Electrostatic attraction governs the dimer assembly of human hemoglobin

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.

Site-Specific Introduction of Negative Charges on the Protein Surface for Improving Global Functions of Recombinant Fetal Hemoglobin

Due to its compatible oxygen-transporting abilities, hemoglobin (Hb) is a protein of interest in the development of artificial oxygen therapeutics. Despite continuous formulation attempts, extracellular Hb solution often exhibits undesirable reactions when applied in vivo. Therefore, protein engineering is frequently used to examine alternative ways of controlling the unwanted reactions linked to cell-free Hb solutions. In this study, three mutants of human fetal hemoglobin (HbF) are evaluated; single mutants αA12D and αA19D, and a double mutant αA12D/A19D. These variants were obtained by site-directed mutagenesis and recombinant production in E. coli, and carry negative charges on the surface of the α-subunit at the designated mutation sites. Through characterization of the mutant proteins, we found that the substitutions affected the protein in several ways. As expected, the isoelectric points (pIs) were lowered, from 7.1 (wild-type) down to 6.6 (double mutant), which influenced the anion exchange chromatographic procedures by shifting conditions toward higher conductivity for protein elution. The biological and physiological properties of HbF could be improved by these small modifications on the protein surface. The DNA cleavage rate associated with native HbF could be reduced by 55%. In addition, the negatively charged HbF mutant had an extended circulation time when examined in a mouse model using top load Hb additions. At the same time, the mutations did not affect the overall structural integrity of the HbF molecule, as determined by small-angle X-ray scattering. In combination with circular dichroism and thermal stability, modest structural shifts imposed by the mutations could possibly be related to changes in secondary structure or reorganization. Such local deformations were too minor to be determined within the resolution of the structural data; and overall, unchanged oxidation and heme loss kinetics support the conclusion that the mutations did not adversely affect the basic structural properties of Hb. We confirm the value of adding negatively charged residues onto the surface of the protein to improve the global functions of recombinant Hb.

The hemoglobin Gly16β1Asp polymorphism in turbot (Scophthalmus maximus) is differentially distributed across European populations

Turbot is an important flatfish widely distributed along the European coasts, whose fishery is centered in the North Sea. The commercial value of the species has boosted a successful aquaculture sector in Europe and China. Body growth is the main target of turbot breeding programs and is also a key trait related to local adaptation to temperature and salinity. Differences in growth rate and optimal growth temperature in turbot have been shown to be associated with a hemoglobin polymorphism reported more than 50 years ago. Here, we identified a Gly16Asp amino acid substitution in the β1 globin subunit by searching for genetic variation in the five functional globin genes within the whole annotated turbot genome. We predicted increased stability of the turbot hemoglobin by the replacement of the conserved Gly with the negative charged Asp residue that is consistent with the higher rate of αβ dimer assembly in the human J-Baltimore Gly16β->Asp mutant than in normal HbA. The turbot Hbβ1-Gly16 variant dominated in the northern populations examined, particularly in the Baltic Sea, while the Asp allele showed elevated frequencies in southern populations and was the prevalent variant in the Adriatic Sea. Body weight did not associate with the Hbβ1 genotypes at farming conditions (i.e., high oxygen levels, feeding ad libitum) after analyzing 90 fish with high growth dispersal from nine turbot families. Nevertheless, all data at hand suggest that the turbot hemoglobin polymorphism has an adaptive significance in the variable wild conditions regarding temperature and oxygen availability.

Gene turnover in the avian globin gene families and evolutionary changes in hemoglobin isoform expression

The apparent stasis in the evolution of avian chromosomes suggests that birds may have experienced relatively low rates of gene gain and loss in multigene families. To investigate this possibility and to explore the phenotypic consequences of variation in gene copy number, we examined evolutionary changes in the families of genes that encode the α- and β-type subunits of hemoglobin (Hb), the tetrameric α2β2 protein responsible for blood-O2 transport. A comparative genomic analysis of 52 bird species revealed that the size and membership composition of the α- and β-globin gene families have remained remarkably constant during approximately 100 My of avian evolution. Most interspecific variation in gene content is attributable to multiple independent inactivations of the α(D)-globin gene, which encodes the α-chain subunit of a functionally distinct Hb isoform (HbD) that is expressed in both embryonic and definitive erythrocytes. Due to consistent differences in O2-binding properties between HbD and the major adult-expressed Hb isoform, HbA (which incorporates products of the α(A)-globin gene), recurrent losses of α(D)-globin contribute to among-species variation in blood-O2 affinity. Analysis of HbA/HbD expression levels in the red blood cells of 122 bird species revealed high variability among lineages and strong phylogenetic signal. In comparison with the homologous gene clusters in mammals, the low retention rate for lineage-specific gene duplicates in the avian globin gene clusters suggests that the developmental regulation of Hb synthesis in birds may be more highly conserved, with orthologous genes having similar stage-specific expression profiles and similar functional properties in disparate taxa.

