Assembly of human hemoglobin. Studies with Escherichia coli-expressed alpha-globin

Development of recombinant hemoglobin-based oxygen carriers

SIGNIFICANCE

The worldwide blood shortage has generated a significant demand for alternatives to whole blood and packed red blood cells for use in transfusion therapy. One such alternative involves the use of acellular recombinant hemoglobin (Hb) as an oxygen carrier.

RECENT ADVANCES

Large amounts of recombinant human Hb can be expressed and purified from transgenic Escherichia coli. The physiological suitability of this material can be enhanced using protein-engineering strategies to address specific efficacy and toxicity issues. Mutagenesis of Hb can (i) adjust dioxygen affinity over a 100-fold range, (ii) reduce nitric oxide (NO) scavenging over 30-fold without compromising dioxygen binding, (iii) slow the rate of autooxidation, (iv) slow the rate of hemin loss, (v) impede subunit dissociation, and (vi) diminish irreversible subunit denaturation. Recombinant Hb production is potentially unlimited and readily subjected to current good manufacturing practices, but may be restricted by cost. Acellular Hb-based O(2) carriers have superior shelf-life compared to red blood cells, are universally compatible, and provide an alternative for patients for whom no other alternative blood products are available or acceptable.

CRITICAL ISSUES

Remaining objectives include increasing Hb stability, mitigating iron-catalyzed and iron-centered oxidative reactivity, lowering the rate of hemin loss, and lowering the costs of expression and purification. Although many mutations and chemical modifications have been proposed to address these issues, the precise ensemble of mutations has not yet been identified.

FUTURE DIRECTIONS

Future studies are aimed at selecting various combinations of mutations that can reduce NO scavenging, autooxidation, oxidative degradation, and denaturation without compromising O(2) delivery, and then investigating their suitability and safety in vivo.

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.

Heme pocket disorder in myoglobin: reversal by acid-induced soft refolding

The protein folding process of heme proteins entails generation of not only a correct global polypeptide structure, but also a correct, functionally competent heme environment. We employed a variety of spectroscopic approaches to probe the structure and dynamics of the heme pocket of a recombinant sperm whale myoglobin. The conformational characteristics were examined by circular dichroism, time-resolved fluorescence spectroscopy, FTIR spectroscopy, and optical absorption spectroscopy in the temperature range 300-20 K. Each of these spectroscopic probes detected modifications confined exclusively to the heme pocket of the expressed myoglobin relative to the native protein. The functional properties were examined by measuring the kinetics of CO binding after flash-photolysis. The kinetics of the expressed myoglobin were more heterogeneous than those of the native protein. Mild acid exposure of the ferric derivative of the recombinant protein resulted in a protein with "nativelike" spectroscopic properties and homogeneous CO binding kinetics. The heme pocket modifications observed in this recombinant myoglobin do not derive from inverted heme. In contrast, when native apomyoglobin is reconstituted with the heme in vitro, the heme pocket disorder could be attributed exclusively to 180 degrees rotation of the bound heme [La Mar, G. N., Toi, H., and Krishnamoorthi, R. (1984) J. Am. Chem. Soc. 106, 6395-6401; Light, W. R., Rohlfs, R. J., Palmer, G., and Olson, J. S. (1987) J. Biol. Chem. 262, 46-52]. We conclude that exposure to low pH decreases the affinity of globin for the heme and allows an extended conformational sampling or "soft refolding" to a nativelike conformation.

