Abstract
Bio-ethanol production from lignocellulosic raw materials could serve as a sustainable potential for improving the supply of liquid fuels in face of the food-to-fuel competition and the growing energy demand. Xylose is the second abundant sugar of lignocelluloses hydrolysates, but its commercial-scale conversion to ethanol by fermentation is challenged by incomplete and inefficient utilization of xylose. Here, we use a coupled strategy of simultaneous maltose utilization and in-situ carbon dioxide (CO2) fixation to achieve efficient xylose fermentation by the engineered Saccharomyces cerevisiae. Our results showed that the introduction of CO2 as electron acceptor for nicotinamide adenine dinucleotide (NADH) oxidation increased the total ethanol productivity and yield at the expense of simultaneous maltose and xylose utilization. Our achievements present an innovative strategy using CO2 to drive and redistribute the central pathways of xylose to desirable products and demonstrate a possible breakthrough in product yield of sugars.
Highlights
XR prefers nicotinamide adenine dinucleotide phosphate (NADPH) to nicotinamide adenine dinucleotide (NADH), while XDH uses only nicotinamide adenine dinucleotide (NAD+) as a cofactor[14]
NADPH for XR is supplied by glucose-6-phosphate (G6P) metabolism through oxidative branch of phosphate pathway (PPP), but gluconeogenesis pathway for conversion of xylose to G6P is limited when S. cerevisiae grows on xylose[23,24]
We found that the xylose consumption rate of YSX4 (0.57 g/L/h) was similar with that of DA24 (0.53 g/L/h) on a glucose-xylose mixture, the key enzymes involved in the xylose metabolic pathway in YSX4 were expressed by using a multi-copy plasmid, while in DA24, they were integrated in genome
Summary
XR prefers nicotinamide adenine dinucleotide phosphate (NADPH) to nicotinamide adenine dinucleotide (NADH), while XDH uses only nicotinamide adenine dinucleotide (NAD+) as a cofactor[14]. NADPH for XR is supplied by glucose-6-phosphate (G6P) metabolism through oxidative branch of PPP, but gluconeogenesis pathway for conversion of xylose to G6P is limited when S. cerevisiae grows on xylose[23,24]. The resulted yeast strains were applied for xylose fermentation with simultaneous utilization of maltose and CO2 (Fig. 1). This strategy may have three major benefits: (i) The mXR-XDH module containing a mutant XR (R276H) preferring NADH, a XR, a XDH and a XK could improve the ethanol productivity and decrease the by-product accumulation. The CO2 produced during fermentation can form an appropriate atmosphere for Rubisco activity and a redox sink for improving the cofactor balance
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