WO2009004310A1 - Modified globin proteins with attenuated electron transport pathway - Google Patents

Abstract

The present invention relates to a modified porphyrin-based oxygen-carrying protein, such as haemoglobin, which has been found, in its unmodified state to have a low affinity site of electron transfer and a high affinity electron transfer between a reductant and ferryl haem iron via one or more protein amino acids. The invention provides such proteins that comprise an attenuating modification in the high affinity pathway.

Description

Modified Globin Proteins With Attenuated Electron Transport Pathway

Field of the Invention

The present invention relates to modified oxygen-carrying compounds such as haemoglobin and their use.

Background to the Invention

Transfusion of a patient with donated blood has a number of disadvantages. Firstly, there may be a shortage of a patient's blood type. Secondly, there is a danger that the donated blood may be contaminated with infectious agents such as hepatitis viruses and HIV. Thirdly, donated blood has a limited shelf life. In addition, there are some situations where blood may not be readily available, such as in a battlefield or civil emergencies.

An alternative to transfusion involves the use of a blood substitute. A blood substitute is an oxygen carrying solution that also provides the oncotic pressure necessary to maintain blood volume. Two types of substitutes have recently been studied, fluorocarbon emulsions and haemoglobin solutions.

Haemoglobin as it exists within the red blood cell is composed of two alpha globin chains and two beta globin chains, each incorporating a haem moiety. One alpha-like globin chain and one beta-like globin chain combine to form a dimer which is very stable. Alpha-like and beta- like globin genes belong to a family of related globin genes which are expressed at different stages of development and regulated by oxygen tension, pH, and the development from embryo to foetus to newborn. Two dimers then line up in anti-parallel fashion to form tetramers. The binding of dimers to form the tetramers is not as strong as in the case of monomers binding to associate into dimers. The tetramers, therefore, have a tendency to fall apart to form dimers and there is always an equilibrium between tetramers, dimers, and monomers. At high concentrations of globin, the predominant form is the tetramer; with dilution, the dimer becomes the predominant form. This equilibrium is also affected by solvent, salts, pH and other factors as the binding forces are partly electrostatic.

There are obstacles however to using native haemoglobin as a blood substitute. Firstly, large dosages are required, requiring large scale production of protein, either by recombinant means or from donated human or recovered non-human blood. Secondly, it is important to obtain haemoglobin that is free from infectious agents and toxic substances. Thirdly, although haemoglobin is normally a tetramer of 68,000 molecular weight, it can dissociate to form alpha- beta dimers. The dimers are rapidly cleared by the kidneys and the residence time is much too short for cell-free haemoglobin to be useful as a blood substitute.

Several approaches have been taken to circumvent these difficulties. These include the expression of haemoglobin via recombinant DNA systems, chemical modification of haemoglobin, and the production of haemoglobin variants. Haemoglobin and variants of it have been expressed in various cellular systems, including E. coli, yeast and mammalian cells such as CHO cells.

A number of naturally-occurring variants of haemoglobin are known. Variants include: variants which autopolymerize, variants which prevent the dissociation of the tetramer, and variants that are stable in alkali. There are also over 30 naturally occurring haemoglobin variants which exhibit lowered oxygen affinity. Several examples of such variants are disclosed in WO 88/091799.

Another approach to improving the use of haemoglobin is the modification of this protein by the addition of further polymers to improve the stability of the protein in the blood. For example, US 5,900,402 describes the use of non antigenic polymers, preferably polyalkylene oxide or polyethylene glycol.

Because haemoglobins (and indeed myoglobins or other oxygen-carrying proteins) are involved in oxygen transport and storage they are, as a consequence of this function (because of the redox properties of the iron ion present in the porphyrin ring of protein), responsible for the generation of reactive oxygen species. Autoxidation of the oxy derivative (Fe(II)) leads to non-functional ferric haem (Fe(III)) and superoxide ion (O₂*^"), which subsequently dismutates to generate H₂O₂. These species can ultimately damage the protein and/or the haem group. An essential intermediate in the pathway leading to this damage is the ferryl haem (Fe(IV)=O^2" ), itself formed through the reaction of the haem with H₂O₂ and lipid peroxides.A protein/porphyrin-based radical cation (P^+') accompanies the formation of the ferryl haem from ferric haem and peroxide as set out in equation (1):

P-Fe(III) + H₂O₂ → P+'Fe(IV)=O^2" + H₂O (1)

Ferryl haem and the radical can also be extremely toxic, notwithstanding their transient existence. These oxidative cascades can be damaging because: (i) peroxide is a powerful oxidant known to produce cellular damage, (ii) both the ferryl haem and protein-based radicals can initiate oxidation of lipids, nucleic and amino acids by abstraction of hydrogen atoms, and (Hi) haem modification can lead to highly toxic haem to protein-cross-linked species and to the loss of haem and the release of the 'free' iron.

The potential for haemoglobin-mediated peroxidative damage exists especially whenever the protein is removed from the protective environment of the erythrocyte. This would occur, for example, during spontaneous erythrocyte haemolysis or in haemolytic anaemias (e.g. sickle- cell anaemia). It has been shown that myoglobin induces kidney damage following crush injury (rhabdomyolysis) by exactly this peroxidative mechanism, rather than by free-iron catalysed Fenton chemistry as was thought previously (Holt et al, (1999) Increased lipid peroxidation in patients with rhabdomyolysis. Lancet 353, 1241 ; Moore, et al (1998) A causative role for redox cycling of myoglobin and its inhibition by alkalinization in the pathogenesis and treatment of rhabdomyolysis-induced renal failure. J. Biol. Chem. 273, 31731-31737).

It has also been shown recently that haemoglobin can cause similar damage in vivo when it is released from the erythrocyte in subarachnoid haemorrhage (Reeder, et al (2002) Toxicity of myoglobin and haemoglobin: oxidative stress in patients with rhabdomyolysis and subarachnoid haemorrhage. Biochem. Soc. Trans. 30, 745-748). Furthermore, uncontrolled haem-mediated oxidative reactions of cell-free haemoglobin (developed as a blood substitute) have emerged as an important potential pathway of toxicity, either directly or via interactions with cell signalling pathways (Alayash, A. I. (2004) Oxygen therapeutics: can we tame haemoglobin? Nat. Rev. Drug Discovery 3, 152-159). The toxicity of ferryl haemoglobin has been demonstrated in an endothelial cell culture model system of ischaemia/reperfusion [McLeod, L. L. and Alayash, A. I. (1999) Detection of a ferryl-haemoglobin intermediate in an endothelial cell model after hypoxia-reoxygenation. Am. J. Physiol. 277, H92-H99] and in cells that lack their antioxidant mechanisms such as glutathione (D'Agnillo & Alayash (2000) Interactions of haemoglobin with hydrogen peroxide alters thiol levels and course of endothelial cell death. Am. J. Physiol. Heart Circ. Physiol. 279, H1880-H1889).