Molecular engineering of a polymer of tetrameric hemoglobins

We have engineered a recombinant mutant human hemoglobin, Hb Prisca beta(S9C+C93A+C112G), which assembles in a polymeric form. The polymerization is obtained through the formation of intermolecular S-S bonds between cysteine residues introduced at position beta9, on the model of Hb Porto Alegre (beta9Ser --> Cys) (Bonaventura and Riggs, Science 1967;155:800-802). Cbeta93 and Cbeta112 were replaced in order to prevent formation of spurious S&bond;S bonds during the expression, assembly, and polymerization events. Dynamic light scattering measurements indicate that the final polymerization product is mainly formed by 6 to 8 tetrameric hemoglobin molecules. The sample polydispersity Q = 0.07 +/- 0.02, is similar to that of purified human hemoglobin (Q = 0.02 +/- 0.02), consistent with a good degree of homogeneity. In the presence of strong reducing agents, the polymer reverts to its tetrameric form. During the depolymerization process, a direct correlation is observed between the hydrodynamic radius and the light scattering of the system, which, in turn, is proportional to the mass of the protein. We interpret this to indicate that the hemoglobin molecules are tightly packed in the polymer with no empty spaces. The tight packing of the hemoglobin molecules suggests that the polymer has a globular shape and, thus, allows estimation of its radius. An illustration of an arrangement of a finite number of tetrameric hemoglobin molecules is presented. The conformational and functional characteristics of this polymer, such as heme pocket conformation, stability to denaturation, autoxidation rate, oxygen affinity, and cooperativity, remain similar to those of tetrameric human hemoglobin.

Allosteric free energy changes at the alpha 1 beta 2 interface of human hemoglobin probed by proton exchange of Trp beta 37

The energetic changes that occur on ligand binding in human hemoglobin have been investigated by measurements of the exchange rates of the indole proton of Trpbeta37(C3). The Trpbeta37 residues are located in helices C of the beta-subunits and are involved in contacts with the segments FG of the alpha-subunits at the interdimeric alpha1beta2 and alpha2beta1 interfaces of the hemoglobin tetramer. In the quaternary structure change that accompanies ligand binding to hemoglobin, these contacts undergo minimal changes in relative orientation and in packing, thereby acting as hinges, or flexible joints. The exchange rates of the indole proton of Trpbeta37(C3) were measured by nuclear magnetic resonance spectroscopy, in both deoxygenated and ligated hemoglobin. The results indicate that, at 15 degrees C, the exchange rate is increased from 9.0. 10(-6) to 3.3. 10(-4) s(-1) upon ligand binding to hemoglobin. This change suggests that the structural units at the hinge regions of the alpha1beta2/alpha2beta1 interfaces containing Trpbeta37(C3) are specifically stabilized in unligated hemoglobin, and experience a change in structural free energy of approximately 4 kcal/(mol tetramer) upon ligand binding. Therefore, the hinge regions of the alpha1beta2/alpha2beta1 interfaces could play a role in the transmission of free energy through the hemoglobin molecule during its allosteric transition.

Expression of functional soluble human alpha-globin chains of hemoglobin in bacteria

Individual, soluble human alpha-globin chains were expressed in bacteria with exogenous heme and methionine aminopeptidase. The yields of soluble alpha chains in bacteria were comparable to those of recombinant non-alpha chains expressed under the same conditions. Molecular mass and gel-filtration properties of purified recombinant alpha chains were the same as those of authentic human alpha chains. Biochemical and biophysical properties of isolated alpha chains were identical to those of native human alpha chains as assessed by UV/vis, circular dichroism (CD), and nuclear magnetic resonance (NMR) spectroscopy which contrasts with previous results of refolded precipitated alpha chains made in the presence of heme in vitro (M. T. Sanna et al., J. Biol. Chem. 272, 3478-3486, 1997). Mixtures of purified, soluble recombinant alpha-globin and native beta-globin chains formed heterotetramers in vitro, and oxygen- and CO-binding properties as well as the heme environment of the assembled tetramers were experimentally indistinguishable from those of native human Hb A. UV/vis, CD, and NMR spectra of assembled Hb A were also the same as those of human Hb A. These results indicate that individual expressed alpha chains are stable in bacteria and fold properly in vivo and that they then can assemble with free beta chains to form hemoglobin heterotetramers in vivo as well as in vitro.

