Hemoglobin function under extreme life conditions

Control of Oxygen Affinity in Mammalian Hemoglobins: Implications for a System Biology Description of the Respiratory Properties of the Red Blood Cell

Hemoglobin and myoglobin have been considered for a long time the paradigmatic model systems for protein function, to the point of being defined the "hydrogen atom[s] of biology". Given this privileged position and the huge amount of quantitative information available on these proteins, the red blood cell might appear as the model system and"hydrogen atom" of system biology. Indeed, since the red cell's main function is O2 transport by hemoglobin, the gap between the protein and the cell may appear quite small. Yet, a surprisingly large amount of detailed biochemical information is required for the modelization of the respiratory properties of the erythrocyte. This problem is compounded if modelization aims at uncovering or explaining evolutionarily selected functional properties of hemoglobin. The foremost difficulty lies in the fact that hemoglobins having different intrinsic properties and relatively ancient evolutionary divergence may behave similarly in the complex milieu of blood, whereas very similar hemoglobins sharing a substantial sequence similarity may present important functional differences because of the mutation of a few key residues. Thus, the functional properties of hemoglobin and blood may reflect more closely the recent environmental challenges than the remote evolutionary history of the animal. We summarize in this review the case of hemoglobins from mammals, in an attempt to provide a reasoned summary of their complexity that, we hope, may be of help to scientists interested in the quantitative exploration of the evolutionary physiology of respiration. Indeed the basis of a meaningful modelization of the red cell requires a large amount of information collected in painstaking and often forgotten studies of the biochemical properties of hemoglobin carried out over more than a century.

Multiplicity and Polymorphism of Fish Hemoglobins

The diversity of fish hemoglobins and the association with oxygen availability and physiological requirements during the life cycle has attracted scientists since the first report on multiple hemoglobin in fishes (Buhler and Shanks 1959). The functional heterogeneity of the fish hemoglobins enables many species to tolerate hypoxic conditions and exhausting swimming, but also to maintain the gas pressure in the swim bladder at large depths. The hemoglobin repertoire has further increased in various species displaying polymorphic hemoglobin variants differing in oxygen binding properties. The multiplicity of fish hemoglobins as particularly found in the tetraploid salmonids strongly contrasts with the complete loss of hemoglobins in Antarctic icefishes and illustrates the adaptive radiation in the oxygen transport of this successful vertebrate group.

Molecular cloning, expression and biochemical characterization of hemoglobin gene from ark shell Scapharca broughtonii

Hemoglobin, the main component of haemolymph, is widely distributed in animals. Although its important oxygen transport functions has been extensively reported, studies on the immune function of hemoglobin in mollusc are few. Research on immune of hemoglobin of ark shell Scapharca broughtonii attracted more and more attention due to its ownership of erythrocyte comparing with many other shellfish. In this study, the hemoglobin cDNA of S. broughtonii was cloned by EST and RACE methods (named as SbHb). Sequence analysis revealed that the cDNA was 946 bp in length, including an open reading frame (ORF) of 459 bp which encoded a polypeptide of 152 amino acid residues, and a 5'-untranslated region (UTR) of 313 bp, a 3'-UTR of 174 bp. Sequence and homology analysis showed that the SbHb shared similarity with that of other related species. The mRNA expression profiles of SbHb in tested tissues analyzed by quantitative real-time PCR (qRT-PCR) revealed that the mRNA of SbHb could be all detected in foot, gill, mantle, adductor muscle, haemocytes and hepatopancreas, and the highest level was found in the haemocytes, which is 163.2 times higher than that in adductor muscle. Vibrio anguillarum stimulation and hypoxia treatment both had significant impact on the expression of SbHb, which showed the same trends as increasing first to the highest at 16 h after treatment and then followed by declining. Recombinant protein of SbHb (rSbHb) was successfully obtained by prokaryotic expression, and further function analysis indicated obviously that the rSbHb had very strong phenoloxidase-like activity (PO-like activity) and it could remarkably inhibit growth of gram-negative bacteria V. anguillarum. All the data suggested that the SbHb plays a significant role in the process of antibacterial and anoxia tolerance reaction in S. broughtonii, providing the evidence that SbHb is a key immune factor.

