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Ethanol production from L-galactose using metabolically engineered Escherichia coli

Ethanol production from L-galactose using metabolically engineered Escherichia coli
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Rising of fossil fuel prices and changing of climate lead to the search for the sustainable fuels. Although land-based biomass is currently used for bioethanol production, marine-based biomass such as seaweed is regarded as an alternative biomass resource for future bioethanol production. In particular, red seaweed is suitable for bioethanol production because it has higher sugar content than brown or green seaweed. In this study, we explored the possibility of using Porphyra tenera, a red seaweed composed of D-galactose and L-galactose called porphyran, as a biomass resource for ethanol production. D-galactose is a fermentable sugar and can be used for ethanol production by ethanologenic microorganisms such as yeast and E. coli KO11. However, L-galactose is non-fermentable because yeast and KO11 are unable to utilize L-galactose. As a result, metabolic engineering technologies are necessary to develop a method for the conversion L-galactose into ethanol. We constructed the recombinant E. coli strain (SB-G1) which utilizes both D-galactose and L-galactose by inserting the Pseudoalteromonas atlantica T6c genes involved in the L-galactose metabolic pathway (L-galactose transporter, L-galactose dehydrogenase, and L-galactonolactonase) in KO11. Activities of L-galactose transporter, L-galactose dehydrogenase, and L-galactonolactonase were confirmed using metabolically engineered E. coli SB-G1 cells and SB-G1 cells were cultured in LB medium supplemented with porphyran hydrolysate. Consumption of L-galactose and production of ethanol was measured using L-galactose dehydrogenase and HPLC, respectively. Comparing the control strain (E. coli KOL2), SB-G1 consumed not only D-galactose but also L-galactose in porphyran hydrolysate. Because of the conversion of L-galactose into ethanol, metabolically engineered SB-G1 strain could produce more ethanol than KOL2.
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