent common origin with the brachyceran glob1 proteins. The mosquito glob1 and glob2 proteins, which separated by gene duplication around 170 million years ago, form a distinct clade of more ancient evolutionary origin within the insects. The glob1 genes have introns in the ancestral globin positions B12.2 and G7.0. An additional intron was observed in Ae. aegypti glob1 helix position E18.0, providing evidence for a recent intron gain event. Both mosquito glob2 genes have lost the B12.2 intron. This pattern must be interpreted in terms of dynamic intron gain and loss events in the globin gene lineage.

Molecular characterization of hemoglobin from the honeybee Apis mellifera

Due to the prevailing importance of the tracheal system for insect respiration, hemoglobins had been considered rare exceptions in this arthropod subphylum. Here we report the identification, cloning and expression analysis of a true hemoglobin gene in the honeybee Apis mellifera (Hymenoptera). The deduced amino acid sequence covers 171 residues (19.5kDa) and harbors all globin-typical features, including the proximal and the distal histidines. The protein has no signal peptide for transmembrane transport and was predicted to localize in the cytoplasm. The honeybee hemoglobin gene shows an ancient structure, with introns in positions B12.2 and G7.0, while most other insect globins have divergent intron positions. In situ hybridization studies showed that hemoglobin expression in the honeybee is mainly associated with the tracheal system. We also observe hemoglobin expression in the Malpighi tubes and testis. We further demonstrated that hemoglobins occur in other insect orders (Hemiptera, Coleoptera, Lepidoptera), suggesting that such genes belong to the standard repertoire of an insect genome. Phylogenetic analyses show that globins evolved along with the accepted insect systematics, with a remarkable diversification within the Diptera. Although insect hemoglobins may be in fact involved in oxygen metabolism, it remains uncertain whether they carry out a myoglobin-like function in oxygen storage and delivery.

Multiple active site conformers in the carbon monoxide complexes of trematode hemoglobins

Sequence alignment of hemoglobins of the trematodes Paramphistomum epiclitum and Gastrothylax crumenifer with myoglobin suggests the presence of an unusual active site structure in which two tyrosine residues occupy the E7 and B10 helical positions. In the crystal structure of P. epiclitum hemoglobin, such an E7-B10 tyrosine pair at the putative helical positions has been observed, although the E7 Tyr is displaced toward CD region of the polypeptide. Resonance Raman data on both P. epiclitum and G. crumenifer hemoglobins show that interactions of heme-bound ligands with neighboring amino acid residues are unusual. Multiple conformers in the CO complex, termed the C, O, and N conformers, are observed. The conformers are separated by a large difference (approximately 60 cm(-1)) in the frequencies of their Fe-CO stretching modes. In the C conformer the Fe-CO stretching frequency is very high, 539 and 535 cm(-1), for the P. epiclitum and G. crumenifer hemoglobins, respectively. The Fe-CO stretching of the N conformer appears at an unusually low frequency, 479 and 476 cm(-1), respectively, for the two globins. A population of an O conformer is seen in both hemoglobins, at 496 and 492 cm(-1), respectively. The C conformer is stabilized by a strong polar interaction of the CO with the distal B10 tyrosine residue. The O conformer is similar to the ones typically seen in mutant myoglobins in which there are no strong interactions between the CO and residues in the distal pocket. The N conformer possesses an unusual configuration in which a negatively charged group, assigned as the oxygen atom of the B10 Tyr side chain, interacts with the CO. In this conformer, the B10 Tyr assumes an alternative conformation consistent with one of the conformers seen the crystal structure. Implications of the multiple configurations on the ligand kinetics are discussed.