Enthalpic partitioning of the reduced temperature sensitivity of O2 binding in bovine hemoglobin

The oxygenation enthalpy of the heme groups of hemoglobin (Hb) is inherently exothermic, resulting in decreased Hb-O2 affinity with rising temperature. However, oxygenation is coupled with endothermic dissociation of allosteric effectors (e.g. protons, chloride ions and organic phosphates) from the protein, which reduces the overall oxygenation enthalpy. The evolution of Hbs with reduced temperature sensitivity ostensibly safeguards O2 unloading in cold extremities of regionally-heterothermic vertebrates permitting energy-saving reductions in heat loss. Ungulate (e.g. bovine) Hbs have long served as a model system in this regard in that they exhibit numerically low oxygenation enthalpies that are thought to correlate with the presence of an additional Cl(-) binding site (compared to human Hb) comprised of three cationic residues at positions 8, 76 and 77 of the β-chains of Hb. However, ungulate Hbs also exhibit distinctive amino acid exchanges at the N-termini of the β-chains that stabilize the low-affinity deoxystructure of the Hb, mimicking the action of organic phosphates. In order to assess the relative contributions from these two effects, we measured the temperature sensitivity of Hb-O2 affinity in bovine and human Hbs in the absence and presence of Cl(-) ions under strictly controlled pH conditions. The data indicate that Cl(-)-binding accounts for a minority (~30%) of the total reduction in the oxygenation enthalpy manifested in bovine compared to human Hb, whereas the majority of this reduction is ascribable to structural differences, including increased β-chain hydrophobicity that would increase the heat of oxygenation-linked conformational change in bovine Hb.

The conserved Phe GH5 of importance for hemoglobin intersubunit contact is mutated in gadoid fish

Background

Functionality of the tetrameric hemoglobin molecule seems to be determined by a few amino acids located in key positions. Oxygen binding encompasses structural changes at the interfaces between the α1β2 and α2β1 dimers, but also subunit interactions are important for the oxygen binding affinity and stability. The latter packing contacts include the conserved Arg B12 interacting with Phe GH5, which is replaced by Leu and Tyr in the α^A and α^D chains, respectively, of birds and reptiles.

Results

Searching all known hemoglobins from a variety of gnathostome species (jawed vertebrates) revealed the almost invariant Arg B12 coded by the AGG triplet positioned at an exon-intron boundary. Rare substitutions of Arg B12 in the gnathostome β globins were found in pig, tree shrew and scaled reptiles. Phe GH5 is also highly conserved in the β globins, except for the Leu replacement in the β1 globin of five marine gadoid species, gilthead seabream and the Comoran coelacanth, while Cys and Ile were found in burbot and yellow croaker, respectively. Atlantic cod β1 globin showed a Leu/Met polymorphism at position GH5 dominated by the Met variant in northwest-Atlantic populations that was rarely found in northeast-Atlantic cod. Site-specific analyses identified six consensus codons under positive selection, including 122β(GH5), indicating that the amino acid changes identified at this position may offer an adaptive advantage. In fact, computational mutation analysis showed that the replacement of Phe GH5 with Leu or Cys decreased the number of van der Waals contacts essentially in the deoxy form that probably causes a slight increase in the oxygen binding affinity.

Conclusions

The almost invariant Arg B12 and the AGG codon seem to be important for the packing contacts and pre-mRNA processing, respectively, but the rare mutations identified might be beneficial. The Leu122β1(GH5)Met and Met55β1(D6)Val polymorphisms in Atlantic cod hemoglobin modify the intradimer contacts B12-GH5 and H2-D6, while amino acid replacements at these positions in avian hemoglobin seem to be evolutionary adaptive in air-breathing vertebrates. The results support the theory that adaptive changes in hemoglobin functions are caused by a few substitutions at key positions.

