Effects of beta 6 amino acid hydrophobicity on stability and solubility of hemoglobin tetramers

The Characteristics of Compound Heterozygosity for Hemoglobin G-Makassar with Hb E in Malaysia

Background

Human hemoglobin of G-Makassar and hemoglobin E (Hb E) are hemoglobin variants that affect Beta (β) globin. Hb G-Makassar is a very rare variant while Hb E is estimated to affect at least one million people worldwide. Both Hb G-Makassar and Hb E can be inherited in the heterozygous, homozygous or compound heterozygous state. This case series describes the characteristics of four individuals with compound heterozygosity for Hb G-Makassar/Hb E cases in Malaysia. To the best of our knowledge, these are the only four individuals with this genotype reported in the literature.

Case Series

We present four cases of compound heterozygosity for Hb G-Makassar/Hb E identified from October 2014 to January 2021. All the cases were incidental findings whereby the screening Hb analysis showed the presence of peaks in both Hb S and Hb E zones on capillary electrophoresis (CE) and cation-exchange high-performance liquid chromatography (HPLC). Molecular analysis confirmed the findings of compound heterozygous Hb G-Makassar/Hb E. Two cases had a history of anemia secondary to unrelated conditions that resolved with treatment of the underlying cause. The other two cases were asymptomatic individuals who were detected through Malaysia's National Thalassemia Screening program. On the last follow-up, all the individuals were well, non-transfusion dependent, and had no reported history of chronic anemia, bleeding, hemolysis or thromboembolism complications.

Conclusion

The cases reported here highlight the possibilities for rare compound heterozygous states in multi-ethnicity populations such as Malaysia. Compound heterozygous Hb G-Makassar/Hb E individuals are clinically silent with laboratory values suggesting microcytic and hypochromic red blood cells. Further local epidemiology or population studies with genotyping tests are required for a better understanding of the diversity of its clinical phenotype.

Clinical and haematological characteristics of 38 individuals with Hb G-Makassar in Malaysia

Haemoglobin (Hb) G-Makassar is a rare Hb variant. It presents a diagnostic challenge as it imitates sickle Hb (Hb S) in standard electrophoresis and high-performance liquid chromatography assays requiring DNA analysis to confirm diagnosis. Both have point mutations in codon 6, exon 1 in the β-globin (HBB) gene with different pathogenicities. This study describes the clinical phenotype, haematology and genotype of Hb G-Makassar. Clinical and laboratory data of 38 cases of Hb G-Makassar over 8 years were analysed. Hb G-Makassar was confirmed by a direct sequencing of HBB gene and co-inheritance of α-thalassaemia determined through multiplex gap-PCR and multiplex Amplification Refractory Mutation System polymerase chain reaction. All cases were Malays, predominantly from Terengganu (n = 20, 52.6%). There were 14 (36.8%) males and 24 (63.2%) females with median age of 25 years. Majority (n = 33, 86.8%) had features of thalassaemia trait with mean ± SD for Hb, mean cell volume (MCV) and mean cell haemoglobin (MCH) as 13.21 g/dL ± 1.69, 73.06 ± 4.48 fL and 24.71 ± 1.82 pg, respectively. None had evidence of haemolysis or thromboembolic complications. Six genotypes were identified; ßG-Makassar/ß,αα/αα (n = 19, 50.0%), ßG-Makassar/ßE,αα/αα (n = 4, 10.5%), ßG-Makassar/ßNewYork,αα/αα (n = 1, 2.6%), ßG-Makassar/ß,αα/-α (n = 11, 28.9%), ßG-Makassar/ß,αα/αAdanaα (n = 2, 5.3%) and ßG-Makassar/ß,αα/-SEA (n = 1, 2.6%). The ßG-Makassar/ß,αα/αα showed that features of thalassaemia trait with mean ± SD for Hb, MCV and MCH were 13.74 g/dL ± 2.40, 76.18 ± 6.02 fL and 25.79 ± 2.41 pg, respectively. This is the largest study reporting a significant number of Hb G-Makassar in Malaysia. Although the mutation is similar to Hb S, the phenotype is benign.

Molecular Dynamics of Hemoglobin Reveals Structural Alterations and Explains the Interactions Driving Sickle Cell Fibrillation

In sickle cell anemia, deoxyhemoglobin deforms RBCs by forming fibrils inside that disintegrate on oxygenation. We studied 100 ns long all-atom molecular dynamics (MD) for sickle and normal hemoglobin fibril models to understand this process, complemented by multiple 1 μs MD for a single tetramer of sickle and normal hemoglobin in deoxy and oxy states. We find that the presence of hydrophobic residues without a bulky side chain at β-6 in hemoglobin is the reason for the stability of the fibrils. Moreover, the free energy landscapes from MD of hemoglobin starting in the tensed (T) state capture the putative transition from T to relaxed (R) state, associated with oxygen binding. The three conformational wells in the landscapes are characterized by the quaternary changes where one αβ dimer rotates with respect to the other. The conformational changes from the oxygenation of sickle hemoglobin hinder the intermolecular contacts necessary for fibril formation.

