Functional properties of beta(NA1)Val-deleted,(NA2)His-->Met hemoglobin synthesized in Escherichia coli

Importance of helices A and H in oxygen binding differences between bovine and human hemoglobins

Human and bovine hemoglobins (Hbs) exhibit several functional differences. They have a similar oxygen affinity in the presence of 2,3-diphosphoglycerate (2,3-DPG); however, bovine Hb has a greatly diminished 2,3-DPG effect, which itself is chloride dependent. The question is to determine whether these differences have a common structural origin, or whether they evolved in an independent fashion. The decreased 2,3-DPG effect can be partially reproduced by mutations at the effector binding sites, substituting the betaNA1 valine-NA2 histidine present in human Hb with a methionine. While changes of human Hb at these sites could provoke the bovine characteristic of the lower 2,3-DPG effect, the oxygen affinities of these mutated Hbs were not as low as that of the bovine Hb. Modifications responsible for tertiary structural modifications of helix A in human Hb might help shift the N-terminal methionine position, thereby locking helix A in place. We replaced the residues proline beta5(A2), arginine beta104(G6), and tyrosine beta130(H8) of human Hb by the residues present in bovine beta-globin, namely alanine, lysine, and phenylalanine, respectively. These mutations did not allow us to obtain a low oxygen affinity recombinant Hb (rHb). This indicates that other factors also influence oxygen binding and the effects are only partially coupled.

[Transfer of functional properties from bovine hemoglobin to human hemoglobin]

Bovine hemoglobin (Hb) has been proposed as a potential blood substitute because of its low intrinsic oxygen affinity in the absence of chloride anions and 2,3-diphosphoglycerate. The use of bovine blood as a source of Hb does not eliminate the risks of viral infections. Biotechnology techniques allow to produce modified recombinant Hbs. We have engineered human Hb mutants with the aim of mimicking the functional properties of bovine Hb. The argument for this work resides in the crystallographic studies and in the comparison of human and bovine beta globin sequences. The mutant recombinant Hbs exhibit the heterotropic effects of bovine Hb do not exhibit the low intrinsic oxygen affinity of bovine Hb.