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.
Highlights
Sugar acids are currently generating considerable interest because of their potential as platform chemicals and their use as precursors in the manufacture of biomass derived plastics
Production of D-xylonate from D-xylose by D-glucose oxidase has been described (Pezzotti and Therisod 2006; Chun et al 2006) and Aspergillus niger produces D-xylonate when cultivated in suitable conditions (Fig. 2)
In addition to the microbial production described in this review, D-xylonate can be produced via enzymatic (Pezzotti and Therisod 2006), electrochemical (Jokic et al 1991) or chemical oxidation (Isbell and Hudson 1932)
Summary
Sugar acids are currently generating considerable interest because of their potential as platform chemicals and their use as precursors in the manufacture of biomass derived plastics. There are some reports of yeast and other fungi producing D-xylonic acid (Suzuki and Onishi 1973; Kiesling et al 1962; Kanauchi and Bamforth 2003), only one gene coding for D-xylose dehydrogenase has been identified in fungal species (Berghäll et al 2007). Production of D-xylonate from D-xylose by D-glucose oxidase has been described (Pezzotti and Therisod 2006; Chun et al 2006) and Aspergillus niger produces D-xylonate when cultivated in suitable conditions (Fig. 2). In addition to the microbial production described in this review, D-xylonate can be produced via enzymatic (Pezzotti and Therisod 2006), electrochemical (Jokic et al 1991) or chemical oxidation (Isbell and Hudson 1932). This review describes the current state in microbial production of D-xylonate with bacteria and fungi
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