Role of β/δ101Gln in regulating the effect of temperature and allosteric effectors on oxygen affinity in woolly mammoth hemoglobin

The oxygen affinity of woolly mammoth hemoglobin (rHb WM) is less affected by temperature change than that of Asian elephant hemoglobin (rHb AE) or human normal adult hemoglobin (Hb A). We report here a biochemical-biophysical study of Hb A, rHb AE, rHb WM, and three rHb WM mutants with amino acid substitutions at β/δ101 (β/δ101Gln→Glu, Lys, or Asp) plus a double and a triple mutant, designed to clarify the role of the β/δ101 residue. The β/δ101Gln residue is important for responding to allosteric effectors, such as phosphate, inositol hexaphosphate (IHP), and chloride. The rHb WM mutants studied generally have higher affinity for oxygen under various conditions of pH, temperature, and salt concentration, and in the presence or absence of organic phosphate, than do rHb WM, rHb AE, and Hb A. Titrations for the O2 affinity of these mutant rHbs as a function of chloride concentration indicate a lower heterotopic effect of this anion due to the replacement of β/δ101Gln in rHb WM. The alkaline Bohr effect of rHb WM and its mutants is reduced by 20-50% compared to that of Hb A and is independent of changes in temperature, in contrast to what has been observed in the hemoglobins of most mammalian species, including human. The results of our study on the temperature dependence of the O2 affinity of rHb WM and its mutant rHbs illustrate the important role of β/δ101Gln in regulating the functional properties of these hemoglobins.

Air-breathing behavior and physiological responses to hypoxia and air exposure in the air-breathing loricariid fish, Pterygoplichthys anisitsi

Hypoxic water and episodic air exposure are potentially life-threatening conditions that fish in tropical regions can face during the dry season. This study investigated the air-breathing behavior, oxygen consumption, and respiratory responses of the air-breathing (AB) armored catfish Pterygoplichthys anisitsi. The hematological parameters and oxygen-binding characteristics of whole blood and stripped hemoglobin and the intermediate metabolism of selected tissue in normoxia, different hypoxic conditions, and after air exposure were also examined. In normoxia, this species exhibited high activity at night and AB behavior (2-5 AB h(-1)). The exposure to acute severe hypoxia elicited the AB behavior (4 AB h(-1)) during the day. Under progressive hypoxia without access to the water surface, the fish were oxyregulators with a critical O2 tension, calculated as the inspired water O2 pressure, as 47 ± 2 mmHg. At water O2 tensions lower than 40 mmHg, the fish exhibited continuous apnea behavior. The blood exhibited high capacity for transporting O2, having a cathodic hemoglobin component with a high Hb-O2 affinity. Under severe hypoxia, the fish used anaerobic metabolism to maintain metabolic rate. Air exposure revealed physiological and biochemical traits similar to those observed under normoxic conditions.

A biochemical--biophysical study of hemoglobins from woolly mammoth, Asian elephant, and humans

This study is aimed at investigating the molecular basis of environmental adaptation of woolly mammoth hemoglobin (Hb) to the harsh thermal conditions of the Pleistocene ice ages. To this end, we have carried out a comparative biochemical-biophysical characterization of the structural and functional properties of recombinant hemoglobins (rHb) from woolly mammoth (rHb WM) and Asian elephant (rHb AE) in relation to human hemoglobins Hb A and Hb A(2) (a minor component of human blood). We have obtained oxygen equilibrium curves and calculated O(2) affinities, Bohr effects, and the apparent heat of oxygenation (ΔH) in the presence and absence of allosteric effectors [inorganic phosphate and inositol hexaphosphate (IHP)]. Here, we show that the four Hbs exhibit distinct structural properties and respond differently to allosteric effectors. In addition, the apparent heat of oxygenation (ΔH) for rHb WM is less negative than that of rHb AE, especially in phosphate buffer and the presence of IHP, suggesting that the oxygen affinity of mammoth blood was also less sensitive to temperature change. Finally, (1)H NMR spectroscopy data indicates that both α(1)(β/δ)(1) and α(1)(β/δ)(2) interfaces in rHb WM and rHb AE are perturbed, whereas only the α(1)δ(1) interface in Hb A(2) is perturbed compared to that in Hb A. The distinct structural and functional features of rHb WM presumably facilitated woolly mammoth survival in the Arctic environment.

