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 paper describing the yeast findings is in the Proceedings of the National Academy of Sciences. 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. 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. 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. That eliminates the costly step of adding a cellobiose-degrading enzyme to the lignocellulose mixture before the yeast consumes it.


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. 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.
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. 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. 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. The cellobiose transporter modification adds the advantage of circumventing the yeast’s own preference for glucose. 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.


Cellobiose is traditionally converted to glucose outside the yeast cell before entering the cell through glucose transporters for conversion to ethanol.
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. Having a cellobiose transporter means that the engineered yeast can bring cellobiose directly into the cell.



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Comments

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