d-Xylose Isomerase from a Marine Bacterium, Vibrio sp Strain XY-214, and d-Xylulose Production from β-1,3-Xylan

The xylA gene from a marine bacterium, Vibrio sp. strain XY-214, encoding d-xylose isomerase (XylA) was cloned and expressed in Escherichia coli. The xylA gene consisted of 1,320-bp nucleotides encoding a protein of 439 amino acids with a predicted molecular weight of 49,264. XylA was classified into group II xylose isomerases. The native XylA was estimated to be a homotetramer with a molecular mass of 190 kDa. The purified recombinant XylA exhibited maximal activity at 60A degrees C and pH 7.5. Its apparent K (m) values for d-xylose and d-glucose were 7.93 and 187 mM, respectively. Furthermore, we carried out d-xylulose production from beta-1,3-xylan, a major cell wall polysaccharide component of the killer alga Caulerpa taxifolia. The synergistic action of beta-1,3-xylanase (TxyA) and beta-1,3-xylosidase (XloA) from Vibrio sp. strain XY-214 enabled efficient saccharification of beta-1,3-xylan to d-xylose. d-Xylose was then converted to d-xylulose by using XylA from the strain XY-214. The conversion rate of d-xylose to d-xylulose by XylA was found to be approximately 40% in the presence of 4 mM sodium tetraborate after 2 h of incubation. These results demonstrated that TxyA, XloA, and XylA from Vibrio sp. strain XY-214 are useful tools for d-xylulose production from beta-1,3-xylan. Because d-xylulose can be used as a source for ethanol fermentation by yeast Saccharomyces cerevisiae, the present study will provide a basis for ethanol production from beta-1,3-xylan.