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ISBN: 978-967-14475-3-6; eISBN: 978-967-14475-2-9 247
Biotechnology for Sustainability
Achievements, Challenges and Perspectives
Biotech Sustainability (2017), P248--261
Microbial Metabolic Engineering: A Key Technology to
Deal with Global Climate and Environmental Challenges
Meerza Abdul Razak1, Pathan Shajahan Begum2, Senthilkumar Rajagopal3, *
1Department of Biotechnology, Rayalaseema University, Kurnool, Andhra Pradesh,
India; 2Department of Zoology, KVR Women’s degree college, Kurnool, Andhra Pra-
desh, India; 3Department of Biochemistry, Rayalaseema University, Kurnool, Andhra
Pradesh, India;*Correspondence: senthilanal@yahoo.com; Tel: +91 9566860390
Abstract: Global climate change and green house effects are very serious and contro-
versial problems that have severe negative impacts on environment, society, energy in-
dustry and government policies and sustainability. Global environment and climate
challenges are directly connected to the accumulation of green house gases which has
caused concerns related to the usage of traditional and fossil fuels as the key energy re-
source. To mitigate climate and environment changes, one solution is to utilize the po-
tential of metabolic engineering of microbes for biofuels production from renewable
sources. For long-term economic sustainability of the energy, industries and transporta-
tion sectors should adopt renewable and sustainable fuels produced by metabolically
engineered microorganisms. The biofuels produced from renewable sources by meta-
bolically engineered microbes carry good energy contents with minimal emission of
greenhouse gases and causes minimal impact on the environment, food chain, water
supply and land use. Toxic organic and inorganic chemicals are also one of the main
reasons for environment contamination and also present major risk for climate change.
Avoiding of upcoming contamination from these chemicals poses a huge technical
challenge. Currently, metabolically engineered microbes have been explored only for
selective and high capacity bioremediation of heavy toxic metals and chemicals. This
chapter will shed light on current trend and developments in metabolic engineering of
microbes for biofuel and bio-based chemicals production from renewable resources.
This chapter also highlights the potential of metabolically engineered microbes for bio-
remediation, a possible futuristic solution for sustainable development for energy and
reduction of global climate change and green house effects.
Keywords: Biofuels; bioremediation; metabolic engineering; synthetic biology; systems
biology
1. Introduction
level and weakening of thermohaline cir-
culation. The atmospheric carbon dioxide
Since the past some decades,
concentration is 400 parts per million
constantly increasing greenhouse gases in
(ppm) and the carbon dioxide released
environment such as carbon dioxide, ni-
from fossil fuels worldwide is 7 Gt of
trous oxide, methane have been linked to
carbon per year (Pacala and Socolow,
global environmental and climate con-
2004; Lewis and Nocera, 2006). If the
cerns (O’Neill and Oppenheimer, 2002;
present upward trend of carbon dioxide
Stocker, 2013). Some of the effects of
continues, by the end of year 2050 the
global climate and environment change
carbon dioxide release rate will be dou-
are abolition of coral reef, rising of sea
bled. It is estimated that the carbon diox-
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Microbial Metabolic Engineering for Sustainability Meerza et al.
ide concentration will reach to 500 ppm
in combination with rising global crises
by doubling of the carbon dioxide emis-
provides the platform for microbial meta-
sion rate without remediation. The carbon
bolic engineering applications and inno-
dioxide concentration of 500 ppm will
vation on a large scale. Scientists from
lead to a global warming of around 2°C
industries and academics strongly believe
above the level in year 1900 (Pacala and
that microbial metabolic engineering in
Socolow, 2004). This level of increase in
combination with other technologies such
temperature would raise the threat of dis-
as synthetic biology and systems biology
integration of the West Antarctic Ice
can help to solve the growing concerns of
Sheet (WAIS) along with other negative
climate and environment (Zhang et al.,
effects. It is estimated that increase in
2011).
temperature by 2°C would lead to disrup-
From the beginning microbial
tive rise of sea level by 4–6 meters
metabolic engineering had aimed the pro-
(Stocker, 2013).
