Engineering filamentous fungi for conversion of d-galacturonic acid to L-galactonic acid

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.