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Metabolic Engineering of a Phosphoketolase Pathway for Pentose Catabolism in Saccharomyces cerevisiae. Marco Sonderegger, 2004.Low ethanol yields on xylose hamper economically viable ethanol production from hemicellulose-rich plant material with Saccharomyces cerevisiae . A major obstacle is the limited capacity of yeast for anaerobic reoxidation of NADH . Net reoxidation of NADH could potentially be achieved by channeling carbon fluxes through a recombinant phosphoketolase pathway . By heterologous expression of phosphotransacetylase and acetaldehyde dehydrogenase in combination with the native phosphoketolase, we installed a functional phosphoketolase pathway in the xylose-fermenting Saccharomyces cerevisiae strain TMB3001c . Consequently the ethanol yield was increased by 25% because less of the by-product xylitol was formed . The flux through the recombinant phosphoketolase pathway was about 30% of the optimum flux that would be required to completely eliminate xylitol and glycerol accumulation . Further overexpression of phosphoketolase, however, increased acetate accumulation and reduced the fermentation rate . By combining the phosphoketolase pathway with the ald6 mutation, which reduced acetate formation, a strain with an ethanol yield 20% higher and a xylose fermentation rate 40% higher than those of its parent was engineered .
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