Alcoholic fermentation of carbon sources in biomass hydrolysates by Saccharomyces cerevisiae: current status

Abstract

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.

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

“…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].…”

“…(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.…”

“…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.…”

Glucose(xylose) isomerase production by Streptomyces sp. CH7 grown on agricultural residues

“…Whereas wild-type S. cerevisiae can ferment xylulose to ethanol via the pentose-phosphate pathway (28), it cannot ferment xylose. Xylose is a major monosaccharide in plant hemicellulosic hydrolysates that can account for up to 30% of total sugars in some plant biomasses such as that of hardwoods *Corresponding Author.…”

“…Hemicelluloses are heterogeneous plant polymers consisting of pentoses, hexoses and sugar acids in which xylan, a backbone of 1,4-linked xylopyranosyl residues, is found as the major polymeric compound (28). As hemicelluloses are abundantly available in nature, they are good renewable resources for biofuel production, and they are cheap substrates for the production of glucose isomerase by microorganisms capable of growing on xylan-containing materials.…”