Engineered Yeast Uses Toxic By-Product to Make Ethanol

Researchers from the University of Illinois at Urbana-Champaign and University of California at Berkeley have used metabolic engineering to develop a yeast strain that is capable of using both xylose sugars and acetic acid for the production of ethanol.

The study aimed to address the limitations of microbial conversion of lignocellulosic biomass or raw materials from wood, grasses and inedible plant parts into ethanol via fermentation process. Most of the lignocellulosic biomass is made up of xylose sugar which is difficult for the baker's yeast (Saccharomyces cerevisiae) to ferment. Also, other biomass components lead to the formation of acetic acid during metabolism, which is toxic to fermenting microorganisms and so further reduces ethanol yields.

The pathway for xylose metabolism was realized by adding the genes xylose reductase and xylitol dehydrogenase from S. stipitis to the metabolic repertoire of S. cerevisiae. The pathway produces excess NADH, an electron-transfer molecule that is part of the energy currency of all cells. The yeast was later induced to consume toxic levels of acetic acid by expressing bacterial enzymes that catalyze this process. The enzymes not only converted acetic acid into ethanol, but also would use the surplus NADH from xylose metabolism.

The innovative strategy, reported in the journal Nature Communications, increases ethanol yield from lignocellulosic sources by about 10 percent. The results demonstrate how an undesirable redox state can be exploited by metabolic engineering to drive desirable reactions—even improving productivity and yield.