Construction Of The Xylan Ethanol Metabolism Engineered Strain Of Saccharomyces Cerevisiae

Saccharomyces cerevisiae is a microorganism that generally recognized as safe(GRAS).It cannot grow nor ferment on xylose,because of lacking the enzymes to converse xylose into xylulose.To make full use of hemicellulose and to reduce the cost of xylan ethanol,this paper modified the metabolic pathway of the industrial diploid S.cerevisiae.AS2.489 through genetic engineering and obtained the recombinant strains which can ferment xylan by consolidated bioprocessing.In this paper,the xyn2,xlnD,xylA and xks1 gene were introduced into S.cerevisiae by episcopal expression vectors carrying the replication origin site of the 2-micrometre plasmid.A recombinant S.cerevisiae AS-KA which can ferment xylose to ethanol was obtained.The other recombinant S.cerevisiae AS-KAXD which can ferment xylan to ethanol by consolidated bioprocessing was also obtained.Two recombinant strains AS-KA1 and AS-KAXD-1 with the highest enzyme activity were selected.Through adaptive evolution,we obtained the other two evolutionary strains AS-KA-27 BAE and AS-KAXD-14 AE.The highest ethanol yields of these two evolutionary strains were 5.752 g/L and 4.485 g/L,respectively.The conversion rates were 69.3% and 63.7% of the theoretical value,respectively.Simultaneous saccharification and fermentation(SSF)of xylan by AS-KA-27 BAE and AS-KAXD-14 AE were conducted and the results showed that when the enzyme dosage were 5 U/g xylan,the ethanol productions were 3.025 g/L and 2.809 g/L respectively;the conversion rates of xylan to ethanol were 26.0% and 24.1% of the theoretical value,respectively.A consolidated bioprocesse(CBP)of xylan fermentation by AS-KAXD-14 AE and AS-KAXD-1 were also conducted in this study.In twenty-day fermentation,the finally ethanol yield were 0.544 g/L and 0.433 g/L respectively;the conversion rates of xylan to ethanol were 4.6% and 3.8% of the theoretical value,respectively.