The structure of α-haemoglobin in complex with a haemoglobin-binding domain from Staphylococcus aureus reveals the elusive α-haemoglobin dimerization interface

Adult haemoglobin (Hb) is made up of two α and two β subunits. Mutations that reduce expression of the α- or β-globin genes lead to the conditions α- or β-thalassaemia, respectively. Whilst both conditions are characterized by anaemia of variable severity, other details of their pathophysiology are different, in part owing to the greater stability of the β chains that is conferred through β self-association. In contrast, α subunits interact weakly, and in the absence of stabilizing quaternary interactions the α chain (α) is prone to haem loss and denaturation. The molecular contacts that confer weak self-association of α have not been determined previously. Here, the first structure of an α2 homodimer is reported in complex with one domain of the Hb receptor from Staphylococcus aureus. The α2 dimer interface has a highly unusual, approximately linear, arrangement of four His side chains within hydrogen-bonding distance of each other. Some interactions present in the α1β1 dimer interface of native Hb are preserved in the α2 dimer. However, a marked asymmetry is observed in the α2 interface, suggesting that steric factors limit the number of stabilizing interactions that can form simultaneously across the interface.

Kinetics of α-globin binding to α-hemoglobin stabilizing protein (AHSP) indicate preferential stabilization of hemichrome folding intermediate

Human α-hemoglobin stabilizing protein (AHSP) is a conserved mammalian erythroid protein that facilitates the production of Hemoglobin A by stabilizing free α-globin. AHSP rapidly binds to ferrous α with association (k'(AHSP)) and dissociation (k(AHSP)) rate constants of ≈10 μm(-1) s(-1) and 0.2 s(-1), respectively, at pH 7.4 at 22 °C. A small slow phase was observed when AHSP binds to excess ferrous αCO. This slow phase appears to be due to cis to trans prolyl isomerization of the Asp(29)-Pro(30) peptide bond in wild-type AHSP because it was absent when αCO was mixed with P30A and P30W AHSP, which are fixed in the trans conformation. This slow phase was also absent when met(Fe(3+))-α reacted with wild-type AHSP, suggesting that met-α is capable of rapidly binding to either Pro(30) conformer. Both wild-type and Pro(30)-substituted AHSPs drive the formation of a met-α hemichrome conformation following binding to either met- or oxy(Fe(2+))-α. The dissociation rate of the met-α·AHSP complex (k(AHSP) ≈ 0.002 s(-1)) is ∼100-fold slower than that for ferrous α·AHSP complexes, resulting in a much higher affinity of AHSP for met-α. Thus, in vivo, AHSP acts as a molecular chaperone by rapidly binding and stabilizing met-α hemichrome folding intermediates. The low rate of met-α dissociation also allows AHSP to have a quality control function by kinetically trapping ferric α and preventing its incorporation into less stable mixed valence Hemoglobin A tetramers. Reduction of AHSP-bound met-α allows more rapid release to β subunits to form stable fully, reduced hemoglobin dimers and tetramers.

The role of alpha-hemoglobin stabilizing protein in redox chemistry, denaturation, and hemoglobin assembly

Hemoglobin biosynthesis in erythrocyte precursors involves several steps. The correct ratios and concentrations of normal alpha (alpha) and beta (beta) globin proteins must be expressed; apoproteins must be folded correctly; heme must be synthesized and incorporated into these globins rapidly; and the individual alpha and beta subunits must be rapidly and correctly assembled into heterotetramers. These events occur on a large scale in vivo, and dysregulation causes serious clinical disorders such as thalassemia syndromes. Recent work has implicated a conserved erythroid protein known as Alpha-Hemoglobin Stabilizing Protein (AHSP) as a participant in these events. Current evidence suggests that AHSP enhances alpha subunit stability and diminishes its participation in harmful redox chemistry. There is also evidence that AHSP facilitates one or more early-stage post-translational hemoglobin biosynthetic events. In this review, recent experimental results are discussed in light of several current models describing globin subunit folding, heme uptake, assembly, and denaturation during hemoglobin synthesis. Particular attention is devoted to molecular interactions with AHSP that relate to alpha chain oxidation and the ability of alpha chains to associate with partner beta chains.

