Microbial D-xylonate production

Abstract

D-Xylonic acid is a versatile platform chemical with reported applications as complexing agent or chelator, in dispersal of concrete, and as a precursor for compounds such as co-polyamides, polyesters, hydrogels and 1,2,4-butanetriol. With increasing glucose prices, D-xylonic acid may provide a cheap, non-food derived alternative for gluconic acid, which is widely used (about 80 kton/year) in pharmaceuticals, food products, solvents, adhesives, dyes, paints and polishes. Large-scale production has not been developed, reflecting the current limited market for D-xylonate. D-Xylonic acid occurs naturally, being formed in the first step of oxidative metabolism of D-xylose by some archaea and bacteria via the action of D-xylose or D-glucose dehydrogenases. High extracellular concentrations of D-xylonate have been reported for various bacteria, in particular Gluconobacter oxydans and Pseudomonas putida. High yields of D-xylonate from D-xylose make G. oxydans an attractive choice for biotechnical production. G. oxydans is able to produce D-xylonate directly from plant biomass hydrolysates, but rates and yields are reduced because of sensitivity to hydrolysate inhibitors. Recently, D-xylonate has been produced by the genetically modified bacterium Escherichia coli and yeast Saccharomyces cerevisiae and Kluyveromyces lactis. Expression of NAD(+)-dependent D-xylose dehydrogenase of Caulobacter crescentus in either E. coli or in a robust, hydrolysate-tolerant, industrial Saccharomyces cerevisiae strain has resulted in D-xylonate titres, which are comparable to those seen with G. oxydans, at a volumetric rate approximately 30% of that observed with G. oxydans. With further development, genetically modified microbes may soon provide an alternative for production of D-xylonate at industrial scale.

Fig. 1

Formation of d-xylonate from d-xylose by NAD(P)⁺ or PQQ-dependent xylose dehydrogenases or glucose oxidase

Fig. 2

Production of d-xylonate and d-gluconate by Aspergillus niger ATCC1015 after 79 h in defined medium with 45 g d-xylose l⁻¹ and 10 g d-glucose l⁻¹ as carbon source. Medium was buffered with 0.1 to 2.0 % (w/v) CaCO₃, and average pH over 79 h is shown

Fig. 3

d-Xylonate production by Gluconobacter oxydans ATCC621 from d-xylose in YE supplemented defined medium with 45 g d-xylose l⁻¹ at pH 5.6 (filled circle) or pH 3.5 (empty circle) and from acid hydrolysed DDGS at pH 5.6 (filled square) or pH 3.5 (empty square). Error bars represent ±SEM for duplicate cultures

Fig. 4

d-Gluconate, d-xylonate, acetate and biomass production, volumetric d-xylonate production rate and specific d-xylonate production rate by Gluconobacter oxydans ATCC621 in chemostat culture with YE supplemented defined medium containing 10 g d-glucose l⁻¹ and 40 g d-xylose l⁻¹ at D = 0.04 h⁻¹, pH 5.5 or 4.5. Error bars represent ±SEM for triplicate (pH 5.5) or duplicate (pH 4.5) samples

Fig. 5

d-Xylonate produced (solid symbols) and d-xylose consumed (open symbols) by Gluconobacter oxydans ATCC621 in pre-treated wheat straw derived hydrolysate (C5 fraction), with (circles) or without (squares) overliming, and supplemented with 5 g yeast extract l⁻¹ at pH 5.6, 30 °C. The hydrolysate contained d-xylose, d-glucose, L-arabinose, and acetate. d-Xylonate measurements in untreated wheat straw hydrolysate are shown in Turkia et al. (2010)