Genetic Analysis Of D-xylose Degradation Pathway In Gluconobacter Oxydans621H

D-xylose, a constituent of the polymer xylan, is the major component of the hemicellulose plant cell wall material and thus one of the most abundant carbohydrates in nature. This work focused on xylose degradation of Gluconobacter oxydans by genetic analysis of related genes due to few studies conducted to understand xylose utilization by this strain.Interestingly, the621H△mgdh strain (deficient in membrane-bound glucose dehydrogenase-mGDH) was greatly inhibited when grown on xylose and no xylonate accumulation was observed in the medium. These experimental observations suggested that the mgdh gene was responsible for the conversion of xylose to xylonate in G. oxydans. Meanwhile, the results of the whole-cell biotransformation experiment showed that mGDH was the only enzyme converting extracellular xylose to xylonate. It was also the first time to reveal that the mgdh gene played a vital role in xylose metabolism in this strain. Most of xylose was converted to xylonate outside the cell, and then xylonate was further metabolized in vivo.However, since a fraction of621H Amgdh could still grow on xylose, two seemingly important genes in the oxo-reductive pathway for xylose metabolism, xylitol dehydrogenase-encoding gox0865(xdh) gene and putative xylulose kinase-encoding gox2214(xk) gene, were studied. The xdh gene was knocked out to investigate its effects on xylose degradation. Though the xdh gene played a less important role in xylose metabolism, it was significant in xylitol utilization in G. oxydans. Meanwhile, the xk gene was both heterologously expressed and knocked out since its biochemical property was not clear, the results showed that the gox2214gene was not involved in xylose metabolism, and there might be other genes encoding xylulose kinase. However, it turned out that gox2214was probably the key gene of glycerol metabolism in G. oxydans.To sum up, when xylose was used as the carbon source, the majority of xylose was directly oxidized to xylonate by membrane-bound glucose dehydrogenase for further degradation in vivo in G. oxydans, whereas only a minor part of xylose was degradated by the oxo-reductive pathway.