Conformational Stability of Legume Lectins Reflect Their Different Modes of Quaternary Association: Solvent Denaturation Studies on Concanavalin A and Winged Bean Acidic Agglutinin

Thermodynamic parameters associated with the unfolding of the legume lectin, WBA II, were determined by isothermal denaturation. The analysis of isothermal denaturation data provided values for conformational stability and heat capacity for WBA II unfolding. To explore the role of intersubunit contact in stability, we carried out similar studies under identical conditions on Concanavalin A, a legume lectin of nearly similar size, buried hydrophobic surface area and tertiary structure to that of WBA II but with a different oligomerization pattern. Both proteins showed a reversible two-state unfolding with guanidine hydrochloride. As expected, the change in heat capacity upon unfolding was similar for both proteins at 3.5 and 3.7 kcal $mol^-^1 K^-^1$ for Concanavalin A and WBA II, respectively. Although the $\bigtriangleup G_{H20}$ at the maximum stability of both proteins is around 16 kcal/mol, Concanavalin A exhibits greater stability at higher temperatures. The Tg obtained for Concanavalin A and WBA II were $21 ^\circ C$ apart at 87.2 and $66.6 ^\circ C$, respectively. The higher conformational stability at higher temperatures and the Tg of Concanavalin A as compared to that of WBA II are largely due to substantial differences in the degree of subunit contact in these dimeric proteins. Ionic interactions and hydrogen bonding between the monomers of the two proteins also seem to play a significant role in the observed stability differences between these two proteins