Role of beta87 Thr in the beta6 Val acceptor site during deoxy Hb S polymerization

Three new Hb S variants containing beta87 Leu, Trp, or Asp instead of Thr were expressed in yeast in order to further define the role of the beta87 position in stability and polymerization of deoxy Hb S. Previous studies showed that hydrophobicity at beta85 Phe and beta88 Leu is critical for stabilization of hemoglobin. Results with the three Hb S beta87 variants, however, showed minimal differences in stability, suggesting that beta87 amino acid hydrophobicity is not critical for stabilization of hemoglobin. Polymerization properties of the variants in the deoxy form, however, were affected by the beta87 amino acid. Polymerization of Hb S beta87 Thr --> Leu and Hb S beta87 Thr --> Trp was preceded by a delay time like Hb S, while Hb S beta87 Thr --> Asp did not show a delay time. In addition, changes in time required for half polymer formation (T1/2) as a function of hemoglobin concentration for Hb S beta87 Thr --> Asp were similar to that for beta87 Thr --> Gln. Hb S beta87 Thr --> Leu polymerized at a lower hemoglobin concentration than Hb S while beta87 Thr --> Trp and Hb S beta87 Thr --> Asp required much higher hemoglobin concentrations for polymer formation. Critical concentration required for deoxy Hb S beta87 Thr --> Asp polymerization was 6- and 2.3-fold greater than that for Hb S beta85 Phe --> Glu and Hb S beta88 Leu --> Glu, respectively. These results suggest that even though beta87 Thr is not a direct interaction site for beta6 Val in deoxy Hb S polymers, it does play a critical role in formation of the hydrophobic acceptor pocket which then promotes protein-protein interactions facilitating formation of stable nuclei and polymers of deoxy Hb S.