Hb Hope [β136(H14)Gly→Asp (GGT→GAT)]: Interactions with Hb S [β6(A3)Glu→Val (GAG→GTG)], Other Variant Hemoglobins and Thalassemia

Hb Hope [beta136(H14)Gly-->Asp (GGT-->GAT)] was first described in an African-American family in 1965. Since then, it has been found in combination with several different globin gene mutations in many other families of divergent ethnic backgrounds. The basis for its relatively frequent occurrences remains unexplained. This variant hemoglobin (Hb) is mildly unstable and has reduced oxygen affinity, but is generally innocuous clinically. This variant Hb can present as a confounding factor in arriving at a correct diagnosis by either electrophoresis or high performance liquid chromatography (HPLC), particularly during the neonatal period. DNA-based diagnostics can help solve this potential problem.

Haematological and clinical features of beta-thalassaemia associated with Hb Dhofar

Hb Dhofar is a variant haemoglobin (beta(29 (GGC-GGT) gly-gly), beta(58 (CCT-CGT) pro-arg)) associated with a thalassaemic phenotype and unique to the Sultanate of Oman. We report clinical and haematological data on 54 subjects with Hb Dhofar (37 heterozygotes, 14 homozygotes and three compound heterozygotes with a different beta-thalassaemia mutation). In heterozygotes, the level of Hb Dhofar ranged from 8.8% to 21.5%. All heterozygotes had Hb A2 > 3.5%, consistent with beta-thalassaemia trait. Hb Dhofar in homozygotes and compound heterozygotes ranged from 26% to 59.7%, with a peripheral film consistent with homozygous beta-thalassaemia. Age at presentation in homozygotes ranged between 6 months and 8 yr, with a majority presenting before 5 yr of age. All had splenomegaly and six (43%) had undergone splenectomy. All had some degree of frontal bossing and in particular, two patients with infrequent transfusions had marked thalassaemic facies and stunting of growth. Hb Dhofar can be mistaken for Hb D as the electrophoretic mobility is similar, but differs from it by a variable and reduced quantity of variant Hb in both heterozygotes and homozygotes. Clinical and haematological data suggest that this mutation behaves like a moderately severe beta(+) thalassaemia allele resulting in a thalassaemia intermedia phenotype.

Assembly of recently translated full-length and C-terminal truncated human gamma-globin chains with a pool of alpha-globin chains to form Hb F in a cell-free system

Assembly of alpha-globin with translated, full-length and C-terminal truncated human gamma-globin to form Hb F was assessed in a cell-free transcription/translation system. Polysome profiles showed two amino acid C-terminal-truncated gamma-chains retained on polysomes can assemble with unlabeled holo alpha-chains only after puromycin-induced chain release. Two amino acid C-terminal truncated gamma-chains encoded from vectors containing a stop codon at the translation termination site were released from polysomes and assembled with alpha-chains in the absence of puromycin addition, while removal of 11 or more amino acids from the gamma-chain carboxy-terminus inhibited assembly with alpha-chains. These results suggest that amino acids in the HC- and H-helix gamma-chain regions including amino acids 135-144 at the C-terminus in the translated gamma-chains play a key role in assembly with alpha-chains, and that assembly occurs soon after exit of translated gamma-chains from the ribosome tunnel and release from polysomes thereby preventing stable gamma(2) homo-dimer formation.

Folding and assembly of hemoglobin monitored by electrospray mass spectrometry using an on-line dialysis system

The native structure of hemoglobin (Hb) comprises two alpha- and two beta-subunits, each of which carries a heme group. There appear to be no previous studies that report the in vitro folding and assembly of Hb from highly unfolded alpha- and beta-globin in a "one-pot" reaction. One difficulty that has to be overcome for studies of this kind is the tendency of Hb to aggregate during refolding. This work demonstrates that denaturation of Hb in 40% acetonitrile at pH 10.0 is reversible. A dialysis-mediated solvent change to a purely aqueous environment of pH 8.0 results in Hb refolding without any apparent aggregation. Fluorescence, Soret absorption, circular dichroism, and ESI mass spectra of the protein recorded before unfolding and after refolding are almost identical. By employing an externally pressurized dialysis cell that is coupled on-line to an ESI mass spectrometer, changes in heme binding behavior, protein conformation, and quaternary structure can be monitored as a function of time. The process occurs in a stepwise sequential manner, leading from monomeric alpha- and beta-globin to heterodimeric species, which then assemble into tetramers. Overall, this mechanism is consistent with previous studies employing the mixing of folded alpha- and beta-globin. However, some unexpected features are observed, e.g., a heme-deficient beta-globin dimer that represents an off-pathway intermediate. Monomeric beta-globin is capable of binding heme before forming a complex with an alpha-subunit. This observation suggests that holo-alpha-apo-beta globin does not represent an obligatory intermediate during Hb assembly, as had been proposed previously. The on-line dialysis/ESI-MS approach developed for this work represents a widely applicable tool for studying the folding and self-assembly of noncovalent biological complexes.

