ENTHALPY OF BINDING OF VARIOUS TRANSITION METAL IONS TO BOVINE APOCARBONIC ANHYDRASE

Reaction calorimetry has been employed to determine the enthalpy changes accompanying the binding of Zn2+, Co2+, Cu2+, Cd2+, and Ni2+ to the apoproteins derived from bovine carbonic anhydrases A and B. Small increases in enthalpy, ranging from 3 to 10 kcal mole-1, were observed. Combination of the data for zinc with the equilibrium measurements of Lindskog and Malmström (Lindskog, S., and Malström, B. G. (1962), J. Biol. Chem. 237, 1129) leads to thermodynamic parameters for the binding process at 25° and pH 7.0 as follows: ∆G′ = -16.4 kcal mole-1 (standard state for hydrogen ions at activity of 10-7 m), ∆S′ = + 88.0 cal deg-1 mole-1, and ∆H = 9.8 kcal mole-1. Comparison of these thermodynamic parameters with those which have been reported for a variety of small ligands suggests that carboxylate ions may contribute significantly to the binding of zinc in the enzyme. It appears that the crevice (Fridborg, K., Kannan, K. K., Liljas, A., Lundin, J., Strandberg, B., Strandberg, R., Tilander, B., and Wiren, G. (1967), J. Mol. Biol. 25, 505) in which the zinc is bound is polar in character in the apoenzyme and becomes relatively nonpolar on addition of the zinc ion. © 1969, American Chemical Society. All rights reserved.