Sunday, April 29, 2007

The fungal path for D-galacturonic acid catabolism

Peter Richard and Satu Hilditch. VTT Technical Research Centre of Finland, Tietotie 2, 02044 VTT, Espoo, Finland

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. The intermediates are L-galactonate, L-threo-3-deoxy-hexulosonate and L-glyceraldehyde. The pathway contains four enzymes, NADPH-utilizing D-galacturonate reductase, L-galactonate dehydratase, L-threo-3-deoxy-hexulosonate aldolase and a glycerol dehydrogenase that converts L-glyceraldehyde to glycerol in the reverse reaction.  All the of the enzymes of this pathway have been cloned, expressed in a heterologous host and their kinetic properties determined. We will present potential biotechnological applications of this novel pathway.