E. Aromatic Compounds
A wide variety of commercially relevant chemicals can be produced starting from aromatic
amino acids. The choice of these chemicals
ranges from small building block molecules
to complicated plant secondary metabolites
such as flavonoids, stilbenes, or the abovementioned opioids (Sua ´stegui and Shao 2016). Most
of these substances are produced in low concentrations in their natural sources, because
many are toxic and the metabolic pathways
generally complicated, often involving cytochrome P450 enzymes. This makes microbial
production, where high concentrations of the
product are required for economic reasons,
challenging. Also in this case, most of the metabolic engineering endeavors have to be been
done so far with S. cerevisiae as cell factory. One
reason for this—apart of the advantageous
availability of many tools for this model organism—is that cytochrome P450 enzymes can be
functionally expressed quite well in baker’s
yeast. There is, however, an intrinsic disadvantage of this microbial host. The shikimate pathway—which is the point of origin for the
aromatic amino acid biosynthesis—combines
two precursor molecules from the primary
metabolism: erythrose-4-phosphate (E4P) and
phosphoenolpyruvate (PEP). PEP derives
directly from glycolysis, while E4P derives
from the pentose phosphate pathway (PPP). In
the Crabtree-positive yeast S. cerevisiae, glycolysis is extremely active, while the carbon flux
via the PPP is at least one order of magnitude
lower. This means that the required precursors
are entirely unbalanced, which leads to very low
yields. Rewiring the entire central carbon
metabolism is therefore a prerequisite for efficient aromatics production. Other yeasts, particularly Crabtree-negative yeasts, such as P.
pastoris (syn. Komagataella spp.) or Scheffersomyces stipitis might be better candidates for
such endeavors. The next challenge is the
deregulation of the amino acid biosynthesis
itself. Aromatic amino acids are the most
“cost intensive” amino acids for the cellular
metabolism. Consequently, their biosynthesis
is strictly regulated on many levels—a hurdle
which needs to be overcome. When all of this is
achieved, the more or less complicated further
pathway has to be clicked in, and finally all
toxicity issues have to be addressed. This
explains why most of the endeavors so far did
not exceed the proof-of-principle level.
Among the few substances which can be
produced in comparatively high amounts is
resveratrol, a plant stilbene, which got a lot of
attention due to alleged health benefits. Following the strategy outlined above, that is starting
with a complete rewiring of the central metabolism, about 800 mg/L of this compound has
been produced with baker’s yeast (Li et al.
2016).
IV. Yeasts for Production of
Recombinant Proteins
The market for recombinant proteins comprises of protein therapeutics, diagnostic proteins, and industrial enzymes. The production
of recombinant protein therapeutics generated
sales of more than US$180 billion in 2017 with
predicted global annual growth rates of 6–8%
(Walsh 2018). In the period of 2014 to July
2018, 129 biopharmaceutically active agents
entered the marked and a total number of 316
distinct biopharmaceutical ingredients are
available in licensed products. In 2017, the
Top 10 biopharma products generated sales of
more than US$80 billion. In addition to that,
the market for industrial enzymes, such as
amylases, lipases, cellulases, proteases, carbohydrases, etc., yielded revenues of US$4.5 billion worldwide in 2012 (Dewan 2014) and is
expected to grow to more than US$5.5 billion
by 2022 (Market Research Engine 2019).
Approximately 20–30% of biopharmaceutical products and industrial enzymes are produced in yeasts (Langer et al. 2018). In contrast
to primary and secondary metabolite production, which is largely dominated by Saccharomyces cerevisiae, the range of yeast hosts used
for recombinant protein production is more
diverse (Gu ¨ndu ¨z Ergu ¨n et al. 2019; Baghban
et al. 2019; Vieira Gomes et al. 2018). Yeast
cell factories for protein production combine
the strengths of microbial and higher eukary328
B. Schmelzer et al.
A wide variety of commercially relevant chemicals can be produced starting from aromatic
amino acids. The choice of these chemicals
ranges from small building block molecules
to complicated plant secondary metabolites
such as flavonoids, stilbenes, or the abovementioned opioids (Sua ´stegui and Shao 2016). Most
of these substances are produced in low concentrations in their natural sources, because
many are toxic and the metabolic pathways
generally complicated, often involving cytochrome P450 enzymes. This makes microbial
production, where high concentrations of the
product are required for economic reasons,
challenging. Also in this case, most of the metabolic engineering endeavors have to be been
done so far with S. cerevisiae as cell factory. One
reason for this—apart of the advantageous
availability of many tools for this model organism—is that cytochrome P450 enzymes can be
functionally expressed quite well in baker’s
yeast. There is, however, an intrinsic disadvantage of this microbial host. The shikimate pathway—which is the point of origin for the
aromatic amino acid biosynthesis—combines
two precursor molecules from the primary
metabolism: erythrose-4-phosphate (E4P) and
phosphoenolpyruvate (PEP). PEP derives
directly from glycolysis, while E4P derives
from the pentose phosphate pathway (PPP). In
the Crabtree-positive yeast S. cerevisiae, glycolysis is extremely active, while the carbon flux
via the PPP is at least one order of magnitude
lower. This means that the required precursors
are entirely unbalanced, which leads to very low
yields. Rewiring the entire central carbon
metabolism is therefore a prerequisite for efficient aromatics production. Other yeasts, particularly Crabtree-negative yeasts, such as P.
pastoris (syn. Komagataella spp.) or Scheffersomyces stipitis might be better candidates for
such endeavors. The next challenge is the
deregulation of the amino acid biosynthesis
itself. Aromatic amino acids are the most
“cost intensive” amino acids for the cellular
metabolism. Consequently, their biosynthesis
is strictly regulated on many levels—a hurdle
which needs to be overcome. When all of this is
achieved, the more or less complicated further
pathway has to be clicked in, and finally all
toxicity issues have to be addressed. This
explains why most of the endeavors so far did
not exceed the proof-of-principle level.
Among the few substances which can be
produced in comparatively high amounts is
resveratrol, a plant stilbene, which got a lot of
attention due to alleged health benefits. Following the strategy outlined above, that is starting
with a complete rewiring of the central metabolism, about 800 mg/L of this compound has
been produced with baker’s yeast (Li et al.
2016).
IV. Yeasts for Production of
Recombinant Proteins
The market for recombinant proteins comprises of protein therapeutics, diagnostic proteins, and industrial enzymes. The production
of recombinant protein therapeutics generated
sales of more than US$180 billion in 2017 with
predicted global annual growth rates of 6–8%
(Walsh 2018). In the period of 2014 to July
2018, 129 biopharmaceutically active agents
entered the marked and a total number of 316
distinct biopharmaceutical ingredients are
available in licensed products. In 2017, the
Top 10 biopharma products generated sales of
more than US$80 billion. In addition to that,
the market for industrial enzymes, such as
amylases, lipases, cellulases, proteases, carbohydrases, etc., yielded revenues of US$4.5 billion worldwide in 2012 (Dewan 2014) and is
expected to grow to more than US$5.5 billion
by 2022 (Market Research Engine 2019).
Approximately 20–30% of biopharmaceutical products and industrial enzymes are produced in yeasts (Langer et al. 2018). In contrast
to primary and secondary metabolite production, which is largely dominated by Saccharomyces cerevisiae, the range of yeast hosts used
for recombinant protein production is more
diverse (Gu ¨ndu ¨z Ergu ¨n et al. 2019; Baghban
et al. 2019; Vieira Gomes et al. 2018). Yeast
cell factories for protein production combine
the strengths of microbial and higher eukary328
B. Schmelzer et al.