Xylose comes from woody plant structures, its a five-carbon atom sugar and is a primary target in lignin and cellulose biomass fermentation.  But most yeast cannot ferment xylose sugar.  Now a collaboration led by researchers at the University of Illinois, the Lawrence Berkeley National Laboratory, the University of California and the oil company BP are reporting a newly engineered yeast strain that can simultaneously consume xylose and the common plant sugar for ethanol production, glucose.

D-xylose 5 Green Balls for the Carbon Atoms. Click image for the largest view.

Glucose 6 Red Balls for the Carbon Atoms. Click image for the largest view.

Glucose, a six-carbon atom sugar is relatively easy to ferment and is the feedstock for most ethanol today.  Xylose, the five-carbon sugar has been much more difficult to utilize in ethanol production. The new yeast strain, made by combining, optimizing and adding to earlier advances, reduces or eliminates several major inefficiencies associated with current biofuel production methods.

The paper describing the yeast findings is in the Proceedings of the National Academy of Sciences. The research project is from Energy Biosciences that’s backed by funding from British Petroleum.  There’s an item to counter an oil spill.

Brewers 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.

But brewers yeast cannot use xylose, a secondary – but significant – component of the lignocellulose that makes up the plant structure in stems and leaves.  Current yeast strains that are engineered to metabolize xylose do so very slowly and add greatly to the cost of production.  This is the problem with the cellulosic ethanol industry, which is years behind popular and political expectations.

Yong-Su Jin, University of Illinois Food Science and Human Nutrition Professor explains the other major problem with getting the xylose fermented.  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.  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.”

Jin and his colleagues, through a painstaking process, converted the base yeast to one that they show in the paper 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.

There’s a heads up moment for you.  Lead author and postdoctoral researcher Suk-Jin Ha said, “If you do the fermentation by using only cellobiose or xylose, it takes 48 hours. 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 paper’s findings are showing the new yeast strain is at least 20 percent more efficient at converting xylose to ethanol than other strains.  Jin believes their yeast is the best xylose-fermenting strain reported in any study to date.

On the technical side the team’s efforts were achieved 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.

That eliminates the costly step of adding a cellobiose-degrading enzyme to the lignocellulose mixture before the yeast consumes it. The cellobiose transporter modification adds the 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. Credit here goes to co-corresponding author Jamie Cate at the Lawrence Berkeley National Laboratory and the University of California at Berkeley.  The work was done with Jonathan Galazka, of UC Berkeley, to clone the transporter and enzyme used in the new strain.

The metabolism or speed matter also presented a challenge.  The team inserted three genes into their modified brewers yeast from a xylose-consuming yeast, Picchia stipitis. That didn’t work so well, and then graduate student Soo Rin Kim at the UI identified a bottleneck in this metabolic pathway.  That set up the team to eliminate the bottleneck and boost the speed and efficiency of xylose metabolism.

Next were two more steps.  First the team engineered an artificial “isoenzyme” that balanced the proportion of two important cofactors so that the accumulation of xylitol, a byproduct in the xylose assimilation pathway, could be minimized.  Wrapping it all up the team used “evolutionary engineering”, a kind selective optimization over generations to optimize the new strain’s ability to utilize xylose.

The payoff could be very important to the lignin and cellulose processing challenge.  Both of the main sugar components can be used in one step. Jin 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.”

Odds are that’s a good possibility.  Brewers yeast works in nearly pure sugar like sugarcane juice and ground corn kernels with an abundance of fiber, protein and other contaminants.  It works in bread.  Brewers yeast is pretty tolerant of those molecules that come along for the ride.
How much tolerance the newly engineered strain has is yet to be seen.  Like lots of ideas to date, lignin and cellulose sugars are very hard to acquire and process and as good as the lab findings are, a 10,000 gallon tank full of raw material with all the expected foreign matter will be a new challenge.  Scale and commercialization can be a huge mountain to climb.

It’s congratulations in any case.  A very promising pathway for modifying brewers yeast is in hand.  It was backed by a nemesis for many, yet for the investment by BP this writer is grateful.  It’s been a very hard year for BP this news has to help measure up to the attacks on the company’s character.

This team’s insight and innovation using the most common tool for ethanol production is worthy of acclaim.  The news swept the news and much of the blogs  Two sugars at once, high speed and better than standard production is certainly big news.  It may also point the way for other sugars as well.  20 million barrels a day of ethanol doesn’t seem so far fetched as just yesterday.


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