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New engineered metabolic pathways in yeast enable efficient fermentation of xylose from biomass

Researchers with the Energy Biosciences Institute (EBI), a partnership that includes Berkeley Lab and the University of California (UC) Berkeley, have introduced new metabolic pathways from the fungus Neurospora crassa into the yeast Saccharomyces cerevisiae to increase the fermentative production of fuels and other chemicals from biomass. An open access paper on the work is publised in the journal eLife.

While S. cerevisiae is the industry mainstay for fermenting sugar from cornstarch and sugarcane into ethanol, it requires substantial engineering to ferment sugars derived from plant cell walls such as cellobiose and xylose. The new metabolic pathways enable the yeast to ferment sugars from both cellulose (glucose) and hemicellulose (xylose)—the two major families of sugar found in the plant cell wall—efficiently, without the need of environmentally harsh pre-treatments or expensive enzyme cocktails.

Jamie Cate, a staff scientist in Berkeley Lab’s Physical Biosciences Division and a professor of biochemistry, biophysics and structural biology at UC Berkeley, and a team of collaborators identified the metabolic pathways in the fungus Neurospora crassa that are used to digest xylose, one of the most abundant sugars in hemicellulose.

In contrast to S. cerevisiae, many cellulolytic fungi including N. crassa naturally grow well on both the cellulose and hemicellulose components of the plant cell wall. By using functional genomics data and N. crassa knockout strains, we identified separate pathways used by N. crassa to consume the cellodextrins and xylodextrins released from plant cell walls by its secreted enzymes.

—Jamie Cate, corresponding author

To enable the N. crassa metabolic pathways to work in yeast, Cate and his collaborators introduced five new genes into the yeast. While the new pathways and genes allow the yeast to directly ferment xylose sugars into a desired biofuel or chemical product, those sugars still have to be released from the plant cell walls.

This can be done, however, with a simple hot water-pretreatment rather than the acids or ionic liquids that current pre-treatment methods deploy. Harsh chemicals like acids and ionic liquids, unlike hot water, must be removed prior to fermentation so as not to harm the microbes. This is another major expense in addition to the expensive enzymes required to break down the xylose sugars.

We’ve discovered new chemicals generated by fungi and bacteria as metabolites in their strategy for consuming the plant cell wall that are a general part of the global carbon cycle. We should now be able engineer biofuel-producing yeast to do what these fungi and bacteria do, opening up many new possible scenarios for making biofuels and other important products. We believe that introducing N. crassa metabolic pathways into yeast could find widespread use in helping to overcome existing bottlenecks to the fermentation of lignocellulosic feedstocks as a sustainable and economical source of biofuels and renewable chemicals.

—Jamie Cate

This research was funded by EBI.

Resources

  • Xin Li, Vivian Yaci Yu, Yuping Lin, Kulika Chomvong, Raíssa Estrela, Annsea Park, Julie M Liang, Elizabeth A Znameroski, Joanna Feehan, Soo Rin Kim, Yong-Su Jin, N Louise Glass, Jamie HD Cate (2015) “Expanding xylose metabolism in yeast for plant cell wall conversion to biofuels” eLife doi: 10.7554/eLife.05896

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