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Poster Presentation 2-16 Xylose Reductase and Xylitol Dehydrogenase Activities on Sugarcane Bagasse Hydrolysate Fermentation Luanne Helena Augusto Lima, Maria das Graças de Almeida Felipe
Faculdade de Engenharia Química de Lorena P.O. Box 116, Lorena -SP, 12608-970, Brazil
Telephone: 55-12-31595000; Fax: 55-12-553-3165; E-mail: luannelima@hotmail.com
Sugarcane bagasse is an agroindustrial residue that contains about 25% hemicellulose (Pandey, 2000). The acid hydrolysis of this residue allows the separation of a xylose-rich hemicellulosic fraction, which is an ideal substrate for the production of xylitol by microbial processes. Xylitol has high commercial value due to its sweetening and anticariogenic properties as well as to its clinical applications. The first two steps in xylose metabolism are catalyzed by NAD(P)H-dependent xylose reductase(XR) (E.C.1.1.1.21) and NAD(P)-dependent xylitol dehydrogenase(XDH) (E.C.1.1.1.9). D-xylose is reduced to xylitol by XR, and then oxidized to D-xylulose by XDH. D-xylulose is converted by xylulokinase to xylulose 5-phosphate, which can then enter the pentose phosphate pathway (IKEUCHI, 2000). This study attempts to examine the enzymes XR and XDH produced by Candida guilliermondii on sugarcane bagasse hemicellulosic hydrolysate. Fermentations were carried out in acid sugarcane bagasse hydrolysate. Enzyme units (IU) were defined as mmol of NADPH or NAD+ oxidized/reduced and determined spectrophotometrically at 340 nm. Specific activities were expressed as IU/mg of protein determined by the Bradford method. The highest activities of XR (0.7228 IU/mg) and XDH (0.8464 IU/mg) occurred after 36 h of fermentation, representing an increase of about 50% in the activities of both enzymes after 24 hours. After 48 h (XR=0.5309 IU/mg, XDH=0.5865 IU/mg) the activities decreased, but the xylitol productivity increased from 0.38 to 0.42 g/L/h. According to these results, the xylitol concentration in the medium did not depend only on its metabolism, but also it depends on its transport across the cell membrane.
Financial support: CAPES
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