Spectrin's E2/E3 ubiquitin conjugating/ligating activity is diminished in sickle cells

Erythrocyte spectrin contains E2/E3 ubiquitin conjugating/ligating activity in its alpha subunit. Ankyrin is a target of spectrin's E2/E3 ubiquitin conjugating/ligating activity in vitro and in vivo. We compare the ubiquitination levels of ankyrin mediated by control and sickle cell spectrin using a biotinylated ubiquitin cell-free assay. Sickle cell spectrin has diminished ability to transfer ubiquitin from an intermediate spectrin-ubiquitin thioester adduct (alpha' spectrin) to ankyrin, which may be due to glutathiolation of spectrin's E2 and/or E3 active site cysteines. There is also a diminished ability of the sickle cell ankyrin to serve as target of spectrin's E2/E3 activity, probably due to oxidative damage to ankyrin. A direct correlation exists between the alpha'/alpha spectrin ratio and spectrin's ability to ubiquitinate ankyrin. There is also an inverse correlation between severity of the disease and the alpha'/alpha spectrin ratio in SS erythrocytes. These results suggest that reduced spectrin E2/E3 activity is an important determinant of sickle cell severity.

A recombinant human hemoglobin with anti-sickling properties greater than fetal hemoglobin

A new recombinant, human anti-sickling beta-globin polypeptide designated beta(AS3) (betaGly(16) --> Asp/betaGlu(22) --> Ala/betaThr(87) --> Gln) was designed to increase affinity for alpha-globin. The amino acid substitutions at beta22 and beta87 are located at axial and lateral contacts of the sickle hemoglobin (HbS) polymers and strongly inhibit deoxy-HbS polymerization. The beta16 substitution confers the recombinant beta-globin subunit (beta(AS3)) with a competitive advantage over beta(S) for interaction with the alpha-globin polypeptide. Transgenic mouse lines that synthesize high levels of HbAS3 (alpha(2)beta(AS3)(2)) were established, and recombinant HbAS3 was purified from hemolysates and then characterized. HbAS3 binds oxygen cooperatively and has an oxygen affinity that is comparable with fetal hemoglobin. Delay time experiments demonstrate that HbAS3 is a potent inhibitor of HbS polymerization. Subunit competition studies confirm that beta(AS3) has a distinct advantage over beta(S) for dimerization with alpha-globin. When equal amounts of beta(S)- and beta(AS3)-globin monomers compete for limiting alpha-globin chains up to 82% of the tetramers formed is HbAS3. Knock-out transgenic mice that express exclusively human HbAS3 were produced. When these mice were bred with knock-out transgenic sickle mice the beta(AS3) polypeptides corrected all hematological parameters and organ pathology associated with the disease. Expression of beta(AS3)-globin should effectively lower the concentration of HbS in erythrocytes of patients with sickle cell disease, especially in the 30% percent of these individuals who coinherit alpha-thalassemia. Therefore, constructs expressing the beta(AS3)-globin gene may be suitable for future clinical trials for sickle cell disease.

Assembly of human hemoglobin (Hb) beta- and gamma-globin chains expressed in a cell-free system with alpha-globin chains to form Hb A and Hb F

Rates of in vitro synthesis of radiolabeled gamma and beta chains made in a cell-free transcription/translation system were similar, but expressed globin chains were unstable. The addition of unlabeled beta or gamma chains at the start of chain synthesis generated radiolabeled beta(4) or gamma(2) and gamma(4) chains, respectively. If unlabeled alpha-globin chains were added at the start of chain synthesis, then approximately equal amounts of radiolabeled alphabeta or alphagamma bands were generated. If unlabeled Hb A or Hb F was added to reactions containing radiolabeled alphabeta or alphagamma prior to electrophoresis, then radiolabeled Hb A or Hb F tetramers, respectively, were generated. If alpha chains were added after synthesis of radiolabeled gamma chains made in the presence of unlabeled gamma chains, then little radiolabeled alphagamma formed. In contrast, if alpha chains were added after synthesis of radiolabeled beta chains made in the presence of unlabeled beta chains, then radiolabeled alpha(2)beta(2) formed. These findings suggest that beta and gamma chains associate with alpha chains during or soon after translation. This would prevent the formation of unstable monomers as well as stable gamma(2) dimers and suggests that alpha chains may bind to nascent non-alpha chains, acting as folding catalysts to promote functional tetrameric hemoglobin formation in vivo.

