Interspecies hybrid HbS: complete neutralization of Val6(beta)-dependent polymerization of human beta-chain by pig alpha-chains

Influence of semisynthetic modification of the scaffold of a contact domain of HbS on polymerization: role of flexible surface topology in polymerization inhibition

A new variant of HbS, HbS-Einstein with a deletion of segment α_23-26 in the B-helix, has been assembled by semisynthetic approach. B-helix of the α chain of cis αβ-dimer of HbS plays dominant role in the quinary interactions of deoxy HbS dimer. This B-helix is the primary scaffold that provides the orientation for the side chains of contact residues of this intermolecular contact domain. The design of HbS-Einstein has been undertaken to map the influence of perturbation of molecular surface topology and the flexibility of surface residues in the polymerization. The internal deletion exerts a strong inhibitory influence on Val-6 (β)-dependent polymerization, comparable to single contact site mutations and not for complete neutralization of Val-6(β)-dependent polymerization. The scaffold modification in cis-dimer is inhibitory, and is without any effect when present on the trans dimer. The flexibility changes in the surface topology in the region of scaffold modification apparently counteracts the intrinsic polymerization potential of the molecule. The inhibition is close to that of Le Lamentin mutation [His-20 (α) → Gln] wherein a mutation engineered without much change in flexibility of the contact domain. Interestingly, the chimeric HbS with swine-human chimeric α chain with multiple non-conservative mutations completely inhibits the Val-6(β)-dependent polymerization. The deformabilities of surface topology of chimeric HbS are comparable to HbS in spite of the multiple contact site mutations in the α-chain. We conclude that the design of antisickling Hbs for gene therapy of sickle cell disease should involve multiple mutations of intermolecular contact sites.

HbS-Savaria: the anti-polymerization effect of a single mutation in human alpha-chains

Recombinant alpha-Savaria globin (alpha(S49R)) was assembled with beta(S) chains by the alloplex intermediate pathway to generate tetrameric rHbS-Sarvaria (alpha (2) (S49R) beta (2) (E6V) ) that exhibited normal O(2) affinity and co-operatively at pH 7.4. Allosteric effectors, 2,3-DPG, L35, and NaCl increased O(2) affinity by 15%. Bohr effects were similar for rHbS-Savaria and HbS (0.38 +/- 0.025 vs. 0.46 +/- 0.03, respectively). The C(SAT) of HbS increased from 16.7 +/- 0.8 to 27.0 +/- 1.0 g/dL. Co-polymerization demonstrated inhibition predominantly by the Cis-dimer. Molecular modeling indicated that the positive charge at alpha-49 generated a strong anion-binding site and reduced flexibility of the CD-region by restricting movement in the E and F helices. The molecular distance between Arg-49 and Asn-78 in the neighboring double strand decreased, and electrostatic repulsion between the inter-double strands increased, resulting in inhibition of polymerization. The Savaria mutation may be useful for the design of super-inhibitory alpha-chains and gene therapy of sickle cell anemia.

Pair-wise interactions of polymerization inhibitory contact site mutations of hemoglobin-S

The linkage of pair-wise interactions of contact site mutations of HbS has been studied using Le Lamentin [His-20 (alpha)-->Gln], Hoshida [Glu-43 (beta)-->Gln] and alpha(2)beta (2) (T87Q) mutations as the prototype of three distinct classes of contact sites of deoxy HbS fiber. Binary mixture experiments established that beta(A)-chain with the Thr-87 (beta)-->Gln mutation is as potent as the gamma-chain of HbF (alpha(2)gamma(2)) in inhibiting polymerization. On combining the influence of Le Lamentin mutation with that of beta (2) (T87Q) mutations; the net influence is only partial additivity. On the other hand, in binary mixture studies, combined influence of Hoshida mutation with that of beta (2) (T87Q) mutations is synergistic. Besides, a significant level of synergistic complementation is also seen when the Le Lamentin and Hoshida mutations are combined in HbS (symmetrical tetramers). Le Lamentin and Hoshida mutation introduced into the cis-dimer of the asymmetric hybrid tetramer completely neutralizes the Val-6 (beta) dependent polymerization. Accordingly, we propose that combining the perturbation of intra-double strand contact site with that of an inter-double strand contact site exhibit synergy when they are present in two different chains of the alphabeta dimer. A comparison of the present results with that of the earlier studies suggest that when the two contact site perturbations are from the same sub-unit of the alphabeta dimer only partial additivity is observed. The map of interaction linkage of the contact site mutations exposes new strategies in the design of novel anti-sickling Hbs for the gene therapy of sickle cell disease.