Soret spectroscopic and molecular graphic analysis of human semi-beta-hemoglobin formation

The interaction of heme-free alpha (alpha(o)) and heme-containing beta (beta(h)) chains of human hemoglobin has been monitored in 0.1 M potassium phosphate buffer, pH 7 or 8, at 5 degrees C. Soret zero and first-derivative spectra were consistent with a uniform association reaction. Stopped-flow investigations demonstrated association rates on the order of 10(7) M(-1) s(-1). This was 100-fold more rapid than the reported rate of combination of alpha(h) and beta(h) proteins. This encounter-like rate of semi-beta-hemoglobin (alpha(o)beta(h)) formation was increased by raising the pH from 7 to 8. pH change is known to affect the spatial arrangement of AB-GH helical entities. Molecular graphic analysis of modeled alpha(o) protein superimposed over native alpha(h) protein revealed an apo Mb-like structure with well-defined AB-GH segments. Repositioning of these core helical segments, resulting in increased conformational freedom of the alpha1beta1 interface, was apparently responsible for the enhanced association properties of the alpha(o) protein.

Structural and functional properties of human hemoglobins reassembled after synthesis in Escherichia coli

Human hemoglobin produced in the Escherichia coli coexpression system of Hernan et al. [(1992) Biochemistry 31, 8619-8628] has been transformed into a functionally homogeneous protein whose properties closely approximate those of normal hemoglobin A. Both of the alpha and beta chains of this hemoglobin contain a valine-methionine substitution at position 1 in order to accommodate the difference in specificity of the protein-processing enzymes of procaryotes. Despite extensive purification, functional homogeneity of the E. coli expressed hemoglobin was achieved only by the complete disassembly of the hemoglobin into its component alpha and beta globins and their reassembly in the presence of hemin. The kinetics of CO combination and the thermodynamics of O2 binding and cooperativity of the reassembled alphaV1M-betaV1M hemoglobin closely approximate those of HbA. The alpha globin obtained from the E. coli expressed hemoglobin was also combined with normal human beta chains and hemin to form the alphaV1M variant. The alpha+M variant of HbA, in which the normal N-terminal valine of the alpha chains is preceded by a methionine residue, was prepared by the same procedure. The kinetics of the reactions of CO with the alphaV1M and alpha+M variants are similar to those for HbA. The equilibria of oxygen binding to alphaV1M and HbA are similar whereas alpha+M exhibits a significantly higher oxygen affinity. The three-dimensional structures of alphaV1M and alpha+M offer an explanation for the latter affinity difference. Although the structures of alphaV1M and HbA, which have been determined by X-ray crystallography, are virtually indistinguishable except at the N-terminal residues, that of alpha+M indicates the displacement of a solvent molecule, possibly a chloride ion, from arginine 141alpha. Such an alteration in an anion binding site could result in increased oxygen affinity.

Cysteines beta93 and beta112 as probes of conformational and functional events at the human hemoglobin subunit interfaces

Three variants of tetrameric human hemoglobin, with changes at the alpha1beta2/alpha2beta1-interface, at the alpha1beta1/alpha2beta2-interface, and at both interfaces, have been constructed. At alpha1beta2/alpha2beta1-interface the beta93 cysteine was replaced by alanine (betaC93A), and at the alpha1beta1/alpha2beta2-interface the beta112 cysteine was replaced by glycine (betaC112G). The alpha1beta2 interface variant, betaC93A, and the alpha1beta1/alpha1beta2 double mutant, beta(C93A+C112G), were crystallized in the T-state, and the structures determined at 2. 0 and 1.8 A resolution, respectively. A comparison of the structures with that of natural hemoglobin A shows the absence of detectable changes in the tertiary folding of the protein or in the T-state quaternary assembly. At the beta112 site, the void left by the removal of the cysteine side chain is filled by a water molecule, and the functional characteristics of betaC112G are essentially those of human hemoglobin A. At the beta93 site, water molecules do not replace the cysteine side chain, and the alanine substitution increases the conformational freedom of beta146His, weakening the important interaction of this residue with beta94Asp. As a result, when Cl- is present in the solution, at a concentration 100 mM, the Bohr effect of the two mutants carrying the beta93Cys-->Ala substitution, betaC93A and beta(C93A+C112G), is significantly modified being practically absent below pH 7.4. Based on the crystallographic data, we attribute these effects to the competition between beta94Asp and Cl- in the salt link with beta146His in T-state hemoglobin. These results point to an interplay between the betaHis146-betaAsp94 salt bridge and the Cl- in solution regulated by the Cys present at position beta93, indicating yet another role of beta93 Cys in the regulation of hemoglobin function.

