LOW OXYGEN AFFINITY MUTANT HEMOGLOBIN

Abstract: A non-naturally occurring mutant hemoglobin ( alpha 96Val}Trp) is provided that has a lower oxygen affinity than that of native hemoglobin, but high cooperativity in oxygen binding. The mutant hemoglobin is preferably obtained by recombinant DNA techniques. Such a mutant hemoglobin may be used as a component of a blood substitute.

Description: LOW OXYGEN AFFINITY MUTANT HEMOGLOBIN ACKNOWLEDGMENT The present invention was developed in part with government support under grant number HL-24525. The government has certain rights in this invention. FIELD OF THE INVENTION This invention relates generally to a novel mutant hemoglobin and more particularly relates to a recombinant mutant hemoglobin "r Hb (cu96Val-»Trp) " that shows a low oxygen affinity, but high cooperativity in oxygen binding. This invention further relates to the preparation of the mutant hemoglobins using recombinant DNA technology that are useful as substitutes for red blood cells. BACKGROUND OF THE INVENTION The prevalence of infectious agents such as HIV and hepatitis in red blood cells of human blood products coupled with blood shortages from lack of suitable donors has led to great interest in the development of red cell substitutes, particularly human hemoglobin ("Hb") and its derivatives. Hemoglobin is the oxygen-carrying component of blood, circulated through the blood stream inside erythrocytes (red blood cells) . Human normal adult hemoglobin ("Hb A") is a tetrameric protein containing two a chains having 141 amino acid residues each and two β chains having 146 amin acid residues each, and also bearing prosthetic groups known as hemes. The ervthrocvtes helD maintain hemoσlobi in its reduced, functional form. The heme-iron atom is susceptible to oxidation, but may be reduced again by one of two systems within the erythrocyte, the cytochrome be, , and glutathione reduction systems. The use of cell-free solutions of hemoglobin as a potential oxygen-carrying red cell substitute has been investigated for a long time. See, for example. Mulder, A.G., et al., J. Cell Comp. Phvsiol . 5:383 (1934) , the disclosure of which is incorporated herein by reference. The use of unmodified cell-free human hemoglobin purified from red blood cells suffers from several limitations in addition to contamination and supply limitations noted above, namely, an increase in oxygen affinity due to loss of the cofactor 2, 3-diphosphoglycerate ("2,3-DPG") , and dissociation of Hb tetramers into aβ dimers which are cleared by renal filtration and which can cause long-term kidney damage. See, for example, Bunn, H.F., et al . , ___ Exp. Med. 129 : 909 (1969) , the disclosure of which is incorporated herein by reference. Hemoglobin is able to alter its oxygen affinity, thereby increasing the efficiency of oxygen transport in the body due to its dependence on 2,3-DPG, an allosteric regulator. 2,3-DPG is present within erythrocytes at a concentration that facilitates hemoglobin to release bound oxygen to tissues. In the absence of 2,3-DPG, hemoglobin binds oxygen more tightly and does not readily release its bound oxygen. Thus, native human normal adult hemoglobin ("Hb A") were it to be introduced into a subject would not be able to properly allow oxygen to be transported through the body due to a lack of 2,3-DPG in the blood plasma. Human globins and hemoglobins have been expressed in the following: transgenic mice, see, for example, Chada, K. , et al . , Nature (London) 314 :377 (1985) and Townes, T. M. , et al . , EMBO J. 4:1715 (1985) , the disclosures of which are incorporated herein by reference; transgenic swine as described by Swanson, M.E., et al., Bio/Technolocrv ___:557 (1992), the disclosure of which is incorporated herein by reference; insect cell cultures as reported by Groebe, D.R., et al. , Protein Expression and Purification 2:134 (1992), the disclosure of which is incorporated herein by reference; yeast as described by Wagenbach, M. , et al. , Bio/Technology 9:57 (1991) and DeLiano, J.J., et al. , Proc. Natl. Acad. Sci. USA 90:918 (1993) , the disclosures of which are incorporated herein by reference, and Escherichia coli ("E. coli") as described by Hoffman, S.J., et al. , Proc. Natl. Acad. Sci. USA 87:8521 (1990), Hernan, R.A. , et al. , Biochemistry __L:8619 (1992), and Shen, T.-J., et al. , Proc. Natl. Acad. Sci. USA S__:8108 (1993), the disclosures of which are incorporated herein by reference. In many respects, the E. coli system is the best choice for such purposes because of its high expression efficiency and the ease of performing sit<i-directed mutagenesis. The first E. coli system to express human a- or / S-globin as a fusion protein was developed as described by Nagai, K., et al. , Nature (London) 309:810 (1984) and Nagai, K. , et al. , Methods Enzvmol. 153 :461 (1987), the disclosures of which are incorporated herein by reference, but the product processing procedure of this system is very laborious and gives a low yield. Thus, this expression system is not desirable, especially when large amounts of recombinant hemoglobin ("r Hb") are required for biochemical, biophysical, and biological studies. Hoffman, S.J., et al. , Proc. Natl. Acad. Sci. USA 6_7:8521 '1990) have reported a coexpression system in which the synthetic human a- and 3-globin genes are organized in a single cistron and are expressed in equal amounts. Both of the expressed α- and β-globins are properly assembled with endogenous hemes into tetrameric Hb molecules in E. coli. Hernan, R.A., et al., Biochemistry 3_1:8619 (1992) have reported the expression of a nonfusion single jβ-globin in E. coli. Although these two nonfusion systems work very well in several aspects, an extra methionine residue is retained at the amino (N) -termini of both α- and 3-globins. The natural N-terminal valine residues of Hb A are known to play important roles in regulating oxygen affinity, the Bohr effect, and interactions with allosteric effectors and anions as reported by Bunn, H.F., et al. , eds. Hemoglobin: Molecular, Genetic and Clinical Aspects (W.B. Saunders, Co., Philadelphia, PA) pp. 37-60 (1986) , the disclosure of which is incorporated herein by reference. The extra methionine can alter the N-terminal conformation of the Hb molecule as reported by Kavanaugh, J.S., et al., Biochemistry 31:8640 (1992) , the disclosure of which is incorporated herein by reference. Hence, the oxygenation properties of Hb depend on the integrity of the N-terminal residue thereby mandating the removal of the extra methionine residues from the N-termini of both α- and 3-globins of the expressed Hb before the E. coli system can be used effectively for the production of desired unmodified and mutant Hbs . Methionine aminopeptidase ("Met-AP" or "MAP") has been found to be responsible for the removal of the N- terminal methionine residue from a nascent peptide chain i E. coli. Salmonella, Bacillus, and yeast, and the Met- APs from a variety of organisms have been purified and characterized. See, for example, Ben-Bassat, A., et al . , J. Bacteriol. 169:751 (1987) and U.S. Patent Nos . 4,865,974, 4,870,017, and 5,013,662, the disclosures of which are incorporated herein by reference. These enzymes possess unique specificity for the N-terminal methionine of a peptide or a protein, and the Met-APs of E. coli and Salmonella tvphimurium have been reported to remove the N- terminal methionine from a number of expressed foreign proteins. Ben-Bassat, A., et al . , J. Bacteriol. 169 :751 (1987) . The cooperative oxygenation of Hb, as measured by the Hill coefficient ("n") is a convenient measure of some of its allosteric properties. See, Dickerson, R.E., et al . , Hemoglobin: Structure, Function, Evolution, and Pathology, (The Benjamin Cummings Publishing Co., Menlo Park, CA) (1983) , the disclosure of which is incorporated herein by reference. Hb A has an n max value of approximately 3 in its binding with O2 under usual experimental conditions. Human abnormal Hbs with amino acid substitutions in the αi (° r «2^l) subunit interface generally show high oxygen affinity and reduced cooperativity in O2 binding compared to Hb A. See, for example. Dickerson, R.E., et al . , Id. (1983) ; Bunn, H.F., et al., J. Biol. Chem. 249:7402 (1974) ; and Perutz, M.F., et al . , Mechanisms of Cooperativity and Allosteric Regulation in Proteins Cambridge University Press (1990) . Thus, it has been suggested that the a β2 subunit interface is important for the functional properties of Hb. For example, human mutant Hbs with an amino acid substitution at the S99Asp possess greatly reduced cooperativity and increased oxygen affinity relative to those exhibited by Hb A. Examples of such mutant hemoglobins are Hb Kempsey (,S99Asp-*Asn) (Reed, C.S., et al. Blood 31:623 (1968)) ; Hb Yakima (099Asp→His) (Jones, R.T., et al., J. Clin. Invest. 46:1840 (1967)) ; and Hb Radcliff (399Asp→Ala) (Weatherall, D.J., et al . , British J. Haematol. 3_5:177 (1977)) , the disclosures of which are all incorporated herein by reference. Fermi, G. , et al . , J. Mol . Biol. 175 :159 (1984) , the disclosure of which is incorporated herein by reference, have performed X-ray crystallographic studies of deoxy-Hb A (Hb A without oxvgen molecules) that show that _99ASP is hvdroσe.n-bonde to both α42Tyr and α:97Asn in the aιβ2 subunit interface. This suggests that the essential role of 399Asp is to stabilize the deoxy-Hb molecule by making intersubunit hydrogen bonds. Recently two recombinant Hb ("r Hb") mutants have been constructed involving mutation of α42Tyr which result in very different properties in the resulting mutants. r Hb (at42Tyr-»His) of Ishimori, K. , et al. , J_ Biol. Chem. 264:14624 (1989) and Imai, K. , et al. , J. Mol. Biol. 218:769 (1991), the disclosures of which are incorporated herein by reference, exhibits some cooperativity in binding oxygen (n = 2 at pH 6.8) and moderate oxygen affinity. r Hb (α42Tyr- * Phe) (Ishimori, K., et al., J. Biol. Chem. 264:14624 (1989) and Imai, K. , et al., J. Mol. Biol. 218:769 (1991)), the disclosures of which are incorporated herein by reference, exhibits almost no cooperativity in binding oxygen (n = 1.2) and possesses very high oxygen affinity. The differences in the properties between these two mutants have been attributed to the presence of a weak hydrogen bond between cκ42His and / S99Asp in the deoxy state of r Hb (o;42Tyr-»His) , but not in deoxy-r Hb (o;42Tyr→Phe) . It is known that abnormal Hbs with an amino acid substitution at either 399Asp or α42Tyr which lose the intersubunit hydrogen bonds in the deoxy form, also lose their functional properties, therefore it has been suggested that these hydrogen bonds may be crucial for the structure and function of the Hb molecule. Hb A in its oxy form (Hb A with oxygen molecules) has a characteristic hydrogen bond between α94Asp and βl02Asn in the 01^2 subunit interface as reported by Shaanan, B., et al. , J. Mol. Biol. 171:31 (1983) , the disclosure of which is incorporated herein by reference. Human Hbs with an amino acid substitution at either the α94Asp position such as Hb Titusville (o94Asp→Asn) (Schneider, R. G. , et al. , Biochim. Biophvs. Acta. 400:365 (1975) , the disclosure of which is incorporated herein by reference) or the /?102Asn position such as Hb Kansas (?102Asn→Thr) (Bonaventura, J. , et al., J. Biol. Chem. 243 :980 (1968), the disclosure of which is incorporated herein by reference) , as well as others, exhibit very low oxygen affinity. However, all these Hb mutants which directly disrupt the hydrogen bond between o;94Asp and ?102Asn in the oxy form of Hb show greatly reduced cooperativity in the binding of oxygen and additionally dissociate easily into dimers when in the ligated state. Low oxygen affinity human mutant Hbs which do not involve either α94Asp or 3l02Asn also exist. For example, Hb Presbyterian ( / SlOδAsn→Lys) (Moo-Penn, W.F., et al., FEBS Lett. 92:53 (1978) and O'Donnell, J.K., et al. , J. Biol. Chem. 269:27692 (1994); Hb Yoshizuka (3l08Asn→Asp) (O'Donnell, J.K., et al. , J. Biol. Chem. 269:27692 (1994) and recombinant Hb Mequon (/341Phe-»Tyr) (Baudin, V., et al., Biochim. Biophvs. Acta. 1159:223 (1992) , the disclosures of which are incorporated herein by reference, all exhibit low oxygen affinity compared to Hb A, but they all exhibit a variable amount of cooperativity as measured by the Hill coefficient, with n varying from 1.8 to 2.9. Molecular dynamics ("MD") simulations have been used to calculate the free energy difference between native and mutant proteins. See, for example, Dang, L.X., et al. , J. Am. Chem. Soc. 111:8505 (1989), and Gao, J., e al., Science 244 :1069 (1989), the disclosures of which ar incorporated herein by reference. MD simulations of Hb Radcliff (,_99Asp→Ala) show close agreement between the measured thermodynamic value and the calculated value as reported by Gao, J., et al. , Science 244:1069 (1989) . MD simulations have been used to successfullv design compensatory amino acid substitutions in an abnormal Hb which restore its allosteric properties as reported by Kim, H.-W., et al. , Proc. Natl. Acad. Sci. USA 91:11547 (1994) , the disclosure of which is incorporated herein by reference. Shen, T.-J., et al. , Proc. Natl. Acad. Sci. USA 9_0:8108 (1993), the disclosure of which is incorporated herein by reference, describe an E. coli expression plasmid (pHE2) in which synthetic human - and 3-globin genes are coexpressed with the E. coli methionine aminopeptidase gene under the control of separate tac promotors. E. coli cells transformed with this plasmid express r Hb A from which the N-terminal methionines have been effectively cleaved by the coexpressed MAP. The resulting recombinant Hb A which lacks an N-terminal methionine is identical to the native Hb A in a number of structural and functional properties. There remains a need, however, for a mutant hemoglobin species that can be used as a component of a hemoglobin-based blood substitute or therapeutic agent. Of particular interest is a mutant hemoglobin that possesses a low oxygen affinity, but high cooperativity in oxygen binding, and which exhibits no unusual subunit dissociation when ligated. There is a further need for such a hemoglobin produced by recombinant methods and an efficient expression system for producing such a mutant hemoglobin in high yield especially for use in a blood substitute product. SUMMARY OF THE INVENTION Accordingly, it is a primary object of the present invention to provide a mutant hemoglobin with low oxygen affinity. Another object of the present invention is to provide a non-naturally occurring mutant hemoglobin with low oxygen affinity. Another object of the present invention is to provide a low oxygen affinity mutant hemoglobin produced artificially, preferably by recombinant means, that also has the correct heme conformation. Yet another object of the present invention is to provide a mutant hemoglobin with a low oxygen affinity but high cooperativity in oxygen binding. Another object of the present invention is to provide a mutant hemoglobin that in a cell-free environment has similar oxygen binding properties as those of human normal adult hemoglobin in red blood cells. Yet another object of the present invention is to provide an expression system for producing such mutant hemoglobins. Still another object of the present invention is to provide an artificial hemoglobin for use as a hemoglobin-based red blood substitute or therapeutic agent. These and other objects of the present invention are achieved by one or more of the following embodiments. In one aspect, the invention features a non- naturally occuring mutant human hemoglobin wherein the valine residue at position 96 of the a chain is replaced by a tryptophan residue. In a preferred embodiment, the hemoglobin possesses low oxygen affinity as compared to normal human adult hemoglobin and high cooperativity in oxygen binding and is produced recombinantly. In another aspect, the invention features a non- naturally occurring low oxygen affinity mutant hemoglobin that has oxygen binding properties comparable to those of human normal adult hemoglobin in the presence of the allosteric effector 2,3-DPG. In a preferred embodiment, in the mutant hemoglobin the valine residue at position 96 of the a chain is replaced by a tryptophan residue. Other features and advantages of the invention will be apparent from the following description of the preferred embodiment, and from the claims. DESCRIPTION OF THE DRAWINGS Figure 1 is a chromatographic elution profile from a Mono S column of r Hb (o;96Val→Trp) obtained from the sonicate of E. coli JM109 cells that were transformed with plasmid pHE202 ( "pHE202/JM109") showing peaks a, b- , and b2. Fractions were monitored at 546 nm. Figures 2A-2D are electrospray mass spectra of r Hbs (α96Val→Trp) from peak a (Figure 2A) ; peak b^ (Figure 2B) ; and peak b2 (Figure 2C) derived from plasmid pHE202; as well as of Hb A (Figure 2D) as a reference. Figures 3A and 3B are 1 H-NMR spectra showing ring-current shifted proton resonances of 4% r Hbs (α96Val- * Trp) derived from pHE202/JM109 and 4% Hb A in the CO forms in 0.1 M phosphate in H 2 0 at pH 7.0 and 29°C. Figure 3A shows purified fractions (a, bη_, b2) from Mono S column purification in their CO forms . Figure 3B shows purified r Hbs (o;96Val→Trp) from peaks a, bτ_, and b2 which were converted from the CO form to the Fe +3 state, and then back to the CO form. Figures 4A and 4B show 1 H-NMR resonances of 4% r Hb (o;96Val-»Trp) from peak a derived from pHE202/JM109 and 4% Hb A in their deoxy forms in 0.1 M phosphate in H2O at pH 7.0 and 29°C (both figures) and 36°C (Figure 4B only) . Figure 4A shows hyperfine-shifted N_H exchangeable proton resonances of the proximal histidine residue; Figure 4B shows hyperfine-shifted and exchangeable resonances. "Deoxy-r Hb" in Figure 4B stands for Deoxy- Figures 5A-5D are ^H-NMR spectra showing the effects of temperature on the 300-MHz proton resonances of 4% r HbCO (o;96Val→Trp) from peak a derived from pHE202/JM109 and 4% HbCO A in the presence of 10 mM inositol hexaphosphate ("IHP") in 0.1 M phosphate at pH 7.0 and at 10, 21, 29 and 36°C. Figures 5A and 5B are exchangeable proton resonances of 4% r HbCO (α96Val→Trp) from peak a and of 4% HbCO A, respectively. Figures 5C and 5D are ring-current shifted resonances of 4% r HbCO (<_96Val→Trp) from peak a, and of 4% HbCO A, respectively. Figures 6A-6D are 1 H-NMR spectra showing the effects of temperature on the 300-MHz proton resonances o 4% r HbCO (α96Val→Trp) from peak a derived from PHE202/JM109 and 4% HbCO A in the absence of IHP in H 2 0 a pH 7.0, and at 10, 15, 21, and 29°C. Figures 6A and 6B are exchangeable proton resonances of 4% r HbCO (α96Val→Trp) from peak a, -=ιnd of 4% HbCO A, respectively. Figures 6C and 6D are ring-current shifted resonances of 4% r HbCO (α96Val→Trp) from peak a and of 4% HbCO A, respectively. Figures 7A and 7B show oxygen affinity with partial O2 pressure at 50% saturation (P50) (mmHg) plotted as a function of pH obtained with a 0.1 mM of Hb A or r Hb (o;96Val→Trp) derived from pHE202/JM109 in 0.1 M sodium phosphate buffer in the pH range 6.3-8.9 at 29°C. Figure 7A shows Hb A (O) and purified fractions of r Hbs (α96Val→Trp ) (peaks a (■) , bι_ (Δ) , and b 2 (+) ) from a Mono S column; Figure 7B shows Hb A (O) and purified fractions of r Hbs (o;96Val→Trp) from a Mono S column (peaks a (■) , b^_ (Δ) , and b2 (+) ) which had been converted from the CO form to the Fe +3 state and then back to the CO form. Figures 8A and 8B show the Hill coefficient (n max ) as a function of pH in which oxygen dissociation data were obtained with 0.1 mM concentration of Hb A or r Hb (α96Val→Trp) derived from pHE202/JM109 in 0.1 M sodium phosphate buffer in the pH range 6.5-8.4 at 29°C. Figure 8A shows Hb A (O) and Mono S-purified fractions (peaks a (■) , bx (Δ) , and b2 (+) ) ; Figure 8B shows Hb A (O) and purified fractions of r Hbs (α96Val→Trp) from a Mono S column (peaks a (■) , b^ (Δ) , and b2 (+) ) which had been converted from the CO form to the Fe +3 state and then back to the CO form. Figure 9 shows oxygen dissociation curves obtained with 0.1 mM Hb in 0.1 M sodium phosphate buffer at pH 7.4 and 29°C of Mono S-purified r Hb (α96Val→Trp) from peak a derived from pHE202/JM109 and Hb A as compared to Mono S-purified r Hb (α96Val→Trp) from peak a in the presence of allosteric effectors: 2 mM 2,3-DPG and 2 mM IHP. Figures 10A and 10B show respectively, the temperature dependence of oxygen affinity (P50) and the Hill coefficient (n max ) obtained with 0.1 mM Hb A or r Hb (cc96Val→Trp) derived from pHE202/JM109 in 0.1 M sodium phosphate buffer at pH 7.4, and at 16, 20, 25, 29, and 37°C. The Hbs used were: Hb A (O) , r Hb (α96Val→Trp) (Δ) , r Hb (α96Val→Trp) in the presence of 2 mM IHP (+) ; and Hb A in the presence of 2 mM IHP (D) . Figures 11A and 11B are 1 H-NMR spectra showing exchangeable proton resonances (Figure 11A) and ring- current shifted proton resonances (Figure 11B) of 4% r HbCO (α96Val→Trp) from peak b2 derived from pHE202/ JM109, 4% r HbCO (o;96Val→Trp) from peak 2 derived from pHE702/vTM109 and Hb A, all in the CO form in 0.1 M phosphate in H2O at pH 7.0 and 29°C. Both r Hb (α96Val→Trp) samples had gone through the oxidation- reduction process before carying out the NMR measurements. DETAILED DESCRIPTION OF THE INVENTION I. DEFINITIONS As used herein, "Hb A" or "native Hb A" means human normal adult hemoglobin as obtained from human subjects. "Recombinant human normal adult hemoglobin, " "r Hb A, " and "unmodified r Hb A" means human normal adult hemoglobin produced through recombinant DNA technology and having essentially the same structure and function as native Hb A as described by Shen, T.