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Lignocellulose model showing lignin, cellulose and hemicellulose SOURCE: USDA AGRICULTURAL RESEARCH SERVICE
Enzyme Requirements for Lignocellulosic Feedstocks A depiction of xylose metabolic pathways SOURCE: USDA AGRICULTURAL RESEARCH SERVICE
obtained from collaborations with Josh LaBaer, director of the Harvard Institute of Proteomics. To further improve these ethanologenic yeast strains for industrial use, scientists at NCAUR are also engineering yeast to express proteins that increase uptake of pentose sugars.
Although S. cerevisiae is a proven industrial ethanol producer in traditional starchbased processes, it will be no easy task to provide this microorganism with the ability to convert lignocellulosic biomass to ethanol. The carbohydrate components of lignocellulose (cellulose and hemicellulose) are tightly bound to lignin, making the sugars largely inaccessible to enzymes. “Before enzymatic hydrolysis, pretreatment with acid or alkali is generally needed to fully
maximize the release of sugars from any lignocellulosic biomass,” says Badal Saha at the NCAUR Fermentation Biotechnology Research Unit. For consolidated bioprocessing, S. cerevisiae must not only ferment both hexoses and pentoses under industrial conditions with high ethanol yield and productivity, it must also express and produce enzymes at sufficient levels to maintain hydrolysis and fermentation of biomass to ethanol. Enzymatic conversion of cellulose to sugars that yeasts can ferment requires the concerted action of three types of cellulase. Due to the heterogeneity and complexity of hemicellulose, its conversion requires an even larger list of enzymes. For robust and complete conversion of polysaccharides locked in biomass, the ultimate ethanologens will need to produce at least a dozen enzymes of different catalytic activities.
Developing New Biocatalysts Producing a yeast strain with optimized sets of cellulases and hemicellulases requires screening thousands of combinations of these biomass-degrading enzymes for enzyme activity. Automation is essential in carrying out these operations. A team of scientists at the NCAUR laboratory has been successful in designing a robotic plat-
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