Rationally Designed Base Editors for Precise Editing of the Sickle Cell Disease Mutation

Base editors are fusions of a deaminase and CRISPR-Cas ribonucleoprotein that allow programmable installment of transition mutations without double-strand DNA break intermediates. The breadth of potential base editing targets is frequently limited by the requirement of a suitably positioned Cas9 protospacer adjacent motif. To address this, we used structures of Cas9 and TadA to design a set of inlaid base editors (IBEs), in which deaminase domains are internal to Cas9. Several of these IBEs exhibit shifted editing windows and greater editing efficiency, enabling editing of targets outside the canonical editing window with reduced DNA and RNA off-target editing frequency. Finally, we show that IBEs enable conversion of the pathogenic sickle cell hemoglobin allele to the naturally occurring HbG-Makassar variant in patient-derived hematopoietic stem cells.

Solubility Challenges in High Concentration Monoclonal Antibody Formulations: Relationship with Amino Acid Sequence and Intermolecular Interactions

The purpose of this work was to elucidate the molecular interactions leading to monoclonal antibody self-association and precipitation and utilize biophysical measurements to predict solubility behavior at high protein concentration. Two monoclonal antibodies (mAb-G and mAb-R) binding to overlapping epitopes were investigated. Precipitation of mAb-G solutions was most prominent at high ionic strength conditions and demonstrated strong dependence on ionic strength, as well as slight dependence on solution pH. At similar conditions no precipitation was observed for mAb-R solutions. Intermolecular interactions (interaction parameter, kD) related well with high concentration solubility behavior of both antibodies. Upon increasing buffer ionic strength, interactions of mAb-R tended to weaken, while those of mAb-G became more attractive. To investigate the role of amino acid sequence on precipitation behavior, mutants were designed by substituting the CDR of mAb-R into the mAb-G framework (GM-1) or deleting two hydrophobic residues in the CDR of mAb-G (GM-2). No precipitation was observed at high ionic strength for either mutant. The molecular interactions of mutants were similar in magnitude to those of mAb-R. The results suggest that presence of hydrophobic groups in the CDR of mAb-G may be responsible for compromising its solubility at high ionic strength conditions since deleting these residues mitigated the solubility issue.

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

Interspecies hybrid HbS (alpha(2)(P)beta(2)(S)), has been assembled in vitro from pig alpha-globin and human beta(S)-chain. The alpha(2)(P)beta(2)(S) retains normal tetrameric structure (alpha(2)beta(2)) of human Hb and an O(2) affinity comparable to that of HbS in 50 mM Hepes buffer; but, its O(2) affinity is slightly higher than that of HbS in the presence of allosteric effectors (chloride, DPG and phosphate). The (1)H-NMR spectroscopy detected distinct differences between the heme environments and alpha(1)beta(1) interfaces of pig Hb and HbS, while their alpha(1)beta(2) interfaces appear very similar. The interspecies hybrid alpha(2)(H)beta(2)(P) resembles pig Hb; the pig beta-chain dictated the conformation of the heme environment of the human alpha-subunit, and to the alpha(1)beta(1) interfaces of the hybrid. In the alpha(2)(P)beta(2)(S) hybrid, beta(S)-chain dictated the conformation of human heme environment to the pig alpha-chain in the hybrid; but the conformation of alpha(1)beta(1) interface of this hybrid is close to, but not identical to that of HbS. On the other hand, the alpha(1)beta(2) interface conformation is identical to that of HbS. More important, the alpha(2)(P)beta(2)(S) does not polymerize when deoxygenated; pig alpha-chain completely neutralizes the beta(S)-chain dependent polymerization. The polymerization inhibitory propensity of pig alpha-chain is higher when it is present in the cis alpha(P)beta(S) dimer relative to that in a trans alpha(P)beta(A) dimer. The semisynthetically generated chimeric pig-human and human-pig alpha-chains by exchanging the alpha(1-30) segments of human and pig alpha-chains have established that the sequence differences of pig alpha(31-141) segment can also completely neutralize the polymerization. Comparison of the electrostatic potential energy landscape of the alpha-chain surfaces of HbS and alpha(2)(P)beta(2)(S) suggests that the differences in electrostatic potential energy surfaces on the alpha-chain of alpha(2)(P)beta(2)(S) relative to that in HbS, particularly the ones involving CD region, E-helix and EF-corner of pig alpha-chain are responsible for the polymerization neutralization activity. The pig and human-pig chimeric alpha-chains can serve as blueprints for the design of a new generation of variants of alpha-chain(s) suitable for the gene therapy of sickle cell disease.

