EFFECTS OF BETA-6 AMINO-ACID HYDROPHOBICITY ON STABILITY AND SOLUBILITY OF HEMOGLOBIN TETRAMERS

The relationship between different amino acids at the beta6 position of hemoglobin and tetramer stability was addressed by a site-directed mutagenesis approach. Precipitation rates during mechanical agitation of oxyhemoglobins with Gln, Ala, Val, Leu and Trp at the beta6 position increased 2, 5, 13, 21 and 53 times, respectively, compared with that for Hb A. There was a linear relationship between the log of the precipitation rate constant and amino acid hydrophobicity at the beta6 position. suggesting that enhanced precipitation of oxy Hb S during mechanical agitation results in part from increased hydrophobicity of beta6 Val. Deoxyhemoglobin solubility increased in the order of beta6 Ile, Leu, Val, Trp, Gln, Ala and Glu suggesting that hydrophobic interactions between beta6 Val and the acceptor site of another hemoglobin molecule during deoxy-Hb S polymerization not only depend on hydrophobicity but also on stereospecificity of the amino acid side chain at the beta6 position. Furthermore, our results indicate that hydrophobic amino acids at the beta6 position which promote tetramer instability in the oxy form do not necessarily promote polymerization in the deoxy form.