Multiphase Bioreactor Design

composition

EMPIRICISM —need for case specific, multi-
parameter optimisation
—difficult process modelling and
control

The biocatalyst forms which can be immobilised range from purified enzymes to
viable microbial cells and animal and plant tissues. Isolated enzymes can give high
activity levels per unit mass or mole, high specificity, and minimum levels of side
reactions. However they are often difficult and costly to prepare. They are, in addition,
frequently unstable and in many cases require parallel cofactor regenerating systems. Due
to their relatively simple chemical nature, as compared to organelles or whole cells,
isolated or partially purified enzymes are the biocatalysts most extensively studied in
relation to immobilisation. Such studies have led to most of the present knowledge of the
nature of interactions between the biocatalysts and the attachment support or confinement
barrier they have also contributed to an understanding of the origin of activity,
specificity, or stability changes upon immobilisation, allowing the optimisation of
process operating conditions. Immobilised, purified enzymes find suitable applications in
developing biosensors and preparing high added-value substances, such as chiral
compounds. In more crude forms, immobilised enzymes are also used in large scale
applications in the carbohydrate, food and pharmaceutical industries.
Multienzyme systems, such as organelles, whole cells, or cell tissues, have some clear
advantages over isolated enzymes, from the view point of immobilisation. Being already
water-insoluble particles, they can be efficiently retained by mild, physical means, thus
preserving, in adequate conditions, the enzyme synthesising and cofactor regenerating
capabilities and providing a suitable microenvironment for single and multiple enzymatic
activities. However, the efficient use of immobilised cells relies on the control of
metabolic and physiological alterations throughout the retention procedure and the
subsquent catalytic process. The major large-scale utilisations of immobilised cell
systems take advantage of the natural tendency of many microbial species to flocculate or
to adhere to solid surfaces. Other applications are restricted to single-enzyme
transformations with non-growing cells in the manufacture of pharmaceuticals and amino
acids.

Effect of Immobilisation on the Enzyme Kinetics and Properties

Although enzyme immobilisation can be very useful, immobilisation may also change the
kinetics and other properties of the enzyme, usually with a decrease of enzyme-specific
activity. This may be ascribed to several factors:

(i) Conformational and steric effects are present—even when the enzyme is bound
without loss of activity—when conformational change of the enzyme molecule occurs
by binding to a carrier or when the interaction of the substrate with the enzyme is
affected by steric hindrance.

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