In industrial fermentation processes, the yeast Saccharomyces cerevisiae is commonly used for ethanol production. However, it lacks the ability to ferment pentose sugars like D-xylose and L-arabinose. Heterologous expression of a xylose isomerase (XI) would enable yeast cells to metabolize xylose. However, many attempts to express a prokaryotic XI with high activity in S. cerevisiae have failed so far. We have screened nucleic acid databases for sequences encoding putative XIs and finally were able to clone and successfully express a highly active new kind of XI from the anaerobic bacterium Clostridium phytofermentans in S. cerevisiae. Heterologous expression of this enzyme confers on the yeast cells the ability to metabolize D-xylose and to use it as the sole carbon and energy source. The new enzyme has low sequence similarities to the XIs from Piromyces sp. strain E2 and Thermus thermophilus, which were the only two XIs previously functionally expressed in S. cerevisiae. The activity and kinetic parameters of the new enzyme are comparable to those of the Piromyces XI. Importantly, the new enzyme is far less inhibited by xylitol, which accrues as a side product during xylose fermentation. Furthermore, expression of the gene could be improved by adapting its codon usage to that of the highly expressed glycolytic genes of S. cerevisiae. Expression of the bacterial XI in an industrially employed yeast strain enabled it to grow on xylose and to ferment xylose to ethanol. Thus, our findings provide an excellent starting point for further improvement of xylose fermentation in industrial yeast strains.

Bioethanol produced by microbial fermentations of plant biomass hydrolysates consisting of hexose and pentose mixtures is an excellent alternative to fossil transportation fuels. However, the yeast Saccharomyces cerevisiae, commonly used in bioethanol production, can utilize pentose sugars like L-arabinose or D-xylose only after heterologous expression of corresponding metabolic pathways from other organisms. Here we report the improvement of a bacterial L-arabinose utilization pathway consisting of L-arabinose isomerase from Bacillus subtilis and L-ribulokinase and L-ribulose-5-P 4-epimerase from Escherichia coli after expression of the corresponding genes in S. cerevisiae. L-Arabinose isomerase from B. subtilis turned out to be the limiting step for growth on L-arabinose as the sole carbon source. The corresponding enzyme could be effectively replaced by the enzyme from Bacillus licheniformis, leading to a considerably decreased lag phase. Subsequently, the codon usage of all the genes involved in the L-arabinose pathway was adapted to that of the highly expressed genes encoding glycolytic enzymes in S. cerevisiae. Yeast transformants expressing the codon-optimized genes showed strongly improved L-arabinose conversion rates. With this rational approach, the ethanol production rate from L-arabinose could be increased more than 2.5-fold from 0.014 g ethanol h ؊1 (g dry weight) Decreasing fossil energy resources and the climate change caused by emissions of CO 2 from their burning have led to a growing interest in renewable-energy alternatives. Bioethanol produced by microbial fermentations of plant biomass is an excellent alternative to fossil fuels. However, for economical ethanol production, it is not enough to convert only the starch and sucrose fractions of plant biomass as is mainly done in conventional ethanol plants. The importance of the conversion of the lignocellulosic fraction as well becomes more and more evident. Lignocellulosic hydrolysates consist of easily fermentable hexose sugars but also significant amounts of pentose sugars. Depending on the choice of raw material, the amounts of pentoses found in lignocellulosic hydrolysates range from, for example, 16% xylan and 5% arabinan in grass and 15% arabinan and 19% xylan in wheat bran (14). These numbers clearly indicate that both hexoses and pentoses must be fermented in an efficient ethanol production process. Even small increases in substrate utilization should significantly improve the overall process costs (28).Although Saccharomyces cerevisiae is the organism most widely used for ethanol production and is able to convert hexoses rapidly and with high ethanol yields, wild-type S. cerevisiae strains are not able to ferment pentose sugars, such as D-xylose and L-arabinose, efficiently. Even though xylose can be slowly metabolized, at least by adapted strains (2), the second most abundant pentose, L-arabinose, cannot be converted at all. Several different genetic-engineering approaches have been used in attempts to enable D-xylose and L-arabinose...

Background: Fermentation of lignocellulosic biomass is an attractive alternative for the production of bioethanol. Traditionally, the yeast Saccharomyces cerevisiae is used in industrial ethanol fermentations. However, S. cerevisiae is naturally not able to ferment the pentose sugars D-xylose and L-arabinose, which are present in high amounts in lignocellulosic raw materials.

Five new linear acetylenic compounds, namely, pentadeca-6,8,10-triynoic acid (1), octadeca-8,10,12-triynoic acid (2), trans-pentadec-10-en-6,8-diynoic acid (3), cis-hexadec-11-en-7,9-diynoic acid (4), and cis-octadec-12-en-7,9-diynoic acid (5), were isolated from the bark of Heisteria acuminata by bioassay-guided fractionation, using cyclooxygenase (COX) and 5-lipoxygenase (5-LO) assays as models for antiinflammatory activity. The structures of compounds 1-5 were established by NMR, MS, IR, and Raman spectroscopy. These isolated compounds were found to be potent inhibitors of COX. Compounds 4 and 5 were the most potent inhibitors of 5-LO, whereas the other compounds only showed a weak inhibition at the same concentration.