Fuel ethanol production from plant biomass hydrolysates by Saccharomyces cerevisiae is of great economic and environmental significance. This paper reviews the current status with respect to alcoholic fermentation of the main plant biomass-derived monosaccharides by this yeast. Wild-type S. cerevisiae strains readily ferment glucose, mannose and fructose via the Embden-Meyerhof pathway of glycolysis, while galactose is fermented via the Leloir pathway. Construction of yeast strains that efficiently convert other potentially fermentable substrates in plant biomass hydrolysates into ethanol is a major challenge in metabolic engineering. The most abundant of these compounds is xylose. Recent metabolic and evolutionary engineering studies on S. cerevisiae strains that express a fungal xylose isomerase have enabled the rapid and efficient anaerobic fermentation of this pentose.L-Arabinose fermentation, based on the expression of a prokaryotic pathway in S. cerevisiae, has also been established, but needs further optimization before it can be considered for industrial implementation. In addition to these already investigated strategies, possible approaches for metabolic engineering of galacturonic acid and rhamnose fermentation by S. cerevisiae are discussed. An emerging and major challenge is to achieve the rapid transition from proof-of-principle experiments under 'academic' conditions (synthetic media, single substrates or simple substrate mixtures, absence of toxic inhibitors) towards efficient conversion of complex industrial substrate mixtures that contain synergistically acting inhibitors.

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Lignocellulosic feedstocks are thought to have great economic and environmental significance for future biotechnological production processes. For cost-effective and efficient industrial processes, complete and fast conversion of all sugars derived from these feedstocks is required. Hence, simultaneous or fast sequential fermentation of sugars would greatly contribute to the efficiency of production processes. One of the main challenges emerging from the use of lignocellulosics for the production of ethanol by the yeast Saccharomyces cerevisiae is efficient fermentation of D-xylose and L-arabinose, as these sugars cannot be used by natural S. cerevisiae strains. In this study, we describe the first engineered S. cerevisiae strain (strain IMS0003) capable of fermenting mixtures of glucose, xylose, and arabinose with a high ethanol yield (0.43 g g ؊1 of total sugar) without formation of the side products xylitol and arabinitol. The kinetics of anaerobic fermentation of glucose-xylose-arabinose mixtures were greatly improved by using a novel evolutionary engineering strategy. This strategy included a regimen consisting of repeated batch cultivation with repeated cycles of consecutive growth in three media with different compositions (glucose, xylose, and arabinose; xylose and arabinose; and only arabinose) and allowed rapid selection of an evolved strain (IMS0010) exhibiting improved specific rates of consumption of xylose and arabinose. This evolution strategy resulted in a 40% reduction in the time required to completely ferment a mixture containing 30 g liter ؊1 glucose, 15 g liter ؊1 xylose, and 15 g liter ؊1arabinose.In recent years, the need for biotechnological manufacturing based on lignocellulosic feedstocks has become evident (6, 10). In contrast to the readily fermentable, mainly starch-or sucrose-containing feedstocks used in current biotechnological production processes, lignocellulosic biomass requires intensive pretreatment and hydrolysis, which yield complex mixtures of sugars (3,7,14,27). For cost-effective and efficient industrial processes, complete and fast conversion of all sugars present in lignocellulosic hydrolysates is a prerequisite. The major hurdles encountered in implementing these production processes are the conversion of substrates that cannot be utilized by the organism of choice and, even more importantly, the subsequent improvement of sugar conversion rates and product yields.The use of evolutionary engineering has proven to be very valuable for obtaining phenotypes of (industrial) microorganisms with improved properties, such as an expanded substrate range, increased stress tolerance, and efficient substrate utilization (16,17). Also, for the yeast Saccharomyces cerevisiae, the preferred organism for large-scale ethanol production for the past few decades, evolutionary engineering has been extensively used to select for industrially relevant phenotypes. For ethanol production from lignocellulose by S. cerevisiae, one of the main challenges is efficient conversion of the pentoses ...

For cost-effective and efficient ethanol production from lignocellulosic fractions of plant biomass, the conversion of not only major constituents, such as glucose and xylose, but also less predominant sugars, such as L-arabinose, is required. Wild-type strains of Saccharomyces cerevisiae, the organism used in industrial ethanol production, cannot ferment xylose and arabinose. Although metabolic and evolutionary engineering has enabled the efficient alcoholic fermentation of xylose under anaerobic conditions, the conversion of L-arabinose into ethanol by engineered S. cerevisiae strains has previously been demonstrated only under oxygen-limited conditions. This study reports the first case of fast and efficient anaerobic alcoholic fermentation of L-arabinose by an engineered S. cerevisiae strain. This fermentation was achieved by combining the expression of the structural genes for the L-arabinose utilization pathway of Lactobacillus plantarum, the overexpression of the S. cerevisiae genes encoding the enzymes of the nonoxidative pentose phosphate pathway, and extensive evolutionary engineering. The resulting S. cerevisiae strain exhibited high rates of arabinose consumption (0.70 g h ؊1 g [dry weight] ؊1 ) and ethanol production (0.29 g h ؊1 g [dry weight] ؊1 ) and a high ethanol yield (0.43 g g ؊1 ) during anaerobic growth on L-arabinose as the sole carbon source. In addition, efficient ethanol production from sugar mixtures containing glucose and arabinose, which is crucial for application in industrial ethanol production, was achieved.In the past decades, it has become clear that for future sustainable and cost-effective production of fuel ethanol from plant biomass, not only the readily degradable starch and sucrose fractions but also the much more resistant lignocellulosic fractions of plant biomass should be used. Although glucose and xylose are often the predominant sugars in these feedstocks, the economically efficient production of ethanol also requires the conversion of smaller carbohydrate fractions, such as L-arabinose, at high rates and yields (9, 23).Saccharomyces cerevisiae is presently the organism of choice for industrial ethanol production. Although wild-type S. cerevisiae strains rapidly ferment hexoses with high efficiency, they cannot grow on or use pentoses, such as D-xylose and L-arabinose (3). In addition to the development of pentose-consuming bacteria such as Zymomonas mobilis, Escherichia coli, and Klebsiella oxytoca as alternative biocatalysts for ethanol production (5), this situation has inspired various studies to expand the substrate range of S. cerevisiae. The combination of metabolic and evolutionary engineering with the heterologous expression of either yeast xylose reductase and xylitol dehydrogenase (14,32,34,35,41) or a fungal xylose isomerase (19)(20)(21)(22) has already enabled the anaerobic fermentation of D-xylose by S. cerevisiae. The next challenge is the fermentation of other pentoses, such as L-arabinose. Although several yeasts and fungi can utilize L-arabinose as ...