Engineering Saccharomyces pastorianus for the co-utilisation of xylose and cellulose from biomass.

BACKGROUND: Lignocellulosic biomass is a viable source of renewable energy for bioethanol production. For the efficient conversion of biomass into bioethanol, it is essential that sugars from both the cellulose and hemicellulose fractions of lignocellulose be utilised. RESULTS: We describe the development of a recombinant yeast system for the fermentation of cellulose and xylose, the most abundant pentose sugar in the hemicellulose fraction of biomass. The brewer's yeast Saccharomyces pastorianus was chosen as a host as significantly higher recombinant enzyme activities are achieved, when compared to the more commonly used S. cerevisiae. When expressed in S. pastorianus, the Trichoderma reesei xylose oxidoreductase pathway was more efficient at alcohol production from xylose than the xylose isomerase pathway. The alcohol yield was influenced by the concentration of xylose in the medium and was significantly improved by the additional expression of a gene encoding for xylulose kinase. The xylose reductase, xylitol dehydrogenase and xylulose kinase genes were co-expressed with genes encoding for the three classes of T. reesei cellulases, namely endoglucanase (EG2), cellobiohydrolysase (CBH2) and ß-glucosidase (BGL1). The initial metabolism of xylose by the engineered strains facilitated production of cellulases at fermentation temperatures. The sequential metabolism of xylose and cellulose generated an alcohol yield of 82% from the available sugars. Several different types of biomass, such as the energy crop Miscanthus sinensis and the industrial waste, brewer's spent grains, were examined as biomass sources for fermentation using the developed yeast strains. Xylose metabolism and cell growth were inhibited in fermentations carried out with acid-treated spent grain liquor, resulting in a 30% reduction in alcohol yield compared to fermentations carried out with mixed sugar substrates. CONCLUSIONS: Reconstitution of complete enzymatic pathways for cellulose hydrolysis and xylose utilisation in S. pastorianus facilitates the co-fermentation of cellulose and xylose without the need for added exogenous cellulases and provides a basis for the development of a consolidated process for co-utilisation of hemicellulose and cellulose sugars.

Challenges for the production of bioethanol from biomass using recombinant yeasts.

Lignocellulose biomass, one of the most abundant renewable resources on the planet, is an alternative sustainable energy source for the production of second-generation biofuels. Energy in the form of simple or complex carbohydrates can be extracted from lignocellulose biomass and fermented by microorganisms to produce bioethanol. Despite 40 years of active and cutting-edge research invested into the development of technologies to produce bioethanol from lignocellulosic biomass, the process remains commercially unviable. This review describes the achievements that have been made in generating microorganisms capable of utilizing both simple and complex sugars from lignocellulose biomass and the fermentation of these sugars into ethanol. We also provide a discussion on the current "roadblocks" standing in the way of making second-generation bioethanol a commercially viable alternative to fossil fuels.

Metabolic engineering of yeasts by heterologous enzyme production for degradation of cellulose and hemicellulose from biomass: a perspective.

This review focuses on current approaches to metabolic engineering of ethanologenic yeast species for the production of bioethanol from complex lignocellulose biomass sources. The experimental strategies for the degradation of the cellulose and xylose-components of lignocellulose are reviewed. Limitations to the current approaches are discussed and novel solutions proposed.