Biomedical Engineering Reference
In-Depth Information
The issue of the regulation of cells and recombinant DNA will undoubtedly undergo
further refinement with time. Both laboratory and manufacturing personnel need to keep
abreast of any such changes.
14.9. METABOLIC ENGINEERING
Metabolic or pathway engineering uses the tools of genetic engineering to endow an
organism to make a totally new pathway, amplify an existing pathway, disable an undesired
pathway, or alter the regulation of a pathway. The principle motivations for metabolic engi-
neering are the production of specialty chemicals (e.g. indigo, biotin, and amino acids), utili-
zation of alternative substrates (e.g. pentose sugars from hemicellulose), or degradation of
hazardous wastes such as benzoates or trichlorethylene. The same concepts form the basis
for gene therapy.
One may ask why genetically engineered organisms should be used instead of natural
isolates. The potential advantages over natural isolates are as follows:
Can put an “odd-ball” pathway under the control of a regulated promoter. The
investigator can turn on the pathway in situations where the pathway might normally be
suppressed (e.g. degradation of a hazardous compound to a concentration lower than
necessary to induce the pathway in the natural isolate).
High levels of enzymes in desired pathways can be obtained with strong promoters; only
low activity levels may be present in the natural isolate.
Pathways moved from lower eukaryotes to bacteria can be controlled by a single
promoter; in lower eukaryotes, each protein has a separate promoter.
Several pathways can be combined in a single organism by recruiting enzymes from more
than one organism.
Can move a pathway from an organism that grows poorly to one that can be more easily
cultured.
The genetically engineered cell can be proprietary property.
Cells that have engineered pathways face many of the same limitations that cells engi-
neered to produce proteins face. Two issues that perhaps assume greater importance with
metabolic engineering are stability and regulatory constraints.
Protein products are of high value and can be made in batch culture. Instability is avoided
by inducing overproduction only at the end of the culture cycle. The productive phase is too
short for nonproducers to grow to a significant level, and cells are not reused. With metabol-
ically engineered cells, the same strategy is untenable. Lower product values necessitate cell
reuse or, at least, extended use. The use of antibiotics as selective agents may be undesirable
because of contamination of product or cost. For a culture with a maximum growth rate of
m
max
¼
0.45 h
1
and a 20-h batch cycle, a continuous system has a 14-fold advantage in
productivity over a batch system. Although the levels of protein overproduction are lower
in metabolically engineered cells, they can experience as high a level of metabolic burden
as “protein producers” because of the diversion of cellular building blocks to nonessential
metabolites. Also, if the cells are used to treat hazardous compounds, the genetically
engineered cells will face competition from a natural flora.
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