Bioconversion of D-galacturonate to keto-deoxy-L-galactonate (3-deoxy-L-threo-hex-2-ulosonate) using filamentous fungi

BACKGROUND The D-galacturonic acid derived from plant pectin can be converted into a variety of other chemicals which have potential use as chelators, clarifiers, preservatives and plastic precursors. Among these is the deoxy-keto acid derived from L-galactonic acid, keto-deoxy-L-galactonic acid or 3-deoxy-L-threo-hex-2-ulosonic acid. The keto-deoxy sugars have been found to be useful precursors for producing further derivatives. Keto-deoxy-L-galactonate is a natural intermediate in the fungal D-galacturonate metabolic pathway, and thus keto-deoxy-L-galactonate can be produced in a simple biological conversion. RESULTS Keto-deoxy-L-galactonate (3-deoxy-L-threo-hex-2-ulosonate) accumulated in the culture supernatant when Trichoderma reesei Δlga1 and Aspergillus niger ΔgaaC were grown in the presence of D-galacturonate. Keto-deoxy-L-galactonate accumulated even if no metabolisable carbon source was present in the culture supernatant, but was enhanced when D-xylose was provided as a carbon and energy source. Up to 10.5 g keto-deoxy-L-galactonate l(-1) was produced from 20 g D-galacturonate l(-1) and A. niger ΔgaaC produced 15.0 g keto-deoxy-L-galactonate l(-1) from 20 g polygalacturonate l(-1), at yields of 0.4 to 1.0 g keto-deoxy-L-galactonate [g D-galacturonate consumed](-1). Keto-deoxy-L-galactonate accumulated to concentrations of 12 to 16 g l(-1) intracellularly in both producing organisms. This intracellular concentration was sustained throughout production in A. niger ΔgaaC, but decreased in T. reesei. CONCLUSIONS Bioconversion of D-galacturonate to keto-deoxy-L-galactonate was achieved with both A. niger ΔgaaC and T. reesei Δlga1, although production (titre, volumetric and specific rates) was better with A. niger than T. reesei. A. niger was also able to produce keto-deoxy-L-galactonate directly from pectin or polygalacturonate demonstrating the feasibility of simultaneous hydrolysis and bioconversion. Although keto-deoxy-L-galactonate accumulated intracellularly, concentrations above ~12 g l(-1) were exported to the culture supernatant. Lysis may have contributed to the release of keto-deoxy-L-galactonate from T. reesei mycelia.