tons per year (Rupilius and Ahma 2006), and
the demand is steadily increasing. They are
currently produced from petroleum (in which
case the term “petrochemicals” is also used) or
biomass-derived (mainly plant) fats. While the
concerns associated with the exploitation of
fossil resources have been known for a long
time, there is an increasing awareness of problems caused by mass cultivation of oily plants
(e.g., palms), for instance, excessive land use
and water consumption (Schmidt 2015). The
biorefinery concept, in which engineered
microbes are used to convert various biomass
feedstocks into desired products (Nielsen et al.
2013), offers a potentially more effective exploitation of resources and thereby reduces the
environmental impact compared to extraction
from plants. Indeed, significant advances in
production of oleochemicals with different
chain lengths and functionalities have been
achieved in diverse microbial hosts (for review,
see Yu et al. 2014; d’Espaux et al. 2015; Pfleger
et al. 2015; Sarria et al. 2017; Marella et al. 2018;
Xue et al. 2018; Zhang et al. 2018). Since rapid
progress is made in the field, we feel that a
critical re-evaluation of the literature is justified. Here, we will focus on strategies developed
in S. cerevisiae, one of the best characterized
and most popular biotechnological host organisms (see also Chap. 13 by Schmelzer et al.), in
which highest yields of microbially produced
FA have been recently reported (Yu et al.
2018). Different approaches for engineering
precursor and cofactor supply, chain length
control, elimination of by-product formation,
and overcoming product toxicity as well as the
development of biosensors for accelerating the
screening for best performing strains are summarized and evaluated. Throughout the manuscript, we consciously do not collect all data on
achieved titers and yields for different FAderived products, as another recent review has
provided a very detailed summary (FernandezMoya and Da Silva 2017). Strategies that have
been developed in other host organisms (e.g.,
Escherichia coli) are occasionally discussed, if
they appear suitable to expand the pathways
already established in S. cerevisiae. Lastly, we
give an overview of the production of FAderived products in S. cerevisiae as well as
methods for optimizing fermentation conditions, enabling the scale-up of current labscale fermentation processes.
II. Providing Carbon, Redox Power,
and Energy for Fatty Acid
Synthesis: The Options for
Precursor Supply Routes
S. cerevisiae, an ethanologenic yeast, only
requires relatively small amounts of FA as
building blocks of membrane lipids. The FA
synthesis naturally occurs in the cytosol by
the FA synthase (FAS) that uses acetyl-CoA
(AcCoA) and its derivative malonyl-CoA (MalCoA) as precursor molecules. AcCoA metabolism in yeast is compartmentalized, whereby
the major part is synthesized inside mitochondria via the pyruvate dehydrogenase (PDH)
complex (Krivoruchko et al. 2015) and the
mitochondrial AcCoA cannot be exported to
the cytosol (van Rossum et al. 2016a, c). The
native pathway for the synthesis of cytosolic
AcCoA in S. cerevisiae is referred to as pyruvate
dehydrogenase (PDH) bypass, and it diverts
only a minor part of the pyruvate produced
from glucose (Pronk et al. 1996). Following
decarboxylation of pyruvate by pyruvate decarboxylases, acetaldehyde is oxidized to acetate
by acetaldehyde dehydrogenases (ALDs), and
acetate is subsequently ligated to CoA by the
acetyl-CoA synthetases (ACS). This reaction is
thermodynamically driven by hydrolysis of ATP
to AMP and inorganic pyrophosphate, which is
subsequently hydrolyzed by the inorganic pyrophosphatase. Thus, each AcCoA molecule is
synthesized at the expense of two ATP equivalents by the PDH bypass. This reaction scheme
implies that the glycolytic ATP supply (net two
ATP moles per mol glucose) is insufficient,
because energy equivalents are also needed for
cell proliferation and maintenance. Thus, a significant proportion of the available (sugar) substrate
must be diverted to the tricarboxylic acid (TCA)
cycle and respiration for the supply of energy
equivalents. This reduces the attainable product
yield (van Rossum et al. 2016b) and, considering
that most FA and derived products have a relatively low commercial value, makes their production through the PDH bypass uneconomical.
