β-Galactosidase - Excipients Interaction by Docking Simulation Studies

Complementary experimental/docking approach for determining chitosan and carboxymethylchitosan ability for the formation of active polymer-β-galactosidase adducts

The spontaneous aggregation of chitosan and carboxymethylchitosan polymers can be advantageous for the enzyme confinement on these colloidal systems during immobilization processes. The initial crucial step involves the polymer-enzyme adduct formation. The objective here is to determine the interactions that drive the adduct formation between these polymers and β-galactosidase from Bacillus circulans. The chemical characterization of chitosan and its carboxymethyl-derivate allowed to explain their colloidal behavior and design the four-unit fragments ligands used for the docking study. The deacetylation degree (0.6 times lower), isoelectric point (5.2 instead 6.4) and substitution degree (DS_O = 1.779 and DS_2N = 0.441) of carboxymenthylchitosan are due to the hydroxide concentration (>25%) and 30 °C modification conditions. Favorable Van der Waals and H-bond interactions between chitosan-β-galactosidase and contribution of electrostatic attraction mediated by calcium ions for carboxymethylchitosan-β-galactosidase explained the zeta potential and dynamic light scattering results at pH 7.0. These interactions occur onto the external surface of this galactosidase, without affecting the catalytic activity. A cross-linked enzyme aggregates-type model was proposed for the formation of the adducts, based on the complementary experimental-docking results. They contribute understanding the behavior of polyelectrolyte chitosan-derived matrices for enzyme immobilization.