Engineering Hexuronic Acid Catabolism

Metabolic engineering targets for the production of high-value compounds. Citrus peel is produced ~15,000,000 t per year worldwide causing environmental problems. Its main constituent is D-galacturonic acid and can be converted to useful chemicals using genetically engineered strains. Engineered Aspergillus niger were used in solid and submerged state fermentation to convert orange peel to L-galactonate in a consolidate process. The gaaB coding for L-galactonate dehydrogenase was deleted (gaaB) and ii) the gaaB was deleted and the gaaA coding for D-galacturonate reductase was overexpressed (gaaB-gaaA). gaaB was able to convert up to 87 % of D-galacturonic acid to L-galactonate. Also, in this work, the eukaryotic D-Glucuronic acid pathway was studied. A decarboxylase that converts 3-keto-L-gulonate to L-xylulose remains poorly characterized and the gene is not known. A coupled enzyme assay was used to detect its activity. In the assay, ammonium sulfate precipitates from bovine liver extract were coupled with engineered L-gulonate-3-dehydrogenase and L-xylulose reductase. A 3-keto-L-gulonate decarboxylase activity could not be detected.