Ferryl haemoglobin can cause cell injury, including apoptotic and necrotic cell death. Perfusion of rat intestine with chemically modified haemoglobin has been shown to cause localized oxidative stress, leading to leakage of the mesentery of radiolabeled albumin (Baldwin et al (2002) Comparison of effects of two haemoglobin-based 02 carriers on intestinal integrity and microvascular leakage. Am. J. Physiol. Heart Circ. Physiol. 283, H1292-H1301). Importantly, the cyanomet derivative of this haemoglobin, in which the haem iron is blocked with cyanide and is unavailable to enter a redox reaction, produced no cellular changes. US 5,606,025 describes the conjugation of haemoglobin to superoxide dismutase and/or catalase as one approach to reduce reperfusion injuries and other free-radical mediated processes associated with haemoglobin blood substitutes.

Disclosure of the invention.

Our studies have investigated in further detail the mechanisms by which the ferryl (IV) species is generated in haemoglobin (Hb) and myoglobin (Mb), and the mechanisms by which this ion is responsible for the generation of oxidative stress.

We have found commonalities in the profiles for the reduction of ferryl Mb by many iron chelators, which we have observed to have anti-oxidant properties. The effect of these reducing agents on the rate constant for ferryl decay plotted as a function of reductant concentration exhibits a complex curve that can be expressed as a double rectangular hyperbola function. We have also now found that more classical reducing agents such as ascorbate also show this double rectangular hyperbola concentration dependence. The two hyperbolae represent two binding sites for the reductant having differing affinities. Through the use of kinetic model, simulations and use of selected native and engineered proteins, we have interpreted this concentration dependence to represent two distinct electron transfer pathways from the reductant to the haem iron.

In particular, we have shown that as well as low affinity electron transfer (close to the haem edge) there is a specific high affinity site via a through-protein electron transfer pathway mediated by specific amino acids including Tyr103 in myoglobin and its equivalent reside in the alpha chain of haemoglobin, Tyr 42.

According to the present invention, we propose that modification of this pathway by substitution of the Tyr 42, or any other residue involved in the high affinity electron transfer pathway, such that the ability of the ferryl ion to oxidise proteins or other substrates in the vascular system (thereby generating radical species) via this pathway is inhibited. Effectively, such modifications "lock in" the ferryl ion such that this ion's ability to damage surrounding tissues or substrates, such as lipids, is inhibited.

Thus the present invention provides a modified porphyrin-based oxygen-carrying protein, said protein in an unmodified state comprising a low affinity site of electron transfer, and a high affinity site via a specific through-protein electron transfer mechanism, wherein said protein comprises an attenuating modification in the high affinity pathway. The protein is preferably a haemoglobin alpha chain (Hbα) or myoglobin. The attenuating modification may be to Tyr 42 of Hbα or the equivalent residue in other oxygen-carrying proteins.

The invention also provides nucleic acids encoding these proteins, means for their production and the use of the proteins in methods of treatment. These and other aspects of the invention are described further herein below.

Brief Description of the Drawings

Figure 1 : Concentration dependence ferryl reduction of different myoglobin species with reducing agent deferiprone. Horse myoglobin (•), but not Ap/ys/a myoglobin (■), shows a double rectangular hyperbola concentration dependence.

Figure 2: Concentration dependence of recombinant sperm whale myoglobin with reducing agent deferiprone. Wild type sperm whale myoglobin (♦), but not the Tyr103>Phe mutant of sperm whale myoglobin (A), shows a double rectangular hyperbola concentration dependence.

Figure 3: The proposed two site model for reduction of a haemoprotein possessing a high affinity through protein electron transfer pathway. Reductant (xH) binds to two possible sites on myoglobin in its ferryl oxidation state (P-[Fe(IV)=O^2"]^2*, where P denotes protein) with affinities K_DI and K_D2. Only from these two sites can electron transfer from the reductant to the ferryl iron take place. The high affinity binding site is situated at, or close to, tyrosine 103 (PxH-[Fe(IV)=O^2"]), allowing the transfer of an electron (^_ax=O.01s^"1) through the protein to the ferryl haem iron to generate the ferric protein (P-Fe(III)). The low affinity site is situated in the haem pocket allowing electron transfer directly between the reductant and the ferryl haem (P-[Fe(I V)=O^2"]xH). This model also allows both sites being filled by reductant (PxH-[Fe(IV)=O^2']xH). For the kinetic simulations it is assumed that the binding on one site will not affect the affinity of binding to the second site. The model also incorporates a step in which the oxidized reductant may be regenerated.

Figure 4: Position of residues that can introduce or eliminate through-protein electron transfer in alpha human haemoglobin (A), beta human haemoglobin (B) with comparison of known electron conduit residue Tyr103 of horse myoglobin (C) from crystal structures. Tyr103 of horse myoglobin is close to the haem and is surface exposed making it ideal to act as an electron conduit from exogenous reductants to the ferryl haem iron. Human haemoglobin alpha subunit has a tyrosine in approximately the same spatial environment (Tyr42), however this residue in human haemoglobin beta is a redox-inactive phenylalanine.

Figure 5: Concentration dependence of wild type ferryl human haemoglobin reduction by ascorbate. Ferryl myoglobin (10μM) was reacted with ascorbate in sodium phosphate pH 7.4. Ferryl reduction rate constants for alpha subunit (•) and beta (O) subunit calculated by fitting to a double exponential function. The alpha subunit, but not the beta subunit shows double rectangular hyperbola concentration dependence.

Figure 6: Concentration dependence of recombinant α-Tyr42>Val ferryl human haemoglobin reduction by ascorbate. Ferryl myoglobin (10μM) was reacted with ascorbate in sodium phosphate pH 7.4. Ferryl reduction rate constants for alpha subunit (•) and beta (O) subunit calculated by fitting to a double exponential function. Both the alpha subunit and beta subunit shows single rectangular hyperbola concentration dependencies.

Figure 7: Concentration dependence of recombinant α-Tyr42>Trp ferryl human haemoglobin reduction by ascorbate. Ferryl myoglobin (10μM) was reacted with ascorbate in sodium phosphate pH 7.4. Ferryl reduction rate constants for alpha subunit (•) and beta (O) subunit calculated by fitting to a double exponential function. Both the alpha subunit and beta subunit shows single rectangular hyperbola concentration dependencies.

Detailed Description of the Invention

Porphyrin-Based Oxygen-Carrying Protein A porphyrin-based oxygen carrying protein refers to any polypeptide chain which in its^' native form carries a porphyrin molecule and which polypeptide, either alone or in a complex, carries and releases oxygen bound to the porphyrin molecule. Variants of such proteins, e.g. naturally occurring or synthetic mutants of wild-type porphyrin-based oxygen-carrying proteins are also contemplated by the invention.

