2009_Book_FoodChemistry

148 2 Enzymes
Fig. 2.47. Enzyme immobilization by covalent binding
to a cellulose matrix
suitable processes can bring about enzyme
encapsulation in a semipermeable membrane
(microencapsulation) or confinement in hollow
fiber bundles.
Kinetics. As a rule, higher substrate concentrations
are required for saturation of an entrapped
enzyme than for a free, native enzyme. This is
due to a decrease in the concentration gradient
which takes place in the pores of the polymer
network. Also, there is an increase in the “apparent”
Michaelis constant for an enzyme bound
covalently to a matrix carrying an electrostatic
charge. This is also true when the substrate and
the functional groups of the matrix carry the
same charge. On the other hand, opposite charges
bring about an increase of substrate affinity for
the matrix. Consequently, this decreases the
“apparent” K m .
pH Optimum. Negatively charged groups on
a carrier matrix shift the pH optimum of the
covalently bound enzyme to the alkaline region,
whereas positive charges shift the pH optimum
towards lower pH values. The change in pH
optimum of an immobilized enzyme can amount
to one to two pH units in comparison to that of
a free, native enzyme.
Thermal Inactivation. Unlike native enzymes, the
immobilized forms are often more heat stable (cf.
example for β-D-glucosidase, Fig. 2.48). Heat stability
and pH optima changes induced by immobilization
are of great interest in the industrial utilization
of enzymes.
2.7.1.2.3 Cross-Linked Enzymes
Derivatization of enzymes using a bifunctional
reagent, e. g. glutaraldehyde, can result in crosslinking
of the enzyme and, thus, formation of
large, still catalytically active insoluble complexes.
Such enzyme preparations are relatively
unstable for handling and, therefore, are used
mostly for analytical work.
2.7.1.2.4 Properties
The properties of an immobilized enzyme are often
affected by the matrix and the methods used
for immobilization.
Fig. 2.48. Thermal stabilities of free and immobilized
enzymes (according to Zaborsky, 1973). 1 β-Dglucosidase,
free, 2 β-D-glucosidase, immobilized