Synthesis of Wild Type and Mutant Human Hemoglobins in Saccharomyces cerevisiae

Introducing Negatively Charged Residues on the Surface of Fetal Hemoglobin Improves Yields in Escherichia coli

Fetal hemoglobin (HbF) has been developed into an important alternative protein for oxygen therapeutics. Such applications require extensive amounts of proteins, which only can be achieved via recombinant means. However, the expression of vertebrate hemoglobins in heterologous hosts is far from trivial. There are several issues that need to be dealt with. These include, among others, the solubility of the globin chains, equimolar expression of the globin chains, and access to high levels of free heme. In this study, we examined the impact of introducing negative charges on the surface of HbF. Three different HbF mutants were examined, carrying four additional negative charges on the α-subunit (rHbFα4), two additional negative charges on the γ-subunit (rHbFγ2) or a combination of these (rHbFα4/γ2). The increase in negative surface charge in these HbF mutants required the development of an alternate initial capture step in the downstream purification procedures. For the rHbFα4 mutant, we achieved a significantly enhanced yield of purified HbF with no apparent adverse effects on Hb functionality. However, the presence of non-functional Hb portions in the rHbFγ2 and rHbFα4/γ2 samples reduced the yields significantly for those mutants and indicated an imbalanced expression/association of globin chains. Furthermore, the autoxidation studies indicated that the rHbFγ2 and rHbFα4/γ2 mutants also were less oxidatively stable than rHbFα4 and wt rHbF. The study further verified the need for an improved flask culture protocol by optimizing cultivation parameters to enable yield-improving qualities of surface-located mutations.

Improved production of human hemoglobin in yeast by engineering hemoglobin degradation

With the increasing demand for blood transfusions, the production of human hemoglobin (Hb) from sustainable sources is increasingly studied. Microbial production is an attractive option, as it may provide a cheap, safe, and reliable source of this protein. To increase the production of human hemoglobin by the yeast Saccharomyces cerevisiae, the degradation of Hb was reduced through several approaches. The deletion of the genes HMX1 (encoding heme oxygenase), VPS10 (encoding receptor for vacuolar proteases), PEP4 (encoding vacuolar proteinase A), ROX1 (encoding heme-dependent repressor of hypoxic genes) and the overexpression of the HEM3 (encoding porphobilinogen deaminase) and the AHSP (encoding human alpha-hemoglobin-stabilizing protein) genes - these changes reduced heme and Hb degradation and improved heme and Hb production. The reduced hemoglobin degradation was validated by a bilirubin biosensor. During glucose fermentation, the engineered strains produced 18% of intracellular Hb relative to the total yeast protein, which is the highest production of human hemoglobin reported in yeast. This increased hemoglobin production was accompanied with an increased oxygen consumption rate and an increased glycerol yield, which (we speculate) is the yeast's response to rebalance its NADH levels under conditions of oxygen limitation and increased protein-production.

Recent Advances in the Microbial Synthesis of Hemoglobin

Hemoglobin is a cofactor-containing protein with heme that plays important roles in transporting and storing oxygen. Hemoglobins have been widely applied as acellular oxygen carriers, bioavailable iron-supplying agents, and food-grade coloring and flavoring agents. To meet increasing demands and overcome the drawbacks of chemical extraction, the biosynthesis of hemoglobin has become an attractive alternative. Several hemoglobins have recently been synthesized by various microorganisms through metabolic engineering and synthetic biology. In this review, we summarize the novel strategies that have been used to biosynthesize hemoglobin. These strategies can also serve as references for producing other heme-binding proteins.

Assembly and dynamics of the U4/U6 di-snRNP by single-molecule FRET

In large ribonucleoprotein machines, such as ribosomes and spliceosomes, RNA functions as an assembly scaffold as well as a critical catalytic component. Protein binding to the RNA scaffold can induce structural changes, which in turn modulate subsequent binding of other components. The spliceosomal U4/U6 di-snRNP contains extensively base paired U4 and U6 snRNAs, Snu13, Prp31, Prp3 and Prp4, seven Sm and seven LSm proteins. We have studied successive binding of all protein components to the snRNA duplex during di-snRNP assembly by electrophoretic mobility shift assay and accompanying conformational changes in the U4/U6 RNA 3-way junction by single-molecule FRET. Stems I and II of the duplex were found to co-axially stack in free RNA and function as a rigid scaffold during the entire assembly, but the U4 snRNA 5' stem-loop adopts alternative orientations each stabilized by Prp31 and Prp3/4 binding accounting for altered Prp3/4 binding affinities in presence of Prp31.

