Professor Eckhard Boles at the Goethe-University Frankfurt, Germany and his colleagues have genetically modified the yeast Saccharomyces cerevisiae to express a newly identified enzyme to enable the fermentation of xylose (a 5-carbon sugar) as well 6-carbon sugars (e.g., glucose). A paper on their work appeared online 13 February in the journal Applied Environmental Microbiology.
Cellulosic biomass, when treated, releases a mixture of hexose (6-carbon) and pentose (5-carbon) sugars, including glucose, galactose, mannose, D-xylose and L-arabinose. S. cerevisiae, the standard industrial yeast, offer fast sugar consumption, high yields and ethanol tolerance&mash;but is unable to ferment pentose sugars.
A lack of microorganisms that will convert both hexoses and pentoses efficiently to ethanol is a constraint to the economical production of cellulosic ethanol by the fermentation pathway.
Boles and his colleagues screened nucleic acid databases and found they could clone and successfully express a highly active new kind of xylose isomerase (XI) from the anaerobic bacterium Clostridium phytofermentans in S. cerevisiae. The new XI offers benefits compared to two other xylose isomerases previously used in modifications of S. cerevisiae.
Heterologous expression of this enzyme confers to the yeast cells the ability to metabolize D-xylose and to use it as the sole carbon and energy source. The new enzyme has low sequence similarities to the XI from Piromyces sp.E2 and Thermus thermophilus, which were the only two xylose isomerases previously functionally expressed in S. cerevisiae. Activity and kinetic parameters of the new enzyme are comparable to the Piromyces xylose isomerase.
Importantly, the new enzyme is far less inhibited by xylitol, which accrues as a side-product during xylose fermentation. Furthermore, expression of the gene could be improved by adapting its codon usage to that of the highly expressed glycolytic genes of S. cerevisiae. Expression of the bacterial XI in an industrially employed yeast strain enabled it to grow on xylose and to ferment it to ethanol. Thus, our findings provide an excellent starting point for further improvement of xylose fermentation in industrial yeast strains.—Brat et al. (2009)
Boles is also the cofounder of biofuel company Butalco gmbH, which seeks to produce biobutanol. (Earlier post.) Boles is thus also developing yeast strains to convert biomass to butanol.