-J., et al. , Proc. Natl. Acad. Sci. USA 90:8108 (1993) . "r Hb (o;96Val→Trp) " refers to a mutant human normal adult hemoglobin produced through recombinant DNA technology in which the valine residue at position 96 of the α chain, located in the otιβ2 (or oi2β ) interface, has been replaced by a tryptophan residue. This hemoglobin shows a low oxygen affinity but high cooperativity in oxygen binding, and exhibits no unusual subunit dissociation when ligated. "Deoxy" and "oxy" refer to the oxygenation state of the heme-iron atom in Hb A and r Hb (α96Val→Trp) . Oxyhemoglobin "Oxy-Hb" or "Hbθ 2 " has four oxygen molecules bound to the heme groups; deoxyhemoglobin ("deoxy-Hb") contains no oxygen molecules. In normal arterial blood, normal adult hemoglobin A ("Hb A") is in the oxy form ("oxy-Hb A") . In venous blood, a portion of Hb A is in the deoxy form ("deoxy-Hb A") . "Carbonmonoxy-Hb" "HbCO A, " "r HbCO (α96Val→Trp) " and "CO form" all refer to hemoglobin bound to carbon monoxide molecules rather than oxygen molecules. "Ferri-hemoglobin, " "ferri-Hb," "ferric form" "methemoglobin, " "met-Hb", and "Fe +3 state" all refer to Hb A and r Hb (o;96Val→Trp) with their respective heme-iron atoms oxidized to the ferric (Fe 3+ ) state. Ferri-Hb does not bind oxygen. "Ferro-hemoglobin, " "ferro-Hb," "Fe +2 state", and "ferrous form" refer to Hb A, r Hb (α;96Val→Trp) , HbCO A, and r HbCO (α96Val-»Trp) with their respective heme-iron atoms in the native, reduced ferrous (Fe +2 ) state. Ferro-Hb is capable of binding oxygen or carbon monoxide. "Control" means a gene coding sequence being subject to another gene in the control region of a DNA molecule, in particular, a promoter, whereby the coding sequence can be expressed and regulated under the control of the promoter. Absent such control, the coding sequences may be expressed at too high or too low a level in the host organism, or at an improper time. "Met-aminopeptidase, " "Met-AP, " and "MAP" refer to the enzyme methionine aminopeptidase which specifically cleaves the amino- (N) terminal methionine residue from a peptide sequence. "Quaternary structural transition" means the structural rearrangement in the subunit interfaces of hemoglobin, such as ctι_β2 upon going from the deoxy form to the oxy form. "Oxygen affinity" means the strength of binding of oxygen to a hemoglobin molecule. High oxygen affinity means hemoglobin does not readily release its bound oxygen molecules. The P5 0 is a measure of oxygen affinity. "Cooperativity" refers to the sigmoidal oxygen- binding curve of hemoglobin, i.e., the binding of the first oxygen to one subunit within the tetrameric hemoglobin molecule enhances the binding of oxygen molecules to other unligated subunits. It is conveniently measured by the Hill coefficient (n max ) . For Hb A, n max = 3.0. R-type or R-like and T-type or T-like structures of hemoglobin refer to those hemoglobins which exhibit characteristic quaternary structural markers, such as the proton resonance at 10.7 ppm from DSS as the R-structural marker and the proton resonance at 14 ppm from DSS as the T-structural marker. Some mutant and chemically modified Hbs could have a T- or R- quaternary structural marker but with altered tertiary structures. II. METHODS AND RESULTS According to the present invention, a non- naturally occurring low oxygen affinity mutant human hemoglobin is provided as well as means for producing such hemoglobin recombinantly. More particularly, the present invention is directed to a recombinantly produced mutant of Hb A, denoted r Hb (o;96Val- * Trp) herein, in which the valine residue at position 96 of the chain, located in the otι_β2 (or α^/Si) subunit interface region, has been replaced by a tryptophan residue. This new artificial hemoglobin, i.e.. derived entirely from sources other than blood, shows a low oxygen affinity but high cooperativity in oxygen binding. Further, it exhibits no unusual subunit dissociation when ligated. In a cell-free environment, the r Hb (<_96Val→Trp) of the present invention has similar oxygen binding properties as those of Hb A in red blood cells. Such a low oxygen affinity hemoglobin mutant produced recombinantly is of value as a component of a hemoglobin-based blood substitute. Such a mutant is also useful in radiation treatments of tumors, trauma, and hemodilution in surgery. It is also within the scope of the present invention to prepare and use other low oxygen affinity hemoglobins with other appropriate mutations. In particular, the methods of the present invention may be used to produce other mutant hemoglobins with additional advantageous properties. It is believed that other mutations in the <tιβ2 interface and the central cavity of hemoglobin may yield other useful low oxygen affinity mutants. Methods for evaluating suitability of such mutants for use in a blood substitute or therapy are described herein below. Molecular dynamics simulations detailed below suggest that the unique oxygen-binding property of r Hb (αt96Val→Trp) of the present invention may be due to an extra hydrogen bond between α96Trp and <599Asp in the a±β2 subunit interface in the deoxy form of r Hb (α96Val→Trp) . Despite the replacement of a small amino acid residue, valine, by a large tryptophan residue in the 01^2 subunit interface, it is shown below that this artificial hemoglobin shows very similar tertiary structure around the heme pockets and quaternary structure in the 01^2 subunit interface compared to those of human normal adult hemoglobin. The tertiary structure of the hemoglobin molecule refers to the steric relationships of amino acid residues that are far apart in the linear chain sequence that cause each chain to fold upon itself, while the quaternary structure refers to how the subunit chains may interact with each other. As will be demonstrated below, the artificial hemoglobin of the present invention possesses the property that the ligated form of this hemoglobin, for example, the carbonmonoxy form, in the oxy-(R-) type quaternary structure, can be converted to the deoxy- (T-) type quaternary structure without changing the ligation state by the addition of an allosteric effector, such as inositol hexaphosphate. This conversion can also be carried out by lowering the temperature in the absence of inositol hexaphosphate. The recombinant hemoglobin of the present invention can therefore be used to gain new insights regarding the nature of subunit interactions in the QΪ3_I_2 interface and the molecular basis for the allosteric mechanism of hemoglobin. Clearly, the methods of the present invention may also be used to produce other mutant artificial hemoglobins with different properties as well as hemoglobins with mutations that compensate for mutants that are naturally occurring. The preferred materials and methods for obtaining r Hb (o;96Val→Trp) are given in the following reference example. While the r Hb (α96Val→Trp) of the present invention is preferably produced recombinantly, it is understood that non-recombinant methods may also be used. Reference Example A. Plasmid PHE202 Construction of Expression Plasmid pHE202 Two plasmids that express r Hb (α-96Val→Trp) have been constructed in accordance with the present invention, plasmid pHE202 and plasmid pHE702. The protocols and results for plasmid pHE202 appear first as follows. The Hb A expression plasmid pHE2 containing synthetic human α- and β- globin genes was used as a starting plasmid. The construction of plasmid pHE2 and properties of the r Hb A produced are fully described in Shen, T.-J., et al.. Proc. Natl. Acad. Sci. USA 90:8108 (1993) , the disclosure of which is incorporated by reference. In constructing plasmid pHE2, the synthetic human - and β- globin genes were obtained from plasmid pDLIII-l3e (a gift from Somatogen, Inc., Boulder CO, and fully disclosed in Hoffman, S.J., et al . , Proc. Natl. Acad. Sci. USA __7:8521 (1993), the disclosure of which is incorporated herein by reference) . The coding sequence of the E. coli Met-AP gene was obtained from plasmid pSYC1174 (pSYC1174 in E. coli MM294 was gift from Cetus Corp., Emeryville, CA) , and this strain was also deposited with the American Type Culture Collection under accession number ATCC 53245 and is available to the public. See also, U.S. Patent Nos . 4,865,974, 4,870,017 and 5,013,662. Plasmid pHE2 coexpresses the synthetic human - and β- globin genes with the E. coli Met-AP gene under the control of separate tac promoters . Site-directed mutagenesis was then carried out to replace the valine residue at position 96 of the a chain by a tryptophan residue as follows. The synthetic human a- and β-globin genes (from plasmid pDLIII-13e) was inserted into phagemid pTZ18U (Bio-Rad Laboratories, Hercules, CA) by methods well known in the art. A synthetic oligonucleotide of sequence 5 ' -TTTGAAGTTCCATG- GATCAAC-3' (mutated codon is underlined) was synthesized at the DNA Synthesizing Facility, University of Pittsburgh, and used as a primer to introduce the mutation, α96Val→Trp, by standard DNA synthesis methodology. Techniques for oligonucleuticle synthesis are well known and this invention is not limited to any particular technique. Site-directed mutagenesis was performed as described by Kunkel, T.M., et al . , Proc. Natl. Acad. Sci. USA 82:488 (1985) and Shen, T.-J., et al., Proc. Natl. Acad. Sci. USA 90:8108 (1993) , the disclosures of which are incorporated herein by reference . The human normal α:-globin gene in plasmid pHE2 was then replaced with the mutated gene to produce plasmid pHE202. Plasmid pHE202 in host cell E. coli JM109 and designated pHE202/JM109 was deposited with the American Type Culture Collection of Rockville, MD on April 26, 1995 under number ATCC 69792. Growth of Cells Plasmid pHE202 was transformed into E. coli JM109 cells (Promega, Madison, WI) by methods well known in the art. The cells were grown in TB medium which contains 1.2% bactotryptone, 2.4% bactoyeast extract, 0.4 glycerol, 17 mM KH 2 P0 4 , 72 mM κ HP0 4 , and 100 μg ampicillin (Sigma, St. Louis, MO) in a 20-liter Microferm Model MF20 fermentor (New Brunswick Scientific, Edison, NJ) at 30°C until the cell density reached 1-2 x 10 9 cells per ml . The expression of r Hb (α96Val→Trp) was induced by adding isopropyl θ-thiogalactopyranoside to a concentration of 0.2 mM. The culture was then supplemented with hemin (20 mg/liter) (Sigma, St. Louis, MO) and glucose (10 g/liter) and the growth was continued for at least another four hours. The cells were then harvested by centrifugation and stored frozen in 100-gram portions at -80°C until needed for further purification. Although E. coli cells are presently preferred for expressing and producing the recombinant mutant hemoglobin of the present invention, the invention is not limited to E. coli cells. Other appropriate expression systems such as yeast, insect cells and transgenic animals such as pigs and cows may also advantageously be used to express mutant hemoglobins. Plasmid pHE202 has been optimized for E. coli cells whereas pHE702, described in detail below, was constructed from human genes and thus other expression systems may be advantageously used with human genes as contained in plasmid pHE702. Isolation and Purification of Recombinant Hbs The r Hb (α;96Val→Trp) obtained from cells transformed with plasmid pHE202 was purified as essentially described by Shen, T.