Disorders of the synthesis of human fetal hemoglobin

Fetal hemoglobin (HbF), the predominant hemoglobin in the fetus, is a mixture of two molecular species (alpha(2)(G)gamma(2) and alpha(2)(A)gamma(2)) that differ only at position 136 reflecting the products of two nonallelic gamma-globin genes. At the time of birth, HbF accounts for approximately 70% of the total Hb. The (G)gamma:(A)gamma globin ratio in the HbF of normal newborn is 70:30 whereas in the trace amounts of HbF that is found in the adult it reverses to 40:60 because of a gamma- to beta-globin gene switch. Alterations of these ratios are indicative of a molecular defect at the level of the HbF synthesis. Qualitative hemoglobinopathies due to (G)gamma and (A)gamma chain structural variants, and quantitative hemoglobinopathies affecting the synthesis of HbF such as gamma-thalassemias, duplications, triplications, and even sextuplications of the gamma-globin genes, which may be detected in newborn blood lysates, have been described. Moreover, several pathological and nonpathological conditions affecting the beta-globin gene cluster, such as beta-thalassemia, sickle cell disease, deltabeta-thalassemia, and hereditary persistence of HbF syndromes, are characterized by the continued synthesis of gamma-globin chains in the adult life. Studies of these natural mutants associated with increased synthesis of HbF in adult life have provided considerable insight into the understanding of the control of globin gene expression and Hb switching.

Body temperature-related structural transitions of monotremal and human hemoglobin

In this study, temperature-related structural changes were investigated in human, duck-billed platypus (Ornithorhynchus anatinus, body temperature T(b) = 31-33 degrees C), and echidna (Tachyglossus aculeatus, body temperature T(b) = 32-33 degrees C) hemoglobin using circular dichroism spectroscopy and dynamic light scattering. The average hydrodynamic radius (R(h)) and fractional (normalized) change in the ellipticity (F(obs)) at 222 +/- 2 nm of hemoglobin were measured. The temperature was varied stepwise from 25 degrees C to 45 degrees C. The existence of a structural transition of human hemoglobin at the critical temperature T(c) between 36-37 degrees C was previously shown by micropipette aspiration experiments, viscosimetry, and circular dichroism spectroscopy. Based on light-scattering measurements, this study proves the onset of molecular aggregation at T(c). In two different monotremal hemoglobins (echidna and platypus), the critical transition temperatures were found between 32-33 degrees C, which are close to the species' body temperature T(b). The data suggest that the correlation of the structural transition's critical temperature T(c) and the species' body temperature T(b) is not mere coincidence but, instead, is a more widespread structural phenomenon possibly including many other proteins.

Evolution of globin genes of the medaka Oryzias latipes (Euteleostei; Beloniformes; Oryziinae)

