Structures of a hemoglobin-based blood substitute: insights into the function of allosteric proteins

Molecular Dynamics of Hemoglobin Reveals Structural Alterations and Explains the Interactions Driving Sickle Cell Fibrillation

In sickle cell anemia, deoxyhemoglobin deforms RBCs by forming fibrils inside that disintegrate on oxygenation. We studied 100 ns long all-atom molecular dynamics (MD) for sickle and normal hemoglobin fibril models to understand this process, complemented by multiple 1 μs MD for a single tetramer of sickle and normal hemoglobin in deoxy and oxy states. We find that the presence of hydrophobic residues without a bulky side chain at β-6 in hemoglobin is the reason for the stability of the fibrils. Moreover, the free energy landscapes from MD of hemoglobin starting in the tensed (T) state capture the putative transition from T to relaxed (R) state, associated with oxygen binding. The three conformational wells in the landscapes are characterized by the quaternary changes where one αβ dimer rotates with respect to the other. The conformational changes from the oxygenation of sickle hemoglobin hinder the intermolecular contacts necessary for fibril formation.

Engineering oxidative stability in human hemoglobin based on the Hb providence (βK82D) mutation and genetic cross-linking

Previous work suggested that hemoglobin (Hb) tetramer formation slows autoxidation and hemin loss and that the naturally occurring mutant, Hb Providence (HbProv; βK82D), is much more resistant to degradation by H₂O₂ We have examined systematically the effects of genetic cross-linking of Hb tetramers with and without the HbProv mutation on autoxidation, hemin loss, and reactions with H₂O₂, using native HbA and various wild-type recombinant Hbs as controls. Genetically cross-linked Hb Presbyterian (βN108K) was also examined as an example of a low oxygen affinity tetramer. Our conclusions are: (a) at low concentrations, all the cross-linked tetramers show smaller rates of autoxidation and hemin loss than HbA, which can dissociate into much less stable dimers and (b) the HbProv βK82D mutation confers more resistance to degradation by H₂O₂, by markedly inhibiting oxidation of the β93 cysteine side chain, particularly in cross-linked tetramers and even in the presence of the destabilizing Hb Presbyterian mutation. These results show that cross-linking and the βK82D mutation do enhance the resistance of Hb to oxidative degradation, a critical element in the design of a safe and effective oxygen therapeutic.

Mechanism of Human Apohemoglobin Unfolding

Removal of heme from human hemoglobin (Hb) results in formation of an apoglobin heterodimer. Titration of this apodimer with guanidine hydrochloride (GdnHCl) leads to biphasic unfolding curves indicating two distinct steps. Initially, the heme pocket unfolds and generates a dimeric intermediate in which ∼50% of the original helicity is lost, but the α₁β₁ interface is still intact. At higher GdnHCl concentrations, this intermediate dissociates into unfolded monomers. This structural interpretation was verified by comparing GdnHCl titrations for adult human hemoglobin A (HbA), recombinant fetal human hemoglobin (HbF), recombinant Hb cross-linked with a single glycine linker between the α chains, and recombinant Hbs with apolar heme pocket mutations that markedly stabilize native conformations in both subunits. The first phase of apoHb unfolding is independent of protein concentration, little affected by genetic cross-linking, but significantly shifted toward higher GdnHCl concentrations by the stabilizing distal pocket mutations. The second phase depends on protein concentration and is shifted to higher GdnHCl concentrations by genetic cross-linking. This model for apoHb unfolding allowed us to quantitate subtle differences in stability between apoHbA and apoHbF, which suggest that the β and γ heme pockets have similar stabilities, whereas the α₁γ₁ interface is more resistant to dissociation than the α₁β₁ interface.

Reaction trajectory revealed by a joint analysis of protein data bank

Structural motions along a reaction pathway hold the secret about how a biological macromolecule functions. If each static structure were considered as a snapshot of the protein molecule in action, a large collection of structures would constitute a multidimensional conformational space of an enormous size. Here I present a joint analysis of hundreds of known structures of human hemoglobin in the Protein Data Bank. By applying singular value decomposition to distance matrices of these structures, I demonstrate that this large collection of structural snapshots, derived under a wide range of experimental conditions, arrange orderly along a reaction pathway. The structural motions along this extensive trajectory, including several helical transformations, arrive at a reverse engineered mechanism of the cooperative machinery (Ren, companion article), and shed light on pathological properties of the abnormal homotetrameric hemoglobins from α-thalassemia. This method of meta-analysis provides a general approach to structural dynamics based on static protein structures in this post genomics era.

