D-galacturonic acid reductase is the first enzyme in the eukaryotic D-galacturonic acid catabolic pathway

There is only limited knowledge about the metabolic route for D-galacturonic acid catabolism in eukaryotic organisms; however such a metabolic route exists since many species of yeast and mould are able to utilise D-galacturonic acid. The metabolic route in bacteria is known since decades and the corresponding genes are identified. The sequencing of several eukaryotic genomes did not reveal genes which were similar to the bacterial pathway suggesting that a eukaryotic pathway exists which is different from the bacterial path. For bacteria a pathway is known consisting of 5 enzymes converting D-galacturonic acid (D-galacturonate) to pyruvate and D-glyceraldehyde 3-phosphate. The intermediate metabolites are D-tagaturonate, D-altronate, 2-dehydro 3-deoxy D-gluconate and 2-dehydro 3-deoxy D-gluconate 6-phosphate. The enzymes are uronate isomerase (EC 5.3.1.12), an NADH utilising D-tagaturonate reductase (EC 1.1.1.5), altronate dehydratase (EC 4.2.1.7), 2-dehydro 3-deoxy D-gluconatekinase (EC 2.7.1.45) and 2-dehydro 3-deoxy D-gluconate 6-phosphatealdolase (EC 4.1.2.14). We could show that in fungi a pathway exists, which is distinctly different from the bacterial pathway. In the filamentous fungus Hypocrea jecorina (Trichoderma reesei) we identified the first enzyme in this pathway, which is a D-galacturonic acid reductase. The D-galacturonic acid reductase is induced when D-galacturonic acid is the carbon source. The enzyme was purified and the amino acid sequences of tryptic fragments were obtained. We cloned the corresponding gene and expressed an active enzyme in S. cerevisiae. The enzyme converts D-galacturonic acid to L-galactonic acid in a reversible reaction. The enzyme is strictly NADPH dependent. Acknowledgements This work was founded through the Maj and Tor Nessling Foundation.