From Cellular Function to Industrial Products
39
identified to produce penicillin. With the introduction of recombinant DNA technology it is,
however, now possible to chose almost any production host for the production. Thus, a strain of
E. coli
has been constructed that can produce ethanol at a high yield, and a recombinant strain of
P. chrysogenum
can now be used to produce 7-ADCA (a precursor used for synthesis of
cephalosporins) directly by fermentation. The choice of strain does, however, often depend much
on tradition within the company, and most of the fermentation industries have a set of favorite
organisms that are used in the production of many different products. For production of a
heterologous protein,
i.e.
expression of a foreign gene in a given organism, it is also necessary to
consider many other aspects,
e.g.
is the protein correctly folded and glycosylated, and Table 2.8
gives an overview of the advantages/disadvantages of different cellular systems for the
production of recombinant proteins. Although optimization of the process continues even after
large scale production has started it is important to choose a good host system from the
beginning, particularly in the production of pharmaceuticals, since the introduction of new strains
requires a new approval of the process, and the associated costs may in some cases prevent the
introduction of a new process even though it gives a better process economy.
As indicated in Table 2.8 the choice of expression system depends on many factors, but the main
factors are:
1
) the desirability of post-translational modification and secretion,
2
) the stability of
the protein in question, and 3) the projected dose of protein per patient (which determines
whether the cost of the drug becomes critical). Thus for proteins used in large doses, such as
Table 2.8 Advantages and disadvantages of different hosts for production of recombinant proteins.
Host
Advantages
Disadvantages
Bacteria (
E. coli)
Wide choice of cloning vectors
Gene expression easy to control
Large yields possible
Good protein secretion
Post-translational modifications
lacking
High endotoxin content
Protein aggregation (inclusion
bodies)
Yeast
(S. cerevisiae)
Generally regarded as safe (GRAS)
No pathogens for humans
Large scale production established
Some post-translational modifications
possible
Less cloning vectors available
Glycosylation not identical to
mammalian glycosylation
Genetics less understood
Filamentous fungi
Experience with large scale production
Source of many industrial enzymes
Excellent protein secretion
High level of heterologous
protein expression has not
been achieved
Genetics not well characterized
Mammalian cells
Same biological activity as natural protein
Expression vectors available
Cells difficult to grow in
bioreactors
Expensive
Slow growth
Low productivity
Cultured insect cell
High level of gene expression possible
Post-translational modification possible
Not always 100% active proteins
Mechanisms largely unknown
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