Hemicelluloses for fuel ethanol: A review.

Production of bioethanol from forest and agricultural products requires a fermenting organism that converts all types of sugars in the raw material to ethanol in high yield and with a high rate. This review summarizes recent research aiming at developing industrial strains of Saccharomyces cerevisiae with the ability to ferment all lignocellulose-derived sugars. The properties required from the industrial yeast strains are discussed in relation to four benchmarks: (1) process water economy, (2) inhibitor tolerance, (3) ethanol yield, and (4) specific ethanol productivity. Of particular importance is the tolerance of the fermenting organism to fermentation inhibitors formed during fractionation/pretreatment and hydrolysis of the raw material, which necessitates the use of robust industrial strain background. While numerous metabolic engineering strategies have been developed in laboratory yeast strains, only a few approaches have been realized in industrial strains. The fermentation performance of the existing industrial pentose-fermenting S. cerevisiae strains in lignocellulose hydrolysate is reviewed. Ethanol yields of more than 0.4 g ethanol/g sugar have been achieved with several xylose-fermenting industrial strains such as TMB 3400, TMB 3006, and 424A(LNF-ST), carrying the heterologous xylose utilization pathway consisting of xylose reductase and xylitol dehydrogenase, which demonstrates the potential of pentose fermentation in improving lignocellulosic ethanol production.

Towards industrial pentose-fermenting yeast strains

We have performed a comparative study of xylose utilization in Saccharomyces cerevisiae transformants expressing two key enzymes in xylose metabolism, xylose reductase (XR) and xylitol dehydrogenase (XDH), and in a prototypic xylose-utilizing yeast, Pichia stipitis. In the absence of respiration (see text), baker's yeast cells convert half of the xylose to xylitol and ethanol, whereas P. stipilis cells display rather a homofermentative conversion of xylose to ethanol. Xylitol production by baker's yeast is interpreted as a result of the dual cofactor dependence of the XR and the generation of NADPH by the pentose phosphate pathway. Further limitations of xylose utilization in S. cerevisiae cells are very likely caused by an insufficient capacity of the non-oxidative pentose phosphate pathway, as indicated by accumulation of sedoheptulose-7-phosphate and the absence of fructose-1,6-bisphosphate and pyruvate accumulation. By contrast, uptake at high substrate concentrations probably does not limit xylose conversion in S. cerevisiae XYL1/XYL2 transformants.

Xylose fermentation by Saccharomyces cerevisiae

Biological conversion of lignocellulosic biomass to ethanol

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

SummaryXylose reductase from the xylose-fermenting yeastPichia stipitis was purified to electrophoretic homogeneity via ion-exchange, gel and affinity chromatography. At physiological pH values the thermodynamic equilibrium constant was determined to be 0.575x1010 (l·mol-1). Product inhibiton studies are reported which clearly show that the kinetic mechanism of the xylose reductase is ordered-bi-bi with isomerisation of a stable enzyme form.

Xylose fermentation by yeasts: 4. Purification and kinetic studies of xylose reductase fromPichia stipitis

Purification and Properties of the NAD^+-Xylitol-Dehydrogenase from the Yeast Pichia stipitis

A kinetic study of the NAD+-xylitol-dehydrogenase from the yeast Pichia stipitis

Kinetic studies are presented for the growth and fermentation of the yeast Pichia stipitis with xylose as the carbon source. Ethanol is produced from xylose under anaerobic as well as under oxygen-limiting conditions but only at dissolved oxygen concentrations up to 3 mumol/L Maximum yields and production rates were obtained under oxygen-limiting conditions, where the xylose metabolism may be considered to be consisted of three different components (assimilation, respiration, fermentation). The contribution of each pathway is determined by the availability of oxygen and the energy yield of each pathway. In order to describe the course of oxygen-limited fermentations, a mathematical model has been developed with the assumption that growth is coupled to the energy production. The resulting model requires only four independent parameters (Y(x/O(2) ), Y(ATP) (max), m(ATP), and P/O). These parameters were estimated on the basis of eight separate batch fermentations.

Xylose fermentation by yeasts. 5. Use of ATP balances for modeling oxygen-limited growth and fermentation of yeast Pichia stipitis with xylose as carbon source.

Ngoc-Anh Bui-Thanh

Papers

Xylose fermentation by yeasts: 4. Purification and kinetic studies of xylose reductase fromPichia stipitis

Purification and Properties of the NAD^+-Xylitol-Dehydrogenase from the Yeast Pichia stipitis

A kinetic study of the NAD+-xylitol-dehydrogenase from the yeast Pichia stipitis

Xylose fermentation by yeasts. 5. Use of ATP balances for modeling oxygen-limited growth and fermentation of yeast Pichia stipitis with xylose as carbon source.