Thesis: D-Galacturonate catabolism in filamentous fungus
A challenge for biotechnology is to convert cheap raw materials to useful and more valuable products. Satu Hilditch from VTT has studied in her thesis how D-galacturonate, the main component of natural polymer pectin, can be converted by eukaryotic microorganisms. Hilditch will present her thesis ”Identification of the fungal catabolic D-galacturonate pathway” at the University of Helsinki on 11 June, 2010 at noon (Address: Viikinkaari 5, Helsinki).
Filamentous fungi are especially useful for the conversion of pectin, since
they are efficient producers of pectinases. Identification of the fungal
D-galacturonate pathway is of fundamental importance for the utilisation of
pectin and its conversion to useful products.
Pectin is a natural polymer consisting mainly of D-galacturonic acid monomers. Microorganisms living on decaying plant material can use D-galacturonic acid for growth. Although bacterial pathways for D-galacturonate catabolism had been described previously, no eukaryotic pathway for D-galacturonate catabolism was known at the beginning of this work. The aim of this work was to identify such a pathway.
In this thesis the pathway for D-galacturonate catabolism was identified in the filamentous fungus Trichoderma reesei. The pathway consisted of four enzymes: NADPH-dependent D-galacturonate reductase (GAR1), L-galactonate dehydratase (LGD1), L-threo-3-deoxy-hexulosonate aldolase (LGA1) and NADPH-dependent glyceraldehyde reductase (GLD1). In this pathway D-galacturonate was converted to pyruvate and glycerol via L-galactonate, L-threo-3-deoxy-hexulosonate and L-glyceraldehyde.
The enzyme activities of GAR1, LGD1 and LGA1 were present in crude mycelial extract only when T. reesei was grown on D-galacturonate. The activity of GLD1 was equally present on all the tested carbon sources. The corresponding genes were identified and cloned. They were functionally expressed in Saccharomyces cerevisiae, and the enzymes were characterised. GAR1 and LGA1 catalysed reversible reactions, whereas only the forward reactions were observed for LGD1 and GLD1. When gar1, lgd1 or lga1 was deleted in T. reesei the deletion strain was unable to grow with D-galacturonate as the only carbon source, demonstrating that all the corresponding enzymes were essential for D-galacturonate catabolism and that no alternative D-galacturonate pathway exists in T. reesei.
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