Metabolic Engineering of Fungal Strains for Conversion of d -Galacturonate to meso -Galactarate

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Abstract
D -Galacturonic acid can be obtained by hydrolyzing pectin, which is an abundant and low value raw material. By means of metabolic engineering, we constructed fungal strains for the conversion of d -galacturonate to meso -galactarate (mucate). Galactarate has applications in food, cosmetics, and pharmaceuticals and as a platform chemical. In fungi d -galacturonate is catabolized through a reductive pathway with a d -galacturonate reductase as the first enzyme. Deleting the corresponding gene in the fungi Hypocrea jecorina and Aspergillus niger resulted in strains unable to grow on d -galacturonate. The genes of the pathway for d -galacturonate catabolism were upregulated in the presence of d -galacturonate in A. niger , even when the gene for d -galacturonate reductase was deleted, indicating that d -galacturonate itself is an inducer for the pathway. A bacterial gene coding for a d -galacturonate dehydrogenase catalyzing the NAD-dependent oxidation of d -galacturonate to galactarate was introduced to both strains with disrupted d -galacturonate catabolism. Both strains converted d -galacturonate to galactarate. The resulting H. jecorina strain produced galactarate at high yield. The A. niger strain regained the ability to grow on d -galacturonate when the d -galacturonate dehydrogenase was introduced, suggesting that it has a pathway for galactarate catabolism.