The worldwide annual ethanol production via microbial fermentation amounted to ca. 40 Mt in 2005 (according to the Renewable Fuel Associa­tion; www. ethanolrfa. org) and is rapidly growing. Although bacteria such as Zymomonas mobilis and engineered Escherichia coli strains are capable of homoethanolic fermentation of sugars [17], the yeast Saccharomyces cere­visiae remains the organism of choice for large-scale industrial production of ethanol. Factors contributing to the popularity of S. cerevisiae as an industrial ethanol producer include its high ethanol tolerance, its ability to grow under strictly anaerobic conditions and — an important characteristic distinguishing it from prokaryotic organisms — its insensitivity to bacteriophage contamina­tions. Moreover, S. cerevisiae grows well at low pH, reducing problems with contamination of industrial processes with, for example, lactic acid bacteria.

Global concern about carbon dioxide emissions and climate change, deple­tion of oil reserves and geopolitical issues all contribute to a drive to increase the production of ethanol as a renewable transport fuel (see the contribution of Otero et al. in this volume). Presently, ethanol is exclusively produced from the starch or the sucrose fraction of a small number of (edible) agricultural crops such as corn, sugar cane, sugar beet and grain. To expand the feed­stock range for large-scale ethanol production and to improve productivity, it is of vital importance to enable efficient ethanol production from agri­cultural residues and other low-value sources of carbohydrates. Feedstocks such as corn stover, bagasse, wheat straw, non-recyclable paper or dedicated crops such as switchgrass represent an enormous potential in terms of avail­able carbohydrates. However, instead of starch and sucrose, the carbohydrates in these feedstocks consist of a complex matrix of cellulose, hemicellulose, pectin and lignin [69].

The use of lignocellulosic raw materials for ethanol production poses a number of major challenges compared to the use of conventional starch — or sucrose-based feedstocks:

(i) Release of monomeric sugars from lignocellulosic biomass requires a mix of physicochemical (extreme pH, high temperature, high pressure) and enzymic polysaccharide (hydrolases) treatments [19,37].

(ii) The resulting lignocellulose hydrolysates contain a wide variety of com­pounds that may inhibit the fermentation process. These compounds are either formed during the pretreatment process (e. g. furfural and hydroxymethylfurfural) or are biomass constituents that are released during hydrolysis (e. g. acetate, formate) [31,37,49,54].

(iii) Whereas starch — and sucrose-based feedstocks yield hexoses upon hydro­lysis, lignocellulosic biomass, and in particular its hemicellulose frac­tion, also contains large amounts of the pentose sugars D-xylose and L-arabinose. D-Xylose, generally the most abundant pentose, comprises up to 25% of the total sugar content in some hydrolysates [24,46,69].

Whereas S. cerevisiae spp. can rapidly ferment hexose sugars such as glucose, fructose, mannose and galactose, they cannot grow on nor ferment D-xylose or L-arabinose [7,69]. Given the importance of xylose fermentation for the efficient production of ethanol from lignocellulosic biomass [24,46,69], it is not surprising that introduction and optimisation of heterologous path­ways for xylose fermentation into S. cerevisiae has long been a hot topic in metabolic engineering of yeast.

Interestingly, it has long been known that S. cerevisiae is able to slowly metabolise the pentose sugar D-xylulose [30,71]. This keto-isomer of xylose is phosphorylated to D-xylulose-5-phosphate by xylulokinase (XKS1, [57]) and subsequently metabolised via the non-oxidative part of the pentose phos­phate pathway and glycolysis. It is therefore logical that strategies for convert­ing D-xylose into D-xylulose are an exhaustively studied topic in the quest for alcoholic fermentation of D-xylose by S. cerevisiae. These strategies will be briefly discussed in Sects. 1.2-1.4.

1.2