The hemoglobin genes of Drosophila

We recently reported the unprecedented occurrence of a hemoglobin gene (glob1) in the fruitfly Drosophila melanogaster. Here we investigate the structure and evolution of the glob1 gene in other Drosophila species. We cloned and sequenced glob1 genes and cDNA from D. pseudoobscura and D. virilis, and identified the glob1 gene sequences of D. simulans, D. yakuba, D. erecta, D. ananassae, D. mojavensis and D. grimshawi in the databases. Gene structure (introns in helix positions D7.0 and G7.0), gene synteny and sequence of glob1 are highly conserved, with high ds/dn ratios indicating strong purifying selection. The data suggest an important role of the glob1 protein in Drosophila, which may be the control of oxygen flow from the tracheal system. Furthermore, we identified two additional globin genes (glob2 and glob3) in the Drosophilidae. Although the sequences are highly derived, the amino acids required for heme- and oxygen-binding are conserved. In contrast to other known insect globin, the glob2 and glob3 genes harbour both globin-typical introns at positions B12.2 and G7.0. Both genes are conserved in various drosophilid species, but only expression of glob2 could be demonstrated by western blotting and RT-PCR. Phylogenetic analyses show that the clade leading to glob2 and glob3, which are sistergroups, diverged first in the evolution of dipteran globins. glob1 is closely related to the intracellular hemoglobin of the botfly Gasterophilus intestinalis, and the extracellular hemoglobins from the chironomid midges derive from this clade.

Modulation of oxygen binding to insect hemoglobins: the structure of hemoglobin from the botfly Gasterophilus intestinalis

Hemoglobins (Hbs) reversibly bind gaseous diatomic ligands (e.g., O2) as the sixth heme axial ligand of the penta-coordinate deoxygenated form. Selected members of the Hb superfamily, however, display a functionally relevant hexa-coordinate heme Fe atom in their deoxygenated state. Endogenous heme hexa-coordination is generally provided in these Hbs by the E7 residue (often His), which thus modulates accessibility to the heme distal pocket and reactivity of the heme toward exogenous ligands. Such a pivotal role of the E7 residue is prominently shown by analysis of the functional and structural properties of insect Hbs. Here, we report the 2.6 A crystal structure of oxygenated Gasterophilus intestinalis Hb1, a Hb known to display a penta-coordinate heme in the deoxygenated form. The structure is analyzed in comparison with those of Drosophila melanogaster Hb, exhibiting a hexa-coordinate heme in its deoxygenated derivative, and of Chironomus thummi thummi HbIII, which displays a penta-coordinate heme in the deoxygenated form. Despite evident structural differences in the heme distal pockets, the distinct molecular mechanisms regulating O2 binding to the three insect Hbs result in similar O(2 affinities (P50 values ranging between 0.12 torr and 0.46 torr).

Characterization of nonsymbiotic tomato hemoglobin

The nonsymbiotic tomato hemoglobin SOLly GLB1 (Solanum lycopersicon) is shown to form a homodimer of approximately 36 kDa with a high affinity for oxygen. Furthermore, our combined ultraviolet/visible, resonance Raman, and continuous wave electron paramagnetic resonance (EPR) measurements reveal that a mixture of penta- and hexacoordination of the heme iron is found in the deoxy ferrous form, whereas the ferric form shows predominantly a bis-histidine ligation (F8His-Fe(2+/3+)-E7His). This differs from the known forms of vertebrate hemoglobins and myoglobins. We have successfully applied our recently designed pulsed-EPR strategy to study the low-spin ferric form of tomato hemoglobin. These experiments reveal that, in ferric SOLly GLB1, one of the histidine planes is rotated 20 degrees (+/-10 degrees ) away from a N(heme)-Fe-N(heme) axis. Additionally, the observed g-values indicate a quasicoplanarity of the histidine ligands. From the HYSCORE (hyperfine sublevel correlation) measurements, the hyperfine and nuclear quadrupole couplings of the heme and histidine nitrogens are identified and compared with known EPR/ENDOR data of vertebrate Hbs and cytochromes. Finally, the ligand binding kinetics, which also indicate that the ferrous tomato Hb is only partially hexacoordinated, will be discussed in relation with the heme-pocket structure. The similarities and differences with other known nonsymbiotic plant hemoglobins will be highlighted.

