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In bacterial system, direct conversion of xylose to xylonic acid is mediated through NAD-dependent xylose dehydrogenase ( xylB ) and xylonolactonase ( xylC ) genes. Heterologous expression of these genes from  Caulobacter crescentus  into recombinant  Corynebacterium glutamicum  ATCC 13032 and  C. glutamicum  ATCC 31831 (with an innate pentose transporter,  araE)  resulted in an efficient bioconversion process to produce xylonic acid from xylose. Process parameters including the design of production medium was optimized using a statistical tool, Response Surface Methodology (RSM). Maximum xylonic acid of 56.32 g/L from 60 g/L xylose, i.e. about 76.67% of the maximum theoretical yield was obtained after 120 h fermentation from pure xylose with recombinant  C. glutamicum  ATCC 31831 containing the plasmid pVWEx1  xylB . Under the same condition, the production with recombinant  C. glutamicum  ATCC 13032 (with pVWEx1  xylB ) was 50.66 g/L, i.e. 69% of the theoretical yield. There was no significant improvement in production with the simultaneous expression of  xylB  and  xylC  genes together indicating xylose dehydrogenase activity as one of the rate limiting factor in the bioconversion. Finally, proof of concept experiment in utilizing biomass derived pentose sugar, xylose, for xylonic acid production was also carried out and obtained 42.94 g/L xylonic acid from 60 g/L xylose. These results promise a significant value addition for the future bio refinery programs.

Heterologous expression of genes for bioconversion of xylose to xylonic acid in Corynebacterium glutamicum and optimization of the bioprocess