The fungal path for D-galacturonate catabolism

D-galacturonate 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-galacturonate to pyruvate and D-glyceraldehyde-3-phosphate. The enzymes of this pathway are uronate isomerase, NADH-utilizing D-tagaturonate reductase, altronate dehydratase, D-erythro-3-deoxy-hexulosonate kinase and D-erythro-3-deoxy-hexulosonate-6-phosphate aldolase. We show that the D-galacturonate pathway is different in the mold Hypocrea jecorina (Trichoderma reesei). In this fungal catabolic pathway D-galacturonate 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 corresponding genes have been identified, cloned, and expressed in the heterologous host S. cerevisiae, and the kinetic properties of the enzymes have been determined.