Inversion of configuration during hydrolysis of α‐1,4‐galacturonidic linkage by three Aspergillus polygalacturonases

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Abstract
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 β‐anomer of triGalUA. The primary products of hydrolysis of triGalU‐ol by ExoPG were diGalU‐ol and the β‐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.