Retention mechanism of β-blockers on an immobilized cellulase.: Relative importance of the hydrophobic and ionic contributions to their enantioselective and nonselective interactions

The adsorption isotherms of the enantiomers of three P-blockers, metoprolol, alprenolol, and propranolol, were measured on cellobiohydrolase I (CBH I) immobilized on silicagel, in the concentration range between 0.25 mu M and 1.7 mM, at pH = 5.0, 5.5, and 6.0, In agreement with previous results, these data are accounted for by a two-sites physical model and fit closely to a Bilangmuir equation. The saturation capacities and the binding constants were determined for each enantiomer on the chiral and the nonchiral sites. The chiral sites are shown to be strongly ionic, in contrast to the nonchiral ones, which are mainly hydrophobic, However, the chiral binding of (S)-propranolol is endothermic, with a high adsorption entropy, in contrast to the chiral interactions of (R)-propranolol and to the nonchiral interactions, which are all exothermic, This indicates that hydrophobic interactions also play a role in the chiral binding. The dependence of the adsorption parameters on the hydrophobicity of the solute is discussed and interpreted in terms of the retention mechanism. The results are compared with the structure of the protein, recently elucidated by X-ray crystallography.