Major Obstacle to Cellulosic Biofuel Overcome

This is a huge step forward.  The newly engineered yeast is able tocoprocess both glucose and the more complex sugar xylose at the same time andas fast as it can do either alone.  theresult is a major hump in efficiency and an important step in processing celluloseitself. 

I anticipate that a processbreaking down cellulose will produce a feedstock rich in these two sugars.  Thus we have a key step in place that is alsohighly efficient.  This will also beuseful in present fermentation processes I am sure.

Ethanol may not end up stayingthe course in terms of general transportation, but it is awfully attractive tothe agricultural industry if one could reduce ag waste into a farm usablebiofuel.  Agriculture will continue toneed heavy equipment with high torque and liquid fuels are pretty good atdelivering.

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 ofsugar from plants to produce ethanol, researchers report. The sugars areglucose, a six-carbon sugar that is relatively easy to ferment; and xylose, afive-carbon sugar that has been much more difficult to utilize in ethanolproduction. The new strain, made by combining, optimizing and adding to earlieradvances, reduces or eliminates several major inefficiencies associated withcurrent biofuel production methods.

The findings, from a collaborative led by researchers at the Universityof Illinois, the Lawrence Berkeley National Laboratory, the University ofCalifornia and the energy company BP, are described in the Proceedings of theNational Academy of Sciences. The Energy Biosciences Institute, a BP-fundedinitiative, supported the research.

Yeasts feed on sugar and produce various waste products, some of whichare useful to humans. One type of yeast, Saccharomyces cerevisiae, has beenused for centuries in baking and brewing because it efficiently ferments sugarsand in the process produces ethanol and carbon dioxide. The biofuel industryuses this yeast to convert plant sugars to bioethanol. And while S. cerevisiaeis very good at utilizing glucose, a building block of cellulose and theprimary sugar in plants, it cannot use xylose, a secondary – but significant –component of the lignocellulose that makes up plant stems and leaves. Mostyeast strains that are engineered to metabolize xylose do so very slowly.

"Xylose is a wood sugar, a five-carbon sugar that is very abundantin lignocellulosic biomass but not in our food," said Yong-Su Jin, aprofessor of food science and human nutrition at Illinois. He also is an affiliate of theU. of I. Institute for Genomic Biology and a principal investigatoron the study. "Most yeast cannot ferment xylose."

A big part of the problem with yeasts altered to take up xylose is thatthey will suck up all the glucose in a mixture before they will touch thexylose, Jin said. A glucose transporter on the surface of the yeast prefers tobind 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 significantlyto the cost of biofuels production.

Jin and his colleagues wanted to induce the yeast to quickly andefficiently consume both types of sugar at once, a process calledco-fermentation. The research effort involved researchers from Illinois, theLawrence Berkeley National Laboratory, the University of California atBerkeley, Seoul National University and BP.

In a painstaking process of adjustments to the original yeast, Jin andhis colleagues converted it to one that will consume both types of sugar fasterand more efficiently than any strain currently in use in the biofuel industry.In fact, the new yeast strain simultaneously converts cellobiose (a precursorof glucose) and xylose to ethanol just as quickly as it can ferment eithersugar alone.

"If you do the fermentation by using only cellobiose or xylose, ittakes 48 hours," said postdoctoral researcher and lead author Suk-Jin Ha."But if you do the co-fermentation with the cellobiose and xylose, doublethe amount of sugar is consumed in the same amount of time and produces morethan double the amount of ethanol. It's a huge synergistic effect ofco-fermentation."

The new yeast strain is at least 20 percent more efficient atconverting xylose to ethanol than other strains, making it "the bestxylose-fermenting strain" reported in any study, Jin said.

The team achieved these outcomes by making several critical changes tothe 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 beforeentering the cell through glucose transporters for conversion to ethanol.Having a cellobiose transporter means that the engineered yeast can bringcellobiose directly into the cell. Only after the cellobiose is inside the cellis it converted to glucose.

This approach, initially developed by co-corresponding author JamieCate at the Lawrence Berkeley National Laboratory and the University ofCalifornia at Berkeley, eliminates the costly step of adding acellobiose-degrading enzyme to the lignocellulose mixture before the yeastconsumes it.

It has the added advantage of circumventing the yeast's own preference forglucose. Because the glucose can now "sneak" into the yeast in theform of cellobiose, the glucose transporters can focus on drawing xylose intothe cell instead. Cate worked with Jonathan Galazka, of UC Berkeley, to clonethe 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-consumingyeast, Picchia stipitis.

Graduate student Soo Rin Kim at the University of Illinoisidentified a bottleneck in this metabolic pathway, however. By adjusting therelative production of these enzymes, the researchers eliminated the bottleneckand boosted the speed and efficiency of xylose metabolism in the new strain.

They also engineered an artificial "isoenzyme" that balancedthe 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'sability to utilize xylose.

The cost benefits of this advance in co-fermentation are verysignificant, 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 prettysure that this research can be commercialized very soon."

Jin noted that the research was the result of a successfulcollaboration among principal investigators in the Energy Biosciences Instituteand a BP scientist, Xiaomin Yang, who played a key role in developing theco-fermentation concept and coordinating the collaboration.

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