Recently we cloned two globin gene clusters from the genome of medaka (Oryzias latipes): one designated the embryonic globin gene cluster (E1; (5')alpha0(3')-(3')beta1(5')-(5')alpha1(3')-(5')beta2(3')-(5')alpha2(3')-(3')alpha3(5')-(5')beta3(3')-(3')beta4(5')-(5')alpha4(3')-(3')psialpha(5')-(5')psibeta(3')) and the other the adult globin gene cluster (A1; (3')ad.alpha1(5')-(5')ad.beta1(3')-(3')ad.alpha2(5')). The E1 and A1 clusters map to linkage groups 8 and 19, respectively. The genes beta1/alpha1, alpha3/beta3, beta4/alpha4, psialpha/psibeta and ad.alpha1/ad.beta1 are organized in head-to-head orientation with respect to transcriptional polarity. The genes alpha0, alpha1 and alpha2 are arranged in tandem with the same orientation. The results suggest that a variety of events occurred in globin gene evolution such as chromosomal translocation, duplication of alpha/beta-paired genes, tandem duplication of single alpha genes and the transformation of one pair of alpha/beta-paired genes into pseudogenes (psialpha/psibeta). Amino acid sequences predicted from the genes were compared with those of 42 alpha and 55 beta teleostean globins using the neighbor-joining or maximum likelihood methods. The phylogenetic trees that were generated classified the teleostean globins into at least four groups, tentatively named 'Embryonic Hb Group (I)', 'Notothenioid Major Adult Hb Group (II)', 'Anodic Adult Hb Group (III)' and 'Cathodic Adult Hb Group (IV)'. The medaka genes alpha0, beta1, alpha1, alpha2, alpha3, beta3, beta4 and alpha4 belong to group I, and ad.alpha1 and ad.beta1 to group II. Further analysis suggests that psialpha/psibeta and beta2/ad.alpha2 belong to groups III and IV, respectively. Thus, globin genes in the medaka probably were diversified from four ancestral genes, one for each group. On the basis of the gene comparisons, we present a hypothetical pathway for globin gene evolution in the medaka.

The Bohr effect of haemoglobin in vertebrates: an example of molecular adaptation to different physiological requirements

The Bohr effect, i.e. the pH dependence of the oxygen affinity of haemoglobins (Hbs) from a variety of vertebrates, and its modulation by temperature and other heterotropic effectors has been reviewed. Haemoglobins from vertebrates were not reviewed following the usual classification (i.e. mammals, birds, etc.); instead we have selected several key examples of animals, which are confronted with a similar environmental situation therefore displaying a similar life style. Hence, the paper starts from a description of the general concepts at the basis of the Bohr effect as exemplified by human HbA and goes towards the analysis of the modulation mechanisms which have been observed in different animals in response to the needs induced by: (i) life in cold environments; (ii) diving behaviour; (iii) flight; and (iv) aquatic life. The emerging picture indicates a complex organization of the information contained in the Hb molecule, the oxygen-binding properties of which depend both on the intrinsic characteristics of the protein and on its heterotropic interactions with ligands such as protons (Bohr effect), small anions like chloride and organic phosphates. In addition, each one of the functional effects induced by binding of a given effector appears to be under the strict control of temperature that enhances or decreases its relative weight with respect to all the others. It is just by this sophisticated network of interactions that the Hb molecule is able to satisfy the physiological requirements of a multitude of organisms without changing dramatically its quaternary structure.

From the Arctic to fetal life: physiological importance and structural basis of an 'additional' chloride-binding site in haemoglobin

Haemoglobins from mammals of sub-Arctic and Arctic species, as well as fetal human Hb, are all characterized by a significantly lower Delta H of oxygenation compared with the majority of mammalian haemoglobins from temperate species (exceptions are represented by some cold-resistant species, such as cow, horse and pig). This has been interpreted as an adaptive mechanism of great importance from a physiological point of view. To date, the molecular basis of this thermodynamic characteristic is still not known. In the present study, we show that binding of extra chloride (with respect to adult human Hb) ions to Hb would significantly contribute to lowering the overall heat of oxygenation, thus providing a molecular basis for the low effect of temperature on the oxygenation-deoxygenation cycle. To this aim, the oxygen binding properties of bovine Hb, bear (Ursus arctos) Hb and horse Hb, which are representative of this series of haemoglobins, have been studied with special regard to the effect of heterotropic ligands, such as organic phosphates (namely 2,3-diphosphoglycerate) and chloride. Functional results are consistent with a mechanism for ligand binding that involves an additional binding site for chloride ion. Analysis of computational chemistry results, obtained by the GRID program, further confirm the hypothesis that the reason for the lower Delta H of oxygenation is mainly due to an increase in the number of the oxygen-linked chloride-binding sites.

