The equilibrium binding properties of ferric Aplysia myoglobin have been studied for a number of anionic ligands in the pH region from neutrality to ~4. For all the ligands studied, the intrinsic affinity of Aplysia metmyoglobin increases by more than one order of magnitude as the pH is lowered well below neutrality. The spectroscopic properties of the ligand-free and the ligand-bound molecules show a pH dependence with apparent pK values of 4.7 and 6.1, respectively. On the basis of temperature-jump experiments, a kinetic scheme has been proposed and rate constants have been measured for the binding of azide at pH 6 and pH 4. Kinetic and thermodynamic features match each other, suggesting that a single ionizing group is responsible for all the observed effects. By inspection of the three-dimensional structure, this group has been tentatively identified as the proximal imidazole. Protonation of the Nε of proximal histidine would be associated to the rupture of the proximal bond, giving rise to the formation of a tetra-co-ordinate, ligand-free and penta-co-ordinate, ligand-bound molecule.
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