Embryonic hemoglobins: dependency of functional characteristics on tetramer composition

1. Nucleated erythroblasts from embryonic rabbits contain two groups of tetrameric hemoglobins (Hbs): Hbs E_I-III consist of embryonic α-type chains (Ξ-chains) and embryonic β-type chains (ε-chains) whilst Hbs L_I-III are composed of adult α-chains and ε-chains. Structural analyses have indicated that the Ξ-chains are evolutionarily older than ε-chains. To obtain informations on possible differences in ligand binding properties associated with these embryonic chains, we have prepared Hbs E_I-III and L_I-III from the erythroblasts of 14-days-old embryonic rabbits and measured their oxygen affinity at various pH values and different concentrations of phosphate compounds. These data were compared with those obtained on the unfractionated embryonic hemolysate and adult rabbit hemoglobin (HbA). 2. We found that Hbs E_I-III have a higher oxygen affinity than Hbs L_I-III at all pH values investigated, the difference becoming larger at more acid pH. As a result, the Bohr effect is smaller in Hbs E_I-III than in Hbs L_I-III, Δ log P₅₀/Δ pH amounting to -0.25 and -0.50, respectively. In the pH range between 6.8 and 7.8 the oxygen affinities of HbA and of Hbs L_I-III are alike but lower in HbA at more acid pH. These results indicate that the presence of embryonic Ξ-chains in hemoglobin tetramers raise the oxygen affinity and lower the Bohr effect of the pigment, whereas the combination of adult α-chains with embryonic ε-chains lead to hemoglobin tetramers with a very similar oxygen affinity to HbA in the physiological pH range. The cooperativity of oxygen binding was smaller both in Hbs E_I-III and L_I-III compared to HbA. 3. The effect of added phosphates notably of 2,3-diphosphoglycerate (2,3-DPG) on the oxygen affinity of Hbs E_I-III and L_I-III was very similar, i.e. the rise in P₅₀ produced by maximal concentrations of 2,3-DPG was not significantly different in the two types of embryonic hemoglobins. In HbA, the increase of P₅₀ produced by comparable concentrations of 2,3-DPG was only slightly higher than in the embryonic hemoglobins. This shows that the embryonic ε-chains are similarly effective in binding phosphate as the adult β-chains.