Info

#-Hydroxy-succinimidyl

Amine

26,27

There are many commercially available supports for immobilization, the best choice in each case requires the consideration of some relevant properties of the catalyst and the intended use. However, it is usually necessary to try more than one approach and then adapt a method to the specific circumstances (20,21).

The covalent reactions commonly employed give rise to enzymes linked to the support through either amide, ether, thio-ether, or carbamate bonds. Therefore, the enzyme is strongly bound to the matrix and, in many cases, it is also stabilized, which will be discussed later in Subheading 6. However, because of the covalent nature of the bond, the matrix has to be discarded together with the enzyme once the enzymatic activity decays. The benefit of obtaining a leak-proof binding between enzyme and matrix resulting from these reactions is partially offset by the cost, in terms of generally low yield of immobilized activity and by the nonreversible character of this binding. Enzymes attached covalently by disulfide bonds to solid supports represent one way to avoid this problem and will be described in Chapter 17.

4.2. Entrapment

The entrapment method is based on the occlusion of an enzyme within a polymeric network that allows the substrate and products to pass through but retains the enzyme (35). This method differs from the coupling methods described above, in that the enzyme is not bound to the matrix or membrane. There are different approaches to entrapping enzymes such as gel (36) or fiber entrapping (37) and micro-encapsulation (38). The practical use of these methods is limited by mass transfer limitations through membranes or gels.

5. Methods of Reversible Immobilization

Because of the type of the enzyme-support binding, reversibly immobilized enzymes can be detached from the support under gentle conditions (see Fig. 2). The use of reversible methods for enzyme immobilization is highly attractive, mostly for economic reasons because when the enzymatic activity decays the support can be regenerated and re-loaded with fresh enzyme. Indeed, the cost of the

Table 5B

Modification of the Polymer Backbone to Produce an Activated Group

Polymer

Group that reacts

Reagent

Activated group produced

Group that reacts (with activated matrix)

Refs.

Cellulose

Diol

Periodate

Aldehyde

Amine

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