The oxygen-carrying proteins to be modified include mammalian haemoglobin subunits and myoglobin proteins, but may include non-mammalian haemoproteins and other genetically engineered proteins where the protein is altered to carry oxygen. These proteins will be recombinant, having altered sequences (substitution of amino acid residues, but may also include deletion or insertion of residues) to modify a high affinity through-protein electron pathway from reductants in the bulk solution to the haem ferryl iron. We have discovered that myoglobin and haemoglobin from certain species (including human) show two distinct pathways of electron transfer from exogenous reductants to the ferryl haem iron. A low affinity pathway (typically >5mM) represents direct electron transfer from the reductant to the ferryl haem iron in a hydrophobic pocket within the protein. A second high affinity pathway (typically <100μM but often <10μM) involves electron transfer between the reductant and ferryl haem iron via one or more protein amino acids. This high affinity through- protein pathway is present in native human myoglobin and haemoglobin alpha subunit, but absent in human haemoglobin beta subunits and Aplysia myoglobin. An example of an amino acid that allows this electron transfer is Tyr103 and Tyr42 in myoglobin and haemoglobin alpha chain respectively.

It is expected that removal of such pathways in proteins such as human haemoglobin alpha subunit will decrease their toxicity by stabilising the highly toxic ferryl oxidation state of these haemoproteins and thus limiting oxidation of substrates such as lipids and DNA until the ferryl is reduced by naturally present antioxidants or by innocuous reductants that are administered with the bloods substitute such as ascorbate, urate or deferiprone.

Thus the invention is applicable to any haemoglobin subunit or myoglobin chain which in its natural state has the high-affinity pathway. In one aspect, the protein is human haemoglobin alpha chain, whose sequence is set out as SEQ ID NO:1 below. However, haemoglobins are highly conserved proteins and thus in principle the invention may be practiced on any haemoglobin which has the high-affinity pathway. Haemoglobin subunit proteins are also numbered by reference to the residues of individual helices or inter-helix resides, as set out in Table 1 below (based on US 5,028,588 the contents of which are incorporated herein by reference). Tyr42 of human haemoglobin alpha chain is thus also identified in the art as residue C7. Accordingly, the equivalent residue in other haemoglobin alpha chains which may be used in the invention will also be in the Tyr42 or C7 position.

Thus other haemoglobin subunits which may be used are those which are vertebrate or non- vertebrate haemoglobin subunits that have a tyrosine residue at a position equivalent to residue 42 / C7.

Vertebrate haemoglobins include mammalian haemoglobins. Mammalian haemoglobins are particularly highly conserved. Non-limiting examples of homologues to the human alpha chain of SEQ ID NO:1 include the human embryonic zeta chain, (Genbank accession number ABD95908) as well as species homologues. Non-limiting examples of such homologues include the mammalian species homologues of Table 2, all of which also have a Tyr42 residue. The sequences may be obtained from on-line databases including via the Research Collaboratory for Structural Bioinformatics protein databank (pdb). These pdb references provide sequences for the haemoglobin alpha subunit proteins and, where applicable, a corresponding beta-family subunit protein. The alpha subunit sequence may be used either with its corresponding beta-family subunit or alone, or in combination with another beta-family subunit protein.

Table 2

Other vertebrate haemoglobins alpha subunit homologues include avian, reptile and fish haemoglobins having a residue equivalent to Tyr42. Non-limiting examples of such subunits include those given in Table 3, which indicates in column 3 the position of the tyrosine residue homologous to Tyr42 of mammalian alpha chain subunits.

Table 3

Other non-vertebrate eukaryote haemoglobins include those of arthropods or other multicellular organisms (e.g. molluscs, nematode worms and non-nematode worms) and those of unicellular organisms. Such haemoglobins include those set out in Table 4, whose columns are in the same format as Table 3 above.

Table 4

In another aspect, the protein may be human myoglobin whose sequence is set out as SEQ ID NO:3 below, or another mammalian myoglobin having tyrosine at position Tyr103 or equivalent, such as Tyr103 of other vertebrate or non-vertebrate eukaryotes. Vertebrates include mammalian myoglobins. Mammalian myoglobins are highly conserved and the include non-limiting examples of which are set out in Table 5 below, all of which have A Tyr103 residue.

Table 5

Attenuating Modification Removal of a key component of the through protein electron transfer pathway, namely Tyr42 in human haemoglobin alpha chain or Tyr103 in sperm whale or human myoglobin collapses the high affinity pathway, as shown kinetically by the absence of double rectangular hyperbolic dependencies on the reductant concentration. This prevents rapid reduction of the ferryl haemoprotein by exogenous substrates such as lipids, key to the pathogenesis of many disease conditions. Thus by "attenuating modification" it is meant any change to the protein which causes the protein to lack the through-protein electron conduits.

The removal of any amino acid, e.g. the tyrosine residues mentioned above, by deletion is one way to provide the attenuating modification. In a preferred aspect, the attenuating modification is brought about by substitution of a residue, particularly the Tyr residues identified above, with another amino acid, particularly a redox inactive residue is contemplated. Such a redox inactive residue may be any other amino acid encoded by the genetic code apart from tryptophan and histidine. Amino acids which are contemplated here include alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine and valine. Other Modifications

In one embodiment, in addition to the attenuating modification of a wild-type oxygen-carrying protein, the protein may comprise one or more (for example from one to five, such as one or two) additional substitutions, or a deletion or insertion of from one to five, such as one, two or three amino acids (which may be contiguous or non-contiguous). These may be variations which affect a further property of the protein, such as its oxygen affinity or cooperativity, enhancements in stability and assembly rates, decreased heme loss rates or autoxidation rates, or resistance to proteolytic degradation and aggregation, its binding to nitric oxide or its ability to be produced in a soluble form by recombinant means. Such modifications are known in the art perse and may be incorporated into the proteins of the present invention.

In a preferred aspect, the modification is one which decreases the binding of nitric oxide (NO). A number of haemoglobin variants which limit NO binding while still permitting oxygen transport are known. A number of variants of haemoglobin alpha chains which have reduced rates of reaction with nitric oxide are disclosed in US 6,455,676, the contents of which are incorporated herein by reference.

In particular, the following changes may be included in the oxygen carrying protein in addition to the attenuating modification:

E11(Val>Leu) and E7(His>Gln); E11(Val>Phe or Trp) and E7(His>Gln); E11(Val>Phe or Trp or Leu) and E7(His>Gln) and G8(Leu>Phe or Trp); B10(Leu>Phe) and E4(Val>Leu); B10(Leu>Trp) and E4(Val>Leu); B10(Leu>Trp) and E7(His>Gln); B10(Leu>Trp) and E11 (Val>Phe); B10(Leu>Trp) and E11 (Val>Trp); B10(Leu>Trp) and E11(Val>Leu) and G8(Leu>Trp); B10(Leu>Trp) and E11(Val>Leu) and G8(Leu>Phe); B10(Leu>Trp) and E11 (Val>Phe) and G8(Leu>Trp); B10(Leu>Trp) and E11 (Val>Phe) and G8(Leu>llc); B10(Leu>Trp) and E7(His>Gln) and E11(Val>Leu) and G8(Leu>Trp); B10(Leu>Trp) and E11 (Val>Trp) and G8(Leu>Trp); E11 (Val>Leu) and G8(Leu>Phe); E11 (Val>Leu) and G8(Leu>Trp); B13(Met>Phe or Trp); G12(Leu>Phe or Trp); or B14(Phe>Trp).