Effect of zeta-globin substitution on the O2-transport properties of Hb S in vitro and in vivo

Hemoglobin zeta(2)beta(2)(S) is generated by substituting embryonic zeta-globin subunits for the normal alpha-globin components of Hb S (alpha(2)beta(2)(S)). This novel hemoglobin has recently been shown to inhibit polymerization of Hb S in vitro and to normalize the pathological phenotype of mouse models of sickle cell disease in vivo. Despite its promise as a therapeutic tool in human disease, however, the basic O(2)-transport properties of Hb zeta(2)beta(2)(S) have not yet been described. Using human hemoglobins purified from complex transgenic-knockout mice, we show that Hb zeta(2)beta(2)(S) exhibits an O(2) affinity as well as a Hill coefficient, Bohr response, and allosteric properties in vitro that are suboptimally suited for physiological O(2) transport in vivo. These data are substantiated by in situ analyses demonstrating an increase in the O(2) affinity of intact erythrocytes from mice that express Hb zeta(2)beta(2)(S). Surprisingly, though, co-expression of Hb zeta(2)beta(2)(S) leads to a substantial improvement in the tissue oxygenation of mice that model sickle cell disease. These analyses suggest that, in the context of sickle cell disease, the beneficial antisickling effects of Hb zeta(2)beta(2)(S) outweigh its O(2)-transport liabilities. The potential structural bases for the antisickling properties of Hb zeta(2)beta(2)(S) are discussed in the context of these new observations.

Aggregation of normal and sickle hemoglobin in high concentration phosphate buffer

Sickle cell disease is caused by a mutant form of hemoglobin, hemoglobin S, that polymerizes under hypoxic conditions. The extent and mechanism of polymerization are thus the subject of many studies of the pathophysiology of the disease and potential treatment strategies. To facilitate such studies, a model system using high concentration phosphate buffer (1.5 M-1.8 M) has been developed. To properly interpret results from studies using this model it is important to understand the similarities and differences in hemoglobin S polymerization in the model compared to polymerization under physiological conditions. In this article, we show that hemoglobin S and normal adult hemoglobin, hemoglobin A, aggregate in high concentration phosphate buffer even when the concentration of hemoglobin is below the solubility defined for polymerization. This phenomenon was not observed using 0.05 M phosphate buffer or in another model system we studied that uses dextran to enhance polymerization. We have used static light scattering, dynamic light scattering, and differential interference contrast microscopy to confirm aggregation of deoxygenated and oxygenated hemoglobins below their solubility and have shown that this aggregation is not observable using turbidity measurements, a common technique for assessing polymerization. We have also shown that the aggregation increases with increasing temperature in the range of 15 degrees -37 degrees C and that it increases as the concentration of phosphate increases. These studies contribute to the working knowledge of how to properly apply studies of hemoglobin S polymerization that are conducted using the high phosphate model.

Hemoglobin Einstein: semisynthetic deletion in the B-helix of the alpha-chain

The influence of the deletion of the tetra peptide segment alpha(23-26) of the B-helix of the alpha-chain of hemoglobin-A on its assembly, structure, and functional properties has been investigated. The hemoglobin with the deletion, ss-Hemoglobin-Einstein, is readily assembled from semisynthetic alpha(1-141) des(23-26) globin and human betaA-chain. The deletion of alpha(23-26) modulates the O2 affinity of hemoglobin in a buffer/allosteric effector specific fashion, but has little influence on the Bohr effect. The deletion has no influence on the thermodynamic stability of the alpha1beta1 and the alpha1beta2 interface. The semisynthetic hemoglobin exhibits normal intersubunit interactions at the alpha1beta1 and alpha1beta2 interfaces as reflected by 1H-NMR spectroscopy. Molecular modeling studies of ss-Hemoglobin-Einstein suggest that the segment alpha(28-35) is in a helical conformation, while the segment alpha(19-22) is the nonhelical AB region. The shortened B-helix conserves the interactions of alpha1beta1 interface. The results demonstrate a high degree of plasticity in the hemoglobin structure that accommodates the deletion of alpha(23-26) without perturbing its overall global conformation.

