Abnormal functional properties of Hb Hope alpha C2A beta2 (H14) Gly-Asp: a low oxygen affinity hemoglobin with decreased DPG effect

Molecular basis of a novel adaptation to hypoxic-hypercapnia in a strictly fossorial mole

Background

Elevated blood O₂ affinity enhances survival at low O₂ pressures, and is perhaps the best known and most broadly accepted evolutionary adjustment of terrestrial vertebrates to environmental hypoxia. This phenotype arises by increasing the intrinsic O₂ affinity of the hemoglobin (Hb) molecule, by decreasing the intracellular concentration of allosteric effectors (e.g., 2,3-diphosphoglycerate; DPG), or by suppressing the sensitivity of Hb to these physiological cofactors.

Results

Here we report that strictly fossorial eastern moles (Scalopus aquaticus) have evolved a low O₂ affinity, DPG-insensitive Hb - contrary to expectations for a mammalian species that is adapted to the chronic hypoxia and hypercapnia of subterranean burrow systems. Molecular modelling indicates that this functional shift is principally attributable to a single charge altering amino acid substitution in the β-type δ-globin chain (δ136Gly→Glu) of this species that perturbs electrostatic interactions between the dimer subunits via formation of an intra-chain salt-bridge with δ82Lys. However, this replacement also abolishes key binding sites for the red blood cell effectors Cl^-, lactate and DPG (the latter of which is virtually absent from the red cells of this species) at δ82Lys, thereby markedly reducing competition for carbamate formation (CO₂ binding) at the δ-chain N-termini.

Conclusions

We propose this Hb phenotype illustrates a novel mechanism for adaptively elevating the CO₂ carrying capacity of eastern mole blood during burst tunnelling activities associated with subterranean habitation.

Hb Hope [β136(H14)Gly→Asp (GGT→GAT)]: Interactions with Hb S [β6(A3)Glu→Val (GAG→GTG)], Other Variant Hemoglobins and Thalassemia

Hb Hope [beta136(H14)Gly-->Asp (GGT-->GAT)] was first described in an African-American family in 1965. Since then, it has been found in combination with several different globin gene mutations in many other families of divergent ethnic backgrounds. The basis for its relatively frequent occurrences remains unexplained. This variant hemoglobin (Hb) is mildly unstable and has reduced oxygen affinity, but is generally innocuous clinically. This variant Hb can present as a confounding factor in arriving at a correct diagnosis by either electrophoresis or high performance liquid chromatography (HPLC), particularly during the neonatal period. DNA-based diagnostics can help solve this potential problem.

Hb Hope, beta 136(H14)Gly----Asp, in a diabetic Japanese female and its functional characterization

A beta-variant hemoglobin, first misjudged as a marked elevation of Hb A1, was found in a 68-year-old Japanese female with diabetes mellitus. This hemoglobin was isolated by Bio-Rex 70 chromatography combined with chromatofocusing, and was found to be Hb Hope, beta 136(H14)Gly----Asp, by classical and high performance liquid chromatographic peptide mapping techniques. Intrinsic oxygen affinity of this hemoglobin was approximately one-third as compared with that of Hb A0. This property was still observed in the constituent beta subunits isolated. Effects of such allosteric effectors as H+ (at a fixed concentration of Cl-), anion (Cl-), 2,3-diphosphoglycerate and carbon dioxide were more or less depressed. Among others, a marked reduction in the carbamate effect should be noted in a structural interpretation of the functional modifications. Subunit cooperativity, on the contrary, was not different from that in Hb A0 (n = 2.8-2.9). Explanation of these altered functions were attempted on the basis of the altered structure. The reduced stability of Hb Hope is also described.

Probing the energetics of proteins through structural perturbation: sites of regulatory energy in human hemoglobin

The sites of energy transduction within the human hemoglobin molecule for the regulation of oxygen affinity have been determined by an extensive study of the molecule's energetic response to structural alteration at individual amino acid residues. For 22 mutant and chemically modified hemoglobins we have determined the total free energy used by the tetrameric molecule for alteration of oxygen affinity at the four binding steps. The results imply that the regulation of oxygen binding affinity is due to energy changes which are mostly localized at the alpha 1 beta 2 interface. They also indicate a high degree of "internal cooperativity" within this contact region--i.e., the structural perturbations at individual residue sites are energetically coupled. Cooperativity in ligand binding is thus a reflection of cooperativity at a deeper level--that of the protein-protein interactions within the alpha 1 beta 2 interfacial domain.

Effect of some physical features and of amino acid substitutions on the mechanical precipitation of hemoglobin

In 1973 Asakura demonstrated that, following mechanical shaking, oxy-Hb S was much less stable than oxy-Hb A (Nature 244: 437, 1973). We have studied the mechanical stability of Hb Crétil (beta 89 Ser leads to Asn), Hb Hope (beta 136 Gly leads to Asp), Hb Strasbourg (beta 23 Val leads to Asp), and the hybrid Hb S/Stanleyville-II (beta 6 Glu leads to Val: alpha 78 Asn leads to Lys) by the method of Roth et al. (Blood 45:377, 1975). Hb Créteil, Hb Hope, and Hb S/St-II were sensitive to mechanical shaking, while Hb Strasbourg was more stable than Hb A, a hitherto undescribed finding. The precise mechanisms responsible for this precipitation are not known. From comparisons with published results, but excluding thermosensitive Hbs, we conclude that: - standard methods for isolation of hemoglobins modify its mechanical stability, - alpha mutation increases the mechanical stability of Hb S in the hybrid Hb S/St-II, - some mutations produce a more stable hemoglobin than Hb A.