Introduction of a new regulatory mechanism into human hemoglobin

Design of recombinant hemoglobins for use in transfusion fluids

Molecular biology has been applied to the development of hemoglobin-based oxygen carrier (HBOC) proteins that can be expressed in bacteria or yeast. The transformation of the hemoglobin molecule into an HBOC requires a variety of modifications for rendering the acellular molecule of hemoglobin physiologically acceptable when transfused in circulation. Hemoglobins with different oxygen affinities can be obtained by introducing mutations at the heme pocket, the site of oxygen binding, or by introducing surface mutations that stabilize the hemoglobin molecule in the low-oxygen-affinity state. Modification of the size of the heme pocket is also used to hinder nitric oxide depletion and associated vasoconstriction. Introduction of cysteine residues on the hemoglobin surface allows formation of intermolecular bonds and formation of polymeric HBOCs. These polymers of recombinant hemoglobin have the characteristics of molecular size, molecular stability, and oxygen delivery to hypoxic tissue suitable for an HBOC.

Recombinant hemoglobins as artificial oxygen carriers

This paper describes the approaches we have taken to construct a) mutant hemoglobins with different oxygen affinities, and b) mutant hemoglobins and myoglobins that polymerize to high molecular weight aggregates in an effort to prevent extravasation and the associated vasoactivity. In vivo testing indicates that exchange transfusion of polymeric hemoglobins in mice does not result in vasoactivity and that polymeric hemoglobins are effective oxygen carriers to ischemic tissues irrespective of their oxygen affinity and cooperativity.

Semihemoglobins, high oxygen affinity dimeric forms of human hemoglobin respond efficiently to allosteric effectors without forming tetramers

Significant reduction in oxygen affinity resulting from interactions between heterotropic allosteric effectors and hemoglobin in not only the unligated derivative but also the fully ligated form has been reported (Tsuneshige, A., Park, S. I., and Yonetani, T. (2002) Biophys. Chem. 98, 49-63; Yonetani, T., Park, S. I., Tsuneshige, A., Imai, K., and Kanaori, K. (2002) J. Biol. Chem. 277, 34508-34520). To further investigate this effect in more detail, alpha- and beta-semihemoglobins, namely, alpha(heme)beta(apo) and alpha(apo)beta(heme), respectively, were prepared and characterized with respect to the impact of allosteric effectors on both conformation and ligand binding properties. Semihemoglobins are dimers characterized by a high affinity for oxygen and lack of cooperativity. We found that, compared with stripped conditions, semihemoglobins responded to effectors (inositol hexaphosphate and L35) by decreasing the affinity for oxygen by 60- and 130-fold for alpha- and beta-semihemoglobins, respectively. 1H NMR and sedimentation velocity experiments carried out with their ligated and unligated forms in the absence and presence of effectors revealed that semihemoglobins always remain as single-heme-carrying dimers. Recombination kinetics of their photolyzed CO derivatives showed that effectors did indeed interact with their ligated forms. Measurements of the Fe-His stretching mode show that the semihemoglobins undergo a large ligand binding-induced conformational shift and that both ligand-free and ligand derivatives respond to the presence of effectors. Contradictions to the Monod-Wyman-Changeaux/Perutz allosteric model arise since 1) the modulation of ligand affinity is not achieved in semihemoglobins by the formation of a low affinity T conformation (quaternary effect) but by direct interaction with effectors, 2) effectors do interact significantly with ligated forms of high affinity semihemoglobins, and 3) modulation of the ligand affinity and the cooperativity are not necessarily linked but instead can be separated into two distinct phenomena that can be isolated.

A recombinant polymeric hemoglobin with conformational, functional, and physiological characteristics of an in vivo O2 transporter

With the objective of developing a recombinant oxygen carrier suitable for therapeutic applications, we have employed an Escherichia coli expression system to synthesize in high-yield hemoglobin (Hb) Minotaur, containing alpha-human and beta-bovine chains. Polymerization of Hb Minotaur through S-S intermolecular cross-linking was obtained by introducing a Cys at position beta9 and substituting the naturally occurring Cys. This homogeneous polymer, Hb Polytaur, has a molecular mass of approximately 500 kDa and was resistant toward reducing agents present in blood. In mice, the circulating half-time (3 h) was fivefold greater than adult human Hb (HbA). The half-time of autooxidation measured in blood (46 h) exceeded the circulating retention time. Hypervolemic exchange transfusion resulted in increased arterial blood pressure similar to that with albumin. The increase in pressure was less than that obtained by transfusion of cross-linked tetrameric Hb known to undergo renovascular extravasation. The nitric oxide reactivity of Hb Polytaur was similar to HbA, suggesting that the diminished pressor response to Hb Polytaur was probably related to diminished extravasation. Transfusion of 3% Hb Polytaur during focal cerebral ischemia reduced infarct volume by 22%. Therefore, site-specific Cys insertion on the Hb surface results in uniform size polymers that do not produce the large pressor response seen with tetrameric Hb. Polymerization maintains physiologically relevant oxygen and heme affinity, stability toward denaturation and oxidation, and effective oxygen delivery as indicated by reduced cerebral ischemic damage.

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.