The numbering used above is based on helix chain numbering, which can be cross-referenced to the primary sequence numbering of Table 1 for the human alpha chain. These modifications at equivalent positions in other oxygen carrying proteins may also be made. Protein Multimers

In one embodiment, oxygen-carrying proteins are present in multimeric forms. Such forms may prolong life of the protein in circulation, improve oxygen-carrying capacity or reduce side- effects.

In the case of haemoglobin alpha subunit proteins such forms include a tetrameric haemoglobin protein. In this form two alpha chains may form a tetramer with two beta chains. Optionally, two or more of the subunits may be covalently linked to each other, e.g. via chemical cross-linking or as a result of recombinant expression.

The beta chains may be wild type beta chains, e.g. a human beta chain of SEQ ID NO:2 or a homologous vertebrate or non-vertebrate beta chain. Such beta chains include those found with their associated with the alpha chains referred to above in Tables 2 to 5 and whose sequences are obtainable from the pdb entries. Vertebrate beta chains include mammalian beta chains. Mammalian beta chains include other members of the beta chain superfamily, such as haemoglobin gamma or delta subunits, present in human HbF (pdb code 1FDH) and human HbA2 (pdb code 1SI4) respectively.

The beta chains may comprise one or more (for example from one to five, such as one or two) additional substitutions, or a deletion or insertion of from one to five, such as one, two or three amino acids (which may be contiguous or non-contiguous). These may be variations which affect a further property of the protein, such as its interaction with other proteins, its binding to nitric oxide or to facilitate its production by recombinant means.

Particular changes to the beta-chain contemplated which modify binding to nitric oxide include B13(Leu>Phe or Trp); G12(Leu>Phe or Trp); B10(Leu>Phe) and E4(Val>Leu); B10(Leu>Trp) and E4(Val>Leu); B14(Leu>Phe or Trp); G8(Leu>Phe) and G12(Leu>Trp); E11 (Val>Leu) and G8(Leu>Trp); E1 1 (Val>Trp) and G8(Leu>Met); E11(Val>Leu) and G8(Leu>Phe); E11 (Val>Leu) and G8(Leu>Met); E11 (Val>Phe) and G8(Leu>lle); E11 (Val>Phe) and G8(Leu>Phe); E11 (Val>Phe) and G8(Leu>Trp); E11 (Val>Phe) and G8(Leu>Met); E11 (Val>Met) and G8(Leu>Trp); E11(Val>Met) and G8(Leu>Trp) and E7(His>Gln); E11 (Val>Trp) and G8(Leu>lle); E7(His>Gln) and E11 (Val>Trp); E7(His>Gln) and E11 (Val>l_eu); E7(His>Gln) and E11 (Val>Phe); E7(His>Gln) and E11(Val>Phe) and G8(Leu>Phe or Trp); E7(His>Gln) and E11 (Val>Leu or Trp) and G8(Leu>Phe or Trp);

E1 1 (Val>Trp or Phe) and G12(Leu>Trp or Met); E11 (Val>Trp or Phe) and B13(Leu>Trp or Met); B10(Leu>Trp) and B13(Leu>Trp or Met); B10(Leu>Phe) and B13(Leu>Trp); B10(Leu>Trp or Phe) and G12(Leu>Trp); B10(Leu>Phe) and G12(Leu>Met); G8(Leu>Trp) and G12(Leu>Trp or Met); or G8(Leu>Trp) and B13(Leu>Trp or Met).

The numbering used above is based on helix chain numbering, which can be cross-referenced to the primary sequence numbering of Table 1 for the human alpha chain. These modifications at equivalent positions in other oxygen carrying proteins may also be made.

Other higher order forms, either covalently or non-covalently associated with each other and/or with other oxygen-carrying proteins may also be provided. For example, polymerized haemoglobin subunit chains or cross-linked chains are known in the art perse and these approaches may be applied to the present invention.

Protecting Groups

In another aspect, the oxygen-carrying proteins of the invention, whether in monomeric or multimeric form, may be conjugated to a protecting group. Various types of protecting groups are known as such in the art and may be used in the present invention. Where the protecting group is a protein, this protecting group may be produced as a fusion, e.g. at the N- or C- terminus of the oxygen-carrying protein. Alternatively, the protein may be co-expressed with the oxygen-carrying protein or expressed separately, and the two proteins joined by chemical means using a cross-linker.

For example, one class of protecting groups are enzymatic anti-oxidant proteins. These include catalase and superoxide dismutase (SOD). Any suitable catalase or SOD may be used, though preferably these are human enzymes. The enzymes may be produced recombinantly or by any other means conventional in the art.

US 5,606,025, the contents of which are incorporated herein by reference, describes the conjugation of such enzymes to a haemoglobin and such methods may be used in the present invention. Thus any suitable inert cross-linking reagent previously reported as suitable for preparing cross-linked haemoglobin for use as an oxygen-carrying resuscitative fluid can be used, for example glutaraldehyde, diasprin derivatives, polyaldehydes including those derived from oxidative ring-opening of oligosaccharides, diphosphate esters, triphosphate esters, etc. The enzymes of interest have chemical groups similar to those on the globin chains of haemoglobin so that they will appropriately chemically bind to the haemoglobin as it cross-links by reaction with the cross-linking reagent. Relative amounts of the oxygen-carrying protein and the enzymatic anti-oxidant protein can vary over wide limits, with the oxygen-carrying protein constituting the major component. The total weight of the enzyme(s) is suitably in the approximate range of 0.1-10% based on the weight of the oxygen-carrying protein, and preferably in the approximate range 0.5-2.5%. When, as in one embodiment, both SOD and catalase are chemically bound to the polyhaemoglobin, the weight ratio of SOD to catalase is suitably from about 1:1 to 5:1 and preferably from about 1.5:1 to 2.5:1.

Another class of protecting group, which may be used as well as the above-described enzymatic groups, or in the alternative, is a non-antigenic polymeric group such as a polyalkylene oxide protecting group. Such groups may also be used on monomeric oxygen- carrying proteins or these proteins when in dimeric or higher form.

For example, US 5,900,402, the contents of which are incorporated herein by reference, describes the conjugation of polyalkylene oxides, most preferably polyethylene glycol (PEG) to oxygen-carrying proteins.

The conjugate is preferably formed by covalently bonding a hydroxyl terminal of the polyalkylene oxide and the free amino groups of lysine residues of the oxygen-carrying protein. See, for example, U.S. Pat. No. 5,234,903, which discloses mPEG-succinimidyl carbonate-Hb conjugates. Other methods for conjugating the polymers with oxygen-carrying proteins are known in the art as such, such as by via an amide or ester linkage, are also suitable for use with the present invention. While epsilon amino group modifications of haemoglobin lysines are preferred, other conjugation methods are also contemplated. Covalent linkage by any atom between the haemoglobin and polymer is possible. Moreover, non-covalent conjugation such as lipophilic or hydrophilic interactions are also contemplated.