Development of recombinant hemoglobin-based oxygen carriers

SIGNIFICANCE

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

RECENT ADVANCES

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

CRITICAL ISSUES

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

FUTURE DIRECTIONS

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

Crystal structure of Prp8 reveals active site cavity of the spliceosome

The active centre of the spliceosome consists of an intricate network formed by U5, U2 and U6 snRNAs, and a pre-mRNA substrate. Prp8, a component of the U5 snRNP, crosslinks extensively with this RNA catalytic core. We present the crystal structure of yeast Prp8 (residues 885-2413) in complex with the U5 snRNP assembly factor Aar2. The structure reveals new tightly associated domains of Prp8 resembling a bacterial group II intron reverse transcriptase and a type II restriction endonuclease. Suppressors of splice site mutations and an intron branchpoint crosslink map to a large cavity formed by the reverse transcriptase thumb, endonuclease-like and the RNaseH-like domains. This cavity is large enough to accommodate the catalytic core of group II intron RNA. The structure provides crucial insights into the architecture of the spliceosome’s active site and reinforces the notion that nuclear pre-mRNA splicing and group II intron splicing have a common origin.

Role of heme in the unfolding and assembly of myoglobin

The unfolding of wild-type holomyoglobin in the ferric state (metMb) appears to be a simple two-state process, even though hemichrome spectra are often observed and apoMb denaturation involves an intermediate. To resolve these discrepancies, we measured GuHCl-induced, equilibrium unfolding of five sperm whale metMb variants, which were selected to examine the relative importance of apoglobin stability and hemin affinity. Combined analysis of CD, Trp fluorescence, and Soret absorbance titration curves for all the variants requires a six-state mechanism containing native (N), intermediate (I), and unfolded (U) states of apoMb and their hemin-bound counterparts, NH (holoMb), IH, and UH, respectively. The unfolding parameters for the apoMbs were obtained in independent experiments and then fixed in the analysis of the holoprotein data, where only the affinities of the apoglobin states for hemin were allowed to vary. This cofactor binding analysis applies generally to all globins and led to three specific conclusions. (1) The stability of holo-metMb is determined primarily by the high affinity (K(d) approximately 10(-13) M) of native apoMb (N) for hemin. (2) The partially unfolded intermediate with hemin bound (IH) has a hemichrome spectrum indicative of a bis-histidyl axial coordination and is seen clearly when the stability of the N state or its affinity for hemin is reduced. (3) Although the affinity of the intermediate for hemin (K(d) approximately 10(-11) M) is approximately 100-fold lower than that for the native state, free hemin can bind to it and promote the assembly of the holoprotein.

Energetic differences at the subunit interfaces of normal human hemoglobins correlate with their developmental profile

A previously unrecognized function of normal human hemoglobins occurring during protein assembly is described, i.e. self-regulation of subunit pairings and their durations arising from the variable strengths of their subunit interactions. Although many mutant human hemoglobins are known to have altered subunit interface strengths, those of the normal embryonic, fetal, and adult human hemoglobins have not been considered to differ significantly. However, in a comprehensive study of both types of subunit interfaces of seven of the eight normal oxy human hemoglobins, we found that the strengths, i.e., the free energies of the tetramer-dimer interfaces, contrary to previous reports, differ by 3 orders of magnitude and display an undulating profile similar to the transitions ("switches") of various globin subunit types over time. The dimer interface strengths are also variable and correlate linearly with their developmental profile. Embryonic hemoglobins are the weakest; fetal hemoglobin is of intermediate strength, and adult hemoglobins are the strongest. The pattern also correlates generally with their different O(2) affinities and responses to allosteric regulatory molecules. Acetylation of fetal hemoglobin weakens its unusually strong subunit interactions and occurs progressively as its level of expression diminishes and adult hemoglobin A formation begins; a causal relationship is suggested. The relative contributions of globin gene order and competition among subunits due to differences in their interface strengths were found to be complementary and establish a connection among genetics, thermodynamics, and development.

Choice of cellular protein expression system

Recombinant protein expression has become a standard laboratory tool, and a wide variety of systems and techniques are now in use. Because there are so many systems to choose from, the investigator has to be careful to use the combination that will give the best results for the protein being studied. This overview unit discusses expression and production choices, including post-translational modifications (e.g., glycosylation, acylation, sulfation, and removal of N-terminal methionine), in vivo and in vitro folding, and influence of downstream elements on expression.

