(107a) Enhanced Biofuel Production Through Coupled Acetic Acid and Xylose Consumption By Engineered Yeast

The anticipation for substituting conventional fossil fuels with cellulosic biofuels fuels is growing in the face of increasing demand for energy and rising concerns of greenhouse gas emissions.  However, commercial production of cellulosic biofuel has been hampered by inefficient fermentation of xylose and toxicity of acetic acid released from hemicellulose in the plant cell wall. While constituting substantial portions of cellulosic biomass, xylose and acetate cannot be fermented to ethanol by Saccharomyces cerevisiae. Here we use a redox balancing strategy to enable efficient xylose fermentation and simultaneous in situ detoxification of plant cell wall-derived feedstocks. By combining an NADH-consuming acetate consumption pathway and an NADH-producing xylose utilization pathway, engineered yeast converted cellulosic sugars and toxic levels of acetate together into ethanol under anaerobic conditions. The engineered yeast strain showed increased ethanol yields, decreased byproduct accumulation, and improved cell growth during co-utilization of xylose and acetic acid compared to the reference strain without the anaerobic acetate-consuming pathway. The results demonstrate a breakthrough in making efficient use of carbon compounds in cellulosic biomass and present an innovative strategy for metabolic engineering whereby an undesirable redox state can be exploited to drive desirable metabolic reactions, even improving productivity and yield.