340
L. Baumann et al.
the demand is steadily increasing. They are
currently produced from petroleum (in which
case the term “petrochemicals” is also used) or
biomass-derived (mainly plant) fats. While the
concerns associated with the exploitation of
fossil resources have been known for a long
time, there is an increasing awareness of problems caused by mass cultivation of oily plants
(e.g., palms), for instance, excessive land use
and water consumption (Schmidt 2015). The
biorefinery concept, in which engineered
microbes are used to convert various biomass
feedstocks into desired products (Nielsen et al.
2013), offers a potentially more effective exploitation of resources and thereby reduces the
environmental impact compared to extraction
from plants. Indeed, significant advances in
production of oleochemicals with different
chain lengths and functionalities have been
achieved in diverse microbial hosts (for review,
see Yu et al. 2014; d’Espaux et al. 2015; Pfleger
et al. 2015; Sarria et al. 2017; Marella et al. 2018;
Xue et al. 2018; Zhang et al. 2018). Since rapid
progress is made in the field, we feel that a
critical re-evaluation of the literature is justified. Here, we will focus on strategies developed
in S. cerevisiae, one of the best characterized
and most popular biotechnological host organisms (see also Chap. 13 by Schmelzer et al.), in
which highest yields of microbially produced
FA have been recently reported (Yu et al.
2018). Different approaches for engineering
precursor and cofactor supply, chain length
control, elimination of by-product formation,
and overcoming product toxicity as well as the
development of biosensors for accelerating the
screening for best performing strains are summarized and evaluated. Throughout the manuscript, we consciously do not collect all data on
achieved titers and yields for different FAderived products, as another recent review has
provided a very detailed summary (FernandezMoya and Da Silva 2017). Strategies that have
been developed in other host organisms (e.g.,
Escherichia coli) are occasionally discussed, if
they appear suitable to expand the pathways
already established in S. cerevisiae. Lastly, we
give an overview of the production of FAderived products in S. cerevisiae as well as
methods for optimizing fermentation conditions, enabling the scale-up of current labscale fermentation processes.
II. Providing Carbon, Redox Power,
and Energy for Fatty Acid
Synthesis: The Options for
Precursor Supply Routes
S. cerevisiae, an ethanologenic yeast, only
requires relatively small amounts of FA as
building blocks of membrane lipids. The FA
synthesis naturally occurs in the cytosol by
the FA synthase (FAS) that uses acetyl-CoA
(AcCoA) and its derivative malonyl-CoA (MalCoA) as precursor molecules. AcCoA metabolism in yeast is compartmentalized, whereby
the major part is synthesized inside mitochondria via the pyruvate dehydrogenase (PDH)
complex (Krivoruchko et al. 2015) and the
mitochondrial AcCoA cannot be exported to
the cytosol (van Rossum et al. 2016a, c). The
native pathway for the synthesis of cytosolic
AcCoA in S. cerevisiae is referred to as pyruvate
dehydrogenase (PDH) bypass, and it diverts
only a minor part of the pyruvate produced
from glucose (Pronk et al. 1996). Following
decarboxylation of pyruvate by pyruvate decarboxylases, acetaldehyde is oxidized to acetate
by acetaldehyde dehydrogenases (ALDs), and
acetate is subsequently ligated to CoA by the
acetyl-CoA synthetases (ACS). This reaction is
thermodynamically driven by hydrolysis of ATP
to AMP and inorganic pyrophosphate, which is
subsequently hydrolyzed by the inorganic pyrophosphatase. Thus, each AcCoA molecule is
synthesized at the expense of two ATP equivalents by the PDH bypass. This reaction scheme
implies that the glycolytic ATP supply (net two
ATP moles per mol glucose) is insufficient,
because energy equivalents are also needed for
cell proliferation and maintenance. Thus, a significant proportion of the available (sugar) substrate
must be diverted to the tricarboxylic acid (TCA)
cycle and respiration for the supply of energy
equivalents. This reduces the attainable product
yield (van Rossum et al. 2016b) and, considering
that most FA and derived products have a relatively low commercial value, makes their production through the PDH bypass uneconomical.
340
L. Baumann et al.