Consequence of beta 16 and beta 112 replacements on the kinetics of hemoglobin assembly

The rates of alpha/beta monomer combination of four beta(A) variants (beta 112C --> S, beta 112C --> D, beta 112C --> T, and beta 112C --> V) in the presence and absence of beta 16G --> D (beta(J)) were measured in an attempt to assess the consequences of amino acid substitution at both a surface (beta 16) and an alpha(1)beta(1) interface (beta 112) residue on oxyhemoglobin assembly. Rates of alpha/beta monomer combination determined spectrally in 0.1 M Tris-HCl, 0.1 M NaCl, 1 mM EDTA, pH 7.4, at 21.5 degrees C differed by over 40-fold (22 +/- 2.0 to 0.49 +/- 0.1 x 10(5) M(-1) s(-1)), and were in the order: HbA beta 112S = HbJ beta 16D, beta 112S > HbA beta 112D = HbJ beta 16D, beta 112D > HbA > Hb J > HbA beta 112T = HbJ beta 16D, beta 112T > HbJ beta 16D, beta 112V > HbA beta 112V. This extensive kinetic investigation of single/double amino acid-substituted recombinant hemoglobin molecules, in conjunction with molecular modeling studies, has allowed examination of an array of unique alpha/beta subunit interactions and assembly processes.

Assembly of gamma- with alpha-globin chains to form human fetal hemoglobin in vitro and in vivo

Soluble gamma-globin chains were expressed in bacteria and purified to assess the mechanism of gamma- and alpha-chain assembly to form Hb F. Formation of Hb F in vitro following incubation of equimolar mixtures of gamma and alpha chains was about 4 x 10(5)-fold slower than assembly of alpha and beta chains to form Hb A in vitro. Results of assembly for gamma(116Ile-->His) and gamma(112Thr-->Asp) chains with alpha chains were similar to that of beta chains, whereas assembly of gamma(112Thr-->Cys) and alpha chains was similar to wild type gamma chains, indicating that amino acid differences at alpha1beta1 and alpha1gamma1 interaction sites between gamma116 Ile and beta116 His are responsible for the different assembly rates in vitro in the formation of Hb F and Hb A. Homoassembly in vitro of individual gamma chains as assessed by size-exclusion chromatography shows that gamma and gamma(112Thr-->Cys) chains form stable dimers like alphabeta and alphagamma that do not dissociate readily into monomers like beta chains. In contrast, gamma(116Ile-->His) chains form monomers and dimers upon dilution. These results are consistent with the slower assembly rate in vitro of gamma and gamma(112Thr-->Cys) with alpha chains, whereas the faster rate of assembly of gamma(116Ile-->His) and gamma(112Thr-->Asp) chains with alpha chains, like beta chains, may be caused by dissociation to monomers. These results suggest that dissociation of gamma(2) dimers to monomers limits formation of Hb F in vitro. However, yields of soluble Hb F expressed in bacteria were similar to Hb A, and no unassembled alpha and gamma chains were detected. These results indicate that gamma chains assemble in vivo with alpha chains prior to forming stable gamma(2) dimers, possibly binding to alpha chains as partially folded nascent gamma-globin chains prior to release from polyribosomes.

Surface and interface beta-chain residues synergistically affect hemoglobin assembly

Homo- and heterotetramer formations of beta112 variants (beta(112Cys-->Asp), beta(112Cys-->Ser), beta(112Cys-->Thr), and beta(112Cys-->Val)) of hemoglobin were characterized in the presence and absence of beta(16Gly-->Asp) in vitro. In all cases an alteration in overall surface charge (beta(16Gly-->Asp)) decreased the beta(4) homotetramer stability (association constants as determined by gel-permeation chromatography) albeit to differing extents. In contrast, competition experiments of hemoglobin subunits showed that heterotetramer formation was promoted by this substitution. Order of increase in tetramer formation by the additional negative surface charge in the beta112 variants was as follows: Hb betaG16D, C112D > Hb betaG16D, C112S > Hb betaG16D > Hb G16D, C112T > Hb betaG16D, C112V. Thus, the overall surface charge of the beta chain and its contribution to electrostatic interaction in these instances appear to act in synergy with alpha(1)beta(1) interface residues to affect the assembly of hemoglobin molecules.

Identification of the Molecular Genetic Defect of Patients With Methemoglobin M Kankakee (M-Iwate), 87 (F8) His → Tyr: Evidence for an Electrostatic Model of M Hemoglobin Assembly

We determined that the molecular defect of 2 patients with hemoglobin (Hb) M-Kankakee [Hb M-Iwate, 87 (F8) His → Tyr] resides in the 1-globin gene. The proportion of Hb M observed is higher than that predicted for an 1-globin variant. Our evidence suggests that the greater-than-expected proportion of Hb M-Kankakee results from preferential association of the electronegative β-globin chains with the M-globin chains that are more electropositive than normal -globin chains.