Protein-induced conformational changes of RNA during the assembly of human signal recognition particle

The human signal recognition particle (SRP) is a large RNA-protein complex that targets secretory and membrane proteins to the endoplasmic reticulum membrane. The S domain of SRP is composed of roughly half of the 7SL RNA and four proteins (SRP19, SRP54, and the SRP68/72 heterodimer). In order to understand how the binding of proteins induces conformational changes of RNA and affects subsequent binding of other protein subunits, we have performed chemical and enzymatic probing of all S domain assembly intermediates. Ethylation interference experiments show that phosphate groups in helices 5, 6 and 7 that are essential for the binding of SRP68/72 are all on the same face of the RNA. Hydroxyl radical footprinting and dimethylsulphate (DMS) modifications show that SRP68/72 brings the lower part of helices 6 and 8 closer. SRP68/72 binding also protects the SRP54 binding site (helix 8 asymmetric loop) from chemical modification and RNase cleavage, whereas, in the presence of both SRP19 and SRP68/72, the long strand of helix 8 asymmetric loop becomes readily accessible to chemical and enzymatic probes. These results indicate that the RNA platform observed in the crystal structure of the SRP19-SRP54M-RNA complex already exists in the presence of SRP68/72 and SRP19. Therefore, SRP68/72, together with SRP19, rearranges the 7SL RNA in an SRP54 binding competent state.

Erythropoiesis and red cell function in vertebrate embryos

All vertebrate embryos produce a specific erythroid cell population--primitive erythrocytes--early in development. These cells are characterized by expression of the specific embryonic haemoglobins. Many aspects of primitive erythropoiesis and the physiological function of primitive red cells are still enigmatic. Nevertheless, recent years have seen intensive efforts to characterize in greater detail the molecular events underlying the initiation of erythropoiesis in vertebrate embryos. Several key genes have been identified that are necessary for primitive and the subsequent definitive erythropoiesis, which differs in several aspect from primitive erythropoiesis. This review gives in its first part a short overview dealing with comparative aspects of primitive and early definitive erythropoiesis in higher and lower vertebrates and in the second part we discuss the physiological function of primitive red cells based mainly on results from mammalian and avian embryos.

N-terminal acetylation and protonation of individual hemoglobin subunits: position-dependent effects on tetramer strength and cooperativity

The presence of alanine (Ala) or acetyl serine (AcSer) instead of the normal Val residues at the N-terminals of either the alpha- or the beta-subunits of human adult hemoglobin confers some novel and unexpected features on the protein. Mass spectrometric analysis confirmed that these substitutions were correct and that they were the only ones. Circular dichroism studies indicated no global protein conformational changes, and isoelectric focusing showed the absence of impurities. The presence of Ala at the N-terminals of the alpha-subunits of liganded hemoglobin results in a significantly increased basicity (increased pK(a) values) and a reduction in the strength of subunit interactions at the allosteric tetramer-dimer interface. Cooperativity in O(2) binding is also decreased. Substitution of Ala at the N-terminals of the beta-subunits gives neither of these effects. The substitution of Ser at the N terminus of either subunit leads to its complete acetylation (during expression) and a large decrease in the strength of the tetramer-dimer allosteric interface. When either Ala or AcSer is present at the N terminus of the alpha-subunit, the slope of the plot of the tetramer-dimer association/dissociation constant as a function of pH is decreased by 60%. It is suggested that since the network of interactions involving the N and C termini of the alpha-subunits is less extensive than that of the beta-subunits in liganded human hemoglobin disruptions there are likely to have a profound effect on hemoglobin function such as the increased basicity, the effects on tetramer strength, and on cooperativity.

Molecular aspects of embryonic hemoglobin function

In order to provide the appropriate level of oxygen transport to respiring tissues, we need to produce a molecular oxygen transporting system to supplement oxygen diffusion and solubility. This supplementation is provided by hemoglobin. The role of hemoglobin in providing oxygen transport from lung to tissues in the adult is well-documented and functional characteristics of the fetal hemoglobin, which provide placental oxygen exchange, are also well understood. However the characteristics of the three embryonic hemoglobins, which provide oxygen transport during the first three months of gestation, are not well recognized. This review seeks to describe the state of our understanding of the temporal control of the expression of these proteins and the oxygen binding characteristics of the individual protein molecules. The modulation of the oxygen binding properties of these proteins, by the various allosteric effectors, is described and the structural origins of these characteristics are probed.

