Purdue Researchers Improve Strain of Yeast for Co-Fermentation of 5 Plant Sugars; Improved Yields of Cellulosic Ethanol
Researchers at Purdue University have added the ability to ferment the sugar l-arabinose to a strain of the industrial yeast Saccharomyces cerevisiae earlier developed by the team at Purdue that itself offered the ability to ferment xylose. (Earlier post.)
The new strain achieved a more than 40% yield in producing ethanol from arabinose. The team achieved overall ethanol production of about 72.5% yield from five-sugar mixtures containing glucose, galactose, mannose, xylose, and arabinose.
Nathan Mosier, an associate professor of agricultural and biological engineering; Miroslav Sedlak, a research assistant professor of agricultural and biological engineering; and Nancy Ho, a research professor of chemical engineering, constructed the new strain by over-expression of two additional genes from fungi l-arabinose utilization pathways.
Natural yeast can ferment three sugars: galactose, mannose and glucose. The original Ho yeast added xylose to that, and now the fifth, arabinose, has been added.
—Dr. Nancy Ho
The addition of new genes to the Ho yeast strain should increase the amount of ethanol that can be produced from cellulosic material. Arabinose makes up about 10% of the sugars contained in those plants.
In addition to creating this new arabinose-fermenting yeast, Mosier, Sedlak and Ho also were able to develop strains that are more resistant to acetic acid. Acetic acid, the main ingredient in vinegar, is natural to plants and released with sugars before the fermentation process during ethanol production. Acetic acid gets into yeast cells and slows the fermentation process, adding to the cost of ethanol production.
Mosier, Sedlak and Ho compared the genes in the more resistant strains to others to determine which genes made the yeast more resistant to acetic acid. By improving the expression of those genes, they increased the yeast’s resistance.
Mosier said arabinose is broken down in the same way as the other four sugars except for the first two steps. Adding the fungus genes allowed the yeast to create necessary enzymes to get through those steps.
The team’s findings on acetic acid were published in the June issue of the journal FEMS Yeast Research. The findings on arabinose were published in the early online version of the journal Applied Microbiology and Biotechnology.
Mosier, Sedlak and Ho will continue to improve the yeast to make it more efficient during industrial ethanol production and more resistant to inhibitors. The US Department of Energy funded their research.
Elizabeth Casey, Miroslav Sedlak, Nancy W.Y. Ho, Nathan S. Mosier (2010) Effect of acetic acid and pH on the cofermentation of glucose and xylose to ethanol by a genetically engineered strain of Saccharomyces cerevisiae FEMS Yeast Research VOl 10, Issue 4 (p 385-393) doi: 10.1111/j.1567-1364.2010.00623.x
Aloke Kumar Bera, Miroslav Sedlak, Aftab Khan and Nancy W. Y. Ho (2010) Establishment of l-arabinose fermentation in glucose/xylose co-fermenting recombinant Saccharomyces cerevisiae 424A(LNH-ST) by genetic engineering. Applied Microbiology and Biotechnology doi: 10.1007/s00253-010-2609-0