The fungal path for D-galacturonic acid catabolism

D-galacturonic acid is the major component of pectin and consequently an important carbon source for microorganisms living on decaying plant material or for biotechnological processes where cheap raw materials such as sugar beet pulp are used. A bacterial catabolic pathway has been described while a eukaryotic pathway has remained unknown. For E. coli a pathway was described consisting of five enzymes converting D-galacturonic acid to pyruvic acid and D-glyceraldehyde-3-phosphate. The enzymes of this pathway are uronate isomerase, NADH-utilizing D-tagaturonate reductase, altronate dehydratase, D-erythro-3-deoxy-D-hexulosonate kinase and D-erythro-3-deoxy-D-hexulosonate-6-phosphate aldolase. We show that a fungal pathway exists that is distinctly different from any previously described pathway. In this pathway D-galacturonic acid is converted to pyruvate and glycerol. Intermediates are L-galactonate and L-threo-3-deoxy-hexulosonate. We have identified most of the enzymes of this pathway, cloned the corresponding genes, expressed them in a heterologus host and determined the kinetic properties of the enzymes. We will present potential biotechnological applications of this novel pathway.