Steric and hydrophobic determinants of the solubilities of recombinant sickle cell hemoglobins

Abstract

Models for the structure of the fibers of deoxy sickle cell hemoglobin (Hb S, β6 Glu → Val) have been obtained from X‐ray and electron microscopic studies. Recent molecular dynamics calculations of polymer formation give new insights on the various specific interactions between monomers. Site‐directed mutagenesis with expression of the Hb S β subunits in Escherichia coli provides the experimental tools to test these models. For Hb S, the β6 Val residue is intimately involved in a specific lateral contact, at the donor site, that interacts with the acceptor site of an adjacent molecule composed predominantly of the hydrophobic residues Phe 85 and Leu 88. Comparing natural and artificial mutants indicates that the solubility of deoxyHb decreases in relation to the surface hydropho‐bicity of the residue at the β6 position with Ile > Val > Ala. We also tested the role of the stereospecific adjustment between the donor and acceptor sites by substituting Trp for Glu at the β36 location. Among these hydrophobic substitutions and under our experimental conditions, only Val and Ile were observed to induce polymer formation. The interactions for the Ala mutant are too weak whereas a Trp residue inhibits aggregation through steric hindrance at the acceptor site of the lateral contact. Increasing the hydrophobicity at the axial contact between tetramers of the same strand also contributes to the stability of the double strand. This is demonstrated by associating the β23 Val → Ile mutation at the axial contact with either the β6 Glu → Val or β6 Glu → Ile substitution in the same β subunit. In comparison to native Hb S the solubility of these deoxyHb double mutants is decreased two‐ and fourfold, respectively.