Biomedical Engineering Reference
In-Depth Information
However, even when an enzyme is identified as being useful for a given reaction, its
application is often hampered by its lack of long-term stability under process conditions
and also by difficulties in recovery and recycling. The instability of enzyme structures was
revealed once they are isolated from their natural environments. Furthermore, unlike con-
ventional heterogeneous chemical catalysts, most enzymes operate dissolved in water in
homogeneous catalysis systems, which is why they contaminate the product and as a rule
cannot be recovered in the active form from reaction mixtures for reuse.
Immobilization is achieved by fixing enzymes to or within solid supports, as a result of
which heterogeneous immobilized enzyme systems are obtained. Biocatalytic process eco-
nomics can be enhanced by enzyme reuse and the improvement in enzyme stability afforded
by immobilization. It can also improve enzyme performance under optimal process reaction
conditions (e.g., acidity, alkalinity, organic solvents, and elevated temperatures), a require-
ment that has often retarded enzyme application in industrial chemical synthesis [3].
There is no general universally applicable method of enzyme immobilization. The main
task is to select a suitable carrier. The selected method should meet both the catalytic needs
(expressed in productivity, space-time yield, stability, and selectivity) and the noncatalytic
needs (e.g., separation, control, and downstreaming process) that are required by a given
application. Cao [4] believes that a rational combination of various immobilization meth-
ods and methodologies is a valuable method to obtain robust immobilized enzymes,
which cannot be obtained by the straightforward immobilization.
Of the many carriers that have been considered and studied for immobilizing enzymes,
organic or inorganic, natural or synthetic, chitin and chitosan are of interest in that they
offer advantages such as high affinity to proteins, availability of reactive functional groups
(-OH and -NH 2 ) for direct reactions with enzymes and for chemical modifications, hydro-
philicity, mechanical stability and rigidity, regenerability, and ease of preparation in dif-
ferent geometrical configurations that provide the system with permeability and surface
area suitable for a chosen biotransformation [2].
Krajewska [2] has reviewed the application of chitin- and chitosan-based materials in
enzyme immobilization and the range of uses of such complexes. Here we will mainly
review the immobilization supports such as chitosan gel (see Section 8.3) , chitosan modi-
fications (see Section 8.4), and chitosan composites (see Section 8.5). Compared with tissue
engineering and drug delivery application in vivo , chitosan supports for enzyme immobi-
lization were more abundant due to little limitation from in vitro . The application of chitin
is not introduced in this chapter. Methods of enzyme immobilization on a chitosan/chitin-
based support are described in Section 8.2 (including the current methods and supports
for enzyme immobilization).
8.2 Enzyme Immobilization and Chitosan-Based Supports
8.2.1 Current Methods of enzyme immobilization
Enzymes may be immobilized by a variety of methods, which may be traditionally classi-
fied as physical, where weak interactions between the support and the enzyme exist, and
chemical, where covalent bonds are formed with the enzyme. Brady [5] presents enzyme
immobilization strategies the can be broadly divided into four groups ( Figure 8.1): entrap-
ment, encapsulation, solid support and self-immobilization.
 
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