Overexpression of mGDH in Gluconobacter oxydans to improve D-xylonic acid production from corn stover hydrolysate

Abstract

Background: D-Xylonic acid is a versatile platform chemical with broad potential applications as a water reducer and disperser for cement and as a precursor for 1,4-butanediol and 1,2,4-tributantriol. Microbial production of D-xylonic acid with bacteria such as Gluconobacter oxydans from inexpensive lignocellulosic feedstock is generally regarded as one of the most promising and cost-effective methods for industrial production. However, high substrate concentrations and hydrolysate inhibitors reduce xylonic acid productivity.

Results: The D-xylonic acid productivity of G. oxydans DSM2003 was improved by overexpressing the mGDH gene, which encodes membrane-bound glucose dehydrogenase. Using the mutated plasmids based on pBBR1MCS-5 in our previous work, the recombinant strain G. oxydans/pBBR-R3510-mGDH was obtained with a significant improvement in D-xylonic acid production and a strengthened tolerance to hydrolysate inhibitors. The fed-batch biotransformation of D-xylose by this recombinant strain reached a high titer (588.7 g/L), yield (99.4%), and volumetric productivity (8.66 g/L/h). Moreover, up to 246.4 g/L D-xylonic acid was produced directly from corn stover hydrolysate without detoxification at a yield of 98.9% and volumetric productivity of 11.2 g/L/h. In addition, G. oxydans/pBBR-R3510-mGDH exhibited a strong tolerance to typical inhibitors, i.e., formic acid, furfural, and 5-hydroxymethylfurfural.

Conclusion: Through overexpressing mgdh in G. oxydans, we obtained the recombinant strain G. oxydans/pBBR-R3510-mGDH, and it was capable of efficiently producing xylonic acid from corn stover hydrolysate under high inhibitor concentrations. The high D-xylonic acid productivity of G. oxydans/pBBR-R3510-mGDH made it an attractive choice for biotechnological production.

Keywords: D-xylonic acid; Gluconobacter oxydans; Lignocellulosic hydrolysate; Membrane-bound glucose dehydrogenase.

Fig. 1

Comparison of specific productivities by different G. oxydans strains. The asterisks indicate a significant difference in the specific productivity compared with G. oxydans/pBBR-R3510-mGDH (*P < 0.05; and **P < 0.01)

Fig. 2

Batch conversion of d-xylose to XA by G. oxydans/pBBR-R3510-mGDH and the control G. oxydans DSM2003

Fig. 3

Fed-batch conversion of 535 g/L d-xylose by G. oxydans/pBBR-R3510-mGDH

Fig. 4

Comparison of biocatalyst performance between CSH and sugar solution. a d-Xylose and d-glucose conversion by G. oxydans/pBBR-R3510-mGDH. b d-xylose and d-glucose conversion by G. oxydans DSM2003. c XA and GA production by G. oxydans/pBBR-R3510-mGDH. d XA and GA production by G. oxydans DSM2003

Fig. 5

Relative specific productivities in synthetic medium with different inhibitor contents. a G. oxydans/pBBR-R3510-mGDH. b G. oxydans DSM2003

Fig. 6

XA and GA production under high concentrations of inhibitors by G. oxydans/pBBR-R3510-mGDH in the fermenter. a Conversion of CSH containing more d-xylose and inhibitors. b Conversion of sugar solution