Structure of Greyhound hemoglobin: origin of high oxygen affinity

This study presents the crystal structure of Greyhound hemoglobin (GrHb) determined to 1.9 Å resolution. GrHb was found to crystallize with an α₁β₁ dimer in the asymmetric unit and belongs to the R2 state. Oxygen-affinity measurements combined with the fact that GrHb crystallizes in the R2 state despite the high-salt conditions used for crystallization strongly indicate that GrHb can serve as a model high-oxygen-affinity hemoglobin (Hb) for higher mammals, especially humans. Structural analysis of GrHb and its comparison with the R2-state of human Hb revealed several regions that can potentially contribute to the high oxygen affinity of GrHb and serve to rationalize the additional stability of the R2-state of GrHb. A previously well studied hydrophobic cluster of bar-headed goose Hb near α119 was also incorporated in the comparison between GrHb and human Hb. Finally, a structural comparison with generic dog Hb and maned wolf Hb was conducted, revealing that in contrast to GrHb these structures belong to the R state of Hb and raising the intriguing possibility of an additional allosteric factor co-purifying with GrHb that can modulate its quaternary structure.

WAXS studies of the structural diversity of hemoglobin in solution

Specific ligation states of hemoglobin are, when crystallized, capable of taking on multiple quaternary structures. The relationship between these structures, captured in crystal lattices, and hemoglobin structure in solution remains uncertain. Wide-angle X-ray solution scattering (WAXS) is a sensitive probe of protein structure in solution that can distinguish among similar structures and has the potential to contribute to these issues. We used WAXS to assess the relationships among the structures of human and bovine hemoglobins in different liganded forms in solution. WAXS data readily distinguished among the various forms of hemoglobins. WAXS patterns confirm some of the relationships among hemoglobin structures that have been defined through crystallography and NMR and extend others. For instance, methemoglobin A in solution is, as expected, nearly indistinguishable from HbCO A. Interestingly, for bovine hemoglobin, the differences between deoxy-Hb, methemoglobin and HbCO are smaller than the corresponding differences in human hemoglobin. WAXS data were also used to assess the spatial extent of structural fluctuations of various hemoglobins in solution. Dynamics has been implicated in allosteric control of hemoglobin, and increased dynamics has been associated with lowered oxygen affinity. Consistent with that notion, WAXS patterns indicate that deoxy-Hb A exhibits substantially larger structural fluctuations than HbCO A. Comparisons between the observed WAXS patterns and those predicted on the basis of atomic coordinate sets suggest that the structures of Hb in different liganded forms exhibit clear differences from known crystal structures.

Hemoglobin redux: combining neutron and X-ray diffraction with mass spectrometry to analyse the quaternary state of oxidized hemoglobins

Improvements in neutron diffraction instrumentation are affording the opportunity to re-examine the structures of vertebrate hemoglobins and to interrogate proton and solvent position changes between the different quaternary states of the protein. For hemoglobins of unknown primary sequence, structural studies of cyanomethemoglobin (CNmetHb) are being used to help to resolve sequence ambiguity in the mass spectra. These studies have also provided additional structural evidence for the involvement of oxidized hemoglobin in the process of erythrocyte senescence. X-ray crystal studies of Tibetan snow leopard CNmetHb have shown that this protein crystallizes in the B state, a structure with a more open dyad, which possibly has relevance to RBC band 3 protein binding and erythrocyte senescence. R-state equine CNmetHb crystal studies elaborate the solvent differences in the switch and hinge region compared with a human deoxyhemoglobin T-state neutron structure. Lastly, comparison of histidine protonation between the T and R state should enumerate the Bohr-effect protons.

