Microcalorimetric indications for ligand binding as a function of the protein for galactoside-specific plant and avian lectins

The process cascade leading to the final accommodation of the carbohydrate ligand in the lectin's binding site comprises enthalpic and entropic contri butions of the binding partners and solvent molecules. With emphasis on lac tose, N-acetyllactosamine, and thiodigalactoside as potent inhibitors of bi nding of galactoside-specific lectins, the question was addressed to what e xtent these parameters are affected as a function of the protein. The micro calorimetric study of carbohydrate association to the galectin from chicken liver (CG-16) and the agglutinin from Viscum album (VAA) revealed enthalpy -entropy compensation with evident protein type-dependent changes for N-ace tyllactosamine. Reduction of the entropic penalty by differential flexibili ty of loops or side chains and/or solvation properties of the protein will have to be reckoned with to assign a molecular cause to protein type-depend ent changes in thermodynamic parameters for lectins sharing the same monosa ccharide specificity. (C) 1999 Elsevier Science B.V. All rights reserved.