Linkage of interactions in sickle hemoglobin fiber assembly: inhibitory effect emanating from mutations in the AB region of the alpha-chain is annulled by a mutation at its EF corner

The AB and GH regions of the alpha-chain are located in spatial proximity and contain a cluster of intermolecular contact residues of the sickle hemoglobin (HbS) fiber. We have examined the role of dynamics of AB/GH region on HbS polymerization through simultaneous replacement of non-contact Ala(19) and Ala(21) of the AB corner with more flexible Gly or rigid alpha-aminoisobutyric acid (Aib) residues. The polymerization behavior of HbS with Aib substitutions was similar to the native HbS. In contrast, Gly substitutions inhibited HbS polymerization. Molecular dynamics simulation studies of alpha-chains indicated that coordinated motion of AB and GH region residues present in native (Ala) as well as in Aib mutant was disrupted in the Gly mutant. The inhibitory effect due to Gly substitutions was further explored in triple mutants that included mutation of an inter-doublestrand contact (alphaAsn(78) --> His or Gln) at the EF corner. Although the inhibitory effect of Gly substitutions in the triple mutant was unaffected in the presence of alphaGln(78), His at this site almost abrogated its inhibitory potential. The polymerization studies of point mutants (alphaGln(78) --> His) indicated that the inhibitory effect due to Gly substitutions in the triple mutant was synergistically compensated for by the polymerization-enhancing activity of His(78). Similar synergistic coupling, between alphaHis(78) and an intra-double-strand contact point (alpha16) mutation located in the AB region, was also observed. Thus, two conclusions are made: (i) Gly mutations at the AB corner inhibit HbS polymerization by perturbing the dynamics of the AB/GH region, and (ii) perturbations of AB region (through changes in dynamics of the AB/GH region or abolition of a specific fiber contact site) that influence HbS polymerization do so in concert with alpha78 site at the EF corner. The overall results provide insights about the interaction-linkage between distant regions of the HbS tetramer in fiber assembly.

Probing the conformation of hemoglobin presbyterian in the R-state

The influence of allosteric effectors on the R-state (liganded) conformation of Tg-HbP (human hemoglobin Presbyterian expressed in transgenic pig) has been probed using a number of biophysical techniques, and the results have been compared with that of liganded of HbA (human normal adult hemoglobin) to gain insight into the molecular basis of Asn-108(beta)->Lys mutation-induced low-oxygen affinity of Hb. The nuclear magnetic resonance studies of Tg-HbP revealed that the conformation of the alpha1beta1 and alpha1beta1 interfaces of the protein in the deoxy state are indistinguishable from that of deoxy HbA, whereas the conformation of the microenvironment of His-103(alpha) of Tg-HbP, a residue of the alpha1beta1 interface, is distinct from that of HbA in the R-state. In addition, the Presbyterian mutation also influences the structure of oxy Hb in other regions of the molecule. First, it facilitates the generation of deoxy (T)-state marker at 14.2 ppm (from 2,2-dimethyl-s-silapentane-5-sulfonate) on the interaction of oxy Hb with inositol hexa-phosphate without changing the ligation state. Second, it increases the geminate yield of the 10 ns photoproduct of CO-Hb. Third, it enhances the propensity of phosphate to increase the geminate yield. Fourth, it potentiates the ability of phosphate to induce deoxy-like features at the heme environment in the R-state. Fifth, it induces T-state-like signatures at the switch and hinge regions of the alpha1beta2 interface. Finally, molecular modeling studies have indicated an increased affinity for the four anion binding sites mapped in the midcentral cavity of Hb caused by the presence of Lys-108(beta). In short, Lys-108(beta) in HbP induces a propensity for oxy Hb to access T-like conformational features in different regions of the oxy Hb molecule and also enhances the T-like signatures in the oxy state on interaction with allosteric effectors without changing its ligation. Interestingly, the intrinsic T-like conformational features of the R-state of HbP, in addition to those induced by the addition of allosteric effectors to liganded HbP, appear to be reminiscent of features of the B-state conformation of Hb found in rHb 1.1 (recombinant hemoglobin). We propose that the lowered oxygen affinity of Tg-HbP in the presence of allosteric effectors is a consequence of an altered R-state conformation of Hb, which reflects the facilitation of switching the R-state of HbP to the T-state compared with the normal R-state of HbA, thereby reducing HbA's affinity to oxygen.