-J., et al . , Proc. Natl. Acad. Sci. USA 9_0:8108 (1993) , with minor modifications. The frozen stored cell paste was put into lysis buffer (40 mM Tris-base/1 mM benzamidine (Sigma) at 3 ml/gm of cell paste) and stirred until it was completely suspended. Lysozyme (Sigma) (1 mg/g cell paste) , dissolved in 10 ml of 40 mM Tris-HCl pH 8.0, was added to the cell suspension and allowed to stand at 4°C for about 45 minutes (solution becomes very viscous) . MgCl 2 and MnCl2 were added to a final concentration of 10 mM and 1 mM, respectively. About 3-5 μg/ml of DNAse (ICN Biochemical, Inc., Costa Mesa, CA) was added and stirred for 30-60 minutes or until viscosity drops. The lysate was then sonicated in a Branson 450 sonifier (Branson, Danbury, CT) in an ice bath at 65-75 W in three 3-minute cycles stopping between cycles to ensure the lysate was kept cold. The lysate was then centrifuged at 4°C for 45 minutes at 14.000 rpm in a Sorvall GSA rotor (E.I. duPont, Wilmington, DE) . The supernatant was then saturated with CO gas and allowed to stand at 4°C for 3-4 days. The pH of the supernatant was adjusted to 8.0 with 1 M Tris base and 10% polye.hyleneimine (Sigma) (vol/vol) was added slowly to a final concentration of 0.5% in order to precipitate out the nucleic acids. This was stirred under CO gas for 15 minutes and then centrifiged at 14,000 rpm as above. The pH of the supernatant was adjusted to 8.3 with 1 M Tris base and put through a Millipore Minitan Acrylic Ultrafiltration System (Millipore Corp., Bedford, MA) until the volume was around 150 ml. The concentrated solution was dialyzed against 20 mM Tris-HCl/0.1 mM triethylenetetraamine ("TETA") pH 8.3 (referred to below as "Q2 buffer") overnight with a change of buffer. The pH and conductivity were adjusted so that they were the same as Q2 buffer, usually by adding 20 mM Tris-base/0.1 mM TET . Two columns were used in the final purification process using a Pharmacia fast protein liquid chromatography ("FPLC") system (Pharmacia Biotech, Inc., Piscataway, NJ) . The first column used was a Q-Sepharose fast-flow column (Pharmacia anion exchanger) (5 cm x 28 cm) which was used to bind Hb. After loading the sample onto the column, it was washed with Q2 buffer (20 mM Tris HCl/0.1 mM TETA, pH 8.3) and monitored at 260 nm until the contaminating nucleic acids had eluted. The Hb fraction was then eluted from the column with 20 mM Tris-HCl at pH 7.2. The eluent was concentrated with 10 mM phosphate/ 0.1 mM ethylenediaminetetraacetic acid ("EDTA") , pH 6.8 (Buffer A) . The second column employed was a Mono S column (Pharmacia cation exchanger, HR6/10) . The equilibrated sample was purified on the Mono S column by running a gradient from 10 mM sodium phosphate/0.1 mM ethylenediaminetetraacetic acid ("EDTA") , pH 6.8 (Buffer A) , to 20 mM sodium phosphate/0.1 mM EDTA, pH 8.3 (Buffer B) . Fractions were monitored at 546 nm. r Hb (α96Val→Trp) was eluted in two major peaks as shown in Figure 1, a symmetrical peak (peak a) followed by two overlapping peaks (peaks b^ and b2> ■ These peaks obtained as described above are referred to hereafter as "peak a", "peak bi", and "peak b2 π , as further shown in Figure 1. The Hb concentration was determined by using published extinction coefficients as described by Antonini, E., et al. , Hemoglobin and Mvoglobin in Their Reactions with Ligands (North Holland, Amsterdam) p. 19 (1971) , the disclosure of which is incorporated herein by reference. Forms of Hb The CO forms of Hbs (e.g., Hb A and r Hb (α96Val→Trp) ) were prepared by passing a stream of CO gas through a Hbθ2 or deoxy-Hb solution contained in a flask. This procedure was carried out inside a ventilated fume hood. Deoxy-Hb was usually prepared by converting HbCO or Hbθ2 to the deoxygenated form in a rotary evaporator under N2 gas at 4°C as fully disclosed in Lindstrom, T.R., et al., Proc. Natl. Acad. Sci. USA 69:1707 (1972) . Oxy-Hb was prepared by exposing deoxy-Hb solution to air or O2 gas. Protein Sequencing Automated cycles of Edman degradation were performed with an Applied Biosystem gas/liquid-phase sequencer (Model 470/900A) that was equipped with an on¬ line phenylthiohydantoin-amino acid analyzer (Model 120A) as described by Hewick, R.M., et al . , ___ Biol . Chem. 256 :7990 (1981) , the disclosure of which is incorporated herein by reference. Mass Spectrometry Hb samples subjected to mass spectrometry were dialyzed extensively against distilled H2O and then lyophilized. Immediately before analysis, the samples were dissolved in water to a concentration of 125 pmol of Hb per μl of H2O (7.8 mg/ml) . Aliquots of these solution were then diluted to give a final concentration of 10 pmol/μl of 50:50 water/acetonitrile containing 0.2% formi acid. Aliquots (10 μl) of these final solutions were introduced into the electrospray ion source at 5 μl/ minute. The electrospray analyses were performed on a V Quattro-BQ (Fisons Instruments, VG Biotech, Altrincham, U.K.) , a triple quadrupole instrument with a mass range for singly charged ions of 4000. Scanning was performed from m/z 980 to 1400 in 10 seconds per scan. The data obtained from 20 scans were summed to give the final spectra. Mass scale calibration used the multiply charge ion peaks from the a chain (M r = 15,126.4) of Hb A as an external reference. The molecular weights calculated fro the amino acid sequences of normal chain and normal β chain are 15,126.4 and 15,367.2, respectively. Such values are based on the following atomic weights of the elements: C = 12.011; H = 1.00794; N = 14.00674; 0 = 15.9994; and S = 32.066, as reported by Commission on Atomic Weights and Isotopic Abundances, Pure Appl. Chem. £1:975 (1991), the disclosure of which is incorporated herein by reference. The resulting electrospray mass spectra are shown in Figures 2A-2D. Conversion of Oxidation State of the Heme-iron Atoms In order to oxidize Hbs, in this case the CO form of Hb A and r Hb (α96Val→Trp) , to the Fe +3 state, the concentration of Hb solution in 0.1 M phosphate at pH 6.5 was determined. A 3-molar excess of K 3 Fe(CN)g (Fisher Scientific) or other suitable oxidizing agent was added and the solution was stirred at room temperature for 1 hour. During this time, the Hb solution changed from red to brownish color. The resulting oxidized Hb (Fe +3 ) was put through a Sephadex G-25 (medium) column (Sigma) using 0.1 M phosphate at pH 6.5 to remove excess K 3 Fe(CN)_ and was left to stand overnight at room temperature. The oxidized Hb (Fe +3 ) was subsequently reduced by first preparing 0.1 M phosphate at about pH 7.0 in a test tube, and deoxygenating the buffer by bubbling nitrogen through the buffer to remove the dissolved oxygen. In another tube, enough sodium dithionite (Fluka Chemie, Switzerland) was weighed out to give a 0.1 M solution of dithionite. The tube was then flushed with nitrogen to remove oxygen. The deoxy phosphate buffer wa then anaerobically transferred to the deoxy dithionite to give a 0.1 M final concentration. A stream of CO was blown across the top of (not bubbled through) the oxidize Hb solution. The deoxy dithionite buffer solution was added to the oxidized Hb solution until the color changed back to bright red. At this point, the optical spectrum can be checked to ensure all the Hb (Fe +3 ) is reduced to Hb (Fe +2 ) . A Sephadex G-25 column was equilibrated with the same CO gassed buffer that was used for reducing the Hb. The reduced Hb solution was then put through the Sephadex G-25 column using the same CO gassed buffer. The resulting oxidized/reduced Hb was then preferably purified through an FPLC Mono S column as described above. The resulting oxidized, reduced, Mono S-purified r Hb (α96Val→Trp) or Hb A was then converted to the carbonmonoxy or oxy form as desired as described above. Other suitable oxidizing agents may be used to oxidize ferro-hemoglobin (Fe +2 ) to ferri-Hb (Fe +3 ) such as, for example, ferricyanide, copper, hydrogen peroxide, hydroxylamine, nitrites, hydrazines, thiols, arylamines or any compound with an electrical chemical potential ("E m ") greater than 0.14 volt. Other suitable reducing agents that may be used to reduce ferri-Hb (Fe +3 ) to ferro- hemoglobin (Fe +2 ) are: (i) for non-enzymatic reduction - dithionite, metabisulfite, cysteine, reduced glutathione, and ascorbic acid (in the presence of methylene blue) ; an (ii) for enzymatic reduction - cytochrome b5 reductase, NADPH-flavin reductase, and any other compound with an E m less than 0.14 volt may be used. See, Bunn, H.F., et al . , Hemoglobin: Molecular, Genetic and Clinical Aspects (W.P. Saunders, Inc., Philadelphia, PA) pp. 634-662 (1986) , the disclosure of which is incorporated herein by reference. NMR Measurements -*-H-NMR spectroscopy has been shown to be an excellent tool to investigate the structural features of Hb A such as the tertiary structure, including the heme pockets, and the quaternary structure, namely, the subuni interfaces. See, Ho, C, Adv. Protein Chem. 43 : 153 (1992) , the disclosure of which is incorporated herein by reference. The ring-current shifted ^H resonances of hemoglobin are known to be sensitive to the heme orientation and environment of r Hb A in the CO form (Ho, C, Adanc. Protein Chem. 41:153 (1992) and Shen, T.-J., al., proc. Natl. Acad. Sci. USA 90:8108 (1993) , the disclosures of which are incorporated herein by reference) . ^-H-NMR spectra were obtained on a Bruker AM-300 spectrometer that was operated at 300 MHz and at temperatures ranging from 10°C-36°C. All of the Hb samples were placed in 0.1 M sodium phosphate buffer (in 100% H2O) at pH 7.0. The Hb concentration range was approximately 4%. The water signal was suppressed by using the "jump-and-return" pulse sequence as reported by Plateau, P., et al . , J. Am. Chem. Soc. 104:7310 (1982) , the disclosure of which is incorporated herein by reference. The -^H-NMR spectra of the particular carbonmonoxy-Hb ("HbCO") and deoxy-Hb samples were obtained by using the proton decoupling coil of a 5-mm multinuclear probe (Bruker) with 90° pulses of 9.7 μseconds, spectral widths of 8 kHz (16 kHz for deoxy-Hb) , and 8000-data points. Typically, 256 or 1024 scans were averaged to improve the signal-to-noise ratio. Proton chemical shifts are referenced to the methyl proton resonance of the sodium salt of 2, 2-dimethyl-2- silapentane-5 sulfonate ("DSS") indirectly by using the water signal, which signal occurs at 4.76 ppm downfield from that of DSS at 29°C, as the internal reference. The resulting -*-H-NMR spectra are shown in Figures 3-6 and 11. Oxygen Binding of Hb A Samples Oxygen-dissociation curves were measured by a Hemox-Analyzer (TCS Medical Products, Huntington Valley, PA) from 16°-37°C in 0.1 M sodium phosphate buffer in the pH range of 6.0-8.3. The methemoglobin reductase system, described by Hayashi, A., et al . , Biochim. Biophvs. Acta 110 . :309 (1973) , the disclosure of which is incorporated herein by reference, was added (30-60 μl) to prevent the formation of ferri-Hb. Partial pressure at 50% oxygenation (P50) and tne Hill coefficient (n maχ ) were determined from each curve. The results of these studies are shown in Figures 7-10. MD Simulations MD simulations were carried out using the stochastic boundary method of Brooks, C.L., et al . , J. Mol. Biol. 208 :159 (1989) , the disclosure of which is incorporated herein by reference, using CHARMM22 (Brooks, B.R., et al . , J. Comp. Chem. 4:187 (1983) , the disclosure of which is incorporated herein by reference) with standard parameters for the polar hydrogen protein model (param 19) . For a comparison of the stability of different hemoglobin conformations, the molecule was partitioned into MD and Langevin regions with radii of 10 angstroms and 15 angstroms, respectively, which were centered on th reference coordinates of the crystal structure of deoxy- Hb A as given by Fermi, G., et al. , J. Mol. Biol. 175:159 (1984) , the disclosure of which is incorporated herein by reference. These