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

OBJECTIVE

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

DESIGN

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

SETTING

University medical center hospital.

PARTICIPANTS

Eighteen patients having surgery under general anesthesia.

INTERVENTIONS

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

MEASUREMENTS AND MAIN RESULTS

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

CONCLUSION

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

A yeast model for classical juvenile Batten disease (CLN3)

The gene involved in the classical juvenile form of Batten disease, CLN3 has been identified as being highly homologous to the Saccharomyces cerevisiae YHC3 gene. To provide a simple model for the biochemical events underlying this disease, several disruptions have been made in YHC3 in three different S. cerevisiae strains. No obvious growth differences were observed, and neither was the previously reported phenotypic difference between wild-type and yeast disrupted in YHC3.

Expression and properties of recombinant HbA2 (alpha2delta2) and hybrids containing delta-beta sequences

Hemoglobin A2 (alpha2delta2), which is present at low concentration (1-2%) in the circulating red cells of normal individuals, has two important features that merit its study, i.e., it inhibits polymerization of sickle HbS and its elevated concentration in some thalassemias is a useful clinical diagnostic. However, reports on its functional properties regarding O2 binding are conflicting. We have attempted to resolve these discrepancies by expressing, for the first time, recombinant hemoglobin A2 and systematically studying its functional properties. The construct expressing HbA2 contains only alpha and delta genes so that the extensive purification required to isolate natural HbA2 is circumvented. Although natural hemoglobin A2 is expressed at low levels in vivo, the amount of recombinant alpha2delta2 expressed in yeast is similar to that found for adult hemoglobin A and for fetal hemoglobin F when the alpha + beta or the alpha + gamma genes, respectively, are present on the construct. Recombinant HbA2 is stable, i.e., not easily oxidized, and it is a cooperative functional hemoglobin with tetramer-dimer dissociation properties like those of adult HbA. However, its intrinsic oxygen affinity and response to the allosteric regulators chloride and 2,3-diphosphoglycerate are lower than the corresponding properties for adult hemoglobin. Molecular modeling studies which attempt to understand these properties of HbA2 are described.

Haemoglobin-based oxygen carriers

Haemoglobin-based oxygen carriers are being developed for use in blood replacement therapies, either for perioperative haemodilution or for resuscitation from haemorrhagic blood loss. There is a high demand for these products because of risks associated with blood transfusions and pending worldwide blood shortages. Development of these products has required new technologies in protein engineering; since the haemoglobin is cell-free in solution, the molecule must be modified to be retained within blood circulation. Three classes of haemoglobin are under development: intramolecular cross-linked, intermolecular polymerised and surface conjugated with polyethylene glycol. Two products based on cross-linking chemistry have been discontinued because of serious adverse events and/or increased mortality rate in Phase III clinical trials. Three products based on polymerisation chemistry are in ongoing Phase III clinical trials. A new product based on surface conjugation is in preclinical evaluation. Although cross-linked and polymerised products have shown to be safe in preclinical and early Phase I/II clinical trials, they have had difficulty in proving efficacy. The primary adverse effect for the majority of cross-linked or polymerised products is a haemodynamic response, leading to increased vascular resistance to blood flow. The physiological mechanisms are still incompletely understood, so that safety and efficacy cannot be completely dissociated. New understandings on the mode of action of these products will help to define their utility and application. New products are under development, designed specifically to maximise blood flow and tissue perfusion and therefore, oxygenation.

Crystal structure of a human embryonic haemoglobin: the carbonmonoxy form of gower II (alpha2 epsilon2) haemoglobin at 2.9 A resolution

The production of recombinant embryonic haemoglobins via a yeast expression system has enabled structural and functional studies to be conducted on these proteins. As part of a programme aimed at understanding the properties of the embryonic haemoglobins we have crystallized the human alpha2 epsilon2 (Gower II) embryonic haemoglobin in its carbonmonoxy form, and determined its structure by X-ray crystallography. The structure was solved by molecular replacement and refined at 2.9 A to give a final model with R-factor=0.185 and Rfree=0.235. The Gower II hemoglobin tetramer is intermediate between the adult R and R2 states, though closer to R2. The tertiary structure of the conserved alpha subunit is essentially identical when compared to that found in the adult (alpha2 beta2) and fetal (alpha2 gamma2) hemoglobins. The embryonic epsilon subunit has a structure very similar to that of the homologous adult beta and fetal gamma subunits, although with small differences at the N terminus and in the A helix. Amino acid substitutions can be identified that may play a role in the altered response of the Gower II haemoglobin to allosteric effectors, in particular chloride ions. The reduced chloride effect is thought to be the primary cause of the higher affinity of this embryonic hemoglobin in comparison to the adult molecule.