Crystal structure of deoxy-human hemoglobin beta6 Glu --> Trp. Implications for the structure and formation of the sickle cell fiber

An atomic-level understanding of the interactions between hemoglobin molecules that contribute to the formation of pathological fibers in sickle cell disease remains elusive. By exploring crystal structures of mutant hemoglobins with altered polymerization properties, insight can be gained into sickle cell hemoglobin (HbS) polymerization. We present here the 2.0-A resolution deoxy crystal structure of human hemoglobin mutated to tryptophan at the beta6 position, the site of the glutamate --> valine mutation in HbS. Unlike leucine and isoleucine, which promote polymerization relative to HbS, tryptophan inhibits polymerization. Our results provide explanations for the altered polymerization properties and reveal a fundamentally different double strand that may provide a model for interactions within a fiber and/or interactions leading to heterogeneous nucleation.

Expression studies of delta-globin gene alleles associated with reduced hemoglobin A2 levels in Greek Cypriots

We previously identified five delta-globin gene alleles associated with reduced hemoglobin (Hb) A2 (Trifillis, P., Ioannou, P., Schwartz, E., and Surrey, S. (1991) Blood 78, 3298-3305). We have now evaluated functional consequences of the changes after expression in COS-1 cells to monitor effects on RNA splicing. In addition, variant Hb A2 tetramers were expressed in yeast to assess effects of amino acid changes on oxygen binding and stability to heat and mechanical agitation. The G --> T change at codon 27 and the A --> G change in IVS-2 both affect RNA splicing, whereas the C --> T change at codon 97 and the AT deletion in IVS-2 have no effect. Oxygen equilibrium curves of the Hb A2 variants expressed in yeast were similar to that of wild type Hb A2. None of the three variant Hb A2 tetramers (Thr --> Ile at codon 4 (Hb deltaT4I), Ala --> Ser at codon 27 (Hb deltaA27S), and Arg --> Cys at codon 116 (Hb deltaR116C)) showed decreased heat stability compared with Hb A2, whereas the Hb deltaT4I variant showed highest instability to mechanical agitation. Co-expression in yeast of alpha-globin chain and the delta-chain variant containing a Leu --> Pro change at codon 141 yielded no identifiable tetramers, suggesting lack of assembly or severe tetramer instability. These studies show the probable cause for decreased Hb A2 for two alleles is due to defective splicing, whereas decreased protein stability, increased tetramer association with red cell membranes, increased interdisulfide bond formation of delta-chains, which inhibits assembly with alpha-chains, and/or reduced assembly is suggested for the other three alleles.

Polymerization of three hemoglobin A2 variants containing Val6 and inhibition of hemoglobin S polymerization by hemoglobin A2

To understand determinants for hemoglobin (Hb) stability and Hb A2 inhibition of Hb S polymerization, three Valdelta6 Hb A2 variants (Hb A2 deltaE6V, Hb A2 deltaE6V,deltaQ87T, and Hb A2 deltaE6V, deltaA22E,deltaQ87T) were expressed in yeast, and stability to mechanical agitation and polymerization properties were assessed. Oxy forms of Hb A2 deltaE6V and Hb A2 deltaE6V,deltaQ87T were 2- and 1.6-fold, respectively, less stable than oxy-Hb S, while the stability of Hb A2 deltaE6V,deltaA22E,deltaQ87T was similar to that of Hb S, suggesting that Aladelta22 and Glndelta87 contribute to the surface hydrophobicity of Hb A2. Deoxy Hb A2 deltaE6V polymerized without a delay time, like deoxy Hb F gammaE6V, while deoxy Hb A2 deltaE6V,deltaQ87T and deoxy Hb A2 deltaE6V,deltaA22E,deltaQ87T polymerized after a delay time, like deoxy Hb S, suggesting that beta87 Thr is required for the formation of nuclei. Deoxy Hb F gammaE6V,gammaQ87T showed no delay time and required a 3.5-fold higher concentration than deoxy Hb S for polymerization, suggesting that Thr effects on Valdelta6 Hb A2 and Valgamma6 Hb F variants are different. Mixtures of deoxy Hb S/Hb A2 deltaE6V,deltaQ87T polymerized, like deoxy Hb S, while polymerization of Hb S/Hb A2 deltaE6V mixtures was inhibited, like Hb S/Hb F gammaE6V mixtures. These results suggest alpha2betaSdelta6 Val, 87 Thr hybrids and Hb A2 deltaE6V,deltaQ87T participate in Hb S nucleation, while only 50% of alpha2betaSdelta6 Val hybrids and none of the Hb A2 deltaE6V participate. These findings are in contrast to those of mixtures of Hb S with Hb F gammaE6V or Hb F gammaE6V,Q87T, which both inhibit Hb S polymerization. Our results also suggest participation in nucleation of some alpha2betaSdelta hybrids in A2S mixtures but not alpha2betaSgamma hybrids in FS mixtures.

