Synergistic modulation by chloride and organic phosphates of hemoglobin from bear (Ursus arctos)

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.

Decrease in the red cell cofactor 2,3-diphosphoglycerate increases hemoglobin oxygen affinity in the hibernating brown bear Ursus arctos

During winter hibernation, brown bears (Ursus arctos) reduce basal O(2) consumption rate to ∼25% compared with the active state, while body temperature decreases moderately (to ∼30°C), suggesting a temperature-independent component in their metabolic depression. To establish whether changes in O(2) consumption during hibernation correlate with changes in blood O(2) affinity, we took blood samples from the same six individuals of hibernating and nonhibernating free-ranging brown bears during winter and summer, respectively. A single hemoglobin (Hb) component was detected in all samples, indicating no switch in Hb synthesis. O(2) binding curves measured on red blood cell lysates at 30°C and 37°C showed a less temperature-sensitive O(2) affinity than in other vertebrates. Furthermore, hemolysates from hibernating bears consistently showed lower cooperativity and higher O(2) affinity than their summer counterparts, regardless of the temperature. We found that this increase in O(2) affinity was associated with a significant decrease in the red cell Hb-cofactor 2,3-diphosphoglycerate (DPG) during hibernation to approximately half of the summer value. Experiments performed on purified Hb, to which DPG had been added to match summer and winter levels, confirmed that the low DPG content was the cause of the left shift in the Hb-O(2) equilibrium curve during hibernation. Levels of plasma lactate indicated that glycolysis is not upregulated during hibernation and that metabolism is essentially aerobic. Calculations show that the increase in Hb-O(2) affinity and decrease in cooperativity resulting from decreased red cell DPG may be crucial in maintaining a fairly constant tissue oxygen tension during hibernation in vivo.

Temperature dependence of haemoglobin-oxygen affinity in heterothermic vertebrates: mechanisms and biological significance

As demonstrated by August Krogh et al. a century ago, the oxygen-binding reaction of vertebrate haemoglobin is cooperative (described by sigmoid O(2) equilibrium curves) and modulated by CO(2) and protons (lowered pH) that - in conjunction with later discovered allosteric effectors (chloride, lactate and organic phosphate anions) - enhance O(2) unloading from blood in relatively acidic and oxygen-poor tissues. Based on the exothermic nature of the oxygenation of the haem groups, haemoglobin-O(2) affinity also decreases with rising temperature. This thermal sensitivity favours oxygen unloading in warm working muscles, but may become detrimental in regionally heterothermic animals, for example in cold-tolerant birds and mammals and warm-bodied fish, where it may perturb the balance between O(2) unloading and O(2) requirement in organs with substantially different temperatures than at the respiratory organs and thus commonly is reduced or obliterated. Given that the oxygenation of haemoglobin is linked with the endothermic release of allosteric effectors, increased effector interaction is an effective strategy that is widely exploited to achieve adaptive reductions in the temperature dependence of blood-O(2) affinity. The molecular mechanisms implicated in heterothermic vertebrates from different taxonomic groups reveal remarkable variability, both as regards the effectors implicated (protons in tunas, organic phosphates in sharks and billfish, chloride ions in ruminants and chloride and phosphate anions in the extinct woolly mammoth, etc.) and binding sites for the same effectors, indicating multiple evolutionary origins, but convergent physiological functionality (reductions in temperature dependence of O(2) -binding affinity that safeguard tissue O(2) supply).

ATP-induced temperature independence of hemoglobin-O2 affinity in heterothermic billfish

The inverse relationship between temperature and hemoglobin-O(2) affinity resulting from the exothermic nature of heme oxygenation favors O(2) unloading from blood to warm, metabolically active tissues. However, this temperature sensitivity is maladaptive, and commonly countered in regional heterotherms, where it may hamper unloading (e.g. in cold extremities of arctic mammals) or increase the diffusive arterio-venous short-circuiting of O(2) (e.g. in counter-current heat exchangers of warm swimming muscles of tuna). We hypothesized analogous blood specializations in heterothermic billfish, whose warm eyes and brains increase the temporal resolution of vision, and measured hemoglobin-O(2) binding properties in three species over a wide pH range, at two temperatures, and in the absence and presence of the major red cell effector, ATP, permitting detailed assessment of overall oxygenation enthalpies (DeltaH') and contributions from oxygenation-linked proton and ATP dissociation. Billfish express multiple isohemoglobins with similar O(2) affinities and pronounced sensitivities to pH and ATP. Compared with the moderate effects associated with proton dissociation upon oxygenation, dissociation of ATP and coupled extra Bohr protons virtually obliterates the temperature sensitivities. At pH 7.4, where this effect is maximal, ATP changes DeltaH' values of blue marlin, striped marlin and shortbill spearfish hemoglobins from -39, -49 and -44 kJ mol(-1) O(2), respectively, to +26, +4 and -7 kJ mol(-1). Thus in addition to allosterically modulating hemoglobin-O(2) affinity, ATP diminishes its temperature sensitivity, reducing deleterious arterio-venous short-circuiting of oxygen in the cranial billfish heat exchangers. The mechanism underlying this reduction in oxygenation enthalpy differs fundamentally from that in tuna, supporting independent evolution of this trait in these scombroid lineages.

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.

Functional characterisation of Eskimo dog hemoglobin: I. Interaction of Cl- and 2,3-DPG and its importance to oxygen unloading at low temperature

The oxygen binding properties of hemoglobin and some hematological parameters in Eskimo dogs (belonging to Canis lupus familiaris) in Ilulissat/Jacobshavn, Greenland were analysed. The average [2,3-DPG] and [Hb] (n = 16) were 3.14 +/- 0.34 mmol l-1 blood and 9.53 +/- 0.65 g dl-1 (1.49 mmol l-1), respectively, giving a stoichiometric ratio of 2.11 mol 2,3-DPG/mol Hb. Oxygen binding analysis carried out on hemolysate in HEPES buffer at 20 and 37 degrees C revealed a high oxygen affinity (1.2 mmHg at pH 7.4, 20 degrees C) in the desalted condition, which decreased markedly in the presence of chloride and 2,3-DPG. A low apparent equilibrium constant for the binding of 2,3-DPG (1.0 x 10(-5) mol l-1) was found at pH 7.2 and 20 degrees C in the absence of chloride. Moreover, we show that chloride ions have an additive effect on oxygen affinity in the concentration range 10-300 mmol l-1 in the presence of 3 mmol l-1 2,3-DPG at low pH and temperature (pH < 7.4 and 20 degrees C). This feature may be of physiological importance to oxygen unloading under acidotic conditions when tissue temperature is low. Thermodynamic analysis reveal that in the presence of 3 mmol l-1 2,3-DPG and 100 mmol l-1 chloride, the Eskimo dog hemoglobin exhibits a low heat of oxygenation, which places this animal close to arctic ruminants with respect to the influence of temperature on oxygen binding in vivo.