Page 164 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

P1: GKY/GLQ/GTK P2: GLM Final Pages
Encyclopedia of Physical Science and Technology en009I-422 July 6, 2001 19:57

392 Metabolic Engineering

Among the techniques applied are metabolic flux analysis, teins, e.g., the production of human insulin by a recom-
metabolic control analysis, DNA arrays, proteome analy- binant E. coli. With the further development in genetic
sis, and metabolite profiling. engineering techniques, the possibility to apply this for
optimization of classical fermentation processes soon be-
came obvious, and through introduction of directed ge-
I. BACKGROUND netic modifications by rDNA technology this has enabled
a far more rational approach to strain improvement than
Fermentation based on microorganisms, plant cells, ani- the classical approach of mutagenesis and screening. In
mal cells, and mammalian cells is currently used to pro- 1991 this led Bailey to discuss the emerging of a new
duce a wide variety of products, ranging from bulk chem- science called metabolic engineering, which he defined
icals and materials to highly valuable pharmaceuticals. as “the improvement of cellular activities by manipula-
Traditionally, optimization of fermentation processes in- tions of enzymatic, transport, and regulatory functions of
volved improved design of the bioreactor and introduction the cell with the use of recombinant DNA technology.”
of advanced schemes for controlling the process. Parallel Initially metabolic engineering was simply the technolog-
to this work, the properties of the organism applied in the ical manifestation of applied molecular biology, but with
process was improved through random mutagenesis fol- the rapid development in new analytical techniques and
lowed by screening for better strains. This is especially in cloning techniques, it has become possible to rapidly
demonstrated with the domestication of the baking and introduce directed genetic changes and subsequently an-
brewing strains of Saccharomyces cerevisiae. Here novel alyze the consequences of the introduced changes at the
strains have been obtained through classical methods of cellular level. Often the analysis will point toward an ad-
mutagenesis, sexual hybridization, and genetic recombi- ditional genetic change that may be required to further im-
nation. Also, in the development of the penicillin produc- prove the cellular performance, and metabolic engineering
tion several rounds of mutagenesis and screening have therefore involves a close integration between analysis of
resulted in strains with improved yield of the secondary the cellular function and genetic engineering as illustrated
metabolite, and Fig. 1 illustrates a typical development in in Fig. 2.
the performance of an industrial strain lineage of Penicil- According to the cycle of metabolic engineering, there
lium chrysogenum applied for penicillin production. is a continuous improvement of the cellular properties
through several rounds of genetic engineering. Depending
on the process and aim, one may start at different loca-
II. INTRODUCTION tions in this cycle. Thus, for production of a heterologous
protein, for production of a new metabolite by pathway
The first successful genetic engineering of Escherichia extension, or for extension of the substrate range for the
coli by Cohen, Boyer, and coworkers in 1973 paved applied microorganism, it is always necessary to start with
the way for a completely new approach to optimization the synthesis step. However, if the aim is to improve the
of existing biotech processes and development of com- yield or productivity in an existing process, it is neces-
pletely new ones. Shortly after were implemented several sary first to analyze the pathway involved in forming the
industrial processes for production of recombinant pro- product, and how this pathway interacts with the overall
cell function, i.e., one should start with the analysis step.
It is always desirable to optimize the yield or productiv-
ity in industrial processes, and the analysis step therefore
always plays a very prominent role—also in those cases
where the first step is to construct a recombinant strain
that produces the product of interest. It is clear that by
passing through the cycle of metabolic engineering one
gains a significant insight into cellular function, and this
is one of the reasons that metabolic engineering today in-
teract closely with the discipline of functional genomics,
where the aim is to assign function to orphan genes in
completelysequencedgenomes(seealsodiscussionlater).
However, since metabolic engineering requires availabil-
ity of the proper tools for genetic modifications and for
FIGURE 1 Increase in productivity (output rate/unit volume, arbi-
trary units) of penicillin G production by Gist-brocades, Delft (now analysis of cellular function, developments in genomics
DSM), in the period between 1962 and 1987. The introduction of and analytics have been one of the main reasons for the
new strains is marked with arrows. rapid expansion of the field of metabolic engineering in