Functional modulation of the Thr72-->Ile mutant from Scapharca inaequivalvis homodimeric hemoglobin

The pH and temperature dependence of both the kinetic and thermodynamic properties of the Thr72-->Ile mutant of Scapharca inaequivalvis homodimeric hemoglobin were investigated between pH 2 and 10 and between 8 degrees C and 36 degrees C, in comparison with the wild-type recombinant protein. Results demonstrate pH-independent O2-binding properties, at least between pH 5 and 10, with the higher affinity of the mutant being related to a less negative entropy change. This observation may relate to a variation in the number of water molecules involved in the intersubunit communication. Furthermore, the kinetic properties of ligand association and dissociation seem to be in keeping with possible structural alterations of water molecules at the subunit interface occurring in the Thr72-->Ile mutant as well as with amino acid residues involved in the modulation of reactivity and cooperativity at the level of (1) the proximal side of the heme pocket and of (2) the heme propionates bridging the two subunits.