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

Novel High Oxygen Affinity Hemoglobin Variant in a Patient with Polycythemia: Hb Kennisis [β85(F1)Phe→Leu (TTT>TTG); HBB: c.258T>G]

We report the case of a 61-year-old Canadian male of Maltese descent investigated for unexplained polycythemia. Decreased p50 suggested the presence of a high oxygen affinity hemoglobin (Hb) variant. Molecular genetic testing demonstrated that he carries a novel missense mutation (HBB: c.258T>G), resulting in a Phe→Leu substitution at position 85 of the β chain. The novel Hb variant has been designated Hb Kennisis in recognition of where the proband resides. Two other missense mutations have been reported at this position [Hb Bryn Mawr or Hb Buenos Aires, β85(F1)Phe→Ser (HBB: c.257T>C); Hb Grantham, β85(F1)Phe→Cys; (HBB: c.257T>G)], both of which have increased oxygen affinity.

Molecular insights into the irreversible mechanical behavior of sickle hemoglobin

Sickle cell disease is caused by the amino acid substitution of glutamic acid to valine, which leads to the polymerization of deoxygenated sickle hemoglobin (HbS) into long strands. These strands are responsible for the sickling of red blood cells (RBCs), making blood hyper-coagulable leading to an increased chance of vaso-occlusive crisis. The conformational changes in sickled RBCs traveling through narrow blood vessels in a highly viscous fluid are critical in understanding; however, there are few studies that investigate the origins of the molecular mechanical behavior of sickled RBCs. In this work, we investigate the molecular mechanical properties of HbS molecules. A mechanical model was used to estimate the directional stiffness of an HbS molecule and the results were compared to adult human hemoglobin (HbA). The comparison shows a significant difference in strength between HbS and HbA, as well as anisotropic behavior of the hemoglobin molecules. The results also indicated that the HbS molecule experienced more irreversible mechanical behavior than HbA under compression. Further, we have characterized the elastic and compressive properties of a double stranded sickle fiber using six HbS molecules, and it shows that the HbS molecules are bound to each other through strong inter-molecular forces.

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.

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.

Role of hydrophobic amino acids at beta85 and beta88 in stabilizing F helix conformation of hemoglobin S

Three Hb S variants containing Glu substitutions at Phe-beta85 and/or Leu-beta88 were expressed in yeast in an effort to evaluate the role of hydrophobic amino acids at these sites in stabilizing F helix conformation of Hb S. Helix stability of tetrameric Hb S betaF85E,betaL88E was measured by CD and compared with those of Hb S betaF85E, Hb S betaL88E, Hb A, and Hb S. The CD spectra of these Hb S variants were similar to those of Hb S and Hb A at 10 degrees C. However, changes in ellipticity at 222 nm for Hb S betaF85E in the CO form at 60 degrees C were about 15-fold greater than that of Hb S, while those for Hb S betaL88E and Hb S betaF85E,betaL88E were similar and about 30-fold greater than Hb S. Thermal stability measured by continuous scanning of spectral changes revealed the three Hb S variants were much more unstable than Hb S, and stability of Hb S betaF85E,betaL88E was similar to that of Hb S betaL88E rather than Hb S betaF85E. These results suggest that Glu insertion at both beta85 and beta88 makes heme insertion into the heme pocket more difficult; however, once inserted, stability of Hb S betaF85E, betaL88E is similar to Hb S betaL88E rather than Hb S betaF85E. Furthermore, these results suggest that both Phe-beta85 and Leu-beta88 are critical for F helix stabilization and that Glu insertion at beta88 leads to more destabilization than insertion at beta85.