Identification of the Molecular Genetic Defect of Patients With Methemoglobin M Kankakee (M-Iwate), 87 (F8) His → Tyr: Evidence for an Electrostatic Model of M Hemoglobin Assembly

We determined that the molecular defect of 2 patients with hemoglobin (Hb) M-Kankakee [Hb M-Iwate, 87 (F8) His → Tyr] resides in the 1-globin gene. The proportion of Hb M observed is higher than that predicted for an 1-globin variant. Our evidence suggests that the greater-than-expected proportion of Hb M-Kankakee results from preferential association of the electronegative β-globin chains with the M-globin chains that are more electropositive than normal -globin chains.

Role of beta112 Cys (G14) in homo- (beta4) and hetero- (alpha2 beta2) tetramer hemoglobin formation

In order to assess the role of beta112 Cys in homo- and hetero-tetrameric hemoglobin formation, we expressed four beta112 variants (beta112Cys-->Asp, beta112Cys-->Ser, beta112Cys-->Thr, and beta112Cys-->Val) and studied assembly with alpha chains in vitro. beta112 Cys is normally present at beta1 beta2 and alpha1 beta1 interaction sites in homo- (beta4) and hetero-tetramers (alpha2 beta2). beta4 formation in vitro was influenced by the amino acid at beta112. beta112 Asp completely inhibited formation of homo-tetramers, whereas beta112 Ser showed only slight inhibition. In contrast, beta112 Thr or Val enhanced homo-tetramer formation compared with betaA chains. Association constants for homo-tetramer formation increased in the order of beta112Cys-->Ser, betaA, beta112Cys-->Thr, and beta112Cys-->Val, whereas the value for beta112Cys-->Asp was zero under the same conditions. These beta112 changes also affected in vitro alpha2 beta2 hetero-tetramer formation. Order of alpha2 beta2 formation under limiting alpha-globin chain conditions showed Hb betaC112S > Hb A > Hb S = Hb betaC112T = Hb betaC112V >>> Hb betaC112D. Hb beta112D can form tetrameric hemoglobin, but this beta112 change promotes dissociation into alpha and beta chains instead of alpha beta dimer formation upon dilution. These results indicate that amino acids at alpha1 beta1 interaction sites such as beta112 on the G helix play a key role in stable alpha beta dimer formation. Our findings suggest, in addition to electrostatic interaction between alpha and beta chains, that dissociation of beta4 homo-tetramers to monomers and hydrophobic interactions of the beta112 amino acid with alpha chains governs stable alpha1 beta1 interactions, which then results in formation of functional hemoglobin tetramers. Information gained from these studies should increase our understanding of the mechanism of assembly of multi-subunit proteins.

Electrostatic enhancement of diffusion-controlled protein-protein association: comparison of theory and experiment on barnase and barstar

The electrostatic enhancement of the association rate of barnase and barstar is calculated using a transition-state theory like expression and atomic-detail modeling of the protein molecules. This expression predicts that the rate enhancement is simply the average Boltzmann factor in the region of configurational space where association occurs instantaneously in the diffusion-controlled limit. Based on experimental evidence, this "transition state" is defined by configurations in which, relative to the stereospecifically bound complex, the two proteins are shifted apart by approximately 8 A (so a layer of water can be accommodated in the interface) and the two binding surfaces are rotated away by 0 degrees to 3 degrees. The values of the average Boltzmann factor, calculated by solving the Poisson-Boltzmann equation, for the wild-type complex and 16 complexes with single mutations are found to correlate well with experimental results for the electrostatic rate enhancement. The predicted rate enhancement is found to be somewhat insensitive to the precise definition of the transition state, due to the long-range nature of electrostatic interactions. The experimental ionic strength dependence of the rate enhancement is also reasonably reproduced.

Effects of Increased Anionic Charge in the β-Globin Chain on Assembly of Hemoglobin In Vitro

Studies on assembly in vitro of α-globin chains with recombinant β16 Gly→Asp, β95 Lys→Glu, β120 Lys→Glu and β16 Gly→Asp, 120 Lys→Glu human β-globin chain variants in addition to human βA- and βS-globin chains were performed to evaluate effects of increased anionic charge in the β chain on hemoglobin assembly using soluble recombinant β-globin chains expressed in bacteria. A β112 Cys→Asp change was also engineered to monitor effects on assembly of increased negative charge at α1β1 interaction sites. Order of tetramer formation in vitro under limiting α-globin chain conditions showed Hb βG16D, K120E = Hb βK120E = Hb βK95E > Hb βG16D > Hb A > Hb S >>> Hb βC112D. In addition, β112 Cys→Asp chains exist as monomers rather than β4tetramers in the absence of α chains, and the β chain in Hb βC112D tetramers was readily exchanged by addition of βs. These results suggest that affinity between α and β chains is promoted by negatively-charged β chains up to a maximum of two additional net negative charges and is independent of location on the surface except at the α1β1 interaction site. In addition, our findings show that β112 Cys on the G helix is critical for facilitating formation of stable αβ dimers, which then form functional hemoglobin tetramers, and that β112 Cys→Asp inhibits formation of stable α1β1 and β1β2 interactions in α2β2 and β4 tetramers, respectively.

