Thermodynamics of concanavalin A dimer-tetramer self-association: sedimentation equilibrium studies

The effects of temperature and pH on the reversible dimer-tetramer association of concanavalin A were studied by the high-speed sedimentation equilibrium technique. Both commercial and highly purified preparations of concanavalin A were used. Equilibrium constants were analyzed as a Wyman linked function by using truncated van't Hoff temperature dependence. In the concentration range 0.1-3.0 mg/mL, from 5 to 35 degrees C and between pH 5.5 and pH 7.5 at 0.5 M ionic strength, only dimer and tetramer species were present in both preparations. For purified concanavalin A, association constants ranged from 1.5 X 10(3) to 8.0 X 10(7) M-1. Constants for our commercial preparation were approximately 10-fold lower due to the decrease competency of some subunits to self-associate. From the fit of the Wyman model to the experimental data, delta G degree, delta H degree, delta S degree, and delta Cp degree were calculated for the association and association-linked ionization reactions. From the values of the ionization thermodynamic parameters, the association is governed by the ionization of a histidine side chain on each subunit, either histidine-51 or histidine-121. The association is characterized by large entropy (66.3 cal.mol-1.deg-1 at 25 degrees C) and heat capacity (-821 cal.mol-1.deg-1) changes in accordance with the large hydrophobic association surface observed in crystallographic studies [Reeke, G. N., Jr., Becker, J. W., & Edelman, G. M. (1975) J. Biol. Chem. 250, 1525-1547]. In addition, there is a large enthalpy change (10.4 kcal.mol-1 at 25 degrees C). We propose a model for the interaction based on a more detailed thermodynamic description than was obtained in an earlier, incomplete study [Huet, M., & Clavarie, J. M. (1978) Biochemistry 17, 236-241].