coordinates refer to roughly in the middle of the Cβ of the two α96Val side chains. For the calculation of the free energy of simulation, the molecul was partitioned into MD and Langevin regions with radii o 10 angstroms and 15 angstroms, respectively, which were centered on the center-of-mass coordinates of the Cβ of the S99Asp side chain in the deoxy (Fermi, G. , et al. , J. Mol. Biol. 175 : 159 1984) and oxy crystal structures (Shannan, B., J. Mol. Biol. 171:31 (1983)) , the disclosur of which is incorporated herein by reference) of Hb A. The inside sphere was filled with charmm-adapted pre- equilibrated TIP3P water molecules as described by Jorgensen, W.L. , et al. , J. Chem. Phys. 79:926 (1983) , t disclosure of which is incorporated herein by reference. The deoxy simulation included 103 water molecules, and the oxy simulation included 83 water molecules. Because there is no available crystal structure for Hb (α96Val→Trp) , several different local conformations for _96Trp were considered for the initial conformation. Four possible side-chain angles of tryptophan from the rotamer library (Ponder, J.W., et al. , J. Mol. Biol. 193 :775 (1987), the disclosure of which is incorporated herein by reference) , which show more than 10% incidence [χ_ = -70.4, χ 2 = -100.5 (37.9%) ; χ λ = 64.8, χ 2 = -88.9 (20.7%) ; X! = -177.3, χ 2 = -95.1 (13.8%) ; χ = -179.5, X2 = 87.5 (10.3%)] were used as the initial orientations for MD simulations . The same number of protein atoms and water molecules were used for each conformation. The transformation between wild-type and mutant proteins can be achieved by using a hybrid potential function V\ = (1-λ) V^+λVg (Gao, J. , et al . , Science 2__4.:1069 (1989) and Tidor, B., et al. , Biochemistry 10:3217 (1991) , the disclosures of which are incorporated herein by reference) , where λ is a coupling factor between 0 and 1. V j ^ and Vg are potential energy functions for Hb A and for mutant Hb, respectively. Simulations were done at nine values of λ± (λ = 0.1, 0.2, 0.9) , with 5- ps of equilibration followed by 5-ps of production dynamics, except at λ = 0.1 and λ = 0.9, where 10-ps of equilibration was employed. Non-bonded interactions were truncated to zero at 8.5 A and a constant dielectric (e=l) was used. All bonds involving hydrogen atoms were constrained with the SHAKE algorithm of Ryckaert, J.-P., et al., J. Comput . Phys . 2.3:327 (1977) , the disclosure of which is incorporated herein by reference. A 10-ps simulation takes about 2 hours on a SunSparc Workstation 10 with 1-fs integration time. The free energy of simulations can be obtained from the trajectory files of MD simulations for both deoxy and oxy forms of Hb using the thermodynamic integration method described by Kirkwood, J.G., J. Chem. Phvs. 1:300 (1935) , the disclosure of which is incorporated herein by reference, with the following equation: ^G *»G_-GA where and the thermodynamic average <ΔV) χ indicates the average of Vχ over the hybrid system. The linear form of the thermodynamic equations shows that the total free energy of the simulations can be decomposed into individual additive contributions. The change in the free energy of cooperativity resulting from the mutations can be indirectly obtained from the thermodynamic cycle as shown previously by Gao, J., et al. , Science 244 :1069 (1989) . Gβoxy dβoxy-HbA ^. deoxy-mutantHb ΔGd- * 0(HbA) 1 1 ΛG→o(mutantHb) oxy-HbA -Gox y oxy-mutantHb For the purpose of structural analysis carried out, average structures for wild-type (λ = 0) and mutant systems (λ = 1) were calculated from the nearest states that were simulated (λ = 0.1 and λ = 0.9, respectively) b using the exponential formulas of Brooks, C.L., et al. , P. Phvs. Chem. S__:6680 (1986) , the disclosure of which is incorporated herein by reference: where X represents the Cartesian coordinates of the system, Xo and Xi are the simulation-average wild-type and mutant coordinates, respectively, β = 1/kgT, and <>χ is an ensemble average under the potential Vχ . B. RESULTS Biochemical Properties of r Hb (o;96Val→Trp) As stated above, the Mono S chromatography of the expressed r Hb (α96Val-*Trp) from the sonicated cells of E. coli JM109 which harbors plasmid pHE202 resulted in two major peaks (peaks a and b) on Mono S column chromatography as shown in Figure 1. A symmetrical peak comes out first (peak a) followed by two overlapping peaks (peaks b^ and b2) • The peaks so obtained by Mono S chromatography are hereinafter referred to as "peak a", "peak bτ_", and "peak b2 n . All of these peaks in the CO form show a visible optical spectrum over the range of 350-700 nm identical to that of Hb A (results not shown) . The electrospray mass spectrometry that was carried out as described above shows that the amino- terminal methionine residues of all three peaks have been effectively cleaved by the coexpressed Met-AP as shown in Figure 2. For peak a, at least 98% of the chain has mass 15,213, which is the calculated value for the replacement of a valine residue by a tryptophan residue, and up to about 90% of the β chain has mass 15,867, which is the calculated mass of the β chain of Hb A. There is a small component (<10%) whose mass (15,998) corresponds to that of the normal β chain plus a methionine residue. However, for peaks bi and b2 / both a and β chains have the correct masses with undetectable (<2%) N-terminal methionine. 1 H-NMR Studies of r Hb (α96Val→Trp) i . Conversion of the CO Form to the Ferric Form and Then Back to the CO Form of Hb A comparison of the ring-current shifted ---H resonances of all three peaks (a, b]_, and b2) of Mono S-purified r Hbs (α96Val→Trp) , derived from pHE202/JM109, in 1 their respective CO forms and HbCO A are shown in Figure 3A. The spectra were obtained using 4% r Hbs (α96Val→Trp) and 4% Hb A in 0.1 M phosphate in H2O at pH 7.0 and at 29°C converted to their respective CO forms as described above. To obtain the spectra of Figure 3B, all the hemoglobins were converted from their CO forms to the Fe +3 states and back to the CO forms. The 1 H resonances from 0 to -2.0 ppm from DSS arise from some of the protons of the amino acid residues located in the vicinity of the heme pockets of the Hb molecule and of the porphyrins (Ho, C, Advanc . Protein Chem. 43:153 (1992) , the disclosure of which is incorporated herein by reference) . As can be seen in Figure 3A, the ring-curren shifted -*-H resonances of r HbCO (c.96Val→Trp) from peaks a, b-ι_, and b2 are all somewhat different from those of HbCO A. Also, there are additional resonances which are not seen in HbCO A as well as changes in the intensities in several resonances over the region from 0 to -1.1 ppm from DSS. However, the resonances from -1.7 to -1.8 ppm, which have been assigned to the Ϊ2 -metn yl groups of the EllVal (distal valine) of the and β chains of HbCO A (Lindstrom, T.R., et al . , Biochemistry 11:1677 (1972) and Dalvit, C, et al . Biochemistry 24:3398 (1985) , the disclosures of which are incorporated herein by reference) from peak a are essentially the same as those of Hb A, while the resonances from peaks b^ and b2 are much broader. The phenomenon of proper insertion of hemes into the expressed globins using Hb expression plasmid pHE2 in E. coli was discussed in Shen, T.-J., et al. , Proc. Natl. Acad. Sci. USA S__*8108 (1993) . They described a conversion of the heme pockets of one of the purification components of r Hb A to those of native Hb A by an oxidation and reduction process. Similarly, the possibility of converting the abnormal 1 H resonances of r Hb (αr96Val→Trp) to those of the native form of hemoglobin was also investigated in the present invention, r Hbs (c.96Val-»Trp ) from all three peaks were first oxidized to their respective ferric states and then reduced back to their respective CO or oxy form as described above. As shown in Figure 3B, such "converted Hbs" from all three peaks after the oxidation and reduction processes show identical NMR spectra (environments of respective distal valines the same) , which are similar to those of r Hb (cr96Val→Trp) from peak a before the conversion (Figure 3A) . This suggests that peak a of r Hb (θf96Val→Trp) has the correct heme conformation and subunit interface. r Hb (α96Val-»Trp) from peak a was therefore used for all further work described herein, unless otherwise specified. ii. Heme Environment and Subunit Interface The 1 H-NMR spectra of Figures 4A and 4B were obtained for 4% Mono S-purified r Hb (α96Val→Trp) from pHE202/JM109 from peak a and 4% Hb A, both in their deoxy forms, as described above, in 0.1 M phosphate in H ? _0 at pH 7.0. The Figure 4A spectra were obtained at 29°C; those of Figure 4B were obtained at 29°C and 36°C as indicated in the figure. Hyperfine-shifted N$H exchangeable proton resonances of the proximal histidine residues are shown in Figure 4A. For Figure 4A the resonance at -63 ppm was assigned to the hyperfine-shifted N_H-exchangeable proton of the proximal histidine residue (α87His) of the β chain of deoxy-Hb A, and the resonance at -77 ppm was assigned to the corresponding residue of the β chain (392His) of deoxy-Hb A (Takahashi, S., et al . , Biochemistry __:5196 (1980) , and La Mar, G.N. , et al. , Biochem. Biophvs. Res. Commun. _6:1172 (1980) , the disclosures of which are incorporated herein by reference.) The chemical shift positions of these two proximal histidyl resonances in peak a of r Hb (α96Val→Trp) are the same as those of Hb A as seen in Figure 4A. This indicates that there are no pertubations around the proximal histidines of r Hb (α96Val→Trp) . The ferrous hyperfine-shifted and exchangeable proton resonances of r Hb (α96Val-»Trp) of peak a- and Hb A in their deoxy form are shown in Figure 4B. The hyperfine-shifted resonances arise from the protons on the heme groups and their nearby amino acid residues due to the hyperfine interactions between protons and unpaired electrons of Fe(II) in the heme iron atoms. The hyperfine-shifted resonances of r Hb (α96Val→Trp) from peak a at 29°C show some chemical shift changes around th resonances at -22.5 and -18.7 ppm from DSS (assigned to the protons associated with the β chains of deoxy-Hb A) (Takahashi, et al. , 1980) and at -17 ppm from DSS (assigned to the protons associated with the chains of deoxy-Hb A) (Takahashi, et al . , 1980) compared to those o Hb A at 29°C. The exchangeable 1 H resonances over the spectral region from 11 to 14 ppm from DSS are known to b excellent markers for the deoxy-quaternary structure of Hb A (Ho, C, Advanc. Protein Chem. 43:153 (1992)) . The resonance at -14 ppm has been assigned to the intersubunit hydrogen bond between cf42Tyr and /S99Asp, a characteristic feature of the deoxy-quaternary (T) structure (Fung, L.W.- M. , et al., Biochemistry 1__:2526 (1975) , the disclosure of which is incorporated herein by reference) . As seen in Figure 4B, there is no noticeable difference in the resonance at -14 ppm from DSS between Hb A and r Hb (α96Val→Trp) from peak a in the deoxy form, which indicates that this d 2 intersubunit hydrogen bond of the deoxy form is unperturbed by the mutation. In r Hb (cc96Val→Trp) , a new resonance appeared at -12.6 ppm from DSS at 29°C as seen in Figure 4B, which is absent in deoxy-Hb A. However, when the !H-NMR spectrum was taken at high temperature (36°C) , the resonance at -12.6 ppm from DSS shifted. The temperature- sensitive chemical shift position indicates that this new resonance is likely to be hyperfine-shifted resonance rather than an exchangeable resonance (Jesson, J.P., J__ Chem. Phvs. 47:579 (1967) ; Kurland, R.J., et al . , J. Magn. Reson. 