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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  Δ lga1  and  Aspergillus niger  Δ gaaC  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  Δ gaaC  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  Δ gaaC , but decreased in  T. reesei .    Conclusions  Bioconversion of D-galacturonate to keto-deoxy-L-galactonate was achieved with both  A. niger  Δ gaaC  and  T. reesei  Δ lga1 , 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.

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