ENHANCED POLYMERIZATION OF RECOMBINANT HUMAN DEOXYHEMOGLOBIN BETA-6 GLU-]ILE

Polymerization of the deoxy form of sickle cell hemoglobin (Hb S; β6 Glu→Val) involves both hydrophobic and electrostatic intermolecular contacts. These interactions drive the mutated molecules into long fibrous rods composed of seven pairs of strands. X-ray crystallography of Hb S and electron microscopy image reconstruction of the fibers have revealed the remarkable complementarity between one of the β6 valines of each molecule (the donor site) and an acceptor site at the EF corner of a neighboring tetramer. This interaction constitutes the major lateral contact between the two strands in a pair. To estimate the relative importance of this key hydrophobic contact in polymer formation we have generated a polymerizing Hb with isoleucine at the β6 position (βE6I) by site-directed mutagenesis. The mutated β chains were produced in Escherichia coli and reassembled into functional tetramers with native α chains. Compared to native Hb S, the βE6I mutant polymerizes faster and with a shortened delay time in 1.8 M phosphate buffer, indicating an increased stability of the nuclei preceding fiber growth. The solubility of the βE6I mutant Hb is half that of native Hb S. Computer modeling of the donor-acceptor interaction shows that the presence of an isoleucine side chain at the donor site induces increased contacts with the receptor site and an increased buried surface area, in agreement with the higher hydrophobicity of the isoleucine residue. The agreement between the predicted and experimental differences in solubility suggests that the transfer of the β6 valine or isoleucine side chain from water to a hydrophobic environment is sufficient to explain the observations.