Combinations of β chain abnormal hemoglobins with each other or with β-thalassemia determinants with known mutations: influence on phenotype

Hematological and hemoglobin (Hb) data are presented for numerous patients with compound heterozygosities for different β chain variants and for a β chain variant with different β-thalassemia (β-thal) alleles. Considerable variations, which result from the type of β chain variant and β-thal mutation, can be noted. The comparison again emphasizes the importance of determining the diagnoses at the molecular level to aid the physician in the management of patients with different combinations of abnormalities. Simplification and commercialization of modern technology may make the introduction of this approach in some clinical chemistry laboratories possible.

Agitoxin footprinting the shaker potassium channel pore

In voltage-dependent K+ channels, each of the four identical subunits contributes one pore loop to the central ion selectivity unit at the interface between the subunits. The pore loop is also the target for scorpion venom peptide inhibitors. These inhibitors bind at the pore entryway between the four subunits and can assume any one of four orientations. The orientations become distinguishable only if the binding site symmetry is disrupted. We have used mutagenesis and site-directed chemical modification to alter pore loop amino acids in either one or four subunits. The effects of these alterations on inhibitor affinity define the eccentricity of amino acids in the pore entryway and imply a different secondary structure for the amino and carboxyl ends of the pore loop.

Monitoring the effect of subunit assembly on the structural flexibility of human alpha apohemoglobin by steady-state fluorescence

A single energy transfer distance, between the sole intrinsic tryptophanyl donor [14(A12)] and a nonfluorescent sulfhydryl acceptor probe (4-phenylazophenylmaleimide, PAPM) attached to the only cysteine [104(G11)], has been employed to examine the effect of subunit assembly on the structure of the heme-free human alpha-hemoglobin. Efficiencies of energy transfer were measured in 0.05 M potassium phosphate buffer, pH 7.0, at 5 degrees C, and the structural flexibility of alpha-apohemoglobin, in the absence and presence of human beta-heme-containing chains, was examined by a steady-state solute quenching technique. The quenched efficiencies (EQ) and Förster distances (R0Q) were analyzed by least-squares to determine the goodness of fit (chi R2) for the assumed distribution parameters: average distance r and half-width hw. Data for alpha-apohemoglobin in the absence and presence of beta h chains yielded values for r of 18 and 22 A and hw of 20 and 8.5 A, respectively. Although the increase in r for alpha-apohemoglobin in the presence of beta h chains was presumably a consequence of additional quenching from the heme moiety, the change in the half-width strongly indicated a decrease in the flexibility of the alpha-apohemoglobin chain within the assembled protein. A transition in structural flexibility similar to that demonstrated here may be an important aspect of human hemoglobin assembly.

Beta-thalassemia intermedia with exceptionally high hemoglobin A2: relationship to mutations in the beta-gene promoter

Small deletions of the 5' portion of the beta-globin gene that remove the promoters but stop 3' to the delta-globin gene are recognized as the sole cause of beta-thalassemia with exceptionally high hemoglobin A2 (HbA2) levels. Two patients with beta-thalassemia intermedia and exceptionally high levels of HbA2 (10.4 and 12.0%) were examined. One patient was a combined heterozygote for the -88 C----T and a novel -87 C----A mutation, while the other was homozygous for the -29 A----G beta(+)-thalassemia mutation. The remainder of the beta genes were normal. There was no evidence for deletions involving the 5' portion of the beta gene or the region between the beta and delta genes. Gene mapping studies excluded the possibility of a beta delta-anti-Lepore hemoglobin gene with beta promoters and delta coding sequences. There were no mutations in the promoters of the G gamma or A gamma-globin genes that have been associated with the hereditary persistence of HbF phenotype. The delta-globin gene promoters were normal from codon 17 to position -145 relative to the mRNA capping site. There appears to be considerable heterogeneity of HbA2 and HbF levels in patients who are homozygous or mixed heterozygotes for mutations in the TATA box and other promoter elements of the beta-globin gene. The capacity for proteolysis within the erythrocyte may vary among individuals. The authors hypothesize that in the exceptionally high HbA2 beta-thalassemia intermedia phenotype, proteolysis of superfluous alpha-globin chains is less efficient than in patients with customary levels of HbA2.(ABSTRACT TRUNCATED AT 250 WORDS)

Visualization of molecular flexibility and its effects on electrostatic recognition