Bishistidyl Heme Hexacoordination, a Key Structural Property in Drosophila melanogaster Hemoglobin

Hemoglobins at high concentration have been isolated long ago from some insect larvae living in hypoxic environments. Conversely, a monomeric hemoglobin has been discovered recently in the fruit fly Drosophila melanogaster as intracellular protein expressed both in larvae and in the adult fly. Such a finding indicates that the oxygen supply in insects may be more complex than previously thought, relying not only on O2 diffusion through the tubular tracheal system, but also on carrier-mediated transport and storage. We present here the crystal structure of recombinant D. melanogaster hemoglobin at 1.20 A resolution. Spectroscopic data show that the protein displays a hexacoordinated heme, whose axial ligands are the proximal and distal His residues. Such bis-His ligation of the heme has sizable effects on the protein local structure. Three protein matrix cavities, comparable in size but not in topological locations with those of sperm whale myoglobin, are spread through the protein matrix; one of these can host a xenon atom. Additionally, D. melanogaster hemoglobin binds one molecule of 3-(cyclohexylamino)propanesulfonic acid (CAPS) buffer at a surface pocket, next to the EF hinge. Despite the high resolution achieved, no sequence/structure features specifically supporting the heme hexa- to pentacoordination transition required for diatomic ligand binding could be recognized.

Invertebrate hemoglobins and nitric oxide: how heme pocket structure controls reactivity

Hemoglobins (Hbs), generally defined as 5 or 6 coordinate heme proteins whose primary function is oxygen transport, are now recognized to occur in virtually all phyla of living organisms. Historically, study of their function focused on oxygen as a reversibly bound ligand of the ferrous form of the protein. Other diatomic ligands like carbon monoxide and nitric oxide were considered "non-physiological" but useful probes of structure-function relationships in Hbs. This investigatory landscape changed dramatically in the 1980s when nitric oxide was discovered to activate a heme protein, cyclic guanylate cyclase. Later, its activation was likened to Perutz' description of Hb's allosteric properties being triggered by a ligand-dependent "out-of-plane/into-plane" movement of the heme iron. In 1996, a functional role for nitric oxide in human and mammalian Hbs was demonstrated and since that time, the interest in NO as a physiologically relevant Hb ligand has greatly increased. Concomitantly, non-oxygen binding properties of Hbs have challenged the view that Hbs arose for their oxygen storage and transport properties. In this focused review we discuss some invertebrate Hbs' functionally significant reactions with nitric oxide and how strategic positioning of a few residues in the heme pocket plays an large role in the interplay of diatomic ligands to ferrous and ferric heme iron in these proteins.

Characterization of Drosophila hemoglobin. Evidence for hemoglobin-mediated respiration in insects

In contrast to previous assumptions, the fruit fly Drosophila melanogaster possesses hemoglobin. This respiratory protein forms a monomer of about 17 kDa that is not exported into the hemolymph. Recombinant Drosophila hemoglobin displays a typical hexacoordinated deoxy spectrum and binds oxygen with an affinity of 0.12 torr. Four different hemoglobin transcripts have been identified, which are generated by two distinct promoters of the hemoglobin (glob1) gene but are identical in their coding regions. Putative binding sites for hypoxia-regulated transcription factors have been identified in the gene. Hemoglobin synthesis in Drosophila is mainly associated with the tracheal system and the fat body. This suggests that oxygen supply in insects may be more complex than thought previously and may depend on hemoglobin-mediated oxygen transport and storage in addition to simple diffusion.