Chimeric beta-EF3-alpha hemoglobin (Psi): energetics of subunit interaction and ligand binding

Among the numerous strategies to design an oxygen carrier, we outline in this work the engineering of a stable homotetrameric hemoglobin, expressed in Escherichia coli. The chimeric globin (Psi) consists of the first 79 residues of human beta globin (corresponding to positions NA1 --> EF3) followed by the final 67 residues of human alpha globin (corresponding to positions EF3 --> HC3). The molecular mass for beta-EF3-alpha (Psi) globin was measured using mass spectrometry to be equal to its theoretical value: 15782 Da. Correct protein folding was assessed by UV/visible and fluorescence spectra. The subunit interaction free energies were estimated by HPLC gel filtration. In the cyanometHb species, the formation of the dimer-tetramer interface is 2 kcal/mol less favorable (Delta G = -7 kcal/mol) than that of Hb A (Delta G = -9 kcal/mol), whereas the dimer-monomer interface is tightly assembled (< -10 kcal/mol) as for the Hb A alpha 1 beta 1 interface. In contrast to Hb A, oxygen binding to Psi Hb is not cooperative. The free energy for binding four oxygen molecules to a Psi homotetramer is slightly increased compared to a Hb A heterotetramer (-28 and -27.5 kcal/4 mol of O2, respectively). The intrinsic O2 affinity of a Psi homodimer is 6-fold higher than that of a homotetramer. The linkage scheme between dimer-tetramer subunit assembly and the noncooperative oxygenation of Psi Hb predicts a stabilization of the tetramer after ligand release. This protein mechanism resembles that of Hb A for which the dimers exhibit a 100-fold higher O2 affinity relative to deoxy tetramers (which are 10(5) times more stable than oxy tetramers). A potent allosteric effector of Hb A, RSR4, binds to Psi Hb tetramers, inducing a decrease of the overall O2 affinity. Since RSR4 interacts specifically with two binding sites of deoxy Hb A, we propose that the chimeric tetramer folding is close to this native structure.

Modification of alpha-chain or beta-chain heme pocket polarity by Val(E11) --> thr substitution has different effects on the steric, dynamic, and functional properties of human recombinant hemoglobin. Deoxy derivatives

The dynamic and functional properties of mutant deoxyhemoglobins in which either the beta-globin Val67(E11) or the alpha-globin Val62(E11) is replaced by threonine have been investigated through the thermal evolution of the Soret absorption band in the temperature range 300 to 20 K and through the kinetics of CO rebinding after flash photolysis at room temperature. The conformational properties of the modified alpha chain and beta chain distal heme pockets were also studied through x-ray crystallography and molecular modeling. The data obtained with the various techniques consistently indicate that the polar isosteric mutation in the distal side of the alpha chain heme pocket has a larger effect on the investigated properties than the analogous mutation on the beta chain. We attribute the observed differences to the presence of a water molecule in the distal heme pocket of the modified alpha chains, interacting with the hydroxyl of the threonine side chain. This is indicated by molecular modeling which showed that the water molecule present in the alpha chain distal heme pocket can bridge by H bonding between Thr62(E11) and His58(E7) without introducing any unfavorable steric interactions. Consistent with the dynamic and functional data, the presence of a water molecule in the distal heme pocket of the modified beta chains is not observed by x-ray crystallography.