Identification of NAD-dependent xylitol dehydrogenase fromGluconobacter oxydansWSH-003
AbstractGluconobacter oxydansplays important role in conversion of D-sorbitol to L-sorbose, which is an essential intermediate for industrial-scale production of vitamin C. In the fermentation process, some D-sorbitol could be converted to D-fructose and other byproducts by uncertain dehydrogenases. Genome sequencing has revealed the presence of diverse genes encoding dehydrogenases inG. oxydans. However, the characteristics of most of these dehydrogenases remain unclear. Therefore, analyses of these unknown dehydrogenases could be useful for identifying those related to the production of D-fructose and other byproducts. Accordingly, dehydrogenases inG. oxydansWSH-003, an industrial strain used for vitamin C production, were examined. An NAD-dependent dehydrogenase, which was annotated as xylitol dehydrogenase 2, was identified, codon-optimized, and expressed inEscherichia coliBL21 (DE3) cells. The enzyme exhibited high preference for NAD+as the cofactor, while no activity with NADP+, FAD, or PQQ was noted. Although this enzyme presented high similarity with NAD-dependent xylitol dehydrogenase, it showed high activity to catalyze D-sorbitol to D-fructose. Unlike the optimum temperature and pH for most of the known NAD-dependent xylitol dehydrogenases (30°C–40°C and about 6–8, respectively), those for the identified enzyme were 57°C and 12, respectively. TheKmandVmaxof the identified dehydrogenase towards L-sorbitol were 4.92 μM and 196.08 μM/min, respectively. Thus, xylitol dehydrogenase 2 can be useful for cofactor NADH regeneration under alkaline conditions or its knockout can improve the conversion ratio of D-sorbitol to L-sorbose.ImportanceProduction of L-sorbose from D-sorbitol byGluconobacter oxydansis the first step for industrial scale production of L-ascorbic acid.G. oxydanscontains a lot of different dehydrogenases, among which only several are responsible for the conversion of D-sorbitol to L-sorbose, while others may responsible for the accumulation of byproducts, thus decreased the yield of L-sorbose on D-sorbitol. Therefore, a new xylitol dehydrogenase has been identified from 44 dehydrogenases ofG.oxydans. Optimum temperature and pH of the xylitol dehydrogenase are different to most of the known ones. Knock-out of the dehydrogenase may improve the conversion ratio of D-sorbitol to L-sorbose. Besides, the enzyme exhibits high preference for NAD+and have potential to be used for cofactor regeneration.