duction of fuels and chemicals as chief
The atmospheric CO2 concentra-
goals of the rising field. The metabolic
tion, CO2 emission and global tempera-
engineering of yeast Saccharomyces
ture are having severe negative effects on
cerevisiae and E. coli are the classic ex-
the environment, climate, economy and
amples of metabolic application in the
society and we need to deal with it. To
field of biofuel production (Kuyper et al.,
avoid the rise in the temperature, it is
2003, Bro et al., 2006, Ingram et al.,
necessary that we should reduce the car-
1987). Microbial metabolic engineering
bon emission from the fossil fuel usage
applications have expanded in the past
and increase the sources of renewable
few years because of the growing atten-
energy and remove the toxic chemicals
tion on biofuels and chemical production
and metals present in the environment.
through biomass conversion. There are
When compared with different renewable
many reports stating that the combination
energy sources, biofuels are well-suited
of microbial metabolic engineering along
with present infrastructure and have an
with combinatorial approaches supported
advanced energy density. Thus there has
by high throughput systems had given
been much curiosity in establishing biore-
good results in biofuel production. The
fineries for the production of fuels and
key advantage of utilizing microorganism
chemicals from renewable resources.
for the production of biofuels from re-
We are in the modern age of
newable sources is the metabolic diversity
microbial metabolic engineering which
of fungi, algae and bacteria facilitate us
comprises of progressively more efforts at
the use of diverse substrates as the start-
cell, and pathway design. One of the rea-
ing point for biofuel generation.
sons for the more success of microbial
Bioremediation is a very cost
metabolic engineering is development of
effective and eco-friendly method and is
additional “omics” tools which provide
steadily making inroads for environmen-
both temporally and spatially analyzing
tal clean-up applications. Bioremediation
opportunity for cellular systems at the
depends on enhanced detoxification and
level of protein, metabolite, RNA and
degradation of toxic metals and degrada-
DNA (Peralta-Yahya et al., 2012). Mi-
tion of toxic pollutants either through en-
crobial metabolic engineering have been
zymatic transformation or intracellular
evolved to solve crucial international
accumulation to less or non-toxic com-
problems such as global warming, biore-
pounds. There are several physio-
mediation, food and human health. If
chemical processes for treating toxic pol-
properly utilized microbial metabolic en-
lutants in environment, but these process-
gineering can play an important role in
es are non specific, very costly and some-
facing the global challenges. By devel-
times they may introduce secondary con-
opment of novel technological innovation
tamination. Microbes naturally have the
ISBN: 978-967-14475-3-6; eISBN: 978-967-14475-2-9 249
Microbial Metabolic Engineering for Sustainability Meerza et al.
capability to transform, degrade and che-
um beijerinckii and Thermoanaerobacte-
late several toxic chemicals. But the mi-
rium thermosaccharolyticum has been
crobial bioremediation process is having
used for the hydrogen production. The
relative slow transformation rates. By
drawback of these microbes is low pro-
metabolically engineering microbes it is
duction of hydrogen (Cai et al., 2011, Oh
possible to remove the toxic inorganic
et al., 2011). Kim et al. overcome the ma-
and organic chemicals from the environ-
jor obstacle of low hydrogen production
ment (Shailendra et al., 2008). The better
by metabolically engineered E. coli
understanding of microbes’ natural trans-
strains (Kim et al., 2009). In one of the
formation ability at genetic level and ad-
investigation a high volumetric productiv-
vance of novel genetic tools are very es-
ity of 2.4 H2/L/h was produced using
sential for metabolic engineering of mi-
immobilized cells of a metabolically en-
croorganism for bioremediation. There
gineered E. coli which had deletion muta-
are several metabolically engineered mi-
tion (Seol et al., 2011). Even though, bio-
croorganisms with superior biotransfor-
logical hydrogen production was consid-
mation capacity and more accumulation
erably increased by metabolically engi-
of toxic wastes. In this chapter we discuss
neered E. coli strain, several vital obsta-
metabolic engineering strategies and suc-
cles involved in the productivity, yield
cessful examples of metabolically engi-
and metabolic robustness are still not up
neered microorganisms for production of
to the mark that would permit commer-
biofuels and chemicals and for bioreme-
cialization.
diation (Brar et al., 2006).
