THERMODYNAMICS OF METAL CATION-BINDING TO APOCONCANAVALIN-A

The thermodynamics of the binding of Mn+2, Co+2, Ni+2, Zn+2, and Cd+2 to demetallized concanavalin A (apoCon A) in 0.02 M dimethylglutaric-NaOH buffer (pH 5.0-5.8 and pH 6.9) was determined from titration calorimetry measurements at 298.2 K and at pH 5.0 from 288.2 to 308.2 K. The apparent binding constants decrease with pH, resulting from competitive protonation of the histidine 24 ligand at the binding site. The binding constants to deprotonated apoCon A increase in the order, Ni+2 > Co+2 > Zn+2 > Cd+2 > Mn+2 at pH less-than-or-equal-to 5.6, the same as their crystal field stabilization energies with the energies of Zn+2, Cd+2, and Mn+2 = 0. At 298.2 K and pH less-than-or-equal-to 5.8, the binding entropies are all approximately 200 J K-1 mol-1 and the binding enthalpies are lower for Co+2 and Ni+2 which results from negative contributions of the crystal field stabilization energies. At pH 6.9, where differential scanning calorimetry shows that apoCon A unfolds at 311.2 K, the apparent binding constants decrease to minimum values. Concanavalin A, however, unfolds at 363.2 K and its free energy difference between the unfolded and folded states is 32 +/- 2 kJ mol-1 greater than that of apoCon A. At 298.2 K, the free energy difference between the unfolded and folded states of concanavalin A and apoCon A is the same as the change in the free energy upon Mn+2 and Ca+2 binding to apoCon A.