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 BP, are described
in the Proceedings of the National Academy of Sciences. The
Energy Biosciences Institute, a BP-funded initiative, supported
Fig. 1. Illinois University
science and human nutrition
professor Yong-Su Jin, center,
and his colleagues
engineered a yeast that
outperforms the industry
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
secondarybut significantcomponent of the
lignocellulose that makes up plant stems and leaves. Most yeast
strains that are engineered to metabolize xylose do so very
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 and a principal investigator on the study. Most
yeast cannot ferment xylose.
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, Dr. Jin said. A glucose
transporter on the surface of the yeast prefers to bind to
Its like giving meat and broccoli to my
kids, he said. They usually eat the meat first and
the broccoli later.
The yeasts extremely slow metabolism of xylose also
adds significantly to the cost of biofuels production.
Dr. 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, Dr. 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 post-doctoral 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. Its a
huge synergistic effect of co-fermentation.
The new yeast strain is at least 20% more efficient at
converting xylose to ethanol than other strains, making it
the best xylose-fermenting strain reported in any
study, Dr. 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 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 yeasts
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
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. The
team identified the bottleneck in this metabolic pathway. By
adjusting the relative production of these enzymes, the
researchers eliminated the bottleneck and boosted the speed of
xylose metabolism in the new strain.
They also engineered an artificial isoenzyme
that balanced the proportion of two important co-factors 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
strains ability to utilize xylose. The cost benefits of
this advance in co-fermentation are very significant, Dr. Jin
We dont 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. HP