Solid phase pegylation of hemoglobin

A solid phase conjugation process was developed for attachment of polyethylene glycol to hemoglobin molecule. Bovine hemoglobin was loaded onto an ion exchange chromatography column and adsorbed by the solid medium. Succinimidyl carbonate mPEG was introduced in the mobile phase after the adsorption. Pegylation took place between the hemoglobin on the solid phase, and the pegylation reagent in the liquid phase. A further elution was carried out to separate the pegylated and the unpegylated protein. Analysis by HPSEC, SDS-PAGE, and MALLS demonstrated that the fractions eluted from the solid phase contained well-defined components. Pegylated hemoglobin with one PEG chain was obtained with the yield of 75%, in comparison to the yield of 30% in the liquid phase pegylation. The P(50) values of the mono-pegylated hemoglobin, prepared with SC-mPEG 5 kDa, 10 kDa and 20 kDa, were 19.97, 20.23 and 20.54 mmHg, which were much closer to the value of red blood cells than that of pegylated hemoglobin prepared with the conventional method.

Interfacial and distal-heme pocket mutations exhibit additive effects on the structure and function of hemoglobin

Protein engineering strategies seek to develop a hemoglobin-based oxygen carrier with optimized functional properties, including (i) an appropriate O 2 affinity, (ii) high cooperativity, (iii) limited NO reactivity, and (iv) a diminished rate of auto-oxidation. The mutations alphaL29F, alphaL29W, alphaV96W and betaN108K individually impart some of these traits and in combinations produce hemoglobin molecules with interesting ligand-binding and allosteric properties. Studies of the ligand-binding properties and solution structures of single and multiple mutants have been performed. The aromatic side chains placed in the distal-heme pocket environment affect the intrinsic ligand-binding properties of the mutated subunit itself, beyond what can be explained by allostery, and these changes are accompanied by local structural perturbations. In contrast, hemoglobins with mutations in the alpha 1beta 1 and alpha 1beta 2 interfaces display functional properties of both "R"- and "T"-state tetramers because the equilibrium between quaternary states is altered. These mutations are accompanied by global structural perturbations, suggesting an indirect, allostery-driven cause for their effects. Combinations of the distal-heme pocket and interfacial mutations exhibit additive effects in both structural and functional properties, contribute to our understanding of allostery, and advance protein-engineering methods for manipulating the O 2 binding properties of the hemoglobin molecule.

Hb(alphaalpha,betabeta): a novel fusion construct for a dimeric, four-domain hemoglobin

Hemoglobin-based blood substitutes are one of the options available to derive a resuscitating fluid taking into account clinical and physiological demands. In this paper we investigated a novel protein, Hb(alphaalpha,betabeta) obtained as a combination of two homodimers alpha(2) and beta(2) both derived from a fusion gene containing two alfa chains or two beta chains, each respectively coupled via a specific linker. The construct here described is thus a novel heterodimeric hemoglobin carrying four heme groups. The protein cannot dissociate into dimers, as demonstrated by its absence of reactivity versus haptoglobin, and is expected to have a relatively long circulating half-life. The modification does not increase the autoxidation rate, but increases the oxygen affinity, due to a destabilization of the T quaternary state. Characterization of the biochemical properties of this protein in comparison with HbA is reported.

Modified haemoglobins and perfluorocarbons

After decades of research activities and product improvements in the field of artificial oxygen carriers based on either haemoglobin modifications or perfluorocarbon emulsions, these products have reached a critical stage of their development. Varieties of haemoglobin-based oxygen carriers and perfluorocarbon emulsions are under current clinical investigation. Although the clinical availability of artificial oxygen carriers may result in profound changes of fluid resuscitation from haemorrhage, the transfusion of human blood components as an integral part of medical trauma management will not be replaced. However, a rapid and effective restoration of tissue oxygenation by the use of artificial oxygen carriers in the treatment of severe haemorrhage may bridge time delays until stored and cross-matched human packed red cells are available. Whether artificial oxygen carriers could provide additional clinical benefits by sustaining tissue oxygenation even under conditions of a disturbed macro- or microcirculation is the subject of current investigations. Therefore, the application of safe and effective artificial oxygen carriers would not only be restricted to the treatment of severe haemorrhage, but additional therapeutic indications of artificial oxygen carriers in emergency medicine, trauma anaesthesia and other medical specialities would emerge.

