Chemistry Reference
In-Depth Information
4.2.1
Outline of Enzyme Immobilisation Techniques
There are many possible techniques available for enzyme immobilisation, however,
finding new and improved techniques is an area of major research [
7
]. Enzyme im-
mobilisation techniques can be classified into four categories: covalent binding, ad-
sorption, cross-linking and entrapment/encapsulation. Although only briefly listed
here, these protocols are discussed in detail in Sect. 4.3. Covalent attachment and
adsorption are of a similar theme since the enzyme becomes attached through some
form of interaction with a supporting material. This is possible because enzymes
contain chemically active, ionic and/or hydrophobic groups. These functionalities
are therefore able to interact with reactive sites on the support. These attachment
methods are most common, however, more recent approaches are now beginning
to take precedence [
7
,
11
]. See Table
4.2
for schematic illustrations of each immo-
bilisation technique.
Cross-linked enzyme aggregates, CLEAs, are a very different approach alto-
gether since they do not necessarily involve a support, and instead make use of a
bi-functional cross-linking agent which reacts with the enzyme to create aggregates.
In entrapment and encapsulation, the enzyme is confined and trapped within a sup-
port, rather than attached through binding (Table
4.2
) [
7
,
11
].
4.2.2 Supports
Care must be taken when choosing a suitable supporting material, such as resins,
glass, cellulose and silica. Materials used depend on the nature of the enzyme,
type of immobilisation and reactions involved. The performance of a biocatalyst
depends on both the enzyme itself, as well as the carrier properties as illustrated in
Fig.
4.2
.
Supporting materials should ideally have the following properties: [
2
,
3
,
7
,
12
]
• A large surface area with good geometric congruence with the enzyme;
• A structure that does not detrimentally affect enzyme performance (i.e. minimal
conformational hindrance);
• Sufficient chemical properties for immobilisation, such as adequate reactive
groups;
• Adequate physical, chemical and biological stability for the required process
conditions;
• Suitable strength and malleability;
• A low cost and be readily available.
Many of the factors mentioned come as a trade-off against each other, and there-
fore optimisation techniques to find optimal immobilisation arrangements are often
