Covalent immobilization of recombinant Rhizobium etli CFN42 xylitol dehydrogenase onto modified silica nanoparticles

Rare sugars have many applications in food industry, as well as pharmaceutical and nutrition industries. Xylitol dehydrogenase (XDH) can be used to synthesize various rare sugars enzymatically. However, the immobilization of XDH has not been performed to improve the industrial production of rare sugars. In this study, silica nanoparticles which have high immobilization efficiency were selected from among several carriers for immobilization of recombinant Rhizobium etli CFN42 xylitol dehydrogenase (ReXDH) and subjected to characterization. Among four different chemical modification methods to give different functional groups, the silica nanoparticle derivatized with epoxy groups showed the highest immobilization efficiency (92%). The thermostability of ReXDH was improved more than tenfold by immobilization on epoxy-silica nanoparticles; the t _1/2 of the ReXDH was enhanced from 120 min to 1,410 min at 40 °C and from 30 min to 450 min at 50 °C. The K _m of ReXDH was slightly altered from 17.9 to only 19.2 mM by immobilization. The immobilized ReXDH had significant reusability, as it retained 81% activity after eight cycles of batch conversion of xylitol into l-xylulose. A ∼ 71% conversion and a productivity of 10.7 g h^-1 l^-1 were achieved when the immobilized ReXDH was employed to catalyze the biotransformation of xylitol to l-xylulose, a sugar that has been used in medicine and in the diagnosis of hepatitis. These results suggest that immobilization of ReXDH onto epoxy-silica nanoparticles has potential industrial application in rare sugar production.