초록
The biotransformation of d-arabitol into xylitol was investigated with focus on the conversion of d-xylulose into xylitol. This critical conversion was accomplished using Escherichia coli to co-express a xylitol dehydrogenase gene from Gluconobacter oxydans and a cofactor regeneration enzyme gene which was a glucose dehydrogenase gene from Bacillus subtilis for system 1 and an alcohol dehydrogenase gene from G. oxydans for system 2. Both systems efficiently converted d-xylulose into xylitol without the addition of expensive NADH. Approximately 26.91g/L xylitol was obtained from around 30g/L d-xylulose within system 1 (E. coli Rosetta/Duet-xdh-gdh), with a 92% conversion yield, somewhat higher than that of system 2 (E. coli Rosetta/Duet-xdh-adh, 24.9g/L, 85.2%). The xylitol yields for both systems were more than 3-fold higher compared to that of the G. oxydans NH-10 cells (7.32g/L). The total turnover number (TTN), defined as the number of moles of xylitol formed per mole of NAD<SUP>+</SUP>, was 32,100 for system 1 and 17,600 for system 2. Compared with that of G. oxydans NH-10, the TTN increased by 21-fold for system 1 and 11-fold for system 2, hence, the co-expression systems greatly enhanced the NADH supply for the conversion, benefiting the practical synthesis of xylitol.