A newly discovered human alpha-globin gene

A previously undefined transcript with significant homology to the pseudo-alpha2 region of the alpha-globin locus on human chromosome 16 was detected as part of an effort to better define the transcriptional profiles of human reticulocytes. Cloning and sequencing of that transcript (GenBank AY698022; named mu-globin) revealed an insert with a 423-nucleotide open reading frame. BLASTP and ClustalW and phylogenetic analyses of the predicted protein demonstrated a high level of homology with the avian alpha-D globin. In addition, the heme- and globin-binding amino acids of mu-globin and avian alpha-D globin are largely conserved. Using quantitative real-time polymerase chain reaction (PCR), mu-globin was detected at a level of approximately 0.1% that measured for alpha-globin in erythroid tissues. Erythroid-specific expression was detected by Northern blot analysis, and maximal expression during the erythroblast terminal differentiation was also detected. Despite this highly regulated pattern of mu-globin gene transcription, mu-globin protein was not detected by mass spectrometry. These results suggest the human genome encodes a previously unrecognized globin member of the avian alpha-D family that is transcribed in a highly regulated pattern in erythroid cells.

Model mice for Presbyterian hemoglobinopathy (Asn(beta108)-->Lys) confer hemolytic anemia with altered oxygen affinity and instability of Hb

Hb Presbyterian is a variant hemoglobin that carries Lys at Asn-108 of beta-globin. This variant Lys(beta108) residue enhances the stability of Hb in the deoxy-state, conferring the low affinity for oxygen-binding in vitro. In the present study, we generated mutant mice carrying the Presbyterian mutation (Asn(beta108)-->Lys) at the beta-globin locus by a targeted knock-in strategy. Heterozygous mice showed the expression of Hb Presbyterian in 27.7% of total peripheral blood without any hematological abnormalities, which well mimicked human cases. On the other hand, homozygous mice exclusively expressed Hb Presbyterian in 100% of peripheral blood associated with hemolytic anemia, Heinz body formation, and splenomegaly. Hb Presbyterian showed instability in an in vitro precipitation assay. Erythrocytes from homozygous mice showed a shortened life span when transfused into wild-type mice, confirming that the knocked-in mutation of Lys(beta108) caused hemolysis in homozygous mice. This is the first report on the hemolytic anemia of unstable hemoglobin in an animal model. These results confirm the notion that the higher ratio of an unstable variant beta-globin chain in erythrocytes triggers the pathological precipitation and induces hemolysis in abnormal hemoglobinopathies.

Intersubunit circular permutation of human hemoglobin

For many years, human hemoglobin (Hb) isolated from erythrocytes has been investigated as a potential oxygen delivery therapeutic. Advantages with respect to the need for blood typing were balanced with various undesirable properties of cell-free Hb, including cost, overall oxygen affinity, alterations in cooperativity, and ready dissociation into toxic dimeric species. The use of total gene synthesis has resulted in very high levels of functional human Hb expression in Escherichia coli, but there remains a desire for effecting the crosslinking of the hemoglobin tetramer and providing for ready means for increasing the globular molecular weight. In this communication, we report a novel method for linking alpha chains. By circularly permuting one alpha sequence, the second alpha chain in the Hb tetramer can be linked with glycine residues to form 2 bridges across the central cavity. The second alpha chain thus presents its amino and carboxyl termini on a solvent exposed surface, providing for additional polymerization of oxygen-carrying subunits or attachment of any other peptide-based therapeutic.

A double-blind study to evaluate the safety of recombinant human hemoglobin in surgical patients during general anesthesia

OBJECTIVE

To evaluate recombinant human hemoglobin (rHb1.1) in patients undergoing surgery involving general anesthesia; examine rHb1.1 for toxicity, including renal dysfunction and hypertension; and measure plasma concentrations of rHb1.1 over time.

DESIGN

Prospective, double-blinded, randomized, placebo-controlled study.

SETTING

University medical center hospital.

PARTICIPANTS

Eighteen patients having surgery under general anesthesia.

INTERVENTIONS

One of 4 escalating doses of rHb1.1 or normal saline (control) was administered by continuous infusion to patients receiving general anesthesia for elective surgical procedures. Total rHb1.1 doses ranged from 4.7 to 25.6 g.