Additional examples of activated polymers which are suitable for covalently conjugating the oxygen carrying proteins are described in U.S. Pat. Nos. 5,349,001 ; 5,321 ,095; 5,324,844 and 5,605,976 as well as PCT Publication Numbers WO95/11924 and WO96/00080, the disclosure of each of which is incorporated herein by reference.

The conjugates preferably include polyethylene glycol (PEG) as the polyalkylene oxide. The polyalkylene oxides include monomethoxy-polyethylene glycol, polypropylene glycol, block copolymers of polyethylene glycol and polypropylene glycol and the like. The polymers can also be distally capped with C_2-4, alkyls instead of monomethoxy groups. To be suitable for use herein, the polyalkylene oxides must be soluble in water at room temperature. Polyalkylene oxide strands having a (number average) molecular weight of from about 200 to about 100,000 Daltons can be used. For example, preferable PAOs have molecular weights of from about 1,000 to about 30,000 while PAOs having a molecular weight of from about 2,000 to about 25,000 are more preferred. Some particularly preferred conjugates of the present invention include polyalkylene oxide strands having a molecular weight of about 5,000 Daltons.

The ratio of the number of strands of the non-antigenic polymeric group to the oxygen-carrying protein may be from about 1:1 to about 20:1 , preferably from about 5:1 to 15:1 , for example about 10:1. The strands may be of the size ranges specified above.

Overall, the molecular weight of a monomer of an oxygen-carrying protein prior to conjugation is about 17,000 Da. Where such a protein is conjugated to a non-antigenic polymeric group as described above, the conjugate will be from about 30% to 60%, such as about 45% to 55% by weight of protein (i.e. the oxygen-carrying protein or a conjugate of this protein and an enzymatic group), the remainder being the non-antigenic polymeric group.

An exemplary embodiment of the invention is thus a conjugate of an oxygen-carrying protein of the invention and 45% to 55% by weight of polyalkylene oxide having a molecular weight of from about 2,000 to about 25,000. In one aspect of this embodiment, the oxygen-carrying protein may be a haemoglobin alpha chain in which the attenuating modification is at Tyr42. In another aspect of this embodiment, the polyalkylene oxide is PEG. In a further aspect, the oxygen-carrying protein is a haemoglobin alpha chain in which the attenuating modification is at Tyr42 and the polyalkylene oxide is PEG.

In the above embodiments, the oxygen-carrying protein may be in the form of a monomer or a polymer of two or more units.

Compositions

The oxygen-carrying proteins of the invention are desirably formulated as a composition comprising a physiologically acceptable carrier, suitable for administration to a mammal, particularly a human. Generally, such a carrier will be a sterile solution which comprises buffers and preservatives used to keep the solution at physiological pH and stable during storage. The carriers may be such physiologically compatible buffers as Hank's or Ringer's solution, physiological saline, a mixture consisting of saline and glucose, and heparinized sodium-citrate-citric acid-dextrose solution. The oxygen-carrying proteins of the present invention can be mixed with colloidal-like plasma substitutes and plasma expanders such as linear polysaccharides (e.g. dextran), hydroxyethyl starch, balanced fluid gelatin, and other plasma proteins. Additionally, the oxygen-carrying proteins may be mixed with water soluble, physiologically acceptable, polymeric plasma substitutes, examples of which include polyvinyl alcohol, poly(ethylene oxide), polyvinylpyrrolidone, and ethylene oxide-polypropylene glycol condensates.

Compositions of the invention may further include one or more compounds with anti-oxidant properties. These compounds may include ascorbate and urate. The anti-oxidant may be included at any suitable concentration, which may vary according to intended use and the nature of the anti-oxidant. For example, a suitable concentration of urate may be in the range of from 50 to 400 micromolar, and for ascorbate of from 50 to 200 micromolar, though lower or higher amounts may be used if need be.

The compositions may also include iron chelating agents which may play a role in sequestering iron released by the breakdown of the oxygen-carrying protein. Examples of such iron chelating agents include desferrioxamine and deferiprone. The iron chelating agent, or mixture thereof, may be present at a concentration of, for example, 10 - 5000 μM.

Administration of Oxygen-Carrying Proteins

Proteins of the invention may be used as blood substitutes. There are numerous conditions in which it will be useful for restoration, maintenance or replacement of oxygen levels is required. These include trauma; ischemia (such as ischemia induced by heart attack, stroke, or cerebrovascular trauma); haemodilution, where a blood substitute is required to replace blood that is removed pre-operatively; septic shock; cancer (e.g. to deliver oxygen to the hypoxic inner core of a tumour mass); chronic anaemia; sickle cell anaemia; cardioplegia; and hypoxia. Thus the oxygen-carrying proteins, and compositions thereof, of the present invention may be used in methods for the treatment of the above-mentioned conditions.

The oxygen-carrying proteins, and compositions thereof, of the present invention may also be used ex vivo in organ perfusion. Blood substitutes may be particularly useful in the organ perfusion, where maintaining oxygen content in an organ ex vivo prior to transplantation is required to sustain the organ in an acceptable condition. Organs include heart, liver, lung, kidneys. The concentration and amount of oxygen-carrying protein of the invention used in any of the above-mentioned methods will be at the discretion of the physician, taking account of the nature of the condition of the patient and the treatment. Typically, the oxygen-carrying protein may be used at a concentration of from 0.1 to 6 g/dl, e.g. from 0.1 to 4 g/dl. The oxygen- carrying protein will usually be administered intravenously.

Co-administration of an innocuous reagent to enhance nitric oxide production (e.g. arginine) is also envisaged.

Nucleic Acids

The invention also provides nucleic acids encoding the modified oxygen-carrying proteins of the invention. The nucleic acid may be DNA or RNA. The DNA may be single- or double- stranded.

The nucleic acid of the invention may in isolated and/or purified form, or free or substantially free of material with which it is naturally associated, such as free or substantially free of nucleic acid flanking the gene in the human genome, except possibly one or more regulatory sequence(s) for expression.

Generally, nucleic acids of the invention may be obtained by modification of wild-type sequences encoding the oxygen-carrying protein. The nucleic acid sequences of wild-type haemoglobins and other oxygen carrying proteins are known in the art and widely available. Generally, recombinant techniques such as site-directed mutagenesis may be used to modify a known wild-type sequence such that the sequence encodes a modified oxygen-carrying protein of the invention.

The wild-type sequence of a mammalian nucleic acid may also be modified to optimize codon usage for expression in a heterologous system, e.g. in bacterial or yeast cells.

