Fuel from Cellulose
Cellulosic
Ethanol – A Magical Biofuel
Due to the rapid rate of depletion
of our natural reserves of oil and natural gas, various government agencies and
private industries are struggling hard to invent technologies which can convert
cellulosic material into motor fuels, in a cost effective manner. Cellulose has
been made the target as; it is the most abundant organic compound present on
our planet Earth. It forms the structural component of all the green plants. It
is rich in glucose, which can be easily converted to sugars and fermented to
produce “cellulosic ethanol” – the magical Biofuel. And above all, if we are
able to produce this biofuel at a reasonable cost and meet the demands of the
entire world, it can act a renewable source of energy, as it is synthesized
from never ending raw materials like, non-edible parts of plants, grasses,
wood, miscanthus, sugarcane bagasse, sorted municipal waste, etc.
Technology
behind Cellulosic Conversion
Even though the conversion of
cellulose into sugars and fermentation into ethanol (biofuel) looks to be
simple on board, we have to cross many hurdles before we are able to device a
user-friendly technology
which can convert cellulose into cellulosic ethanol. Currently, cellulose
conversion technologies have been broadly classified into two categories, which
are, thermal and biochemical (enzyme based processes) technologies. Under
enzyme based biochemical processes, the bonds between the sugars are broken
with the help
of microbe-made enzymes. The enzymes capable of breaking these bonds are known
as glycoside hydrolases, or GHs. The enzymes of various bacteria like Saccharophagus
degradans, Clostridium thermocellu and Caldanaerobius polysaccharolyticus, are
being studied for their efficiency in conversion of cellulose to ethanol.
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What
makes cellulosic conversion difficult?
Currently, even though the raw
material required for the conversion is very abundant, the use of this
cellulose conversion technology is very limited, as it is highly expensive. The
main reason behind this is that, apart from cellulose the cell walls also
contain abundant quantities of hemicellulose.
The microorganisms are generally able to convert cellulose into sugars, but
fail to make efficient use of hemicellulose. The bonds which join the sugar
units of cellulose are different from those which join the sugar units of
hemicellulose. In cellulose, the glucose units are linked together with the
help of β-1,4-glycosidic linkages, whereas in some feedstock, hemicellulose
mainly consists of β-1,4-linked xylose backbones with arabinose side chains. In
some cases, the hemicellulose of feedstock also consists of larger variety of
sugars, including galactose and mannose. These sugars exist in different forms
of mannans. The presence of such complex strong bonds makes it difficult for
the microbes to produce ethanol in comparable quantities.
Amongst the different varieties of
mannans found in nature, some are made up of a linear chain of mannose residues
linked with the help of β-1,4-linkages. These mannans include glucomannan,
galactomannan, and glucogalactomannan. The galactose residues are further
linked to the mannan backbone with the help of α-1,6-linkages in galactomannans
or glucogalactomannnans. Hence, hydrolysis of mannans to its component sugars
is a very crucial step and requires endo-1,4-β-mannanases enzymes. These
enzymes can hydrolyze the backbone linkages and produce short-chain
manno-oligosaccharides. These are further degraded with the help of
1,4-β-mannosidases to generate monosaccharides. Hence, only the microbes
containing the enzymes capable of breaking the backbone of mannans can prove to
be economically useful.
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Can Caldanaerobius polysaccharolyticus, the “garbage bug” produce biofuel economically?
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Can Caldanaerobius polysaccharolyticus, the “garbage bug” produce biofuel economically?
The discovery of Caldanaerobius
polysaccharolyticus by Rod Mackie in 1993 has come up as a solution to the
above mentioned problem.
It is a thermophilic bacterium and was isolated from the garbage dump of a
canning plant. When the genome of this bacterium was sequenced and analyzed, it
was found that its genome possessed a gene cluster containing both hydrolytic
enzymes and enzymes key to the pentose-phosphate pathway. Man5A enzyme encoded
by its genome showed mannanase/endoglucanase activities. The recently
discovered man5B gene products in Caldanaerobius polysaccharolyticus, have
also shown the hydrolytic activity on mannan-containing polysaccharides. As
these genes involved in the conversion are clustered together in a single place
within the genome, it makes the job of genetic engineers easier.
Another advantage associated with
this bacterium is that the enzymes of this bacterium can tolerate high
temperatures (as high as 70 degrees Celsius) as Caldanaerobius
polysaccharolyticus is a thermophilic bacterium. According to the current
practice, the biofuel fermentation is carried at 37°C and this temperature
favors the growth of multiple microbes which can contaminate the fermentation
vats easily. However, as Caldanaerobius polysaccharolyticus can be used at
very high temperatures, the risk of contamination is negated.
Future
of Cellulosic Ethanol
Cellulosic ethanol has the potential
to act as a fuel which can power our vehicles
and perform multiple activities. Hence, research promoting the design of
convenient technology involving Caldanaerobius polysaccharolyticus bacterium
has a promising future. It will not only clear huge amounts of plant waste
thrown in the landfills but also generate a highly useful product for
day-to-day use.
1. Brief: Converting cellulose into
ethanol and other biofuels, retrieved from
2. Han, Y., Agarwal, V, et al.
(2012). Biochemical and Structural Insights into Xylan Utilization by the
Thermophilic Bacterium Caldanaerobius polysaccharolyticus, The Journal of
Biological Chemistry, 287, 34946-34960.
3. Han, Y., Dodd, B., et al. (2010).
Comparative Analyses of Two Thermophilic Enzymes Exhibiting both β-1,4
Mannosidic and β-1,4 Glucosidic Cleavage Activities from Caldanaerobius
polysaccharolyticus, J. Bacteriol. 192,16 4111-4121.
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