This is a huge step forward. The newly engineered yeast is able to
coprocess both glucose and the more complex sugar xylose at the same time and
as fast as it can do either alone. the
result is a major hump in efficiency and an important step in processing cellulose
itself.
I anticipate that a process
breaking down cellulose will produce a feedstock rich in these two sugars. Thus we have a key step in place that is also
highly efficient. This will also be
useful in present fermentation processes I am sure.
Ethanol may not end up staying
the course in terms of general transportation, but it is awfully attractive to
the agricultural industry if one could reduce ag waste into a farm usable
biofuel. Agriculture will continue to
need heavy equipment with high torque and liquid fuels are pretty good at
delivering.
This is a bit of a read, but worth the effort.
Team overcomes major obstacle to cellulosic biofuel production
Jan 10, 2011
A newly engineered yeast strain can simultaneously consume two types of
sugar from plants to produce ethanol, researchers report. The sugars are
glucose, a six-carbon sugar that is relatively easy to ferment; and xylose, a
five-carbon sugar that has been much more difficult to utilize in ethanol
production. The new strain, made by combining, optimizing and adding to earlier
advances, reduces or eliminates several major inefficiencies associated with
current biofuel production methods.
The findings, from a collaborative led by researchers at the University
of Illinois, the Lawrence Berkeley National Laboratory, the University of
California and the energy company BP, are described in the Proceedings of the
National Academy of Sciences. The Energy Biosciences Institute, a BP-funded
initiative, supported the research.
Yeasts feed on sugar and produce various waste products, some of which
are useful to humans. One type of yeast, Saccharomyces cerevisiae, has been
used for centuries in baking and brewing because it efficiently ferments sugars
and in the process produces ethanol and carbon dioxide. The biofuel industry
uses this yeast to convert plant sugars to bioethanol. And while S. cerevisiae
is very good at utilizing glucose, a building block of cellulose and the
primary sugar in plants, it cannot use xylose, a secondary – but significant –
component of the lignocellulose that makes up plant stems and leaves. Most
yeast strains that are engineered to metabolize xylose do so very slowly.
"Xylose is a wood sugar, a five-carbon sugar that is very abundant
in lignocellulosic biomass but not in our food," said Yong-Su Jin, a
professor of food science and human nutrition at Illinois
A big part of the problem with yeasts altered to take up xylose is that
they will suck up all the glucose in a mixture before they will touch the
xylose, Jin said. A glucose transporter on the surface of the yeast prefers to
bind to glucose.
"It's like giving meat and broccoli to my kids," he said.
"They usually eat the meat first and the broccoli later."
The yeast's extremely slow metabolism of xylose also adds significantly
to the cost of biofuels production.
Jin and his colleagues wanted to induce the yeast to quickly and
efficiently consume both types of sugar at once, a process called
co-fermentation. The research effort involved researchers from Illinois, the
Lawrence Berkeley National Laboratory, the University of California at
Berkeley, Seoul National University and BP.
In a painstaking process of adjustments to the original yeast, Jin and
his colleagues converted it to one that will consume both types of sugar faster
and more efficiently than any strain currently in use in the biofuel industry.
In fact, the new yeast strain simultaneously converts cellobiose (a precursor
of glucose) and xylose to ethanol just as quickly as it can ferment either
sugar alone.
"If you do the fermentation by using only cellobiose or xylose, it
takes 48 hours," said postdoctoral researcher and lead author Suk-Jin Ha.
"But if you do the co-fermentation with the cellobiose and xylose, double
the amount of sugar is consumed in the same amount of time and produces more
than double the amount of ethanol. It's a huge synergistic effect of
co-fermentation."
The new yeast strain is at least 20 percent more efficient at
converting xylose to ethanol than other strains, making it "the best
xylose-fermenting strain" reported in any study, Jin said.
The team achieved these outcomes by making several critical changes to
the organism. First, they gave the yeast a cellobiose transporter. Cellobiose,
a part of plant cell walls, consists of two glucose sugars linked together.
Cellobiose is traditionally converted to glucose outside the yeast cell before
entering the cell through glucose transporters for conversion to ethanol.
Having a cellobiose transporter means that the engineered yeast can bring
cellobiose directly into the cell. Only after the cellobiose is inside the cell
is it converted to glucose.
This approach, initially developed by co-corresponding author Jamie
Cate at the Lawrence Berkeley National Laboratory and the University of
California at Berkeley, eliminates the costly step of adding a
cellobiose-degrading enzyme to the lignocellulose mixture before the yeast
consumes it.
It has the added advantage of circumventing the yeast's own preference for
glucose. Because the glucose can now "sneak" into the yeast in the
form of cellobiose, the glucose transporters can focus on drawing xylose into
the cell instead. Cate worked with Jonathan Galazka, of UC Berkeley, to clone
the transporter and enzyme used in the new strain.
The team then tackled the problems associated with xylose metabolism.
The researchers inserted three genes into S. cerevisiae from a xylose-consuming
yeast, Picchia stipitis.
Graduate student Soo Rin Kim at the University of Illinois
They also engineered an artificial "isoenzyme" that balanced
the proportion of two important cofactors so that the accumulation of xylitol,
a byproduct in the xylose assimilitary pathway, could be minimized. Finally,
the team used "evolutionary engineering" to optimize the new strain's
ability to utilize xylose.
The cost benefits of this advance in co-fermentation are very
significant, Jin said.
"We don't have to do two separate fermentations," he said.
"We can do it all in one pot.
And the yield is even higher than the industry standard. We are pretty
sure that this research can be commercialized very soon."
Jin noted that the research was the result of a successful
collaboration among principal investigators in the Energy Biosciences Institute
and a BP scientist, Xiaomin Yang, who played a key role in developing the
co-fermentation concept and coordinating the collaboration.
Source: University of Illinois
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