Industrial Biotransformations

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5 Basics of Bioreaction Engineering
5.2
Biocatalyst Kinetics
5.2.1
Types of Biocatalysts
A biocatalyst is always described as a whole cell or an enzyme. In the first case we are
looking at a mini-reactor with all the necessary cofactors and sequences of enzymes concentrated
in one cell. In the second case, the main catalytic unit is isolated and purified.
In both cases optimization is possible. Furthermore, multistep biosynthetic pathways
can be changed to prevent degradation of the desired product or to produce precursors
not normally prioritized in the usual pathway. All these changes for the whole cell lead to
an optimized mini-plant. The optimization of the main catalyst is comparable to catalyst
development inside a reaction system.
Whole cells can be bacteria, fungi, plant cells or animal cells. They are subdivided into
the two groups: prokaryotic cells and eukaryotic cells.
5.2.1.1 Prokaryotic Cells
Prokaryotic cells are the “lowest microorganisms” and do not possess a true nucleus. The
nuclear material is contained in the cytoplasm of the cell. They reproduce by cell division,
are relatively small in size (0.2–10 µm) and exist as single cells or as mycelia. When
designing bioreactors, an adequate supply of nutrients as well as oxygen to the bioreactor
must be assured, as the cells, e.g., bacteria, grow rapidly. Parameters such as pH, oxygen
feed rate and temperature in the bioreactor must be optimized. Perhaps the most widely
used prokaryotic microorganism in industrial biotransformations is Escherichia coli,
which is a native to the human intestinal flora.
5.2.1.2 Eukaryotic Cells
Eukaryotic cells are higher microorganisms and have a true nucleus bounded by a
nuclear membrane. They reproduce by an indirect cell division method called mitosis, in
which the two daughter nuclei normally receive identical compliments of the number of
chromosomes characteristic of the somatic cells of the species. They are larger in size
(5–30 µm) and have a complex structure. When eukaryotic cells are used as biocatalysts,
high or low mechanical stress must be avoided by using large stirrers at slow speed and
by eliminating dead zones in the fermenter. Saccharomyces cerevisiae and Zymomonas
mobilis represent the most important eukaryotic cells used in industrial biotransformations.
5.2.2
Enzyme Structure
An enzyme is an accumulation of one or more polypeptide chains in the form of a protein.
It is unique in being capable of accelerating or producing by catalytic action a trans-