17-14: Rapid xylose and glucose co-fermentation by S. cerevisiae – an engineered yeast strain for commercial production of cellulosic ethanol

The yeast Saccharomyces cerevisiae is an efficient and widely used ethanol producer, but does not naturally ferment xylose, which is the dominant pentose sugar in lignocellulosic materials. By identifying and alleviating a previously unexplored bottleneck in the xylose metabolic pathway, Terranol has developed a polyploid industrial S. cerevisiae strain for fermentation of xylose at increased rates. In addition to up-regulation of the pentose phosphate pathway, and an efficient bacterial xylose isomerase, the strain expresses a xylose 1-epimerase. The epimerase catalyzes an otherwise slow conversion between the β- and α-epimers of D-xylose, of which only the α-epimer is a substrate of the xylose isomerase.

All inserted genes are stably integrated in the genome of the yeast and the strain has undergone extensive evolutionary engineering, resulting in rapid xylose consumption, high resistance towards biomass derived inhibitors, as well as very low formation of the byproduct xylitol.

Fermentations of various lignocellulosic hydrolyzates by Terranol’s strain have shown high ethanol yields and concentrations. Furthermore, by development of automatic feed controls, the sugar concentrations can be optimized during the fed-batch fermentation providing better co-consumption of glucose and xylose, resulting in a more rapid, and hence shorter, fermentation. For instance, in a fed-batch fermentation of wheat straw hydrolyzate liquid (about 120 g/L total sugars), with a total yeast loading of 1 g dw/L, both glucose and xylose was completely fermented in as little as 48 hours resulting in an ethanol yield around 90 % of total available sugars and a final ethanol concentration above 50 g/L.