title: Engineering filamentous fungi for conversion of D-galacturonic acid to L-galactonic acid.
abstract: D-Galacturonic acid, the main monomer of pectin, is an attractive substrate for bioconversions, since pectin-rich biomass is abundantly available and pectin is easily hydrolyzed. l-Galactonic acid is an intermediate in the eukaryotic pathway for d-galacturonic acid catabolism, but extracellular accumulation of l-galactonic acid has not been reported. By deleting the gene encoding l-galactonic acid dehydratase (lgd1 or gaaB) in two filamentous fungi, strains were obtained that converted d-galacturonic acid to l-galactonic acid. Both Trichoderma reesei Δlgd1 and Aspergillus niger ΔgaaB strains produced l-galactonate at yields of 0.6 to 0.9 g per g of substrate consumed. Although T. reesei Δlgd1 could produce l-galactonate at pH 5.5, a lower pH was necessary for A. niger ΔgaaB. Provision of a cosubstrate improved the production rate and titer in both strains. Intracellular accumulation of l-galactonate (40 to 70 mg g biomass(-1)) suggested that export may be limiting. Deletion of the l-galactonate dehydratase from A. niger was found to delay induction of d-galacturonate reductase and overexpression of the reductase improved initial production rates. Deletion of the l-galactonate dehydratase from A. niger also delayed or prevented induction of the putative d-galacturonate transporter An14g04280. In addition, A. niger ΔgaaB produced l-galactonate from polygalacturonate as efficiently as from the monomer.
authors.affiliation: VTT Technical Research Centre of Finland, Espoo, Finland.
authors.forename: Joosu
authors.initials: J
authors.lastname: Kuivanen
authors.forename: Dominik
authors.initials: D
authors.lastname: Mojzita
authors.forename: Yanming
authors.initials: Y
authors.lastname: Wang
authors.forename: Satu
authors.initials: S
authors.lastname: Hilditch
authors.forename: Merja
authors.initials: M
authors.lastname: Penttilä
authors.forename: Peter
authors.initials: P
authors.lastname: Richard
authors.forename: Marilyn G
authors.initials: MG
authors.lastname: Wiebe
mesh_terms: D001234:Aspergillus niger / Q000235:genetics / Q000378:metabolism*; D001711:Biotransformation; D003470:Culture Media / Q000737:chemistry; D017353:Gene Deletion; D006603:Hexuronic Acids / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D060847:Metabolic Engineering; D053858:Metabolic Networks and Pathways / Q000235:genetics*; D013400:Sugar Acids / Q000378:metabolism*; D014242:Trichoderma / Q000235:genetics / Q000378:metabolism*
chemical_list: D003470:Culture Media; D006603:Hexuronic Acids; D013400:Sugar Acids; C012991:galactonic acid; C007819:galacturonic acid