P140: Conversion of pectin rich residues to bio-based chemicals by metabolically engineered filamentous fungi: production of L-galactonic acid

Pectin is a structural heteropolysaccharide found in the cell walls of the non-woody plant parts. Two abundantly available pectin rich residues are citrus fruit peel and sugar beet pulp. The annual worldwide production of citrus fruit and sugar beet is about 115 x 10⁶ and 250 x 10⁶ metric tons respectively. After processing citrus peel and sugar beet pulp are mainly used as cattle feed or dumped. From that perspective it is desirable to find new ways for converting these residues to value-added products.

Aspergillus niger has an extensive set of enzymes for the hydrolysis of pectin. D-galacturonic acid, the main component of pectin, is catabolised further to pyruvate and glycerol through a reductive pathway. By disrupting the native D-galacturonic acid catabolic pathway it is possible to engineer fungal strains for alternative D-galacturonic acid conversions. L-galactonic acid is the first intermediate in the pathway. It is currently expensive and not widely used, but has the potential to be used more widely once it is available at a low price. The physico-chemical properties are similar to those of D-gluconic acid, which is widely used as a chelator, in the pharmaceutical, cosmetic, and industrial sectors and as an acidifier in food. L-galactonic acid is also a precursor for L-ascorbic acid (vitamin C) synthesis.

In the present work we have engineered fungal strains that convert D-galacturonic acid, pectin and pectin rich residues to L-galactonic acid which is excreted into the medium.