Physical Adsorption Mode

        This method for the immobilization of an enzyme is based on the physical adsorption of enzyme protein on the surface of water-insoluble carriers. Hence, the method causes little or no conformational change of the enzyme or destruction of its active center. If a suitable carrier is found, this method can be both simple and cheap. However, it has the disadvantage that the adsorbed enzyme may leak from the carrier during use due to a weak binding force between the enzyme and the carrier. The earliest example of enzyme immobilization using this method is the adsorption of beta-D-fructo-furanosidase onto aluminum hydroxide. The processes available for physical adsorption of enzymes are:         Of the four techniques, the most frequently used in the lab is Mixing-Bath Loading . For commercial purposes the preferred method is Reactor Loading .

        A major advantage of adsorption as a general method of immobilizing enzymes is that usually no reagents and only a minimum of activation steps are required. Adsorption tends to be less disruptive to the enzymatic protein than chemical means of attachment because the binding is mainly by hydrogen bonds, multiple salt linkages, and Van der Waal's forces. In this respect, the method bears the greatest similarity to the situation found in natural biological membranes and has been used to model such systems.

        Because of the weak bonds involved, desorption of the protein resulting from changes in temperature, pH, ionic strength or even the mere presence of substrate, is often observed. Another disadvantage is non-specific, further adsorption of other proteins or other substances as the immobilized enzyme is used. This may alter the properties of the immobilized enzyme or, if the substance adsorbed is a substrate for the enzyme, the rate will probably decrease depending on the surface mobility of enzyme and substrate.

        Adsorption of the enzyme may be necessary to facilitate the covalent reactions described later in this presentation. Stabilization of enzymes temporarily adsorbed onto a matrix has been achieved by cross-linking the protein in a chemical reaction subsequent to its physical adsorption.



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