Relationship between tetramer-dimer assembly and the stability of Hb Malmö ( α2β297Gln)

Abstract

The effects of the mutation at the α 1 β 2 contact in Hb Malmö α 2β 2 97( FG4) His a ̊ Gln ) on oxygen-binding properties, ease of dissociation into dimeric hemoglobin and stability were studied. The P 50 value of Hb Malmö in the absence of organic phosphates was 1.9. mmHg, in contrast to 8.8 mmHg of Hb A. The n-value of Hb Malmö was 1.6. The overall free energy of interaction of oxygen with Hb Malmö was about 25% that of Hb A. The Adair constant, K 1, of Hb Malmö was about 10-times larger than that of Hb A, but the K 4 of Hb Malmö was similar to that of Hb A. The liganded form of Hb Malmö was found to dissociate into dimers more readily than Hb A by gel filtration on Sephadex G-100. Dissociation into dimeric hemoglobin was enhanced in dilute solutions. Increased instability during mechanical agitation of diluted samples was greater for Hb Malmö than for Hb A. The denaturation rate constants of tetramers of the oxyform of Hb A and Hb Malmö were about 20-times greater than those of dimers of these hemoglobins. The instability of Hb Malmö depends on a greater α 1 β 2 dissociation constant compared with that of Hb A. These findings allow an examination of the role of the intersubinit contact in determining the functional properties and the stability of the hemoglobin molecule.