2.2. Bioethanol production
2. Biofuels production by metabol-
Bioethanol is the major renewable
ically engineered microorgan-
liquid energy source comprising 90% of
isms
the global world Biofuel market. It is es-
timated that annual production of bioeth-
With the increasing costs of
anol throughout the world is more than
energy and the challenges of global
105 billion liters. Most of the Bioethanol
warming that arise due to the usage of
production is by yeast and it is based on
petroleum based feedstock, the scientific
the sugarcane and starch, this type of pro-
community throughout the globe is
duction competes with feed and food
searching for energy substitutes without
(Geddes et al., 2011b). In one of the study
adding up to the existing carbon footprint.
E. coli was metabolically engineered to
Biofuels can be an exciting substitute to
efficiently convert glycerol to ethanol.
solve both the climate and environmental
This strain was able to convert 40 g/L
issues since they are produced from the
glycerol to ethanol in 48 h with 90% of
renewable resources. Microbial produc-
the ethanol yield (Trinh and Srienc,
tion of hydrogen as future fuel is a hope-
2009). By introducing the adh B and pdc
ful possibility as an alternative for petro-
genes from Zymomonas mobilis which
leum based fuels. Hydrogen is a more
encodes alcohol dehydrogenase and py-
energy dense source and its conversion to
ruvate decarboxylase into E. coli redi-
power or heart is very simple and clean.
rected the carbon flux into the ethanol
Hydrogen when combusted with oxygen
production and the obtained metabolically
only H2O is formed without the formation
engineered E. coli produced ethanol upto
of toxic pollutants (Kim and Lee 2010,
1.28% (v/v) using xylose as carbon
Panagiotopoulos et al., 2009).
source within 36 hour fermentation (San-
ny et al., 2010). Metabolic engineering
2.1. Hydrogen production
approaches were implemented to engineer
Large number of microbes such as
E. coli for ethanol production from mixed
Sporoacetigenium mesophilum, Clostridi-
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Microbial Metabolic Engineering for Sustainability Meerza et al.
sugars, glucose and xylose (Sanny et al.,
oped an E. coli strain which can fermenta-
2010, Wang et al., 2008).
tively produce 1- butanol by transferring a
group of six genes from C. acetobutyli-
2.3. Isopropanol production
cum 1- butanol pathway and removal of
E. coli do not have some of the
the competing pathway. This metabolical-
necessary pathways which are very much
ly engineered E. coli strain produced 552
necessary pathways for the production of
mg/L of 1- butanol under semi-anaerobic
advanced fuels, metabolic engineering
conditions from a rich medium (Atsumi
has provided the chance to produce non-
and Liao, 2008a). 1- Propanol is univer-
traditional biofuels by the construction of
sal solvent with several industrial applica-
non native biosynthesis pathways (Atsumi
tions which can be converted to propylene
et al., 2008b). The microorganisms such
and diesel and it is a promising gasoline
as Clostridium can naturally produce Iso-
substitute. The wild strain organisms can-
propanol which is one of the secondary
not produce the 1-propanol is considera-
alcohols. Isopropanol has several diverse
ble amounts (Shen and Liao, 2008). 2-
applications. In Clostridium several at-
ketobutyrate is a precursor of 1-propanol
tempts have been made to enhance the
and isoleucine. It is also can be converted
production ability, but product inhibition
to 2-methy 1-butanol and 1- butanol
and low titer. As a result, metabolic engi-
through several multi steps enzymes reac-
neering of E. coli for Isopropanol produc-
tions. E. coli was metabolically engi-
tion become a promising substitute for
neered to produce 1-propanol and 1-
industrial production of Isopropanol.
butanol through 2-ketobutyrate (Shen and
Metabolically engineered strain of E. coli
Liao, 2008).
was constructed that produced a 13.6 g/L
Metabolically
engineered
isopropanol from glucose under vigorous
strain was developed by overexpres-
aerobic culture conditions (Jojima et al.,
sion of the genes such as thrAfbBC,
200