earticle

영어

Lignocellulosic biomass from agricultural and agro-industrial residues represents one of the most important energy-rich resources that can be utilized for the production of bioethanol by fermentation. The yeast Saccharomyces cerevisiae is widely used for commercial production of ethanol from sucrose or starch-derived glucose. While glucose can be fermented to ethanol efficiently, the major pentose sugar xylose remains unutilized. In most natural xylose-ssimilating yeasts and fungi, xylose is metabolized via two consecutive redox reactions catalyzed by the predominantly NADPH-dependent xylose reductase (XR) followed by the NAD+-dependent xylitol dehydrogenase (XDH), with xylitol as the pathway intermediate. Such yeasts and recombinant S. cerevisiae strains harboring these genes can ferment xylose to ethanol anaerobically, but also produce considerable amounts of xylitol as byproduct due to an impairment of the cofactor balance resulting from the different coenzyme specificities of XR and XDH. In contrast, bacteria convert xylose directly to xylulose using the enzyme xylose isomerase which requires divalent metal ions as cofactors. However, bacterial xylose isomerases (XylA) are mostly thermophilic in nature and inactive or misfolded when expressed in yeast. In our studies, we have constructed a recombinant xylose-utilizing S. cerevisiae strain by co-expressing the genes for xylose isomerase (xylA) from the anaerobic fungus Orpinomyces, the endogenous S. cerevisiae xylulokinase (XKS) and the SUT1 sugar transporter from Pichia stipitis. The recombinant strain, designated as INVSc1/ pRS406XKS/pILSUT1/pWOXYLA, consumed 15.55 g l-1 xylose to produce 6.05 g l-1 ethanol, during 140 h fermentation, with an ethanol yield of 0.39 g (g xylose consumed)-1. The accumulation of byproduct xylitol (1.28 g l-1) was significantly low compared to other recombinant strains of S. cerevisiae expressing the XR-XDH pathways1. Additionally, this strain was adapted for improved xylose utilization by serial transfer in aerobic shake flask cultures in xylose containing minimal medium. The xylose-adapted strain, designated as ADAP8, was used for subsequent fermentation studies. In order to improve the ethanol productivity from xylose using the strain ADAP8, we studied the effect of temperature and addition of sugar-complexing agent sodium tetraborate on the fermentation performance of the recombinant yeast. Consistent with previous studies2, fermentation at 35 ºC led to a marked improvement in the ethanol yield and productivity. Xylose was also fermented efficiently in non-selective complex yeast extract-peptone medium. The addition of borate resulted in further enhancement of ethanol yield and productivity. During fermentation in complex medium at 35 ºC and in the presence of an optimal concentration of borate, 15.99 g l-1 xylose was consumed to produce 7.63 g l-1 ethanol after 40 h.