2.:286 (1970) ; Johnson, M.E., et al . , J. Am. Chem. Soc . 9_9:1245 (1977) , the disclosures of which are incorporated herein be reference) . The foregoing results indicate that MD simulations can be applied successfully for the transformation from the wild-type (HbA) to mutant Hb (α96Val→Trp) . iii . Switching of Quaternary Structure without Changing the Ligation State The exchangeable proton resonances of r Hb (α96Val→Trp) in the CO form in the presence of 10 mM inositol hexaphosphate ("IHP") (Sigma) exhibit some very interesting and unique features compared to those of Hb A ---H-NMR spectra were obtained of 4% r HbCO (α96Val→Trp) derived from pHE202/JM109 peak a and 4% HbCO A at 300 MHz in the presence of 10 mM IHP in 0.1 M phosphate in H2O at pH 7.0. Spectra were obtained at 10, 21, 29, and 36°C. Figures 5A and 5B show exchangeable proton resonances of r HbCO (α96Val→Trp) and HbCO A, respectively. Figures 5C and 5D show ring-current shifted resonances of r HbCO (α96Val→Trp) and HbCO A, respectively. The resonance at -10.7 ppm from DSS, a characteristic feature of the oxy-quaternary (R) structure (the intersubunit hydrogen bond between α94Asp and |3l02Asn) (Fung, L. W.-M., et al. , Biochemistry 14:2526 (1975); Shaanan, B., J. Mol. Biol. 171:31 (1983), the disclosure of which is incorporated herein by reference) , disappears in the presence of IHP in the 1 H-spectrum of r HbCO (α96Val→Trp) at 29°C (Figure 5A) . Instead, Figure 5A shows the resonance at -14 ppm from DSS, a characteristic feature of the deoxy-quaternary (T) structure (the intersubunit hydrogen bond between o;42Tyr and / 399Asp) (Fung, et al. , 1975; Fermi, G., et al. , J__ Mol. Biol. 175:159 (1984), the disclosure of which is incorporated herein by reference) appears as compared to HbCO A (Figure 5B) . As will be shown below, with reference to Figure 5, the presence of a strong allosteric effector, IHP, appears to stabilize the deoxy form of r Hb (α;96Val-»Trp) , and thus can shift the equilibrium toward the deoxy form. The presence of new hydrogen bonds in the 01^ 2 and α^i subunit interfaces can add extra stability and make the quaternary structural transition toward the deoxy- quaternary structure much easier in the presence of IHP, especially at lower temperatures. In Figure 5A, the intensity of the resonance at -14 ppm from DSS gradually increases in the spectrum of r Hb (ct96Val→Trp) as the temperature is decreased from 29 to 10°C. At 36°C, the resonance at -14 ppm from DSS disappears, and the resonance at -10.7 ppm from DSS, a characteristic feature of the oxy-quaternary (R) structure, reappears as a shoulder. The relative intensity of the resonance at -14 ppm at 10°C is similar to that of the exchangeable resonance in the deoxy form of r Hb (o;96Val→Trp) . The chemical shift position of this resonance at -14 ppm from DSS does not change with temperature, suggesting that the origin of this resonance is exchangeable rather than hyperfine shifted. The appearance of the resonance at -14 ppm from DSS is not accompanied by the appearance of the hyperfine-shifted resonances from -11 to -24 ppm, indicating that the heme iron is still in the low-spin, diamagnetic state (i.e., ligand-bound form) . The change of intensity with temperature of the resonances which are characteristics of the deoxy- (T-) and oxy- (R-) quaternary structures is reversible. Figure 5B shows that there is very little variation of the exchangeable proton resonances of HbCO A in 10 mM IHP as a function of temperature, contrary to that observed for r HbCO (α96Val→Trp) in Figure 5A. Even at 10°C, it appears that there is only a very small signal at -14 ppm for HbCO A, very different in intensity compared to that for r HbCO (α96Val→Trp) . At intermediate temperatures, such as 21 and 29°C, the exchangeable proton resonances of r HbCO (α96Val→Trp) in the presence of IHP show that the resonance at -10.7 ppm, a characteristic feature of the oxy-quaternary structure, essentially disappears, but the resonance at -14 ppm, a characteristic feature of the deoxy-quaternary structure, exhibits only very weak intensity. These results suggest that an intermediate quaternary structure may exist which loses a hydrogen bond between α94Asp and βl02Asn, manifested by the resonance at -10.7 ppm, but which still does not form a hydrogen bond between α42Asp and β99Asp, manifested by the resonance at -14 ppm. This r HbCO (α96Val→Trp) of the present invention can be gradually converted to the deoxy- like quaternary structure without changing the ligation state by just the addition of a strong allosteric effector, IHP, and/or by lowering the temperature. The ring-current shifted proton resonances of r Hb (α:96Val→Trp) and Hb A in the CO form in the presence of 10 mM IHP were also compared at different temperatures as shown in Figures 5C and 5D. The relative intensity and the chemical shift position of the resonances over the region from 0 to -1.1 ppm from DSS of r HbCO (α96Val→Trp) were seen to vary a great deal with temperature, while those of Hb A vary relatively little. The resonances from -1.7 to -1.8 ppm from DSS, assigned to the J2-methyl groups of the ΞllVal (distal valine) of the a and β chains of HbCO A (Liπdstrom, T.R. , et al. , Biochemistry 11:1677 (1972) ; Dalvit, C, et al . , Biochemistry 24:3398 (1985) , the disclosures of which are incorporated herein by reference) , merge into one peak at low temperature in the CO form of both r Hb (α96Val→Trp) and Hb A. The resonances from -11.7 to -1.8 ppm from DSS or r Hb (c.96Val→Trp) are seen to be much broader than those of Hb A. It is believed that change in the relative intensity and the chemical shift position of the resonances over the region from 0 to -1.1 ppm from DSS and the broadening of the resonances from -1.7 to -1.8 ppm of r Hb (ct96Val→Trp) may reflect structural transitions which are not observed in Hb A. 1 H-NMR spectra were also obtained of 4% r HbCO (α96Val→Trp) from peak a and 4% HbCO A at 300 MHz, without any IHP present, in 0.1 M phosphate in H2O at pH 7.0. Spectra obtained at the exchangeable proton resonances of r Hb (α96Val→Trp) and Hb A in the CO form in the absence of IHP at different temperatures are compared in Figures 6A and 6B. As the temperature is decreased, the resonanc at -14 ppm from DSS appears in r Hb (o;96Val→Trp) in the absence of IHP, while Hb A shows no appearance of the resonance at -14 ppm from DSS, even at 10°C. Figures 6C and 6D show the ring-current shifted resonances of r Hb (α;96Va_→Trp) and HbCO A in the CO form, respectively, in the absence of IHP. These figures also show spectral changes depending on the temperature. The relative intensity and the chemical shift position of the resonances over the region from 0 to -1.1 ppm from DSS of r HbCO (o;96Val→Trp) were seen to vary greatly with temperature, while those of HbCO A were seen to vary little. The appearance of the exchangeable resonance at -14 ppm from DSS, a characteristic feature of the deoxy-quaternary structure, at a low temperature (Figures 6A and 6B) , and the changes in the relative intensity and the chemical shift position of the resonances over the ring-current shifted region from 0 to -1.1 ppm from DSS of r HbCO (α96Val→Trp) (Figures 6C and 6D) suggest that a deoxy-quaternary structural conversion occurs even in the absence of IHP. Oxygen-Binding Properties of r Hb (α?96Val→Trp) 0.1 mM of each of the Hbs of the three peaks of r HbC0 2 (α96Val→Trp) derived from pHE202/JM109 (peaks a, b-_, and b2) in 0.1 M phosphate at pH 7.0 were applied to a column of Sephadex G-75. Each eluted as a symmetrical peak in the position corresponding to that of Hbθ2 A, indicating that all three forms of r Hb (α96Val→Trp) in the oxy form are tetrameric under the present experimental conditions (results not shown) . The oxygen-binding properties of peaks a, b ] _, and b2 of r Hb (α;96Val→Trp) as a function of pH were investigated as described above. Briefly, oxygen dissociation data were obtained with 0.1 mM concentration of each particular Hb used in 0.1 M sodium phosphate buffer in the pH range 6.3-8.9 at 29°C. Partial O2 pressure at 50% saturation (P50) wa s determined from each curve. The data for Figure 7A were obtained from pHE202/ JM109-derived Mono S-purified fractions, peaks a, b ] ., and b2 from pHE202/JM109 and Hb A. The data for Figure 7B were obtained from pHE202/JM109-derived Mono S-purified fractions, peaks a, b-j_, and b2 and Hb A that were converted from their respective CO forms to their Fe +3 forms, and then back to their CO forms as described above For both Figures 7A and 7B, Hb A is (O) ; r Hb (α96Val→Trp from peak a is (■) ; r Hb (α96Val→Trp) from peak b^ is (Δ) and r Hb (<_96Val-»Trp) from peak b2 is (+) . As seen in Figures 7A and 7B, r Hbs (α96Val→Trp) from peaks b^ and b shows similar P5 0 (partial O2 pressure at 50% saturation) values to that of Hb A before the oxidation and reduction process, whereas, r Hb (α96Val→Trp) from peak a and "converted" Hbs from peaks b]_ and b 2 exhibit a significantly lower oxygen affinity. The Hill coefficient (n m aχ) was determined for Mono S-purified peaks a, b_, and b2 of r Hb (α96Val→Trp) derived from pHE202/JM109 and Hb A both for their respective unmodified forms as well as for their forms which were converted from their CO respective forms to their Fe +3 states and then back to their CO forms. Oxyge dissociation data were obtained as described above. Briefly, the data for each Hb sample was obtained with 0.1 mM concentration of Hb in 0.1 M sodium phosphate in the pH range 6.5-8.4 at 29°C. The Hill coefficient was determined from each curve. Figure 8A shows data obtaine from the various unmodified hemoglobins; Figure 8B shows data obtained from the hemoglobins that were converted from CO forms to Fe +3 states and back to CO forms. In both figures, Hb A is (O) ; r Hb (α96Val→Trp) from peak a is (■) ; r Hb (α;96Val→Trp) from peak bi is (Δ) ; and r Hb (oι96Val→Trp) from peak b 2 is (+) . Figures 8A and 8B show that r Hb (α96Val→Trp) from peak a and "converted" Hbs exhibit about 90% of the Bohr effect (ΔlogP5 0 /ΔpH) of Hb A and exhibit cooperativity with n max values of about 2.2 to 2.6 depending on the pH, compared to about 3 for Hb A. Table 1 below compares the values of P50 and n max f° r r Hb (o;96Val-*Trp) from peak a derived from pHE202/JM109, r Hb A (prepared in our laboratory according to Shen, T.