MEASUREMENTS AND MAIN RESULTS

Clinical and laboratory data, including vital signs monitoring, hematology (white blood cell and reticulocyte count, hemoglobin, hematocrit, erythrocyte sedimentation rates, and coagulation values), renal function (serum creatinine and blood urea nitrogen), hepatic function (mean and indirect bilirubin), pancreatic function (serum amylase and lipase), and antibodies (IgG and IgM) to Escherichia coli protein, were collected at specified intervals for 7 days after infusion of rHb1.1. No serious adverse events occurred. The most frequently observed clinical event occurred during the first 24 hours after infusion and was primarily associated with surgery and anesthetic administration. A slightly higher incidence of hypertension, symptoms suggestive of pyrogenicity, mildly elevated total and indirect bilirubin, and elevated pancreatic enzymes was observed in rHb1.1 treatment groups when compared with control. Hypertension resolved within 7 hours, and laboratory values returned to normal levels by day 7.

CONCLUSION

Although the elevations in pancreatic enzymes seen in some rHb1.1-treated patients remain unexplained, the safety profile of rHb1.1 appears to be acceptable. These results support the continued clinical evaluation and development of rHb1.1.

The X-ray structure determination of bovine carbonmonoxy hemoglobin at 2.1 A resoultion and its relationship to the quaternary structures of other hemoglobin crystal froms

Crystallographic studies of the intermediate states between unliganded and fully liganded hemoglobin (Hb) have revealed a large range of subtle but functionally important structural differences. Only one T state has been reported, whereas three other quaternary states (the R state, B state, and R2 or Y state) for liganded Hb have been characterized; other studies have defined liganded Hbs that are intermediate between the T and R states. The high-salt crystal structure of bovine carbonmonoxy (CO bovine) Hb has been determined at a resolution of 2.1 A and is described here. A detailed comparison with other crystallographically solved Hb forms (T, R, R2 or Y) shows that the quaternary structure of CO bovine Hb closely resembles R state Hb. However, our analysis of these structures has identified several important differences between CO bovine Hb and R state Hb. Compared with the R state structures, the beta-subunit N-terminal region has shifted closer to the central water cavity in CO bovine Hb. In addition, both the alpha- and beta-subunits in CO bovine Hb have more constrained heme environments that appear to be intermediate between the T and R states. Moreover, the distal pocket of the beta-subunit heme in CO bovine Hb shows significantly closer interaction between the bound CO ligand and the Hb distal residues Val 63(E11) and His 63(E7). The constrained heme groups and the increased steric contact involving the CO ligand and the distal heme residues relative to human Hb may explain in part the low intrinsic oxygen affinity of bovine Hb.

Activation of the low oxygen affinity-inducing potential of the Asn108(beta)-->Lys mutation of Hb-Presbyterian on intramolecular alpha alpha-fumaryl cross-bridging