Hemoglobin based oxygen carriers: how much methemoglobin is too much?

The oxidized form of hemoglobin, methemoglobin, is unable to deliver oxygen to tissues. Hemoglobin based oxygen carriers generally lack the natural oxidative-reductive machinery present within the red blood cell that converts methemoglobin to hemoglobin. This study examines tolerable levels of methemoglobin that can be present in solutions of polyethylene glycol (PEG) conjugated bovine hemoglobin without compromising its ability to deliver oxygen. Rodents were exchange-transfused to 30% of their estimated blood volume with solutions of six grams percent PEG-hemoglobin containing varying concentrations of PEG-methemoglobin. Tissue oxygenation was measured by an oxygen dependant phosphorescence quenching method. This study also looked at the level of methemoglobin formation following a top loaded infusion of low methemoglobin containing PEG-hemoglobin. Results of the oxygenation study showed that PEG-methemoglobin levels at or below 10% did not significantly alter the ability of solutions to deliver oxygen to intestines, liver, spleen and kidney. However, PEG-methemoglobin levels greater than 10% resulted in a significant decrease in PEG-hemoglobin's ability to oxygenate tissues. In addition, methemoglobin levels remain low (< 10%) for a substantial period of time following PEG-hemoglobin administration.

A hemoglobin-based blood substitute: transplanting a novel allosteric effect of crocodile Hb

Recombinant DNA technology has enabled the large scale production of human hemoglobin in bacteria and yeast. This has opened up a way to produce a hemoglobin-based blood substitute which could replace conventional blood transfusion in some situations. Using our understanding of the structure-function relationships and evolutionary history of hemoglobin it has been possible to improve the oxygen transport properties of the molecule and solve a number of problems associated with the use of natural hemoglobin as a cell-free blood substitute.

Resuscitation with increasing doses of diaspirin crosslinked hemoglobin in swine

This study examined the effects of administering 0.5, 4, 10, and 30 mL/kg of Diaspirin Crosslinked Hemoglobin (DCLHb) in a swine model of non-lethal hemorrhagic shock. Thirty unanesthetized animals were bled (30 mL/kg, 1 mL/kg/min) and either recovered without treatment (Untreated Control, UC) or infused with 10 g/dL DCLHb (0.5, 4.0, 10 or 30 mL/kg at 1 mL/kg/min) or Lactated Ringer (LR, 90 mL/kg at 3 mL/kg/min). DCLHb caused dose-related increases in MAP. Both the 10 and 30 mL/kg doses of DCLHb increased MAP more than UC or LR. Lower doses of DCLHb and LR had effects on MAP similar to UC. After hemorrhage, CO increased in all groups. The effect of DCLHb on CO was dose-related. Only LR and 30 mL/kg of DCLHb transiently (through 90 min) increased CO more than UC. CO in animals given lower doses of DCLHb was comparable to UC. DCLHb (10 and 30 mL/kg) improved base excess and lactate concentrations, two indices of global perfusion, more rapidly and to a greater extent than either UC or LR. In this swine model of hemorrhage, even small doses of DCLHb exerted measurable beneficial effects on blood pressure and perfusion.

Synthesis of human globin polypeptides mediated by recombinant adeno-associated virus vectors

Adeno-associated virus, serotype 2 (AAV2)-based chimeric plasmids that harbored a near-full-length human alpha- or beta-globin cDNA were constructed. The cDNAs were spliced into an AAV plasmid, pAAV delta K, downstream from the viral P40 promoter, substituting the capsid gene region. The correctness of the insertion with regard to the transcription polarity was ascertained by both restriction enzyme analysis and DNA sequencing. One of the constructs, pAAVcHBBLCR, contained the erythroid-specific enhancer elements, the locus control region, HS1 and HS2, to ensure an efficient and tissue-specific gene expression. Use of a defective complementing helper, pAVXB (Dixit, M.; et al. Gene 1991, 104, 253-257.) and adenovirus 2 made it possible to prepare recombinant AAVs (rAAVs). Infection of human 293 cells (embryonal kidney cell line) with the resultant rAAV (AAVcHBB) and cotransfection of mouse erythroleukemia (MEL) cells with the beta-globin construct (pAAVcHBBLCR) and an alpha-globin construct (pAAVcHAB) triggered efficient synthesis of human globin polypeptides in the cells, as analyzed by biochemical and immunohistochemical means. The LCR made the construct respond to an inducer, N,N-hexamethylenebisacetamide, the amount of expressed human beta-globin reaching a similar level as the endogenous mouse beta-globin in MEL cells. Electrotransfection of mouse bone marrow hematopoietic stem/progenitor cells with the constructs dramatically increased the number of benzidine-positive cells in liquid suspension culture, indicating expression and synthesis of a human hemoglobin in these cells. Thus, the rAAV constructs may be useful for gene therapy of hemoglobinopathies.