The 109 residue nerve tissue minihemoglobin from Cerebratulus lacteus highlights striking structural plasticity of the alpha-helical globin fold

A very short hemoglobin (CerHb; 109 amino acids) binds O(2) cooperatively in the nerve tissue of the nemertean worm Cerebratulus lacteus to sustain neural activity during anoxia. Sequence analysis suggests that CerHb tertiary structure may be unique among the known globin fold evolutionary variants. The X-ray structure of oxygenated CerHb (R factor 15.3%, at 1.5 A resolution) displays deletion of the globin N-terminal A helix, an extended GH region, a very short H helix, and heme solvent shielding based on specific aromatic residues. The heme-bound O(2) is stabilized by hydrogen bonds to the distal TyrB10-GlnE7 pair. Ligand access to heme may take place through a wide protein matrix tunnel connecting the distal site to a surface cleft located between the E and H helices.

The primary structure of globin and linker chains from the chlorocruorin of the polychaete Sabella spallanzanii

Annelid hemoglobins are organized in a very complex supramolecular network of interacting polypeptides, the structure of which is still not wholly resolved. We have separated by two-dimensional electrophoresis the 4-MDa chlorocruorin of Sabella spallanzanii and identified its components by amino-terminal sequencing. This work reveals a high rate of heterogeneity of constituent chains in a single animal as well as in the Sabella population. Using a cDNA library prepared from the hematopoietic tissue of this worm, we have isolated and fully sequenced most globin and linker cDNAs. The primary structure features of these polypeptides have been characterized by comparison with model globin and linker sequences.

Very high resolution structure of a trematode hemoglobin displaying a TyrB10-TyrE7 heme distal residue pair and high oxygen affinity

Monomeric hemoglobin from the trematode Paramphistomum epiclitum displays very high oxygen affinity (P(50)<0.001 mm Hg) and an unusual heme distal site containing tyrosyl residues at the B10 and E7 positions. The crystal structure of aquo-met P. epiclitum hemoglobin, solved at 1.17 A resolution via multiwavelength anomalous dispersion techniques (R-factor=0.121), shows that the heme distal site pocket residue TyrB10 is engaged in hydrogen bonding to the iron-bound ligand. By contrast, residue TyrE7 is unexpectedly locked next to the CD globin region, in a conformation unsuitable for heme-bound ligand stabilisation. Such structural organization of the E7 distal residue differs strikingly from that observed in the nematode Ascaris suum hemoglobin (bearing TyrB10 and GlnE7 residues), which also displays very high oxygen affinity. The oxygenation and carbonylation parameters of wild-type P. epiclitum Hb as well as of single- and double-site mutants, with residue substitutions at positions B10, E7 and E11, have been determined and are discussed here in the light of the protein atomic resolution crystal structure.

Nonvertebrate hemoglobins: functions and molecular adaptations

Hemoglobin (Hb) occurs in all the kingdoms of living organisms. Its distribution is episodic among the nonvertebrate groups in contrast to vertebrates. Nonvertebrate Hbs range from single-chain globins found in bacteria, algae, protozoa, and plants to large, multisubunit, multidomain Hbs found in nematodes, molluscs and crustaceans, and the giant annelid and vestimentiferan Hbs comprised of globin and nonglobin subunits. Chimeric hemoglobins have been found recently in bacteria and fungi. Hb occurs intracellularly in specific tissues and in circulating red blood cells (RBCs) and freely dissolved in various body fluids. In addition to transporting and storing O(2) and facilitating its diffusion, several novel Hb functions have emerged, including control of nitric oxide (NO) levels in microorganisms, use of NO to control the level of O(2) in nematodes, binding and transport of sulfide in endosymbiont-harboring species and protection against sulfide, scavenging of O(2 )in symbiotic leguminous plants, O(2 )sensing in bacteria and archaebacteria, and dehaloperoxidase activity useful in detoxification of chlorinated materials. This review focuses on the extensive variation in the functional properties of nonvertebrate Hbs, their O(2 )binding affinities, their homotropic interactions (cooperativity), and the sensitivities of these parameters to temperature and heterotropic effectors such as protons and cations. Whenever possible, it attempts to relate the ligand binding properties to the known molecular structures. The divergent and convergent evolutionary trends evident in the structures and functions of nonvertebrate Hbs appear to be adaptive in extending the inhabitable environment available to Hb-containing organisms.

