ABSTRACT
The increased concern for the security of the oil supply and the negative impact of fossil fuels on the environment, particularly greenhouse gas emissions, has put pressure on society to find renewable fuel alternatives. The most common renewable fuel today is ethanol produced from sugar or grain (starch); however, this raw material base will not be sufficient. Consequently, future large-scale use of ethanol will most certainly have to be based on production from lignocellulosic materials. This review gives an overview of the new technologies required and the advances achieved in recent years to bring lignocellulosic ethanol towards industrial production. One of the major challenges is to optimize the integration of process engineering, fermentation technology, enzyme engineering and metabolic engineering.
Subject(s)
Agriculture/methods , Biotechnology/trends , Cellulose/metabolism , Energy-Generating Resources , Ethanol/isolation & purification , Ethanol/metabolism , Industrial Microbiology/trends , Biomass , Biotransformation , Fermentation , ForecastingABSTRACT
Introduction of the xylose pathway from Pichia stipitis into Saccharomyces cerevisiae enables xylose utilization in recombinant S. cerevisiae. However, xylitol is a major by-product. An endogenous aldo-keto reductase, encoded by the GRE3 gene, was expressed at different levels in recombinant S. cerevisiae strains to investigate its effect on xylose utilization. In a recombinant S. cerevisiae strain producing only xylitol dehydrogenase (XDH) from P. stipitis and an extra copy of the endogenous xylulokinase (XK), ethanol formation from xylose was mediated by Gre3p, capable of reducing xylose to xylitol. When the GRE3 gene was overexpressed in this strain, the xylose consumption and ethanol formation increased by 29% and 116%, respectively. When the GRE3 gene was deleted in the recombinant xylose-fermenting S. cerevisiae strain TMB3001 (which possesses xylose reductase and XDH from P. stipitis, and an extra copy of endogenous XK), the xylitol yield decreased by 49% and the ethanol yield increased by 19% in anaerobic continuous culture with a glucose/xylose mixture. Biomass was reduced by 31% in strains where GRE3 was deleted, suggesting that fine-tuning of GRE3 expression is the preferred choice rather than deletion.
Subject(s)
Aldehyde Reductase/metabolism , NADP/metabolism , Saccharomyces cerevisiae/metabolism , Xylose/metabolism , Aldehyde Reductase/genetics , DNA, Fungal/chemistry , DNA, Fungal/genetics , Fermentation , Gene Expression Regulation, Enzymologic , Gene Expression Regulation, Fungal , Mutagenesis, Insertional , Pentose Phosphate Pathway/physiology , Pichia/genetics , Polymerase Chain Reaction , Recombination, Genetic , Saccharomyces cerevisiae/enzymology , Saccharomyces cerevisiae/geneticsABSTRACT
The stereoselective reduction of the bicyclic diketone bicyclo[2.2.2]octane-2,6-dione, to the ketoalcohol (1R,4S,6S)-6-hydroxybicyclo[2.2.2]octane-2-one, was used as a model reduction to optimize parameters involved in NADPH-dependent reductions in Saccharomyces cerevisiae with glucose as co-substrate. The co-substrate yield (ketoalcohol formed/glucose consumed) was affected by the initial concentration of bicyclic diketone, the ratio of yeast to glucose, the medium composition, and the pH. The reduction of 5 g l(-1) bicyclic diketone was completed in less than 20 h in complex medium (pH 5.5) under oxygen limitation with an initial concentration of 200 g l(-1) glucose and 5 g l(-1) yeast. The co-substrate yield was further enhanced by genetically engineered strains with reduced phosphoglucose isomerase activity and with the gene encoding alcohol dehydrogenase deleted. Co-substrate yields were increased 2.3-fold and 2.4-fold, respectively, in these strains.
