long been a playground for the field of metabolic engineering. The first reports about successful engineering of baker’s yeast for betacarotene and lycopene production date back
to the beginning of the 1990s. Many studies
have since then been published transferring
the synthesis of various carotenoids into many
yeasts species, among them Saccharomyces cerevisiae, Pichia pastoris (syn Komagataella spp),
and Yarrowia lipolytica. While most of these
studies show a proof-of-principle, the titers
and productivities reached with natural producers have not been exceeded (Mata-Go ´mez et al.
2014). Very successful was a recent attempt to
reroute the pathway from astaxanthin to zeaxanthin production in X. dendrorhous (Breitenbach et al. 2019).
B. Omega-3 Fatty Acids
The first example of a commercial product to be
produced by metabolically engineered yeasts
was the omega-3 long-chain polyunsaturated
fatty acid eicosapentaenoic acid (EPA; Xue
et al. 2013). EPA and docosahexaenoic acid
(DHA) are natural products of high commercial
value. They are essential for human health and
have therefore broad applications as nutritional supplements, but they are also of great
importance as animal feed, particularly in
aquaculture. In nature EPA and DHA are
mainly produced by marine microorganisms
and a few plants. They then accumulate in
other organisms through the food chain. The
demand for EPA and DHA exceeds the natural
reserves, and increasing pollution of the sea
and concomitant accumulation of pollutants
with the desired acids in fish oil, for example,
led to the strong request for other sources. First
attempts to produce EPA in baker’s yeast were
not successful due to very low yields. The successful approach started from Y. lipolytica,
which is an oleaginous yeast, naturally accumulating high amounts of lipids under conditions of carbon excess. Expression of the
enzymes required to modify the natural fatty
acids to EPA led to a moderately productive
strain. Interestingly, the final step to success
happened by chance: when the strain was constructed by random integration of the expression cassettes, the authors found one “jackpot”
clone with decisively increased EPA accumulation. They then found out, that a pex gene had
been interrupted, which led to compromised
peroxisome morphology and abolished betaoxidation. Reconstruction of the same modification in another background confirmed this
finding. The final strain produces up to 30%
of the cell dry weight as lipids, of which 56.6%
are EPA. This strain is used for two commercial
products—one nutritional supplement for
humans and one for the feeding of cultured
salmon (Xue et al. 2013). The first production
approach was a batch culture. In a two-stage
continuous fermentation, productivity could be
improved by 80% and EPA concentration in the
reactor by 40% without compromising the yield
or EPA concentration in the biomass (Xie et al.
2017).
Fatty acids and derivatives are generally an
important product of many yeast species. Further examples and metabolic engineering
approaches to increase yield and modify the
accumulated products are highlighted in
Chap. 14 (by Baumann et al.).
C. Artemisinic Acid
Another success story for metabolic engineering of yeast is the market entrance of semisynthetic artemisinin in 2013. Artemisinin is a
plant-derived antimalarial drug, which has
been used by traditional Chinese medicine for
a long time. After a recommendation of this
drug by the World Health Organization, production of the drug by isolation from plants in
the required amounts became difficult—so
other production routes had to be explored.
Extensive metabolic engineering of baker’s
yeast allowed for the production of artemisinic
acid, which can be chemically converted into
artemisinin. Three modules had to be optimized for efficient production of the desired
acid: firstly, to provide the isoprenoid precursors efficiently, the enzymes of the mevalonate
pathway needed to be overexpressed. Sec326
B. Schmelzer et al.
to the beginning of the 1990s. Many studies
have since then been published transferring
the synthesis of various carotenoids into many
yeasts species, among them Saccharomyces cerevisiae, Pichia pastoris (syn Komagataella spp),
and Yarrowia lipolytica. While most of these
studies show a proof-of-principle, the titers
and productivities reached with natural producers have not been exceeded (Mata-Go ´mez et al.
2014). Very successful was a recent attempt to
reroute the pathway from astaxanthin to zeaxanthin production in X. dendrorhous (Breitenbach et al. 2019).
B. Omega-3 Fatty Acids
The first example of a commercial product to be
produced by metabolically engineered yeasts
was the omega-3 long-chain polyunsaturated
fatty acid eicosapentaenoic acid (EPA; Xue
et al. 2013). EPA and docosahexaenoic acid
(DHA) are natural products of high commercial
value. They are essential for human health and
have therefore broad applications as nutritional supplements, but they are also of great
importance as animal feed, particularly in
aquaculture. In nature EPA and DHA are
mainly produced by marine microorganisms
and a few plants. They then accumulate in
other organisms through the food chain. The
demand for EPA and DHA exceeds the natural
reserves, and increasing pollution of the sea
and concomitant accumulation of pollutants
with the desired acids in fish oil, for example,
led to the strong request for other sources. First
attempts to produce EPA in baker’s yeast were
not successful due to very low yields. The successful approach started from Y. lipolytica,
which is an oleaginous yeast, naturally accumulating high amounts of lipids under conditions of carbon excess. Expression of the
enzymes required to modify the natural fatty
acids to EPA led to a moderately productive
strain. Interestingly, the final step to success
happened by chance: when the strain was constructed by random integration of the expression cassettes, the authors found one “jackpot”
clone with decisively increased EPA accumulation. They then found out, that a pex gene had
been interrupted, which led to compromised
peroxisome morphology and abolished betaoxidation. Reconstruction of the same modification in another background confirmed this
finding. The final strain produces up to 30%
of the cell dry weight as lipids, of which 56.6%
are EPA. This strain is used for two commercial
products—one nutritional supplement for
humans and one for the feeding of cultured
salmon (Xue et al. 2013). The first production
approach was a batch culture. In a two-stage
continuous fermentation, productivity could be
improved by 80% and EPA concentration in the
reactor by 40% without compromising the yield
or EPA concentration in the biomass (Xie et al.
2017).
Fatty acids and derivatives are generally an
important product of many yeast species. Further examples and metabolic engineering
approaches to increase yield and modify the
accumulated products are highlighted in
Chap. 14 (by Baumann et al.).
C. Artemisinic Acid
Another success story for metabolic engineering of yeast is the market entrance of semisynthetic artemisinin in 2013. Artemisinin is a
plant-derived antimalarial drug, which has
been used by traditional Chinese medicine for
a long time. After a recommendation of this
drug by the World Health Organization, production of the drug by isolation from plants in
the required amounts became difficult—so
other production routes had to be explored.
Extensive metabolic engineering of baker’s
yeast allowed for the production of artemisinic
acid, which can be chemically converted into
artemisinin. Three modules had to be optimized for efficient production of the desired
acid: firstly, to provide the isoprenoid precursors efficiently, the enzymes of the mevalonate
pathway needed to be overexpressed. Sec326
B. Schmelzer et al.