A nucleic acid of the invention may be incorporated into a recombinant replicable vector. The vector may be used to replicate the nucleic acid in a compatible host cell. Thus in a further embodiment, the invention provides a method of making a nucleic acid of the invention by introducing a nucleic acid of the invention into a replicable vector, introducing the vector into a compatible host cell, and growing the host cell under conditions which bring about replication of the vector. Preferably, a nucleic acid of the invention in a vector is operably linked to a control sequence which is capable of providing for the expression of the coding sequence by the host cell, i.e. the vector is an expression vector.

The term "operably linked" refers to a juxtaposition wherein the components described are in a relationship permitting them to function in their intended manner. A control sequence

"operably linked" to a coding sequence is ligated in such a way that expression of the coding sequence is achieved under condition compatible with the control sequences.

Suitable vectors can be chosen or constructed, containing appropriate regulatory sequences, including promoter sequences, terminator fragments, polyadenylation sequences, enhancer sequences, marker genes and other sequences as appropriate. Vectors may be plasmids, viral e.g. 'phage phagemid or baculoviral, cosmids, YACs, BACs, or PACs as appropriate. Vectors include gene therapy vectors, for example vectors based on adenovirus, adeno- associated virus, retrovirus (such as HIV or MLV) or alpha virus vectors.

The vectors may be provided with an origin of replication, optionally a promoter for the expression of the oxygen-carrying protein and optionally a regulator of the promoter. The vectors may contain one or more selectable marker genes, for example an ampicillin resistance gene in the case of a bacterial plasmid or a neomycin resistance gene for a mammalian vector. Vectors may be used in vitro, for example for the production of RNA or used to transfect or transform a host cell. Systems for cloning and expression of a polypeptide in a variety of different host cells are well known. Suitable host cells include bacteria, eukaryotic cells such as mammalian and yeast, and baculovirus systems. Mammalian cell lines available in the art for expression of a heterologous polypeptide include Chinese hamster ovary cells, HeLa cells, baby hamster kidney cells, COS cells and many others.

Promoters and other expression regulation signals may be selected to be compatible with the host cell for which the expression vector is designed. For example, yeast promoters include S. cerevisiae GAL4 and ADH promoters, S. pombe nmt1 and adh promoter. Mammalian promoters include the metallothionein promoter which is can be induced in response to heavy metals such as cadmium. Viral promoters such as the SV40 large T antigen promoter or adenovirus promoters may also be used. All these promoters are readily available in the art.

The vectors may include other sequences such as promoters or enhancers to drive the expression of the inserted nucleic acid, nucleic acid sequences so that the oxygen-carrying protein is produced as a fusion and/or nucleic acid encoding secretion signals so that the polypeptide produced in the host cell is secreted from the cell.

Vectors for production of polypeptides of the invention for use in gene therapy include vectors which carry a mini-gene sequence of the invention.

Host Cells and Production of Oxygen-Carrying Proteins

Host cells according to the invention such as those mentioned herein above may be cultured under conditions to bring about expression of the oxygen-carrying protein, followed by recovery of the protein to provide the protein in substantially isolated form.

The protein may be produced with a source of haem or may be mixed with a suitable source of haem such as ferro-protoporphyrin or ferri-protoporphyrin (haemin) during or after recovery in order to provide a functional oxygen-carrying protein.

For example, US 5,801 ,019, the contents of which are incorporated herein by reference, describes expression and recovery of various modified haemoglobins, including multimers of haemoglobin subunits, in yeast cells. Such methods may be used in the present invention for the production of the oxygen-carrying proteins.

Where the oxygen-carrying protein is a haemoglobin alpha chain subunit, it may be co- expressed with complementary subunits, e.g. a beta chain subunit. The co-expressed protein may be in the form of a separate protein or a fusion with the alpha chain subunit.

Following expression, the proteins are recovered using standard methods including but not limited to chromatography (e.g., ion exchange, affinity, and sizing column chromatography), centrifugation, differential solubility, or by any other standard technique for the purification of proteins. If one globin chain is expressed, the expressed globin chain may be combined with another globin chain and a source of haem to form haemoglobin. If haemoglobin is expressed in the yeast cell, no further steps are necessary.

EXAMPLES

The present invention is illustrated further by the following examples. Example 1 :

Myoglobin from Horse, but not Aplysia, possesses a high affinity through-protein electron transfer pathway as measured by ferryl reduction kinetics

In this example the kinetics of the reduction of ferryl haem from myoglobin from different species to ferric haem protein were examined kinetically to determine the presence or absence of a high affinity-through protein pathway. Following the procedure, ferric horse myoglobin from Sigma-Aldrich (Poole, Dorset, UK, further purified by gel filtration), or recombinant ferric Aplysia myoglobin Mb (20μM) in 5mM sodium phosphate pH 7.4 was reacted with H₂O₂ (20μM) at 25⁰C for 15 min. At this time conversion of ferric myoglobin to ferryl myoglobin was greater than 95% as measured by addition of sodium sulfide (1mM). Catalase (1OnM) was added to remove unreacted H₂O₂ and was left to react for a further 1 min. At the pH used the ferryl haem protein is stable for several hours. Reductant was then added in 0.1 M sodium phosphate pH 7.4 in a 1 :1 volume ratio so that final concentration of ferryl myoglobin was 10μM. The pH may be 'jumped' to other values where the ferryl haem is unstable using other strong buffers (e.g. 0.1 M sodium acetate, pH 5). The optical spectrum was followed until reaction was complete. The time course (425nm-408nm) was fitted to a single exponential function using the least squares method. These rate constants were then plotted as a function of reductant concentration and this profile fitted (least squares method) to a double rectangular hyperbola (Fig 1):

k_α[S] k_b[S] k_oh = ^α + ^b-^L-^Λ — + A_R o^bs [S] ₊ K _m [S] ₊ K₀₂ ^R

Where k_a and k_b are the maximum rates for each hyperbola and K_D1 and K₀₂ are the dissociation constants, S is the concentration of the reductant and A_R is the rate constant for ferryl auto-reduction.

The dependence of the concentration of reductant on the changes in the observed rate constant for ferryl myoglobin reduction from horse shows a double rectangular hyperbola dependence that is not observed in Aplysia myoglobin. Myoglobin from horse has two tyrosine residues at positions 103 and 146, while Aplysia myoglobin has no tyrosine residues. Example 2:

A tyrosine residue close to the haem is key to the high affinity through-protein electron transfer pathway.

Wild-type and recombinant sperm whale myoglobin and recombinant Tyr103>Phe sperm whale myoglobin were reacted with peroxide and the kinetics of ferryl haem reduction by deferiprone determined as described in example 1. The through-protein electron transfer pathway, evident in wild type sperm whale myoglobin, is not observed in the Tyr103>Phe mutant (Fig 2). This demonstrates that the presence of a redox active tyrosine, interfacing the haem and the external environment, is a key component for the high affinity pathway.

