Development of Efficient Xylose Fermentation in Saccharomyces cerevisiae: Xylose Isomerase as a Key Component

Metabolic engineering of Saccharomyces cerevisiae for ethanol production from D-xylose, an abundant sugar in plant biomass hydrolysates, has been pursued vigorously for the past 15 years. Whereas wild-type S. cerevisiae cannot ferment D-xylose, the keto-isomer D-xylulose can be metabolised slowly. Conversion of D-xylose into D-xylulose is therefore crucial in metabolic engineering of xylose fermentation by S. cerevisiae. Expression of heterologous xylose reductase and xylitol dehydrogenase does enable D-xylose utilisation, but intrinsic redox constraints of this pathway result in undesirable byproduct formation in the absence of oxygen. In contrast, expression of xylose isomerase (XI, EC 5.3.1.5), which directly interconverts D-xylose and D-xylulose, does not have these constraints. However, several problems with the functional expression of various bacterial and Archaeal XI genes have precluded successful use of XI in yeast metabolic engineering. This changed with the discovery of a fungal XI gene in Piromyces sp. E2, expression of which led to high XI activities in S. cerevisiae. When combined with over-expression of the genes of the non-oxidative pentose phosphate pathway of S. cerevisiae, the resulting strain grew anaerobically on D-xylose with a doubling time of ca. 8 h, with the same ethanol yield as on glucose. Additional evolutionary engineering was used to improve the fermentation kinetics of mixed-substrate utilisation, resulting in efficient D-xylose utilisation in synthetic media. Although industrial pilot experiments have already demonstrated high ethanol yields from the D-xylose present in plant biomass hydrolysates, strain robustness, especially with respect to tolerance to inhibitors present in hydrolysates, can still be further improved.