Some effects of post-translational N-terminal acetylation of the human embryonic zeta globin protein

Using site-directed mutagenesis we have produced the first mutant form of a human embryonic haemoglobin. We have mutated the N-terminal Ser residue of the zeta-chain of haemoglobin Portland, zeta 2 gamma 2, (which is normally acetylated) to a Val (which possesses a free amine terminus). The protein spontaneously assembles into a fully functional tetramer which shows cooperative oxygen binding. Determination of the reactivity of the mutant protein with 2,3-diphosphoglycerate indicates that the mutation process does not lead to any major disruption of the protein structure. A comparison of the properties of the mutant and wild-type proteins identifies a significant role for the normal N-terminal acetylation of the zeta-chain with regard to the alkaline Bohr effect and the sensitivity of the oxygen affinity of the protein towards chloride ions. The possible physiological significance of this modification is discussed.

Mutational analysis of phenylalanine beta 85 in the valine beta 6 acceptor pocket during hemoglobin S polymerization

Hemoglobin (Hb) S containing Leu, Ala, Thr, or Trp substitutions at beta 85 were made and expressed in yeast in an effort to evaluate the role of Phe-beta 85 in the acceptor pocket during polymerization of deoxy Hb S. The four Hb S variants have the same electrophoretic mobility as Hb S, and these beta 85 substitutions do not significantly affect heme-globin interactions and tetramer helix content. Hb S containing Trp-beta 85 had decreased oxygen affinity, whereas those with Leu-, Ala-, and Thr-beta 85 had increased oxygen affinity. All four supersaturated beta 85 variants polymerized with a delay time as does deoxy Hb S. This is in contrast to deoxy Hb S containing Phe-beta 88, Ala-beta 88, Glu-beta 88, or Glu-beta 85, which polymerized with no clear delay time (Adachi K, Konitzer P, Paulraj CG, Surrey S, 1994, J Biol Chem 269:17477-17480; Adachi K, Reddy LR, Surrey S, 1994, J Biol Chem 269:31563-31566). Leu substitution at beta 85 accelerated deoxy Hb S polymerization, whereas Ala, Thr, or Trp substitution inhibited polymerization. The length of the delay time and total polymer formed for these beta 85 Hb S variants depended on hemoglobin concentration in the same fashion as for deoxy Hb S: the higher the concentration, the shorter the delay time and the more polymer formed. Critical concentrations required for polymerization of deoxy Hb SF veta 85L, Hb SF beta 85A, Hb SF beta 85T, and Hb SF beta 85W are 0.65-, 2.2-, 2.5- and 3-fold higher, respectively, than Hb S.(ABSTRACT TRUNCATED AT 250 WORDS)

Allosteric modulation of oxygen binding to the three human embryonic haemoglobins

Plasmid based yeast expression systems have been developed for the high-level expression of the three human embryonic haemoglobins Gower I (zeta 2 epsilon 2), Gower II (alpha 2 epsilon 2) and Portland (zeta 2 gamma 2). Physiochemical characterization of the three product haemoglobins show them to be in the 'native' state. Oxygen-binding studies show that, under what are usually considered physiological conditions, each of the embryonic haemoglobins shows a high oxygen affinity, coupled to a high degree of co-operativity. Allosteric modulation of the oxygen-binding properties of the three haemoglobins in response to organic phosphates and protons has been investigated. The various responses exhibited by the three haemoglobins are rationalized in terms of their amino acid sequences.