Polymerization of hemoglobin S. Quinary interactions of Glu-43(beta)

Hemoglobin S (HbS) Hoshida and three substituted forms of HbS Hoshida (the substituents being on the amide nitrogen of Gln-43(beta) have been prepared by the amidation of Glu-43(beta) of HbS with ammonia, methylamine, glycine ethyl ester, and galactosamine. The O2 affinity of HbS is increased slightly on amidation of Glu-43(beta). All the four amidated derivatives exhibited nearly the same oxygen affinity. On the other hand, the influence of amidation on the solubility exhibits some sensitivity to the chemical nature of the substituent on the Gln-43(beta). The solubility of HbS Hoshida (a case with no substitution on Gln-43(beta), and the methyl-substituted derivatives are about 33 and 36% higher than that of HbS. The solubility of the HbS modified with the glycine ethyl ester or galactosamine is increased to 41 and 47%, respectively. The first derivative UV spectra of HbS Hoshida and its methyl derivative reflect very little perturbations in their alpha 1 beta 2 interface as compared with that of HbS, whereas the amidated derivatives with larger substituents on Gln-43(beta) reflected noticeable difference. Thus, the increase in the solubility and the oxygen affinity of HbS on the amidation of Glu-43(beta) is primarily a consequence of the loss of the negative charge at 43(beta), a residue proximal to the alpha 1 beta 2 interface. The copolymerization studies of amidated HbS with HbA, and HbS with amidated HbA demonstrate that cis Glu-43(beta) is the "active" residue. This assignment is discrepant with the earlier implication of a trans configuration for this residue in the polymer (Edelstein, S. J. (1981) J. Mol. Biol. 150, 557-575). However, it is consistent with the solution studies of Nagel et al. (Nagel, R. L., Bookchin, R. M., Johnson, J., Labie, D., Wajcman, H.., Isaac-Sodeye, W. A., Honig, G. R., Schiliro, G., Crookstan, J. H., and Matsutomo, K. (1979) Proc. Nat. Acad. Sci. U.S.A. 76, 670-672) and McCurdy et al. (McCurdy, P. R., Lorkin, P. A., Casey, R., Lehmann, H., Uddin, D. E., and Dickson, L. G. (1974) Am. J. Med. 57, 665-760).

Role of hydrophobic and hydrophilic forces in peptide-protein interaction: new advances

A considerable part of important biological processes is governed by the noncovalent association of peptides and proteins. Various types of intermolecular forces may be involved in the formation of these molecular assemblies. This review gives a brief account of the physicochemical bases of interactive forces, with special emphasis on their impact on various peptide-protein interactions; summarizes the newest biochemical and biophysical methods for the study of such interactions; and discusses the role of various hydrophilic and hydrophobic forces in peptide-protein interactions in various fields of life sciences, such as immunology, enzymology, receptor binding, and toxicology.

Properties of a recombinant human hemoglobin double mutant: sickle hemoglobin with Leu-88(beta) at the primary aggregation site substituted by Ala

A recombinant double mutant of hemoglobin (Hb), E6V/L88A(beta), was constructed to study the strength of the primary hydrophobic interaction in the gelation of sickle Hb, i.e., that between the mutant Val-6(beta) of one tetramer and the hydrophobic region between Phe-85(beta) and Leu-88(beta) on an adjacent tetramer. Thus, a construct encoding the donor Val-6(beta) of the expressed recombinant HbS and a second mutation encoding an Ala in place of Leu-88(beta) was assembled. The doubly mutated beta-globin gene was expressed in yeast together with the normal human alpha-chain, which is on the same plasmid, to produce a soluble Hb tetramer. Characterizations of the Hb double mutant by mass spectrometry, by HPLC, and by peptide mapping of tryptic digests of the mutant beta-chain were consistent with the desired mutations. The absorption spectra in the visible and the ultraviolet regions were practically superimposable for the recombinant Hb and the natural Hb purified from human red cells. Circular dichroism studies on the overall structure of the recombinant Hb double mutant and the recombinant single mutant, HbS, showed that both were correctly folded. Functional studies on the recombinant double mutant indicated that it was fully cooperative. However, its gelation concentration was significantly higher than that of either recombinant or natural sickle Hb, indicating that the strength of the interaction in this important donor-acceptor region in sickle Hb was considerably reduced even with such a conservative hydrophobic mutation.