Multiphase Bioreactor Design

(ii) Partitioning effects, related to the chemical nature of the support material, may arise
from electrostatic or hydrophobic interactions between the matrix and low molecular
weight species present in the solution, leading to a modified microenvironment.
(iii) Mass transfer or diffusional effects arise from diffusional resistance to the
translocation of substrate from the bulk solution to the catalytic sites and from the
diffusion of products of the reaction back to the bulk solution. These diffusional
resistances may be classified as internal or intraparticle mass transfer effects when the
enzyme is located in a porous medium, and external or interparticle mass transfer
effects when they occur between the bulk solution and the outer surface of the
enzyme-matrix particle.

When the kinetic behaviour of immobilised enzyme can be controlled by one or more of
the above effects, it is useful to distinguish among (i) intrinsic rate parameters, the kinetic
parameters inherent in a particular immobilised enzyme and that are different from those
of free enzyme (because of conformational change and steric effects); (ii) inherent rate
parameters, the kinetic parameters that are observed in the absence of any diffusional
effects; (iii) effective rate parameters, the kinetic parameters when mass transfer effects
are present and in the presence or the absence of partition effects.

Conformational and steric effects

The decrease of specific activity of enzymes, which occurs on their binding either to solid
supports or upon intermolecular crosslinking, is usually attributed to conformational
changes in the tertiary structure of the enzymes. For instance, co valent bonds between
the enzyme and the matrix can stretch the enzyme molecule and thus the three-
dimensional structure at the active site. The specific activity decrease may also be
attributed to steric hindrance resulting in limits on the accessibility of the substrate. In
these two cases the decrease in enzyme activity can be reduced or prevented by choosing
suitable conditions for immobilisation. Thus the active center of the enzyme can be
protected with a specific inhibitor, substrate, or product, and the shielding effect of the
matrix that causes steric hindrances can be reduced by the introduction of “spacers” that
keep the enzyme at a definite and certain distance from the matrix. In addition to these
problems, denaturation of the enzyme can arise by the action of reagents used in
entrapment methods.
In addition to their influence on the enzyme activity, any physical or chemical
matrixenzyme interactions may additionally modify the selectivity and stability of the
bound enzyme from that which it normally possesses in free solution.

Partition effects

In the carrier binding method, when the support matrix is charged the kinetic behaviour
of the immobilised enzyme may differ from that of the free enzyme even in the absence
of mass transfer effects. This difference is commonly attributed to partition effects that
cause different concentrations of charged species, substrates, products, hydrogen ions,
hydroxyl ions, and so on, in the domain of the immobilised enzyme and in the domain of
the bulk solution, owing to electrostatic interactions with fixed charges on the support.

Design and modelling of immobilised biocatalytic reactors 93