A recombinant human hemoglobin with asparagine-102(beta) substituted by alanine has a limiting low oxygen affinity, reduced marginally by chloride

A recombinant (r) mutant hemoglobin (Hb) with Asn-102(beta) replaced by an Ala (N102A(beta)) has been prepared by PCR amplification of a mutagenic DNA fragment and expression of the recombinant protein in yeast. The side chain of Asn-102(beta) is part of an important region of the alpha 1 beta 2 interface that undergoes large structural changes in the transition between the deoxy and oxy conformations. Three natural mutant Hbs with neutral substitutions of Thr, Ser, or Tyr at this site have low oxygen affinities because a hydrogen bond between Asn-102(beta) and Asp-94(alpha) in normal HbA was considered to be absent in these mutants, thereby destabilizing the oxy conformation in favor of the deoxy conformation. This proposal has been tested by expression of an rHb containing alanine at position 102(beta); alanine was chosen because its methyl side chain cannot participate in hydrogen bond formation, yet it is small enough not to disrupt the subunit interface. The nature of the desired replacement was established by sequencing the entire mutated beta-globin gene as well as the tryptic peptide containing the substitution. Further characterization by SDS-PAGE, isoelectric focusing, HPLC analysis, mass spectrometry, amino acid analysis, and sequencing of the mutant tryptic peptide confirmed the purity of the rHb. Its oxygen binding curve (2.4 mM in heme) in the absence of chloride showed that it had a very low oxygen affinity with a P50 of 42 mm Hg.(ABSTRACT TRUNCATED AT 250 WORDS)

[Expression of recombinant human hemoglobin in plants]

Human utilization of recombinant proteins of therapeutical interest, as hemoglobin, implies that the transgenic host allows a low cost production of the active proteins with minimal risks of pathogen contamination. In this regard, the use of transgenic plants could be of great interest. In particular, the systems based on plants could be one of the most economical transgenic system, compared with the others, because biomass obtention in fields is not expensive.

[Recombinant human hemoglobin with low oxygen affinity: additional effects of two mutations]

The search for human Hb variants exhibiting a low oxygen affinity without requiring 2,3-diphosphoglycerate, together with a low oxygation rate, is of an increased interest in the view of producing an artificial oxygen carrier. We have synthesized the recombinant Hb beta 41Phe-->Tyr (rHb beta F41Y) which exhibits a low oxygen affinity due to the stabilization of the deoxy state of tetrameric Hb [1]. Interestingly, the autooxydation rate for this mutant is similar to that for Hb A. We have associated the mutation beta F41Y with the naturally occurring beta 82Lys-->Asp substitution (Hb Providence) known to be responsible for a low oxygen affinity [2]. The second-site mutation further decreases the oxygen affinity of the rHb beta F41Y. The effects of the beta F41Y and K82D mutations are additive, resulting in a four fold decrease in oxygen affinity of the artificial mutant Hb beta F41Y-K82D, compared to Hb A. In spite of the marked decrease in oxygen affinity, the autooxydation rate is 2- to 3 fold larger than that of Hb A. These data show that it is possible to adjust the oxygen binding properties of human Hb by using protein engineering methods. Because of the low oxygen affinity coexisting with a moderately increased autooxydation rate, this variant is a good candidate for the development of a Hb-based oxygen carrier.

The role of hemoglobin based blood substitutes in transfusion medicine

A cell-free oxygen transporting blood substitute would obviate many of the current concerns about conventional red cell transfusion therapy. Moreover, a stable oxygen-carrying solution could have benefits and applications not possible with red cell transfusions, such as the treatment of acute hypovolemic shock in acute care settings, the treatment of patients such as Jehovah's Witnesses who refuse blood transfusions, the priming of blood oxygenation pumps, ex vivo organ perfusion prior to transplantation, and in vivo perfusion in order to enhance sensitivity to radiation therapy. Among potential blood substitutes that transport oxygen, attention has focused on perfluorocarbons and a variety of hemoglobin preparations, either in free solution or encapsulated into lipid vesicles. In the design and production of hemoglobin solutions the following criteria must be met: low toxicity and antigenicity; efficacy as a plasma expander; prolonged survival in the circulation; adequate oxygen carrying capability and efficient oxygen unloading to tissues; long shelf life. Extensive preclinical testing and recent clinical trials have been performed on human and bovine hemoglobin chemically crosslinked to present rapid leakage of hemoglobin through the kidneys. Bovine hemoglobin has intrinsically low oxygen affinity simulating that of human hemoglobin in red cells. An alternative and attractive strategy is the production of human hemoglobin in E. Coli, thus enabling appropriate genetic mutations to optimize function. These include creation of peptide linkers to enhance plasma survival and amino acid replacements that permit a finely regulated lowering of oxygen affinity.