Circular dichroism spectra of human hemoglobin reveal a reversible structural transition at body temperature

Previously we have shown that human red blood cells (RBCs) undergo a sudden change from blocking to passing through a 1.3+/-0.2-microm micropipette when applying an aspiration pressure of 2.3 kPa at a critical transition temperature (Tc = 36.4+/-0.3 degrees C). Low-shear viscosity measurements suggested that changes in the molecular properties of hemoglobin might be responsible for this effect. To evaluate structural changes in hemoglobin at the critical temperature, we have used circular dichroism (CD) spectroscopy. The thermal denaturation curves of human hemoglobin A (HbA) and hemoglobin S (HbS) upon heating between 25 and 60 degrees C were non-linear and showed accelerated denaturation between 35 and 39 degrees C with a midpoint at 37.2+/-0.6 degrees C. The transition was reversible below 39 degrees C and independent of solution pH (pH 6.8-7.8). It was also independent of the oxygenation state of hemoglobin, since a sample that was extensively deoxygenated with N2 showed a similar transition by CD. These findings suggest that a structural change in hemoglobin may enable the cellular passage phenomenon as well as the temperature-dependent decrease in viscosity of RBC solutions.

[Hemoglobin, from microorganisms to man: a single structural motif, multiple functions]

Haemoglobins from unicellular organisms, plants or animals, share a common structure, which results from the folding, around the heme group, of a polypeptide chain made from 6-8 helices. Nowadays, deciphering the genome of several species allows one to draw the evolutionary tree of this protein going back to 1800 millions of years, at a time when oxygen began to accumulate in the atmosphere. This permits to follow the evolution of the ancestral gene and of its product. It is likely that, only in complex multicellular species, transport and storage of oxygen became the main physiological function of this molecule. In addition, in unicellular organisms and small invertebrates, it is likely that the main function of this protein was to protect the organism from the toxic effect of O2, CO and NO*. The very high oxygen affinity of these molecules, leading them to act rather as a scavenger as an oxygen carrier, supports this hypothesis. Haemoglobins from microorganisms, which may probably be the closest vestiges to the ancestral molecules, are divided into three families. The first one is made from flavohaemoglobins, a group of chimerical proteins carrying a globin domain and an oxido-reduction FAD-dependant domain. The second corresponds to truncated haemoglobins, which are hexacoordinated with very high oxygen-affinity molecules, 20-40 residues shorter than classical haemoglobins. The third group is made from bacterial haemoglobins such as that of Vitreoscilla. Some specific structural arrangements in the region surrounding the heme are cause of their high oxygen affinity. In plants, two types of haemoglobins are present (non-symbiotic and symbiotic), that arose from duplication of an ancestral vegetal gene. Non-symbiotic haemoglobins, which are probably the oldest, are scarcely distributed within tissues having high energetic consumption. Conversely, symbiotic haemoglobins (also named leghaemoglobins) are present at a high concentration (mM) mostly in the rhizomes of legumes, where they are involved in nitrogen metabolism. In some species, haemoglobin was proposed to be an oxygen sensor bringing to the organism information to adjust metabolism or biosynthesis to the oxygen requirement. Elsewhere haemoglobin may act as final electron acceptors in oxido-reduction pathways. Evolution of haemoglobin in invertebrates followed a large variety of scenarios. Some surprising functions as sulphide acquisition in invertebrates living near hydrothermal vents, or a role in the phototrophism of worm need to be mentioned. In invertebrates, the size of haemoglobin varies from monomers to giant molecules associating up to 144 subunits, while in vertebrates it is always a tetramer. In some species, several haemoglobins, with completely different structure and function, may coexist. This demonstrates how hazardous may be to extrapolate the function of a protein from only structural data.