The data from Figs 1 and 2 can be rationalised by a two site model where a reductant (xH) binds to two possible sites on myoglobin in its ferryl oxidation state (P-[Fe(IV)=O^2"]²⁺, where P denotes protein, Fig 3) with affinities K_Di and K_D2- Only from these two sites can electron transfer from the reductant to the ferryl iron take place. The high affinity binding site is situated at, or close to, TyM 03 (PxH-[Fe(IV)=O^2"]), allowing the transfer of an electron (k_max=0.01s^"1) through the protein to the ferryl haem iron to generate the ferric protein (P-Fe(III)). The low affinity site is situated in the haem pocket allowing electron transfer directly between the reductant and the ferryl haem (P-[Fe(IV)=O^2"]xH). This model also allows both sites being filled by reductant (PxH-[Fe(IV)=O^2~]xH). For the kinetic simulations it is assumed that the binding on one site will not affect the affinity of binding to the second site. The model incorporates a step in which the oxidized reductant may be regenerated. This model also allows predictions on other haem proteins based on crystal structures. The structure of alpha subunit of human haemoglobin shows a tyrosine in a similar spatial position (Tyr42) compared to myoglobin, close to the haem and is surface exposed making it ideal to act as an electron conduit from exogenous reductants to the ferryl haem iron (Fig 4). The corresponding residue in human haemoglobin beta is a redox-inactive phenylalanine. Thus the model predicts that human haemoglobin alpha subunit, but not the beta subunit, will possess the high affinity through- protein electron transfer pathway.

Example 3.

The heterogeneous subunits of human haemoglobin exhibit different mechanisms of ferryl reduction. Recombinant human haemoglobin was reacted with peroxide and the kinetics of ferryl haem reduction by ascorbate determined as described in example 1. The time course of ferryl haem reduction is not single exponential, as observed with myoglobin, but can be described using a double exponential function that generates two rate constants, one representing the observed rate constant for ferryl reduction of the alpha subunit and one representing the observed rate constant for ferryl reduction of the beta subunit. The kinetics of ferryl reduction (Fig 5) shows that the subunits behave very differently towards reductants with only one of the haemoglobin subunits, assigned to the alpha subunit, exhibiting a high affinity electron transfer pathway.

Example 4.

Site directed mutagenesis of α-Tyr42 eliminates the high affinity through-protein electron transfer pathway decreasing the rate of ferryl haem reduction.

Mutant variants of the alpha genes were created using site-directed mutagenesis. Primer sequences can be found in Table 6 (below). A high-fidelity enzyme, either Phusion™ (Finnzymes) or Pfu Ultra™ (Stratagene), was used according to the suppliers' specifications in the PCR reactions with the following PCR-program: 95⁰C 2min; 95°C 30s 55°C 1min 72°C 5min 16 cycles; 72°C 10min. The template DNA was then digested using Dpnl (Fermentas) and mutated plasmid transformed into Escherichia coli BL21 DE3 using standard procedures. Resulting clones were sequenced with BigDye™ terminator v.3.0 (Applied Biosystems) to confirm correct sequence.

Escherichia coli harbouring a plasmid encoding the modified alpha chains were grown and the modified alpha chains recovered using standard methods.

Table 6. Primer sequences used for site-directed mutagenesis.

Name Sequence (5'-3') aY42V for CCTTCCCAACCACCAAAACCGTGTTCCCACACTTTGATCTG (SEQ ID NO:4) aY42V rev CAGATCAAAGTGTGGGAACACGGTTTTGGTGGTTGGGAAGG (SEQ ID NO:5) aY42random for CCTTCCCAACCACCAAAACCNNKTTCCCACACTTTGATCTG (SEQ ID NO:6) aY42random rev CAGATCAAAGTGTGGGAAMNNGGTTTTGGTGGTTGGGAAGG (SEQ ID NO:7)

Recombinant human haemoglobin with α-Tyr42>Val (Fig 6) or α-Tyr42>Trp (Fig 7) mutation was reacted with peroxide and the kinetics of ferryl haem reduction by ascorbate determined as described in example 1. In both mutants the effect on the rate of ferryl reduction of the alpha subunit is dramatic with loss of the high affinity through-protein pathway as shown by the absence of the first hyperbola. Thus mutation of α-Tyr42 to a partially redox active tryptophan decreases the rate of ferryl reduction of the alpha subunit 2 fold at 10μM ascorbate concentration and mutation of α-Tyr42 to a redox inactive valine decreases the rate of ferryl reduction of the alpha 12 fold at 10μM ascorbate concentration.

Table I.^¬

Amino Acid Sequence and Helical Residue Notation for Human Haemoglobin Ao

Helix a Helix (3 Helix a Helix 13

NAl 1 VaI NAl 1 VaI E17 68 Asn El 7 73 Asp

- - NA2 2His E18 69 Ala E18 74GIy

NA2 2 Leu NA3 3 Leu E19 70VaI E19 75 Leu

Al 3Ser Al 4Thr E20 71 Ala E20 76AIa

A2 4 Pro A2 5 Pro EFl 72His EFl 77His

A3 5AIa A3 6GIu EF2 73VaI EF2 78 Leu

A4 6 Asp A4 7GIu EF3 74 Asp EF3 79 Asp

A5 7Lys A5 8 Lys EF4 75 Asp EF4 80 Asn

A6 8Thr A6 9Ser EF5 76 Met EF5 81 Leu

A7 9 Asn A7 10 Ala EF6 77 Pro EFό 82 Lys

A8 10 VaI A8 11 VaI EF7 78 Asn EF7 83GIy

A9 11 Lys A9 12Thr EF8 79 Ala EF8 84Thr

AlO 12 Ala AlO 13AIa Fl 80 Leu Fl 85Phe

All 13AIa All 14 Leu F2 81 Ser F2 86 Ala

A12 14Trp A12 15Trp F3 82AIa F3 87Thr

A13 15GIy A13 16GIy F4 83 Leu F4 88 Leu

A14 16 Lys A14 17 Lys F5 84 Ser F5 89 Ser

A15 17VaI A15 18VaI F6 85 Asp F6 90GIu

A16 18GIy - - F7 86 Leu F7 91 Leu

ABl 19 Ala - - F8 87His F8 92His

Bl 20His Bl 19 Asn F9 88AIa F9 93 Cys

B2 21 Ala B2 20VaI FGl 89His FGl 94 Asp

B3 22GIy B3 21 Asp FG2 90 Lys FG2 95 Lys

B4 23GIu B4 22GIu FG3 91 Leu FG3 96 Leu

B5 24Tyr B5 23VaI FG4 92 Arg FG4 97His

B6 25GIy B6 24GIy FG5 93 VaI FG5 98VaI

B7 26 Ala B7 25GIy Gl 94 Asp Gl 99 Asp

B8 27GIu B8 26GIu G2 95 Pro G2 100 Pro

B9 28 Ala B9 27AIa G3 96VaI G3 101 GIu

BlO 29 Leu BlO 28 Leu G4 97 Asn G4 102 Asn

BIl 30GIu BIl 29GIy G5 98Phe G5 103 Phe

B12 31 Arg B12 30 Arg G6 99 Lys G6 104 Arg

B13 32 Met B13 31 Leu G7 100 Leu G7 105 Leu

B14 33Phe B14 32 Leu G8 101 Leu G8 106 Leu

B15 34 Leu B15 33VaI G9 102 Ser G9 107GIy

B16 35Ser Blό 34VaI GlO 103His GlO 108 Asn

Cl 36Phe Cl 35Tyr GIl 104 Cys GIl 109VaI

Cl 37 Pro C2 36 Pro G12 105 Leu G12 110 Leu

C3 38Thr C3 37Trp G13 106 Leu G13 111 VaI

C4 39Thr C4 38Thr G14 107VaI G14 112 Cys Amino Acid Sequence and Helical Residue Notation for Human Haemoglobin A₀

