Influence of the A helix structure on the polymerization of hemoglobin S

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.

Heme as an optical probe of a conformational transition of ovine recPrP

The prion protein occurs as a globular domain and a leading fragment whose structure is not well-defined. For the ovine species, all of the tryptophan residues are in the initial fragment, while the globular domain is rich in tyrosine residues. Using heme as a spectroscopic probe, we have studied the recombinant prion protein before and after a temperature-induced conformational change. As for most heme proteins, the absorption spectrum of heme-CO displays a red shift upon binding to the protein, and both the Y and W fluorescence are highly quenched. Flash photolysis kinetics of the PrP-heme-CO complex shows a low yield for the bimolecular phase, indicating a pocket around the hemes. By comparing the holoprotein and the truncated sequence corresponding to the globular domain, the stoichiometry was determined to be five hemes for the globular domain and two hemes for the leading fragment. At high temperature, the hemes are released; upon cooling, only two hemes bind, and only the tryptophan fluorescence is quenched; this would indicate that the globular domain has formed a more compact structure, which is inert with respect to the hydrophobic probe. The final state of polymerization is perturbed if the synthetic peptide "N3" (PrP residues 142-166, which include the first helix) is added to the prion protein solution; the temperature cycle no longer reduces the number of heme binding sites. This would indicate that the peptide may alter or inhibit the polymer formation.

Beta7E-beta132K salt bridge and sickle haemoglobin stability and conformation

The liganded (R-state) form of sickle cell haemoglobin (HbS) is of particular relevance at non-polymerizing concentrations as oxy HbS exhibits unusual properties compared with oxy HbA: mechanical precipitability (resulting from surface denaturation), greater unfolding at an air-water interface and a tendency to oxidize more readily. In human haemoglobins, the beta7 (A4) Glu residue forms an intrachain salt bridge with beta132 (H10) Lys in both liganded and deoxy structures. In the present study, recombinant haemoglobins with substitutions in the beta7 and beta132 sites were studied in order to determine the role of the beta7-beta132 salt bridge on Hb conformational integrity and stability. The elimination of this interhelix bridge correlates with enhanced surface denaturation and conformational alterations in the central cavity 2,3-diphosphoglycerate (DPG) cleft and alpha1beta2 interface. The A-helix beta7 Ala substitution generates a class of conformational change at the DPG pocket and alpha1beta2 interface that is distinct from that dictated by the H-helix beta132 Ala substitution. These results are significant with regard to the communication pathway between the alpha1beta1 and alpha1beta2 interfaces, and the new understanding of Hb allostery dependent upon tertiary structural constraints caused by effector binding to the R-state.

Conformational changes in hemoglobin S (betaE6V) imposed by mutation of the beta Glu7-beta Lys132 salt bridge and detected by UV resonance Raman spectroscopy

The impact upon molecular structure of an additional point mutation adjacent to the existing E6V mutation in sickle cell hemoglobin was probed spectroscopically. The UV resonance Raman results show that the conformational consequences of mutating the salt bridge pair, betaGlu(7)-betaLys(132), are dependent on which residue of the pair is modified. The betaK132A mutants exhibit the spectroscopic signatures of the R --> T state transition in both the "hinge" and "switch" regions of the alpha(1)beta(2) interface. Both singly and doubly mutated hemoglobin (Hb) betaepsilon7Alpha exhibit the switch region signature for the R --> T quaternary state transition but not the hinge signature. The absence of this hinge region-associated quaternary change is the likely origin of the observed increased oxygen binding affinity for the Hb betaepsilon7Alpha mutants. The observed large decrease in the W3 alpha14beta15 band intensity for doubly mutated Hb betaepsilon7Alpha is attributed to an enhanced separation in the A helix-E helix tertiary contact of the beta subunits. The results for the Hb A betaGlu(7)-betaLys(132) salt bridge mutants demonstrate that attaining the T state conformation at the hinge region of the alpha(1)beta(2) dimer interface can be achieved through different intraglobin pathways; these pathways are subject to subtle mutagenic manipulation at sites well removed from the dimer interface.