Free vs chitosan-immobilized urease: Microenvironmental effects on enzyme inhibitions

Chitosan gel membranes were prepared by a solvent-evaporation method and used as a support for covalent immobilization of jack bean urease. The effects of the local microenvironment created by both the electrostatic potential of the polycationic support and the enzyme reaction on the inhibition of urease by phosphate buffer were investigated as a function of pH and compared with other urease competitive inhibitions. It was found that the kinetic behaviour of chitosan-immobilized urease in the inhibition is resultant of structural, diffusion limitation-related and microenvironmental factors. More importantly, it was shown that this behaviour is microenvironment-dependent when either a pH-dependence of enzyme inhibition (phosphate and F^-) or electrostatic inhibitor-support repulsion (Ni^2?+?) prevailed in the system. Other inhibitors uncharged that are do not show pH-dependented inhibition (boric and acetohydroxamic acids), the inhibition only depends on factors other than microenvironmental. Knowledge of such microenvironmental effects is of practical and theoretical importance in designing applications of immobilized enzymes and in clarifying the mode of action of enzymes in biological membranes in their native milieu.