-J., et al., Proc. Natl. Acad. Sci. USA 90:8108 (1993)) , Hb A, and Hb A in the presence of 5 mM of 2,3- DPG. r Hb (_96Val-»Trp) from peak a shows similar oxygen- binding properties to those of Hb A in the presence of 2,3-DPG. Thus, it is believed that due to the unique structure of r Hb (c.96Val-»Trp) , even in the absence of an allosteric effector, it has properties similar to those of Hb A in the presence of an allosteric effector. Table 1 r Hb(α96Val-y_rp) from peak a r Hb A Hb A Hb A+2,3-DPG pH P 50 n max P 50 n max P 50 n max P 50 n max (mm Hg) (mm Hg) (mm Hg) (mm Hg) 6.5 26.6 2.4 19.8 3.0 20.0 3.0 26.3 2.7 6.8 22.0 2.5 17.3 2.9 16.5 3.0 23.4 2.8 7.4 11.6 2.6 7.3 2.9 8.0 3.0 11.1 2.9 8.0 5.6 2.4 3.6 2.9 3.5 3.0 3.8 2.7 * Experimental Error ±10% Oxygen-dissociation data were also obtained with .0.1 mM of each of the following hemoglobin samples in 0.1 M sodium phosphate buffer at pH 7.4 and 29°C. Curves were determined for Hb A, r Hb (α96Val→Trp) (pHE202/ JM109) , r Hb (α96Val→Trp) in the presence of 2 mM 2,3-DPG, and r Hb (cϋ96Val-»Trp) in the presence of 2 mM IHP. The results are shown in Figure 9. In addition to low oxygen affinity and high cooperativity, the oxygen-binding curve of r Hb (α96Val→Trp) from peak a shows a unique oxygen binding property. At very low oxygen pressure, oxygen binding of r Hb (α96Val→Trp) from peak a in the presence of an allosteric effector, such as 2,3-DPG or IHP, shows a biphasic curve. The effect of temperature on the oxygen affinity (P 50 ) and Hill coefficient (n max ) of r Hb (α96Val→Trp) from peak a and Hb A are compared in Figures 10A and 10B, respectively. The oxygen dissociation data for the various Hb samples were obtained as described above with 0.1 mM concentration of each Hb sample in 0.1 M sodium phosphate buffer at pH 7.4. The temperatures were 16, 20, 25, 29, and 37°C. The P50 and Hill coefficient (n ma χ) were determined from each curve. Data was obtained for Hb A (O) ; r Hb (α96Val→Trp) (pHE202/JM109) from peak a (Δ) ; r Hb (o;96Val→Trp) (pHE202/JM109) in the presence of 2 mM IHP (+) and Hb A in the presence of 2 mM IHP (D) . Figure 10A shows oxygen affinities (P50) ; Figure 10B shows Hill coefficients (n max ) . As shown in Figure 10A, the oxygen affinity is increased significantly in r Hb (α96Val→Trp) and Hb A, both in the presence of IHP and in the absence of IHP, as the temperature is decreased. However, Figure 10B shows that the n max values of r Hb (α96Val→Trp) and Hb A do not change much as a function of temperature. Over the temperature range from 15 to 37°C, both r Hb (α96Val→Trp) and Hb A exhibit cooperativity with n max values of about 2.0 to 2.9. B. Plasmid PHE702 Construction of Expression Plasmid pHE702 A similar α96Val→Trp mutation was introduced into plasmid pHE7 with oligonucleotide 5 * -AGCTTGAAGTTCCAT- GGGTCCACCC-3 ' (DNA International, Inc., Lake Oswego, OR) to form plasmid pHE702 according to published methods (Kunkel, T.M. 1985, Proc. Natl. Acad. Sci. USA 82:488-492; Shen, T.-J., et al . , 1993 Proc. Natl. Acad. Sci. USA j__:8108) . The plasmid pHE7 is a plasmid coexpressing human a- and β-cDNAs and the E. coli Met-AP gene. It contains the two expression cassettes arranged in tandem in the same orientation as for pHE2 described above: (i) tac promoter - E. coli Met-AP coding sequence - 5ST1T2 terminator and (ii) tac promoter - α-globin cDNA coding sequence - β-globin cDNA coding sequence - 5ST1T2 terminator. The plasmid pHE702 was constructed in a manner similar to that used to construct pHE2 as set forth above, except: (1) the synthetic human α-globin gene was replaced by the human α-globin cDNA; (2) the synthetic human β-globin gene was replaced by the human β-globin cDNA; and (3) the E. coli Met-AP coding sequence was directly obtained from E. coli genomic DNA rather than from plasmid pSYC1174. The human α-globin cDNA coding sequence was obtained by PCR with pKT218-αc (a gift, kindly provided by B.G. Forget, Yale University and available on request) as the template and two synthesized primers 5' -GAGAACCCCATAT- GGTGCTGTCTCCTGCC-3' and 5' -CCGAGGCACTAGTTTAACGGTATTTG- GAGGTC-3' (both from DNA International, Inc.), which are complementary to the 5'- and 3'- end sequences of the human α-globin cDNA and respectively contain a Ndel site and a Spel site. The human β-globin cDNA coding sequence was obtained by PCR with pβ which was constructed according to the protocol of Groebe, et al . , Protein Expression and Purification 1:134 (1992) as template and two synthesized primers 5' -CAAACAGACCATATGGTGCACCTGACTCCTGAGGAG-3' and 5' -AGCAAGAATGCATGCTTAGTGATACTTGTGGGCCAGG-3' (both from DN International, Inc.) , which are complementary to the 5'- and 3'- end sequences of the human β-globin cDNA and respectively contain a Ndel site and a Nsil site. The coding sequence of E. coli Met-AP gene was obtained by PCR with E. coli genomic DNA as template and two synthesized primers, 5' -TGGACAGAATTCCATGGCTATCTC- AATCA-3' and 5' -TGGCTTAAGCTTATTCGTCGTGCGAG-3 ' (both from DNA Synthesizing Facility, University of Pittsburgh) , which are complementary to the 5'- and 3'- end sequences of the E. coli Met-AP gene and respectively contain an EcoRI site and a HindiII site. Plasmid pHE702 in host cell E. coli JM109 and designated pHE702/JM109 was deposited with the American Type Culture Collection of Rockville, MD on April 26, 1995 under number ATCC 69793. Comparison of r Hb (α96Val→Trp) from plasmids pHE202 and pHE702 The growth conditions for E. coli JM109 harboring pHE702 are identical to those set forth above for pHE202. The yield of r Hb (α96Val→Trp) derived from pHE702 is about 50% that of r Hb (α96Val-»Trp) derived fro pHE202 set forth above. As noted above, pHE702 was constructed from human genes and thus other expression systems may be advantageously used. The elution profile for r Hb (α96Val→Trp) derived from pHE702 gives two peaks after Mono S chromatography, labeled peak 1 and peak 2 (results not shown) . Similarly, all other analytical procedures as described above for r Hb (α96Val→Trp) derived from pHE202 were performed the same way for the pHE702-derived hemoglobin. A comparison of both the functional and structural properties of the low oxygen affinity r Hb (α96Val→Trp) derived from the two different plasmids, pHE202 and pHE702 was carried out. A detailed description of r Hb (α96Val→Trp) derived from pHE202 is described above. r Hb (α96Val→Trp) derived from pHE202 and pHE702 were oxidized and reduced as described above and their P50 and n max values were measured in 0.1 M sodium phosphate at 29°C at pH 6.8 and 7.4 and compared to those for Hb A. Table 2 below summarizes the oxygen-binding properties of Hb A, r Hb (α96Val-*Trp) derived from pHE202, and r Hb (α96Val→Trp) derived from pHE702 at pH 6.8 and 7.4. It is clear that the functional properties as exhibited by r Hb (α96Val→Trp) derived from both plasmids are essentially identical . Table 2 Figure 11 shows the -^H-NMR spectra of Hb A and r Hb (α96Val-*Trp) derived from pHE202 and pHE702 in the CO form in 0.1 M phosphate at pH 7.0 and 29°C, carried out as described in detail above. Figure 11A shows the exchangeable proton resonances and Figure 11B shows the ring-curent shifted proton resonances of Hb A and r Hb (o*96Val→Trp) derived from pHE202 (peak b2) and pHE702 (peak 2) in the CO form. Both sets of ---H-NMR results indicate that there is no observable difference in the 1 H- NMR spectra between these two r Hb (α96Val→Trp) . Thus, it is concluded that there is no observable structural or functional differences between r Hb (α96Val→Trp) derived from pHE202 and pHE702. C. MD Simulations The distance between the Cα of the two α96Val residues in the crystal structure of deoxy Hb A is 10.3 angstroms (Fermi, G., et al. , J. Mol. Biol. 175 :159 (1984) , the disclosure of which is incorporated herein by reference) . The size of the tryptophan residue located in the α96 position of r Hb (α96Val→Trp) would lead one to expect that the stability of r Hb (α96Val→Trp) would be highly dependent on the side-chain angle orientation of the tryptophan residue. Initial computer modeling of the deoxy form indicates that out of the four side-chain angles of tryptophan which show more than 10% incidence from the rotamer library (Ponder, J.W., et al. , J. Mol. Biol. 193 :775 (1987) , the disclosure of which is incorporated herein by reference) , only the tryptophan with the side-chain angles χ x = 64.8 and χ 2 = -88.9 (20.7%) gives a stable conformation. The other three tryptophan conformations all direct toward each other, giving unfavorable non-bonding interactions. The transformation between wild-type and mutant Hbs was carried out with all four possible side-chain angles of the tryptophan residue. The distance between the Neτ_ Trp96 residues in the two α-chains and the side- chain angles of each α96Trp residue in the average simulated structure in the deoxy form are summarized in Table 3 below. Table 3 Rotamer Library* αiTr sε α 2 Trp96 Distance Xi X2 Xi X2 Xi X 2 Ne^ ^-Ne^ ^) (deg) (deg) (deg) (deg) (deg) (deg) (angstroms) -70.4±7.0 100.5_18.2 -63.3 ■ 17β.3 -88.S 128.9 7.7 (37.9%) 64.θ±13.0 -88.9±5.3 92.3 -71.0 96.2 -78.7 17.3 (20.7%) -177.3±7.9 -95.1±7.6 -86.5 89.9 •174.2 -116.3 10.2 (13.8%) -179.5±3.4 87.5±3.8 -118.0 89.5 -177.1 71.8 5.6 (10.3%) *From Ponder, J. . , et al., J. Mol. Biol. 193:775 (1987) Only the simulated structure from the initial tryptophan side-chain angles of χ x = 64.8 and χ 2 = -88.9 shows both _96Trp with a relatively symmetrical conformation. The side-chain angles of both α96Trp residues in this conformation are relatively well preserved, and the distance between the e^ of the two <_96Trp residues indicates that the α96Trρ residues are sufficiently separated so as not to affect each other. These results -suggest that the tryptophan side-chain angles of χ_ = 64.8 and χ 2 = -88.9 may be the closest to the natural conformation. Thus, in order to make the analysis of the free energy of simulation easier, for the calculation of the free energy of cooperativity, MD simulations were carried out with side-chain angles of x = 64.8 and χ 2 = -88.9 on a sphere centered on the center of mass coordinate of Cβ of β99Asp of the crystal structures of both deoxy and oxy forms of this r Hb. This sphere includes only one α96Trp residue for calculation purposes. The average simulated structure of Hb A and r Hb (α96Val→Trp) in the deoxy form was prepared by the molecular graphics program, GRAPHX, developed at the Pittsburgh Supercomputing Center, Pittsburgh, PA. The average simulated structure in the deoxy form from these side-chain angles suggests new interfacial hydrogen bonds between α96Trp and β99Asp in addition to the existing hydrogen bonds between ?99Asp and α42Tyr and between β99Asp and α97Asn. Further 100-ps MD simulations of the transformed mutant Hb (α96Val→Trp) in the deoxy form suggest that these hydrogen bonds in the α^p^ subunit interface are very stable, i.e., with average simulated distances of 2.25 ± 0.19 angstroms and 2.32 ± 0.22 angstroms between α42Tyr and β99Asp, and 2.22 ± 0.34 angstroms between α97Asn and β99Asp, respectively. Although there is no experimentally measured value for the free energy of cooperativity for r Hb (α96Val-*Trp) , the relatively high Hill coefficient of r Hb (α96Val→Trp) (n max = 2.6 at pH 7.4) , which is quite comparable to that of Hb A, suggests that the free energy of cooperativity for r Hb (α96Val-»Trp) is close to that of Hb A (n max = 3.0 at pH 7.4) . The calculated value for the difference in the free energy of cooperativity between r Hb (α96Val→Trp) and Hb A is -1.5 kcal/mol per interface, as shown in Table 4 below, which is about the expected value for an Hb which has a slightly lower cooperativity than Hb A (Turner, G.J., et al., Proteins: 14 333 (1992), the disclosure of which is incorporated herein by reference) . Table 4 Contribution