Sickle hemoglobin polymer stability probed by triple and quadruple mutant hybrids

The Sickle-Cell Fiber Revisited

Sickle cell disease is the consequence of a single point mutation on the surface of the β chains of the hemoglobin molecule leading to the formation of rigid polymers that disrupt circulation. It has long been established that the polymers are comprised of seven pairs of double strands that are twisted replicas of the double strands found in crystals. Here, we review several newer developments that elaborate on that simple model and provide deeper insights into the process.

Free energy of sickle hemoglobin polymerization: a scaled-particle treatment for use with dextran as a crowding agent

Fundamental to the analysis of protein polymerization is the free energy of association, typically determined from solubility. It has been previously shown that concentrated 70 kDa dextran lowers the solubility of sickle hemoglobin, due to molecular crowding, and provides a useful ranking tool for the effects of inhibitors and molecular modifications. Because hemoglobin occupies a substantial volume as well, crowding effects of both hemoglobin and dextran contribute to the nonideality of the solution. We show how scaled-particle theory can be used to account for both types of crowding, thus allowing the determination of solubility in the absence of dextran, given data measured in its presence. The approach adopted approximates dextran as a sphere with a volume that decreases as the concentration of dextran increases. We use an asymptotic relation to describe the volume, which decreases nearly linearly by a factor of two over the range studied, from 60 to 230 mg/ml. This compression is similar to previously observed compression of sephadex beads and ficoll solutions. In the limit of low hemoglobin concentrations, the theory reduces to the previously-used approach of Ogston. Our method therefore provides a means of measuring the free energy of association of molecules that occupy significant volume fractions, even when assisted by the crowding of dextran and we present a tabulation of all known free energies of polymerization of sickle hemoglobin measured in the presence of dextran.

Modification of axial fiber contact residues impact sickle hemoglobin polymerization by perturbing a network of coupled interactions

The identity of intermolecular contact residues in sickle hemoglobin (HbS) fiber is largely known. However, our knowledge about combinatorial effects of two or more contact sites or the mechanistic basis of such effects is rather limited. Lys16, His20, and Glu23 of the alpha-chain occur in intra-double strand axial contacts in the sickle hemoglobin (HbS) fiber. Here we have constructed two novel double mutants, HbS (K16Q/E23Q) and (H20Q/E23Q), with a view to delineate cumulative impact of interactions emanating from the above contact sites. Far-UV and visible region CD spectra of the double mutants were similar to the native HbS indicating the presence of native-like secondary and tertiary structure in the mutants. The quaternary structures in both the mutants were also preserved as judged by the derivative UV spectra of liganded (oxy) and unliganded (deoxy) forms of the double mutants. However, the double mutants displayed interesting polymerization behavior. The polymerization behaviour of the double mutants was found to be non-additive of the individual single mutants. While HbS (H20Q/E23Q) showed inhibitory effect similar to that of HbS (E23Q), the intrinsic inhibitory propensity of the associated single mutants was totally quelled in HbS (K16Q/E23Q) double mutant. Molecular dynamics (MD) simulations studies of the isolated alpha-chains as well as a module of the fiber containing the double and associated single mutants suggested that these contact sites at the axial interface of the fiber impact HbS polymerization through a coupled interaction network. The overall results demonstrate a subtle role of dynamics and electrostatics in the polymer formation and provide insights about interaction-linkage in HbS fiber assembly.

Two-step mechanism of homogeneous nucleation of sickle cell hemoglobin polymers

Sickle cell anemia is a debilitating genetic disease that affects hundreds of thousands of babies born each year worldwide. Its primary pathogenic event is the polymerization of a mutant, sickle cell, hemoglobin (HbS); and this is one of a line of diseases (Alzheimer's, Huntington's, prion, etc.) in which nucleation initiates pathophysiology. We show that the homogeneous nucleation of HbS polymers follows a two-step mechanism with metastable dense liquid clusters serving as precursor to the ordered nuclei of the HbS polymer. The evidence comes from data on the rates of fiber nucleation and growth and nucleation delay times, the interaction of fibers with polarized light, and mesoscopic metastable HbS clusters in solution. The presence of a precursor in the HbS nucleation mechanism potentially allows low-concentration solution components to strongly affect the nucleation kinetics. The variations of these concentrations in patients might account for the high variability of the disease in genetically identical patients. In addition, these components can potentially be utilized for control of HbS polymerization and treatment of the disease.

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.