The Asn108 beta-->Lys mutation in hemoglobin (HbPresbyterian mutation) endows a low O(2) affinity-inducing propensity to the protein. Introduction of a fumaryl cross-bridge between its two alpha 99 lysine residues also induces a low O(2) affinity into HbA. We have now engineered an alpha alpha-fumaryl cross-bridge into Hb-Presbyterian to determine the synergy or additivity, if any, that can be achieved between these two low O(2) affinity-inducing structural perturbations. Despite the presence of the additional epsilon-amino group of Lys108(beta) within the central cavity, the epsilon-amino group of Lys99(alpha alpha) of deoxy Hb-Presbyterian retained high selectivity for alpha alpha-fumaryl cross-bridging, with an overall efficiency comparable to that with HbA. The alpha alpha-fumaryl cross-linking of Hb-Presbyterian reduced its O(2) affinity much more significantly than that observed with HbA, indicating a synergy between the two low O(2) affinity-inducing structural perturbations. Apparently, the alpha alpha-fumaryl cross-bridge in Hb-Presbyterian activates part of the latent low O(2) affinity-inducing potential of Lys108(beta) that is generally activated in the presence of chloride. The synergy between the Asn108(beta)-->Lys mutation and the alpha alpha-fumaryl cross-bridging was conserved in the presence of chloride, but not in the presence of DPG. Furthermore, in the presence of chloride and DPG, alpha alpha-fumaryl Hb-Presbyterian accessed a low O(2) affinity T-state that is accessed by HbA, alpha alpha-HbA and Hb-Presbyterian only in the presence of IHP. Isoelectric focusing analysis suggested that the alpha alpha-fumaryl cross-linking of Hb-Presbyterian induces changes in the ionization behavior of one or more of the functional groups neighboring Lys99(alpha) and Lys108(beta) [presumably His103(alpha) and/or Glu101(beta)] to compensate for the extra positive charge of Lys108(beta). Molecular modeling studies identified two potential chloride binding sites per alpha beta dimer within the middle of the central cavity of alphaalpha-fumaryl HbA involving residues His103(alpha), Arg104(beta) and Asn108(beta). The affinity of these sites is increased in alpha alpha-fumaryl Hb-Presbyterian as a result of the Asn108(beta)-->Lys mutation. Thus, the results of the present study suggest that the enhanced neutralization of the positive charges in the middle of the central cavity of Hb achieved by these two electrostatic modifications, one (the alpha alpha-fumaryl cross-bridge) acting directly and the other (the Presbyterian mutation) acting indirectly through the mediation of chloride ion binding, facilitates the alpha alpha- fumaryl-Hb Presbyterian to access a low O(2) affinity T-state structure much more readily than either Hb-Presbyterian or alpha alpha-fumaryl HbA.

High-resolution crystal structure of deoxy hemoglobin complexed with a potent allosteric effector

The crystal structure of human deoxy hemoglobin (Hb) complexed with a potent allosteric effector (2-[4-[[(3,5-dimethylanilino)carbonyl]methyl]phenoxy]-2-methylpropionic acid) = RSR-13) is reported at 1.85 A resolution. Analysis of the hemoglobin:effector complex indicates that two of these molecules bind to the central water cavity of deoxy Hb in a symmetrical fashion, and that each constrains the protein by engaging in hydrogen bonding and hydrophobic interactions with three of its four subunits. Interestingly, we also find that water-mediated interactions between the bound effectors and the protein make significant contributions to the overall binding. Physiologically, the interaction of RSR-13 with Hb results in increased oxygen delivery to peripheral tissues. Thus, this compound has potential therapeutic application in the treatment of hypoxia, ischemia, and trauma-related blood loss. Currently, RSR-13 is in phase III clinical trials as a radiosensitizing agent in the treatment of brain tumors. A detailed structural analysis of this compound complexed with deoxy Hb has important implications for the rational design of future analogs.

Assessing the relative stabilities of engineered hemoglobins using electrospray mass spectrometry

An ion trap mass spectrometer equipped with an electrospray source was used to examine the relative thermodynamic stabilities of various hemoglobins with respect to both tetramer dissociation and hemin dissociation. The results demonstrated that the stability of hemoglobin molecules can be differentiated by the amount of applied collision-induced dissociation (CID) energy necessary to break up the intact tetramer into its constituent globins. The stability of the intact tetramer was affected by single mutations in the beta-globins. The stabilities of the constituent hologlobins were assessed via trap CID of selected ions. The results demonstrated the importance of the contributions of the hologlobin components to the stability of the intact tetramer. Genetic fusion of two alpha-globins, through the introduction of a single glycine residue between the C-terminus of one alpha-chain and the N-terminus of the second, significantly increased the stability of the hemoglobin pseudo-tetramer. Chemical crosslinking of the beta-globins in addition to genetic fusion of alpha-globins further stabilized the hemoglobin molecule. A dihemoglobin molecule produced by the genetic fusion of two di-alpha-globins with a flexible linker demonstrated a decreased stability relative to the corresponding monohemoglobin.

Hemoglobin-based oxygen carriers: development and clinical potential

This article addresses issues involved in the development of hemoglobin-based oxygen carriers and provides a focused overview of the 4 hemoglobin-based oxygen carriers with emergency medicine application currently in clinical trials.