Low-temperature sensitivity and enhanced Bohr effect in red deer (Cervus elaphus) haemoglobin: a molecular adaptive strategy to life at high altitude and low temperature

A study of the functional properties of haemoglobin from red deer (Cervus elaphus) whose habitat varies over a wide range of latitude, was performed. The oxygen-binding properties of the most common haemoglobin phenotype from the species living in Sardinia were examined with particular attention to the effect of pH, chloride, 2, 3-bisphosphoglycerate and temperature. Results indicate that red deer haemoglobin, like all haemoglobins from ruminants so far examined, is characterized by a low intrinsic oxygen affinity, with chloride being its main physiological modulator in vivo. The functional results and the low temperature sensitivity of the oxygen affinity are discussed in the light of the amino acid sequence of closely related ruminant haemoglobins.

Circe's haemoglobins, pig-human hybrids: functional characterization and structural considerations

We report the isolation and the functional characterization of alpha and beta chains from pig (Sus scropha domesticus) haemoglobin, as well as of the pig-human hybrid haemoglobins, alpha2(h)beta2(p) and alpha2(p)beta2(h) (i.e. Circe's haemoglobins), obtained by mixing the purified alpha and beta pig chains respectively with the corresponding partner human chains. Their functional properties have been compared with those of both parental haemoglobins in order to obtain information on the role of the different subunits and of their inter-relationships, both at the structural and functional levels. The results indicate that the functional properties of both hybrids are closer to those of the parental haemoglobin that provides the beta chains, confirming the major role of the beta chains in determining the oxygen affinity and the modulation mechanisms of the tetrameric molecule. This is supported by the thermodynamic properties, since the very low DeltaH of oxygen binding that characterizes pig haemoglobin and the alpha2(h)beta2(p) hybrid haemoglobin may be taken as the reflection of specific structural properties of pig beta chain.

A comparative study on earthworm hemoglobins: an amino acid sequence comparison of monomer globin chains of two species, Pontodrilus matsushimensis and Pheretima communissima that belong to the family Megascolecidae

The monomer subunits of giant extracellular hemoglobins from earthworms Pontodrilus matsushimensis and Pheretima communissima that belong to the family Megascolecidae, Oligochaeta, were purified by a reversed-phase column, Resource RPC, and sequenced. The complete amino acid sequences of the two monomeric globin chains were determined: 141 amino acid residues with a molecular weight of 16,366 Da for Pontodrilus matsushimensis and 140 amino acid residues with a molecular weight of 16,000 for Pheretima communissima, respectively. The Pontodrilus matsushimensis monomer globin has three cysteine residues, and the two located at positions 2 and 131 are conserved as those observed in all annelids and contribute to form a disulfide-bonded interchain. The third cysteine residue at position 73 is the first evidence for the annelid monomer globin subunits. The physiological functions of the third cysteine residue, however, are still unknown. The monomer sequences of the two species were aligned with those of five known sequences from annelids, including a polychaete, Tylorrhynchus heterochaetus, and four oligochaetes, Pheretima hilgendorfi, Pheretima sieboldi, Lumbricus terrestris and Tubifex tubifex. Using computer analysis, a 87.9% identity of the amino acid sequences between two monomeric subunits of Pheretima communissima and Pheretima hilgendorfi hemoglobins showed the highest degree of sequence similarity. A molecular phylogenetic tree of seven species of annelids has constructed, suggesting that the divergence times among the three species of Pheretima and between Pheretima and Pontodrilus were 50 to 100 and about 209 million years ago, respectively.

The multiple functions of hemoglobin

The aim of this review is to focus and discuss several parallel biological functions of hemoglobin besides its basic function of oxygen transport. In light of the information present in the literature the following possible physiological roles of hemoglobin are discussed: (1) hemoglobin as molecular heat transducer through its oxygenation-deoxygenation cycle, (2) hemoglobin as modulator of erythrocyte metabolism, (3) hemoglobin oxidation as an onset of erythrocyte senescence, (4) hemoglobin and its implication in genetic resistance to malaria, (5) enzymatic activities of hemoglobin and interactions with drugs, and (6) hemoglobin as source of physiological active catabolites.