Properties of a recombinant human hemoglobin with aspartic acid 99(beta), an important intersubunit contact site, substituted by lysine

Site-directed mutagenesis of an important subunit contact site, Asp-99(beta), by a Lys residue (D99K(beta)) was proven by sequencing the entire beta-globin gene and the mutant tryptic peptide. Oxygen equilibrium curves of the mutant hemoglobin (Hb) (2-15 mM in heme) indicated that it had an increased oxygen affinity and a lowered but significant amount of cooperativity compared to native HbA. However, in contrast to normal HbA, oxygen binding of the recombinant mutant Hb was only marginally affected by the allosteric regulators 2,3-diphosphoglycerate or inositol hexaphosphate and was not at all responsive to chloride. The efficiency of oxygen binding by HbA in the presence of allosteric regulators was limited by the mutant Hb. At concentrations of 0.2 mM or lower in heme, the mutant D99K(beta) Hb was predominantly a dimer as demonstrated by gel filtration, haptoglobin binding, fluorescence quenching, and light scattering. The purified dimeric recombinant Hb mutant exists in 2 forms that are separable on isoelectric focusing by about 0.1 pH unit, in contrast to tetrameric hemoglobin, which shows 1 band. These mutant forms, which were present in a ratio of 60:40, had the same masses for their heme and globin moieties as determined by mass spectrometry. The elution positions of the alpha- and beta-globin subunits on HPLC were identical. Circular dichroism studies showed that one form of the mutant Hb had a negative ellipticity at 410 nm and the other had positive ellipticity at this wavelength. The findings suggest that the 2 D99K(beta) recombinant mutant forms have differences in their heme-protein environments.

Expression of the outer capsid proteins VP2 and VP5 of bluetongue virus in Saccharomyces cerevisiae

cDNAs transcribed from bluetongue virus serotype 1 (Australia) ds RNA 2 and ds RNA 6 coding for the major neutralising antigen VP2 and the outer capsid protein VP5, respectively, were amplified in polymerase chain reactions and ligated downstream of the copper-inducible metallothionein promoter in the yeast expression plasmid pYELC5. Saccharomyces cerevisiae transformed with the recombinant plasmid pYELC5-VP2 expressed full-length VP2 only following induction with 1 mM CuSO4 and reached the maximum level after 6 h. In contrast, S. cerevisiae transformants harboring the recombinant plasmid pYELC5-VP5 expressed VP5 constitutively, although induction increased the level to a maximum after 4 h. A sheep trial was done testing the recombinant proteins, however it was shown that none of these were effective immunogens for eliciting a protective response against a subsequent challenge with bluetongue virus. An analysis of the yeast expression products for the VP2 outer coat protein using a panel of monoclonal antibodies showed that the yeast expressed VP2 was in a conformation different from native VP2 and hence probably unable to elicite an appropriate protective immune response.

Production of human embryonic haemoglobin (Gower II) in a yeast expression system

The cDNA coding for a human embryonic globin protein has been obtained from an erythroleukaemic cell line. A plasmid expression system for human embryonic haemoglobin Gower II containing cDNA copies of the appropriate pair of globin genes coupled to synthetic galactose-regulated hybrid promoters has been engineered. Transformation of Saccharomyces cerevisiae with this plasmid yields a cellular system capable of high-level production of fully functional tetrameric embryonic haemoglobin. We have developed a purification scheme which gives high yields of pure human embryonic haemoglobin suitable for structural and functional studies. Preliminary characterization studies are reported.

Recombinant hemoglobin A produced in transgenic swine: structural equivalence with human hemoglobin A

Recombinant human hemoglobin A produced by coexpressing human alpha and beta globin genes in swine, and purified from the lysate of transgenic swine has been subjected to detailed protein chemical analysis. These structural studies involving laser desorption mass spectrometry, separation of globin chains by RPHPLC, amino terminal sequence analysis of the isolated globin chains, the tryptic peptide mapping of the purified globin chains and the amino acid composition analysis of the purified tryptic peptides of globin chains have established the primary structural equivalence of the globin chains of the transgenic swine derived hemoglobin A with that of human hemoglobin A. These results demonstrate that the transgenic swine system correctly translates the human alpha and beta globin m-RNA; carries out the correct cotranslational processing of globin chains, and does not introduce any unwanted post translational modifications into the mature chains. Thus, the transgenic swine expression system is an excellent approach for the production of HbA for developing an effective hemoglobin based oxygen carrier.