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

• Main Authors: Wiebe Marilyn G, Mojzita Dominik, Hilditch Satu, Ruohonen Laura, Penttilä Merja
• Format: Article
• Language: English
• Published: BMC 2010-08-01
• Series: BMC Biotechnology
• Online Access: http://www.biomedcentral.com/1472-6750/10/63

<p>Abstract</p> <p>Background</p> <p>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-<it>threo</it>-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.</p> <p>Results</p> <p>Keto-deoxy-L-galactonate (3-deoxy-L-<it>threo</it>-hex-2-ulosonate) accumulated in the culture supernatant when <it>Trichoderma reesei </it>Δ<it>lga1 </it>and <it>Aspergillus niger </it>Δ<it>gaaC </it>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^-1was produced from 20 g D-galacturonate l^-1and <it>A. niger </it>Δ<it>gaaC </it>produced 15.0 g keto-deoxy-L-galactonate l^-1from 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^-1intracellularly in both producing organisms. This intracellular concentration was sustained throughout production in <it>A. niger </it>Δ<it>gaaC</it>, but decreased in <it>T. reesei</it>.</p> <p>Conclusions</p> <p>Bioconversion of D-galacturonate to keto-deoxy-L-galactonate was achieved with both <it>A. niger </it>Δ<it>gaaC </it>and <it>T. reesei </it>Δ<it>lga1</it>, although production (titre, volumetric and specific rates) was better with <it>A. niger </it>than <it>T. reesei</it>. <it>A. niger </it>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^-1were exported to the culture supernatant. Lysis may have contributed to the release of keto-deoxy-L-galactonate from <it>T. reesei </it>mycelia.</p>