ΔG(deoxy) ΔG(Oxy) ΔΔG (kcal/mole) •5.0 • 5.3 Self (α*96Val- * Trp) -24.9 •32.0 •7.8 Total -37.6 ■39.1 -1.5 All values are given for one a-i_p"2 interface. Only residues that contribute more than 0.5 kcal in either the deoxy or oxy form are listed. The positive sign given in ΔG signifies a contribution destabilizing the Hb (_96Val→Trp) relative to Hb A. ΔΔG is ΔG(oxy) - ΔG(deoxy), which corresponds to the difference of free energy of cooperativity between Hb (α96Val-»Trp) and Hb A. Thus, the MD simulation results may be used advantageously to obtain information about the specific interactions which contribute to the total free energy of cooperativity. The contributions of each individual amino acid to the total free energy of cooperativity are also shown in Table 4. Most individual amino acids contribute less than 1.0 kcal, indicating that the tryptophan introduced into the cx- β2 interface does not cause large conformational changes. The largest contribution to the total free energy of cooperativity comes from the interaction with β99Asp (14.8 kcal) , which most likely results from the stabilization in the deoxy-structure (-12.2 kcal) . These results suggest that the new hydrogen bond between α96Trp and β99Asp may be responsible for the stabilization of the deoxy structure of this r Hb. The r Hb (α96Val→Trp) of the present invention must be cross-linked before it can be used as a component of a blood substitute or therapeutic agent in order to prevent the dissociation of the molecule into dimers. While practically all the hemoglobin in the red cell is in the tetrameric state, circulating cell-free hemoglobin, such as in a blood substitute, would dissociate into dimers to such an extent that would lead to blockage of the glomerular membrane of the kidney, as well as the continuous drainage of the infused tetrameric hemoglobin from circulation. The hemoglobin must be cross-linked to prevent this dissociationary clearance. Hemoglobin may be cross-linked between the α chains, such as by diisothiocyanato benzene sulfonate, within the hemoglobin tetramer, such as by pyridoxal and derivatives and acylphosphates. See, Winslow, R.M., et al. , eds. Blood Substitutes Physiological Basis of Efficacy (Birkhauser, Boston, MA) pp. 82-84 (1995) , the disclosure of which is incorporated herein by reference. See also, Winslow, R.M. Hemoglobin-Based Red Cell Substitutes (Johns Hopkins University Press, Baltimore, MD) (1992) ; Manning. L.R., et al . , Biochemistry 27:6640 (1988) ; Benesch, R.E., et al. , Biochem. Biophvs. Res. Comm. 156:9 (1988) ; Bucci, E., et al. , J. Biol. Chem. 264 :6191 (1989) ; Chang, T.M.S., et al. , eds. Proceeding o II International Symposium on Blood Substitutes, Biomater. Artif. Cells Artif. Organs (1988) ; Kluger, R. , et al., Biochemistry 31:7551 (1992); and DeVenuto, F. , et al. , Surg. Gynecol. Obstet. 155:342 (1982), the disclosures of which are incorporated herein by reference. Purified cross-linked r Hb (α96Val→Trp) that is appropriately cross-linked can then be incorporated into a hemoglobin-based blood substitute that is physiologically acceptable according to methods known in the art. See, R.M. Winslow, et al., Eds. Blood Substitutes Physiological Basis of Efficacy (Birkhauser, Boston, MA) (1995) , the disclosure of which is incorporated herein by reference. Although the invention has been described in detail for the purposes of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims. SEQUENCE LISTING (1) GENERAL INFORMATION (i) APPLICANT: Ho, Chien Kim, Hyun-Won Shen, Tong-Jian (ii) TITLE OF INVENTION: Low Oxygen Affinity Mutant Hemoglobin (iii) NUMBER OF SEQUENCES: EIGHT (8) (iv) CORRESPONDENCE ADDRESS: (A) ADDRESSEE: Carnegie Mellon University (B) STREET: 4400 Forbes Avenue (C) CITY: Pittsburgh (D) STATE: Pennsylvania (E) COUNTRY: USA (F) ZIP: 15213 (v) COMPUTER READABLE FORM: (A) MEDIUM TYPE: 5-1/4 low density diskette (B) COMPUTER: IBM PC or compatibles (C) OPERATING SYSTEM: MS-DOS (D) SOFTWARE: ASCII (vi) CURRENT APPLICATION DATA: (A) APPLICATION NUMBER: Not available (B) FILING DATE: 30-APR-96 (C) CLASSIFICATION: Not available (vii) PRIOR APPLICATION DATA: (A) APPLICATION NUMBER: US08/432, 071 (B) FILING DATE: 01-MAY-95 (viii) ATTORNEY/AGENT INFORMATION: (A) NAME: Mary-Elizabeth Buckles (B) REGISTRATION NUMBER: 31,907 (C) REFERENCE/DOCKET NUMBER: 95-243PCT (ix) TELECOMMUNICATION INFORMATION: (A) TELEPHONE: 202/414-9200 (B) TELEFAX: 202/414-9299 (C) TELEX: 64711 (2) INFORMATION FOR SEQ ID NO: 1 (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 21 nucleotides (B) TYPE: nucleic acid (C) STRANDEDNESS : single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: synthetic DNA (x) PUBLICATION INFORMATION: (A) AUTHORS: Hyun-Won Kim Tong-Jian Shen Dazhen Philip Sun Nancy T. Ho Marcella Madrid Chien Ho (B) TITLE: A Novel Low Oxygen Affinity Recombinant Hemoglobin (a96Val_Trp) : Switching Quaternary Structure Without Changing the Ligtion State (C) JOURNAL: Journal of Molecular Biology (D) VOLUME: 248 (E) ISSUE: 4 (F) PAGES: 867-882 (G) DATE: 00-MAY-95 (K) RELEVANT RESIDUES IN SEQ ID NO: 1: 1 to 21 (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 1: TTTGAAGTTC CATGGATCAA C 21 (3) INFORMATION FOR SEQ ID NO: 2 (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 25 nucleotides (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: synthetic DNA (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 2: AGCTTGAAGT TCCATGGGTC CACCC 25 (4) INFORMATION FOR SEQ ID NO: 3 (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 29 nucleotides (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: synthetic DNA (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 3: GAGAACCCCA TATGGTGCTG TCTCCTGCC 29 (5) INFORMATION FOR SEQ ID NO: 4 (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 32 nucleotides (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: synthetic DNA (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 4: CCGAGGCACT AGTTTAACGG TATTTGGAGG TC 32 (6) INFORMATION FOR SEQ ID NO: 5 (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 36 nucleotides (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: synthetic DNA (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 5: CAAACAGACC ATATGGTGCA CCTGACTCCT GAGGAG 36 (7) INFORMATION FOR SEQ ID NO: 6 (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 37 nucleotides (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: synthetic DNA (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 6: AGCAAGAATG CATGCTTAGT GATACTTGTG GGCCAGG 37 (8) INFORMATION FOR SEQ ID NO: 7 (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 29 nucleotides (B) TYPE: nucleic aciα (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: synthetic DNA (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 7: TGGACAGAAT TCCATGGCTA TCTCAATCA 29 (9) INFORMATION FOR SEQ ID NO: 8 (i) SEQUENCE CHARACTERISTICS: (ii) MOLECULE TYPE: synthetic DNA (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 8: TGGCTTAAGC TTATTCGTCG TGCGAG 26

Claims: We Claim : 1. A non-naturally occuring mutant human hemoglobin wherein the valine residue at position 96 of the α chain is replaced by a tryptophan residue. 2. The hemoglobin of Claim 1 possessing low oxygen affinity as compared to normal human adult hemoglobin. 3. The hemoglobin of Claim 2 further possessing high cooperativity in oxygen binding. 4. The hemoglobin of Claim 3 produced recombinantly. 5. r Hb (α96Val→Trp) . 6. A method of treating a human subject, comprising administering a nontoxic composition comprising an artificial mutant hemoglobin. 7. The method of Claim 6 wherein said mutant hemoglobin possesses low oxygen affinity compared to human normal adult hemoglobin. 8. The method of Claim 7 wherein said mutant hemoglobin also possesses high cooperativity in oxygen binding. 9. The method of Claim 8 wherein said mutant hemoglobin is produced recombinantly. 10. The method of Claim 9 wherein said mutant hemoglobin contains a mutation of the valine residue at position 96 of the α chain. 11. The method of Claim 10 wherein said mutant hemoglobin is r Hb (α96Val-*Trp) . 12. A recombinant DNA expression plasmid, comprising expressible human α- and β-globin genes, wherein said α-globin gene contains a mutation, and a Met-AP gene from E. coli arranged in tandem under the control of separate tac promoters, said sequences being expressed separately for said human α- and β-globin genes and said Met-AP gene, and incorporating heme to form a recombinant hemoglobin with essentially the same amino acid sequence, but for the mutation, and heme conformation as native Hb A. 13. The plasmid of Claim 12 wherein said α- globin gene is mutated such that the valine residue at position 96 of the α chain is replaced by a tryptophan residue in the expressed protein. 14. A method of producing artificial hemoglobin, comprising: introducing the expression plasmid of Claim 13 into a suitable host other than an erythrocyte and growing the transformed cells; expressing said DNA to produce said artificial hemoglobin; and recovering and purifying said hemoglobin. 15. The method of Claim 14, wherein said host cells are E. coli. 16. The method of Claim 15, wherein said plasmid is pHE202. 17. The method of Claim 15, wherein said plasmid is pHE702. 18. A non-toxic pharmaceutical composition comprising a non-naturally occurring mutant hemoglobin in a pharmaceutically acceptable carrier. 19. The composition of Claim 18 wherein said hemoglobin in a cell-free envirnoment has oxygen binding properties lower than those of human normal adult hemoglobin. 20. The composition of Claim 19 wherein said hemoglobin is r Hb (α96Val→Trp) . 21. Plasmid pHE202. 22. Plasmid pHE702. 23. An artificial mutant human hemoglobin which in a cell-free environment has oxygen binding properties comparable to those of human normal adult hemoglobin in red blood cells. 24. The hemoglobin of Claim 23 wherein said hemoglobin contains a mutation of the valine residue at position 96 of the α chain. 25. The hemoglobin of Claim 24 wherein said valine residue is replaced by a tryptophan residue. 26. The hemoglobin of Claim 25 which is produced recombinantly. 27. The hemoglobin of Claim 26 which is r Hb (α96Val→Trp) . 28. A non-naturally occurring low oxygen affinity mutant hemoglobin containing a mutation in the αι-?2 subunit interface region of the molecule wherein said mutant hemoglobin possesses oxygen affinity lower than human normal adult hemoglobin. 29. A non-naturally occurring low oxygen affinity mutant hemoglobin that has oxygen binding properties comparable to those of human normal adult hemoglobin in the presence of the allosteric effector 2,3- DPG. 30. The mutant hemoglobin of Claim 29, wherein the valine residue at position 96 of the α chain is replaced by a tryptophan residue. 31. A non-naturally occurring mutant hemoglobin possessing the properties shown in Table 1. 32. r Hb (α96Val→Trp) derived from cells transformed with plasmid pHE202. 33. r Hb (α96Val→Trp) derived from cells transformed with plasmid pHE702.

Inventors: HO CHIEN, KIM HYUN-WON, SHEN TONG-JIAN, HO, CHIEN, KIM, HYUN-WON, SHEN, TONG-JIAN
Applicants: UNIV CARNEGIE MELLON, CARNEGIE MELLON UNIVERSITY
Application year: 1996
Main classification: C12P21
IPC codes: A61K38, A61P7, C07K14, C12N1, C12N15, C12P21, C12R1