Protein and gene structure of a chlorocruorin chain of Eudistylia vancouverii

The polychaete annelid, Eudistylia vancouverii, contains as oxygen carrier a hexagonal bilayer (HBL) chlorocruorin. One of the globin chains, chain a1, has 142 amino acids (Mr 16,054.99) and its sequence deviates strongly from other nonvertebrate globin sequences. Unprecedented, it displays a Phe at the distal position E7 as well as at position B10, creating a very hydrophobic heme pocket probably responsible for the low oxygen affinity of the native molecule. Phylogenetic analysis of annelid globin chains clearly proves that globin chain a1 belongs to type I of globin chains having a pattern of 3 cysteine residues essential for the aggregation into a HBL structure. The gene coding for globin chain a1 is interrupted by 2 introns at the conserved positions B12.2 and G7.0. Based on protein and gene structure it can therefore be concluded that the globin chains of chlorocruorins are not fundamentally different from other annelid globin chains.

The hemoglobin gene of the deep-sea clam Calyptogena soyoae has a novel intron in A-helix

The cDNAs encoding two dimeric hemoglobins, Hbs I and II, of the deep-sea clam Calyptogena soyoae were amplified by PCR and the complete nucleotide sequences determined. The cDNA-derived amino acid sequences agreed completely with those determined chemically. Many of the molluscan intracellular globin genes have a characteristic four-exon/three-intron structure, with the precoding and two conventional introns conserved widely in animal globin genes. In this work we have determined the exon/intron organization of two hemoglobin genes of the deep-sea clam C. soyoae. Surprisingly, this gene has no precoding intron but instead contains an additional intron in the A-helix (A3.1), together with the two conventional introns (B12.2 and G6.3). This observation suggests that the precoding intron has been lost and the insertion of intron in A-helix occurred in the genes of Calyptogena. Alternatively, the sliding of intron from precoding to A-helix might have occurred.

A novel two-over-two alpha-helical sandwich fold is characteristic of the truncated hemoglobin family

Small hemoproteins displaying amino acid sequences 20-40 residues shorter than (non-)vertebrate hemoglobins (Hbs) have recently been identified in several pathogenic and non-pathogenic unicellular organisms, and named 'truncated hemoglobins' (trHbs). They have been proposed to be involved not only in oxygen transport but also in other biological functions, such as protection against reactive nitrogen species, photosynthesis or to act as terminal oxidases. Crystal structures of trHbs from the ciliated protozoan Paramecium caudatum and the green unicellular alga Chlamydomonas eugametos show that the tertiary structure of both proteins is based on a 'two-over-two' alpha-helical sandwich, reflecting an unprecedented editing of the classical 'three-over-three' alpha-helical globin fold. Based on specific Gly-Gly motifs the tertiary structure accommodates the deletion of the N-terminal A-helix and replacement of the crucial heme-binding F-helix with an extended polypeptide loop. Additionally, concerted structural modifications allow burying of the heme group and define the distal site, which hosts a TyrB10, GlnE7 residue pair. A set of structural and amino acid sequence consensus rules for stabilizing the fold and the bound heme in the trHbs homology subfamily is deduced.

A hemoglobin with an optical function

Hemoglobins are best known as oxygen transport proteins. Here we describe a hemoglobin from the parasitic nematode Mermis nigrescens (Mn-GLB-E) that has an optical, light shadowing function. The protein accumulates to high concentration as intracellular crystals in the ocellus of mature phototactic adult females while also being expressed at low concentration in other tissues. It differs in sequence and expression pattern from Mn-GLB-B, a second Mermis globin. It retains the structure and oxygen-binding and light-absorbing properties typical of nematode hemoglobins. As such, recruitment to a shadowing role in the eye appears to have occurred by changes in expression without modification of biochemistry. Both globins are coded by genes interrupted by two introns at the conserved positions B12.2 and G7.0, which is in agreement with the 3exon/2intron pattern model of globin gene evolution.