CRITERIA for SELECTING SUPPORT for IMMOBILIZATION of ENZYMES

The criteria that should be taken into account for selecting a support for enzyme immobilization are:

• Surface properties of the support.
• Interactions between support and enzyme.
• Interactions between support and reaction mixture.
• Physical and Mechanical properties of the support.

Surface properties of the support
A variety of support materials are used for immobilization of enzymes.

Activated charcoal, silica, alumina, clay, glass etc., are used as support materials for adsorption.
Cellulose, carboxymethyl cellulose, dextrose, agarose are some of the important supports used in covalent immobilization.
The choice of the support depends on the surface properties of the support material.
The surface should have the capacity to adsorb the enzyme or have functional groups essential for attachment of enzymes.
If the support doesn't have the functional groups essential for attachment of enzymes, they should be modified/activated.
In many covalent immobilization processes the first step is the surface modification or activation. Two commonly used for surface modification or activation are silanisation and attachment of flexible spacer arm moeties .

Interactions between support and the enzyme
The support binds the enzyme through certain functional groups.
If the support binds to the residues with in the active site and neat the active site then the enzyme activity will be reduced.
Attachment of support near active site of the enzyme renders it unavailable for the catalytic process.

Interactions between support and reaction mixture
The support will interact with the reaction mixture also.
For example:
A charged support causes the concentration of the opposite charged ions near its surface. This affects the pH of the local environment around the enzyme.
Similarly the hydrophobicity or hydrophilicity of the support will influence the local concentrations of the solutes and solvents according to their hydrophobicity or hydrophilicity.
Thus the interaction between the support and the reaction mixture creates a local environment around the enzyme that differs from the bulk solution.

Physical and mechanical properties of the support
The physical properties of the support that are significant are:

• Porosity
• Mechanical strength
• Compressibility
• Swelling properties.

Porosity of the material determines the amount of enzyme that can be immobilized and the amount of substrate that would be available to the enzymes immobilized on the internal surfaces of the support.
High mechanical strength is requires if the immobilized catalysis are to be used in agitated reactors. High compressibility is undesirable if the enzymes are to be used for large scale packed column applications.