To study the effect of protein flexibility on electrostatic recognition, we have devised two novel computer graphic representations of the changes in the electrostatic field of a protein resulting from its internal motions. The atomic structure of Cu, Zn superoxide dismutase was minimized, and the 200 lowest frequency normal modes of the enzyme were determined. Individual and combined normal-mode vibrations were visualized interactively with the program Flex. Normal-mode motions are fast enough (approximately 10(-11) s cycle-1) to evade solvent damping, thus allowing long-range electrostatic interactions to dominate. The changing electrostatic environment of the protein was examined by animating precalculated frames of electrostatic field vectors with GRAMPS. With Vu, changes in electrostatic potential were displayed as variations in the color-coding of dots lying on a consensus surface that maintains the protein's shape. The consensus surface was calculated with the program Sphinx, and was derived from spherical harmonic approximations of expanded molecular surfaces. The ability to view the effects of molecular motions interactively should be useful in understanding the relationships of protein structure to function.

Two families with hemoglobin Sogn, beta(A11)14 Leu----Arg, in Minnesota and Indiana: hematologic, functional, and biosynthetic features

Sogn hemoglobinopathy was identified in a young American woman and in a young American man of apparently unrelated families of Norwegian ancestry. Both persons were asymptomatic and without clinical or hematologic manifestations. Hemoglobin Sogn, beta(A11)14 Leu----Arg, is an unstable hemoglobin that may easily be mistaken for hemoglobin S, G, or D by alkaline hemoglobin electrophoresis. These are the first known instances of hemoglobin Sogn outside of Norway. Oxygen affinity is normal. Sogn hemoglobinopathy is an incidental finding that has no adverse implication for the health of heterozygotes.

Inactivation-reactivation of two-electron reduced Escherichia coli glutathione reductase involving a dimer-monomer equilibrium

Glutathione reductase from Escherichia coli is inactivated when incubated with either NADPH or NADH. The process is inversely dependent on the enzyme concentration. Inactivation is rapid and monophasic with 1 microM NADPH and 1 nM enzyme FAD giving a t1/2 of 1 min. Complex formation between NADPH and the two-electron reduced enzyme (EH2) at higher levels of NADPH protects against rapid inactivation. NADP+, produced in a side reaction with oxygen, also protects by forming a complex with EH2. These complexes make analysis of the concentration dependence of the inactivation process difficult. Inactivation with NADH, where complexes do not interfere, is slower but can be analyzed more readily. With 152 microM NADH and 5.4 nM enzyme FAD, the time required for 50% inactivation is 17 min. The process is markedly biphasic, reaching the final inactivation level after 5-7 h. Analysis of the relationship between the final level of inactivation with NADH and the enzyme concentration indicates that inactivation is due to dissociation of the normally dimeric enzyme. Thus, the position of the dimer-monomer equilibrium between an active dimeric two-electron reduced species and an inactive monomeric two-electron reduced form determines the enzyme activity. An apparent equilibrium constant (Kd) for dissociation of dimer obtained from the anaerobic concentration dependent inactivation curves is 220 nM. Enzyme inactivated with NADH can be reactivated with glutathione, and the reactivation kinetics are second order, monomer-monomer over 75% of the reaction with an average apparent association rate constant (ka) of 13.1 (+/- 5.5) X 10(6) M-1 min-1.

Influence of gamma-chain amino-terminal acetylation on subunit assembly of human fetal hemoglobin

A human hemoglobin F subunit recombination study was performed to determine the relative efficiency of recombination of amino-terminally acetylated gamma-chains and non-acetylated chains with alpha-chains. The results of this work suggested that the acetylated gamma Ic-chains combined more readily with the alpha-chains than the non-acetylated gamma o-chains. An important factor in the function and assembly of multi-subunit macromolecules is the interaction of the unlike subunits. A model system for the study of such interactions has been the protein hemoglobin. With respect to the hemoglobin molecule, it has been noted that relative affinities of normal and mutant subunits for the unlike subunits can have a significant influence on hemoglobin synthesis at the post-translation level, i.e., subunit assembly [1,2]. A similar mechanism may control the formation of human Hb FIc, a hemoglobin with NH2-terminally acetylated gamma- (gamma Ic)-chains. In this instance acetylation may occur on the ribosomes before subunit assembly. A previous report from our laboratory showed a slight increase in the relative proportion of Hb FIc in cord blood samples with over 5% Hb Bart's [3]. The data were interpreted to be due to an influence of alpha-chain deficiency (alpha-thalassemia) on the formation of Hb FIc and Hb Fo tetramers by a preference of alpha-chains for gamma Ic-chains over gamma o- (non-acetylated gamma)-chains. The present study involves an examination of the relative affinity of the gamma Ic- and gamma o-chains for the normal alpha-chains in an in vitro recombination system.