Effects of saccharide and salt binding on dimer-tetramer equilibrium of concanavalin A.

The effect of the binding of saccharide ligands on the reversible dimer-tetramer equilibrium on concanavalin A was studied by the high-speed sedimentation equilibrium technique. Both commercial and highly purified fragment-free concanavalin A preparations were used. In the case of the fragment-free preparation, there was no effect of the binding of alpha-methyl mannoside or alpha-methyl glucoside at 35 degrees C and at a variety of conditions of pH and ionic strength. This implies no difference in ligand binding activity between dimeric and tetrameric Con A, in contrast to an earlier report [McKenzie, G. H., & Sawyer, W. H. (1973) J. Biol. Chem. 248, 549-556]. There was a profound effect in the case of the commercial preparation. Dimers that contain hydrolyzed subunits appear to be incompetent to self-associate in the presence of alpha-methyl mannoside or alpha-methyl glycoside, while alpha-methyl galactoside, which does not bind to Con A, had no effect. The effects of very high concentrations of CaCl2 (to 2.5 m) and NaCl (60 6.2 m) were also studied. The data were analyzed by an integrated form of the Tanford extension [Tanford, C. (1969) J. Mol. Biol. 39, 539-544] of the Wyman linked function theory, which includes preferential interactions with salt and water. The integrated form allows preferential interactions to be described as the sum of salt binding and water binding. The data were well described by salt binding alone; it was unnecessary to invoke any water binding effect. The CaCl2 data did indicate that one calcium per subunit of the dimer binds to a site that is buried in the tetramer. This suggests a site on the dimmer-dimer interface which is consistent with Reeke's identification of the protomers composing the solution dimer [Reeke, G. N., Jr., Becker, J. W., & Edelman, G. M. (1975) J. Biol. Chem. 250, 1525-1547].