building blocks for cellular components.
Important examples of primary metabolites
are amino acids for biosynthesis or ethanol
and lactic acid as products of fermentation
processes in many different cell types. In contrast to secondary metabolites (see Sect. III)
primary metabolites do not have direct pharmacological effects in humans. Biotechnologically produced primary metabolites rather
play important roles in everyday human life,
serving as preservative or flavoring components in food, beverages, cosmetics and pharmaceuticals, as biofuels, cleaning agents,
precursor for biopolymers, dyes, etc. Due to
its robustness and well-established metabolic
engineering tools, yeast is often the preferred
production host for primary metabolites in
today’s industrial biotechnology. Below you
find several examples of industrially produced
primary metabolites in different kinds of yeast
species.
A. Ethanol
With over 80 million tons globally produced in
2017 (Renewable Fuels Association 2017), ethanol is one of the most important products in
industrial biotechnology. Bioethanol is used as
renewable fuel and widely applied as additive in
gasoline (e.g., E10: 10% ethanol + 90% gasoline). Currently, most of the industrial ethanol
is produced by fermenting substrates such as
hydrolyzed corn starch or cane sugar (“first
generation” biofuel). Due to its rapid substrate
consumption and ethanol fermentation ability,
and its robustness against harsh conditions,
Saccharomyces cerevisiae is ideal for industrial
ethanol production (Nielsen et al. 2013). Over
the last two decades, a lot of research effort was
aimed at using cheap and abundant forestry
and agricultural substrates (“second generation” biofuel) with the goal of reducing the
carbon footprint of ethanol production and
avoiding the ethical dilemma of using farmland
for biofuels and not for food (Rude and Schirmer 2009). Such non-food substrates primarily
include lignocellulosic plant biomass derived
from agricultural waste such as straw, fruit
pulp, leaves, unused parts of food crops, wood
chips, and others. Lignocellulose consists of
carbohydrate polymers (cellulose, hemicellulose) bound to cross-linked phenolic polymers
called lignin. By a series of pre-treatment steps
including heat, acid, and enzymatic methods,
the sugar monomers trapped inside the lignocellulose are extracted and used as substrate in
ethanol fermentations. Rational metabolic
engineering and adaptive laboratory evolution
strategies further improved the performance of
S. cerevisiae in bioethanol production (Jansen
et al. 2017).
B. Butanol
Butanol isomers (1-butanol, 2-butanol, isobutanol, 2-methyl-2-propanol) are very attractive
biofuel alternatives with several advantages
over ethanol, which is currently the most
important biofuel. These advantages include
higher energy density, better blending ability,
lower moisture adsorption, and lower corrosiveness (Du ¨rre 2007). Furthermore, butanol is
an important chemical building block for
paints, coatings, plastic polymers, lubricants,
fragrances, etc. In the first half of the twentieth
century, butanol was mainly produced by the
acetone-butanol-ethanol (ABE) fermentation of
Clostridia species. However, high purification
and raw material costs, growth inhibition by
the products, low butanol recovery yield, and
the availability of cheap oil led the butanol
production away from the bioprocess to the
more economical petrochemical industry. In
the recent decades, modern metabolic engineering and advanced fermentation technologies helped to revive the fermentation route.
High butanol production was achieved by metabolic engineering and fermentation of Escherichia coli (Dellomonaco et al. 2011; Shen et al.
2011). Due to its butanol tolerance and wellestablished genetic tools, S. cerevisiae shows
great potential as butanol production host. By
extensive metabolic engineering of S. cerevisiae,
butanol titers and yields could be improved
significantly (Generoso et al. 2015; Buijs et al.
2013). Further research and process optimiza322
B. Schmelzer et al.
Important examples of primary metabolites
are amino acids for biosynthesis or ethanol
and lactic acid as products of fermentation
processes in many different cell types. In contrast to secondary metabolites (see Sect. III)
primary metabolites do not have direct pharmacological effects in humans. Biotechnologically produced primary metabolites rather
play important roles in everyday human life,
serving as preservative or flavoring components in food, beverages, cosmetics and pharmaceuticals, as biofuels, cleaning agents,
precursor for biopolymers, dyes, etc. Due to
its robustness and well-established metabolic
engineering tools, yeast is often the preferred
production host for primary metabolites in
today’s industrial biotechnology. Below you
find several examples of industrially produced
primary metabolites in different kinds of yeast
species.
A. Ethanol
With over 80 million tons globally produced in
2017 (Renewable Fuels Association 2017), ethanol is one of the most important products in
industrial biotechnology. Bioethanol is used as
renewable fuel and widely applied as additive in
gasoline (e.g., E10: 10% ethanol + 90% gasoline). Currently, most of the industrial ethanol
is produced by fermenting substrates such as
hydrolyzed corn starch or cane sugar (“first
generation” biofuel). Due to its rapid substrate
consumption and ethanol fermentation ability,
and its robustness against harsh conditions,
Saccharomyces cerevisiae is ideal for industrial
ethanol production (Nielsen et al. 2013). Over
the last two decades, a lot of research effort was
aimed at using cheap and abundant forestry
and agricultural substrates (“second generation” biofuel) with the goal of reducing the
carbon footprint of ethanol production and
avoiding the ethical dilemma of using farmland
for biofuels and not for food (Rude and Schirmer 2009). Such non-food substrates primarily
include lignocellulosic plant biomass derived
from agricultural waste such as straw, fruit
pulp, leaves, unused parts of food crops, wood
chips, and others. Lignocellulose consists of
carbohydrate polymers (cellulose, hemicellulose) bound to cross-linked phenolic polymers
called lignin. By a series of pre-treatment steps
including heat, acid, and enzymatic methods,
the sugar monomers trapped inside the lignocellulose are extracted and used as substrate in
ethanol fermentations. Rational metabolic
engineering and adaptive laboratory evolution
strategies further improved the performance of
S. cerevisiae in bioethanol production (Jansen
et al. 2017).
B. Butanol
Butanol isomers (1-butanol, 2-butanol, isobutanol, 2-methyl-2-propanol) are very attractive
biofuel alternatives with several advantages
over ethanol, which is currently the most
important biofuel. These advantages include
higher energy density, better blending ability,
lower moisture adsorption, and lower corrosiveness (Du ¨rre 2007). Furthermore, butanol is
an important chemical building block for
paints, coatings, plastic polymers, lubricants,
fragrances, etc. In the first half of the twentieth
century, butanol was mainly produced by the
acetone-butanol-ethanol (ABE) fermentation of
Clostridia species. However, high purification
and raw material costs, growth inhibition by
the products, low butanol recovery yield, and
the availability of cheap oil led the butanol
production away from the bioprocess to the
more economical petrochemical industry. In
the recent decades, modern metabolic engineering and advanced fermentation technologies helped to revive the fermentation route.
High butanol production was achieved by metabolic engineering and fermentation of Escherichia coli (Dellomonaco et al. 2011; Shen et al.
2011). Due to its butanol tolerance and wellestablished genetic tools, S. cerevisiae shows
great potential as butanol production host. By
extensive metabolic engineering of S. cerevisiae,
butanol titers and yields could be improved
significantly (Generoso et al. 2015; Buijs et al.
2013). Further research and process optimiza322
B. Schmelzer et al.