Osmolyte effects on the self-association of concanavalin A: testing theoretical models

The formation and stability of protein-protein interfaces are of obvious biological importance. While a large body of literature exists describing the effect of osmolytes on protein folding, very few studies address the effect of osmolytes on protein association and binding. The plant lectin concanavalin A (ConA), which undergoes a reversible tetramer-to-dimer equilibrium as a function of pH, was used as a model system to investigate the influence of nine osmolytes on protein self-association. The stabilizing or destabilizing impacts of the osmolytes were evaluated from pH titrations combined with circular dichroism spectroscopy. Relative to the dimer, trimethylamine N-oxide, betaine, proline, sarcosine, sorbitol, sucrose, and trehalose all stabilized the ConA tetramer to varying extents. Glycerol had a negligible effect, and urea destabilized the tetramer. From multiple titrations in different osmolyte concentrations, an m-value (a thermodynamic parameter describing the change in the association free energy per molar of osmolyte) was determined for each osmolyte. Experimental m-values were compared with those calculated using two theoretical models. The Tanford transfer model, with transfer free energies determined by Bolen and co-workers, failed to accurately predict the m-values in most cases. A model developed by Record and co-workers, currently applicable only to urea, betaine, and proline, more accurately predicted our experimental m-values, but significant discrepancies remained. Further theoretical work is needed to develop a thermodynamic model to predict the effect of osmolytes on protein-protein interfaces, and further experimental work is needed to determine if there is a general stabilization by osmolytes of such interfaces.