Abstract
BackgroundLignocellulosic 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.ResultsWe 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 β-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.ConclusionsReconstitution 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12934-015-0242-4) contains supplementary material, which is available to authorized users.
Highlights
Lignocellulosic biomass is a viable source of renewable energy for bioethanol production
While S. cerevisiae remains the host of choice for heterologous expression of cellulases and xylose utilising enzymes, we recently demonstrated that up to ten times higher levels of cellulase activity is attained from recombinant proteins expressed in the brewer’s yeast Saccharomyces pastorianus, when compared to the levels produced in S. cerevisiae strains [35]
Choice of host We previously observed that expression of recombinant cellulase genes in the brewer’s yeasts, S. pastorianus, produced significantly higher enzyme activities when compared to the activity observed in S. cerevisiae [35]
Summary
Lignocellulosic biomass is a viable source of renewable energy for bioethanol production. A β-glucan linear polymer of D-glucose linked by β-1,4-glycosidic bonds, is the most abundant polysaccharide on earth [4]. In plants, it is organised into crystalline fibers containing tightly packed microfibrils of approximately 30 β-glucan chains containing some disorganised amorphous regions [5]. The composition of hemicellulose is variable and depends upon the plant source glucose and xylose are generally the major hexose and pentose sugars respectively [7]. The third component of lignocellulose, lignin, is a polymer of phenylpropanoid monomers Lignin links both hemicellulose and cellulose together forming a physical barrier in the plant cell wall [8]. Hydrolysis of lignin does not generate fermentable sugars
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