Industrial Biotransformations

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7 Quantitative Analysis of Industrial Biotransformations
oxidizing cells
Rreducingcells
isomerases
lyases
transferases
hydrolases
oxidoreductases
lipases
proteases
glycosidases
Fig. 7.4 Enzyme types used in industrial biotransformations.
amidases
other
Processes involving redox biotransformations are very important. Some of these
involve two isolated oxidoreductases, one for the actual biotransformation and one for
the regeneration of the cofactor NAD(P)H. However, in the majority of redox biotransformations,
for example for all steroid biotransformations, metabolizing cells are applied.
These cells can use their primary metabolism to regenerate redox cofactors. They often
use glucose as the source of electrons, or oxygen as the sink for electrons.
Whole cells are also used in many non-redox biotransformations. In the absence of
cofactor regeneration cycles, only the key biotransformation enzymes will be active. Cells
can be used merely as crude enzyme preparations, in order to save on enzyme purification
costs. Figure 7.5 indicates that whole cells are more popular than (partly) isolated
enzymes. When whole cells are used, immobilization is less common than when isolated
enzymes are used. For redox biotransformations with whole cells, immobilization will be
avoided when oxygen is used. Owing to the very low aqueous solubility of oxygen, oxygen
diffusion limitation in particles of several millimeters can not really be prevented at reasonable
reaction rates. When redox biotransformations by whole cells are not taken into
account, immobilization is performed in almost half of the industrial biotransformations,
not only for enzymatic processes but also for the remaining whole cell processes.
In a few cases ultrafiltration membranes are used to retain the biocatalysts, but binding
on or in particles is much more common.
Retention of biocatalysts by these methods is a prerequisite for using continuous reactors
in industrial biotransformations. On the other hand, in almost half of the cases
free cells
immobilized
cells
immobilized
enzymes
not reported
free enzymes
Fig. 7.5 Use of enzymes or whole cells in industrial biotransformations.