Inversion of Configuration During Hydrolysis of Alpha-1,4-galacturonidic Linkage by Three Aspergillus Polygalacturonases

Endopolygalacturonases I and II (PGI and PGII) of Aspergillus niger and an exopolygalacturonase (ExoPG) of A. tubingensis were investigated to reveal the stereochemistry of their hydrolytic action. Reduced pentagalacturonic acid (pentaGalU-ol) and reduced trigalacturonic acid (triGalU-ol) were used as non-reducing substrates for the enzymes. The configuration of the reducing ends in the products formed in D2O reaction mixtures was followed by 1H-NMR spectroscopy. It has been unambiguously established that primary cleavage of pentaGalU-ol by both PGI and PGII leads to diGalU-ol and the beta-anomer of triGalUA. The primary products of hydrolysis of triGalUA-ol by ExoPG were diGal-ol and the beta-anomer of GalUA. Thus, all three Aspergillus polygalacturonases belong to the so-called inverting glycanases, i.e. they utilize the single displacement mechanism of hydrolysis of the glycosidic linkage.