Product-conformation-driven ligation of peptides by V8 protease

Organic co-solvent-induced secondary conformation of alpha(17-40) of human hemoglobin facilitates the splicing of E30-R31 in a mixture of its complementary segments by V8 protease. The amino acid sequence of alpha(17-40) has been conceptualized by the general structure FR(I)-EALER-FR(II) and the pentapeptide sequence EALER playing a major role in inducing the alpha-helical conformation. The primary structure of alpha(17-40) has been engineered in multiple ways to perturb one, two, or all three regions and the influence of the organic co-solvent-induced conformation and the concomitant resistance of E30-R31 peptide bond to V8 protease digestion has been investigated. The central pentapeptide (EALER), referred to here as splicedon,(3) appears to dictate a primary role in facilitating the splicing reaction. When the same flanking regions are used, (1) splicedons that carry amino acid residues of low alpha-helical potential, for example G at position 2 or 3 of the splicedon, generate a conformational trap of very low thermodynamic stability, giving an equilibrium yield of only 3%-5%; (2) splicedons with amino acid residues of good alpha-helical potential generate a conformational trap of medium thermodynamic stability and give an equilibrium yield of 20%-25%; (3) the splicedons with amino residues of good alpha-helical potential and also an amino acid that can generate an i, i + 4 side-chain carboxylate-guanidino (amino) interaction, a conformational trap of maximum thermodynamic stability is generated, giving an equilibrium yield of 45%-50%; and (4) the thermodynamic stability of the conformational trap of the spliced peptide is also influenced by the amino acid composition of the flanking regions. The V8 protease resistance of the spliced peptide bond is not a direct correlate of the amount of alpha-helical conformation induced into the product. The results of this study reflect the unique role of the splicedon in translating the organic co-solvent-induced product conformation as a site-specific stabilization of the spliced peptide bond. It is speculated that the splicedon with higher alpha-helical potential as compared to either one of the flanking regions achieves this by integrating its potential with that of the flanking region(s). Exchange of flanking regions with the products of other V8 protease-catalyzed splicing reactions will help to establish the general primary structural requirements of this class of splicing reactions and facilitate their application in modular construction of proteins.

A role for the alpha 113 (GH1) amino acid residue in the polymerization of sickle hemoglobin. Evaluation of its inhibitory strength and interaction linkage with two fiber contact sites (alpha 16/23) located in the AB region of the alpha-chain

A cluster of amino acid residues located in the AB-GH region of the alpha-chain are shown in intra-double strand axial interactions of the hemoglobin S (HbS) polymer. However, alphaLeu-113 (GH1) located in the periphery is not implicated in any interactions by either crystal structure or models of the fiber, and its role in HbS polymerization has not been explored by solution experiments. We have constructed HbS Twin Peaks (betaGlu-6-->Val, alphaLeu-113-->His) to ascertain the hitherto unknown role of the alpha113 site in the polymerization process. The structural and functional behavior of HbS Twin Peaks was comparable with HbS. HbS Twin Peaks polymerized with a slower rate compared with HbS, and its polymer solubility (C(sat)) was found to be about 1.8-fold higher than HbS. To further authenticate the participation of the alpha113 site in the polymerization process as well as to evaluate its relative inhibitory strength, we constructed HbS tetramers in which the alpha113 mutation was coupled individually with two established fiber contact sites (alpha16 and alpha23) located in the AB region of the alpha-chain: HbS(alphaLys-16-->Gln, alphaLeu-113-->His), HbS(alphaGlu-23-->Gln, alphaLeu-113-->His). The single mutants at alpha16/alpha23 sites were also engineered as controls. The C(sat) values of the HbS point mutants involving sites alpha16 or alpha23 were higher than HbS but markedly lower as compared with HbS Twin Peaks. In contrast, C(sat) values of both double mutants were comparable with or higher than that of HbS Twin Peaks. The demonstration of the inhibitory effect of alpha113 mutation alone or in combination with other sites, in quantitative terms, unequivocally establishes a role for this site in HbS gelation. These results have implications for development of a more accurate model of the fiber that could serve as a blueprint for therapeutic intervention.

A coiled coil with a fluorous core

The design, synthesis, and structural characterization of a highly fluorinated peptide system based on the coiled coil region of the yeast transcription factor GCN4 is described. All four leucine residues (a position) and three valine residues (d position) were replaced by the unnatural amino acids 5,5,5-trifluoroleucine and 4,4,4-trifluorovaline, respectively. The peptide is highly alpha-helical at low micromolar concentrations as judged by circular dichroism spectra, sediments as a dimeric species in the 5-30 microM concentration range, and exhibits a dimer melting temperature that is 15 degrees C higher than a control peptide with a hydrocarbon core. Furthermore, the apparent free energy of unfolding as calculated from guanidinium hydrochloride denaturation experiments is larger by approximately 1.0 kcal/mol for the fluorinated peptide than its hydrocarbon counterpart. We conclude that additional stability is derived from sequestering the more hydrophobic trifluoromethyl groups from aqueous solvent. These studies introduce a new paradigm in the design of molecular self-assembling systems, one based on orthogonal solubility properties of liquid phases.