Glutaraldehyde modification of recombinant human hemoglobin alters its hemodynamic properties

Many cell-free hemoglobin solutions designed as oxygen-carrying therapeutics produce a hypertensive effect in animals. The response is likely due to oxidation of nitric oxide by hemoglobin. Since the site of oxidation may lie outside the vascular compartment, we tested the hypothesis that polymerization of hemoglobin, rHb1.1, by glutaraldehyde would attenuate the hypertensive response. Two products of the cross-linking reaction were isolated, a glutaraldehyde-derivatized monomer (mono-glxrHb) and a glutaraldehyde cross-linked polymer (poly-glxrHb), and evaluated for their effects on systemic hemodynamics in conscious rats. Administration of rHb1.1 caused a mean arterial pressure elevation of approximately 20 mm Hg and an increase in total peripheral resistance of approximately 30%. Administration of mono-glxrHb induced changes in mean arterial pressure and vascular resistance that were significantly diminished relative to those observed with rHb1.1. Poly-glxrHb elicited a mean arterial pressure response that was further reduced compared with that obtained with mono-glxrHb and a change in vascular resistance that was the same as the response to mono-glxrHb. These results suggest that rHb peripheral vasoconstriction elicited by rHb1.1 is significantly attenuated by glutaraldehyde modification of the hemoglobin monomer and that the effect of glutaraldehyde polymerization is likely due to surface modification and/or intramolecular cross-linking, rather than an increase in molecular size.

Rate constants for O2 and CO binding to the alpha and beta subunits within the R and T states of human hemoglobin

Despite a large amount of work over the past 30 years, there is still no universal agreement on the differential reactivities of the individual alpha and beta subunits in human hemoglobin. To address this question systematically, we prepared a series of hybrid hemoglobins in which heme was replaced by chromium(III), manganese(III), nickel(II), and magnesium(II) protoporphyrin IXs in either the alpha or beta subunits to produce alpha2(M)beta2(Fe)1 and alpha2(Fe)beta2(M) tetramers. None of the abnormal metal complexes react with dioxygen or carbon monoxide. The O2 affinities of the resultant hemoglobins vary from 3 microM-1 (Cr(III)/Fe(II) hybrids) to 0.003 microM-1 (Mg(II)/Fe(II) hybrids), covering the full range expected for the various high (R) and low (T) affinity quaternary conformations, respectively, of human hemoglobin A0. The alpha and beta subunits in hemoglobin have similar O2 affinities in both quaternary states, despite the fact that the R to T transition causes significantly different structural changes in the alpha and beta heme pockets. This functional equivalence almost certainly evolved to maintain high n values for efficient O2 transport.

Preclinical biology of recombinant human hemoglobin, rHb1.1

Historically, the development of hemoglobin based oxygen carriers, HBOCs, were confounded by issues related to activation of the complement cascade and other inflammatory processes, renal toxicity, and significant systemic vasoconstriction. However, with shortages in the blood supply, the risk of infectious agent contamination, and delays associated with complete crossmatch as well as transfusion reactions, HBOC development has assumed greater importance. A successful HBOC in addition to having favorable oxygen binding parameters and colloid oncotic properties, must also have a low toxicity profile, be nonimmunogenic, have positive rheologic properties, and have an adequate in vivo half life. In addition, it must also be stable in vivo and not undergo significant oxidation to methemoglobin or release heme or iron in the vasculature. The preclinical studies which have been designed and executed to address these requirements for recombinant human hemoglobin rHb1.1 serve as the focus of this review. Recombinant Hb1.1 represents the first HBOC to enter clinical trials as a recombinant product in distinction to other HBOCs which are derived from bovine or outdated human blood. While currently in phase II clinical trials, the preclinical biology which has increased our understanding of this molecule are the subject of this review.

Allosteric intermediates indicate R2 is the liganded hemoglobin end state

Hemoglobin has been a long-standing paradigm for understanding protein allostery. Here, the x-ray structures of two chemically crosslinked, fully liganded hemoglobins, alpha2beta82CA82beta and alpha2beta82ND82beta, are described at 2.3 A and 2.6 A resolution, respectively. Strikingly, these crosslinked hemoglobins assume intermediate conformations that lie between those of R and the controversial liganded hemoglobin state R2 rather than between R and T. Thus, these structures support only a T left and right arrow R left and right arrow R2 allosteric pathway and underscore the physiological importance of the R2 conformation.