Helix a Helix P Helix a Helix B

C5 40Lys C5 39GIn G15 108 Thr G15 113VaI

C6 41 Thr C6 40 Arg G16 109 Leu G16 114 Leu

C7 42Tyr C7 41 Phe G17 110 Ala G17 115AIa

CEl 43Phe CDl 42Phe G18 111 Ala G18 116 His

CE2 44 Pro CD2 43GIu G19 112 His GI9 117 His

CE3 45His CD3 44Ser GHl 113 Leu GHl 118 Phe

CE4 46Phe CD4 45 Phe GH2 114 Pro GH2 119 GIy

- - CD5 46GIy GH3 115AIa GH3 120 Lys

CE5 47 Asp CD6 47 Asp GH4 116GIu GH4 121 GIu

CE6 48 Leu CD7 48 Leu GH5 117 Phe GH5 122 Phe

CE7 49Ser CD8 49Ser Hl 118 Thr Hl 123 Thr

CE8 50His Dl 50 Thr H2 119 Pro H2 124 Pro

- - D2 51 Pro H3 120AIa H3 125 Pro

- - D3 52 Asp H4 121 VaI H4 126VaI

- - D4 53 Ala H5 122His H5 127GIn

- - D5 54VaI H6 123 Ala H6 128AIa

- - D6 55 Met H7 124Ser H7 129 Ala

CE9 51 GIy D7 56GIy H8 125 Leu H8 130Tyr

El 52Ser El 57Asn H9 126 Asp H9 131 GIn

E2 53 Ala E2 58 Pro HlO 127 Lys HlO 132 Lys

E3 54GIn E3 59 Lys HlI 128 Phe HIl 133VaI

E4 55 VaI E4 60VaI H12 129 Leu H12 134VaI

E5 56 Lys E5 61 Lys H13 130AIa H13 135AIa

E6 57GIy E6 62AIa H14 131 Ser H14 136GIy

E7 58His E7 63His H15 132VaI H15 137VaI

E8 59GIy E8 64GIy H16 133 Ser H16 138AIa

E9 60 Lys E9 65 Lys H17 134 Thr H17 139 Asn

ElO 61 Lys ElO 66 Lys H18 135VaI H18 140AIa

Ell 62VaI Ell 67VaI H19 136 Leu H19 141 Leu

E12 63AIa E12 68 Leu H20 137 Thr H20 142AIa

E13 64 Asp E13 69GIy H21 138 Ser H21 143His

E14 65 Ala E14 70 Ala HCl 139 Lys HCl 144 Lys

E15 66 Leu E15 71 Phe HC2 140Tyr HC2 145 Tyr

E16 67 Thr E16 72Ser HC3 141 Arg HC3 146His

Sequences:

SEQ ID NO:1 (Hb Alpha)

1 vlspadktnv kaawgkvgah ageygaeale rmflsfpttk tyfphfdlsh gsaqvkghgk 61 kvadaltnav ahvddmpnal salsdlhahk lrvdpvnfkl lshcllvtla ahlpaeftpa 121 vhasldkfla svstvltsky r

SEQ IDNO:2 (Hb Beta)

1 vhltpeeksa vtalwgkvnv devggealgr llvvypwtqr ffesfgdlst pdavmgnpkv 61 kahgkkvlga fsdglahldn lkgtfatlse lhcdklhvdp enfrllgnvl vcvlahhfgk 121 eftppvqaay qkvvagvana lahkyh SEQ ID NO:3 (Myoglobin)

1 glsdgewqlv lnvwgkvead ipghgqevli rlfkghpetl ekfdkfkhlk sedemkased 61 lkkhgatvlt alggilkkkg hheaeikpla qshatkhkip vkylefisec iiqvlqskhp 121 gdfgadaqga mnkalelfrk dmasnykelg fqg

Claims

Claims

1. A modified porphyrin-based oxygen-carrying protein, said protein in an unmodified state comprising a low affinity site of electron transfer, and a high affinity electron transfer between a reductant and ferryl haem iron via one or more protein amino acids, wherein said protein comprises an attenuating modification in the high affinity pathway.

2. The protein of claim 1 which is a haemoglobin alpha chain subunit.

3. The protein of claim 1 or 2 which is a vertebrate haemoglobin subunit protein.

4. The protein of claim 3 which is a mammalian haemoglobin subunit protein.

5. The protein of claim 4 which is modified by substitution of Tyr42.

6. The protein of claim 5 wherein the modification is Tyr42 to a redox inactive amino acid.

7 The protein of claim 6 wherein the redox inactive amino acid is valine.

8. The protein of claim 1 which is myoglobin.

9. The protein of claim 8 wherein said myoglobin is a mammalian myoglobin.

10. The protein of claim 8 or 9 which is modified by substitution of Tyr103.

11. The protein of claim 10 which is Tyr103>Val.

12. The protein of any one of the preceding claims which comprises non-wild-type residue which decreases NO binding.

13. The protein of any one of the preceding claims conjugated to a protecting group.

14. The protein of claim 13 wherein the protecting group is an anti-oxidant enzyme.

15. The protein of claim 13 wherein the protecting group is a polyalkylene oxide.

16. A protein comprising a dimer, tetramer or multimer of the protein of any one of claims 1 to 15.

17. The protein of claim 16 wherein the dimer, tetramer or multimer is cross-linked.

18. The protein of claim 16 or 17 which is a tetramer comprising two beta haemoglobin subunits and two subunits of the protein of any one of claims 1 to 12.

19. A composition comprising the protein of any one of the preceding claims in a physiologically acceptable carrier.

20. A method of treatment of a human subject comprising administering to a subject in need of treatment an effective amount of the composition of claim 16.

21. A nucleic acid encoding the protein of any one of claims 1 to 14.

22. An expression vector comprising the nucleic acid of claim 21 operably linked to a promoter.

23. A host cell comprising the expression vector of claim 22.

24. A method of making the protein of any one of claims 1 to 14 which comprises expressing a protein in the host cell of claim 22 to recover the protein of any one of claims 1 to 15 and optionally modifying the protein to provide a protein as defined in any one of claims 13 to 18.

25. The method of claim 24 which further comprises forming a multimer with a second haemoglobin subunit protein.