Purification and characterisation of two exo-polygalacturonases from Aspergillus niger able to degrade xylogalacturonan and acetylated homogalacturonan

Two exo-polygalacturonases (EC 3.2.1.67) were purified from a commercial Aspergillus niger enzyme preparation by ammonium sulfate precipitation, preparative electrofocusing, anion-exchange and size-exclusion chromatographies. The enzymes had molar masses of 82 kDa (exo-PG1) and 56 kDa (exo-PG2). Exo-PG1 was stable over wider pH and temperature ranges than exo-PG2. Addition of 0.01 mM HgCl(2) increased the exo-PG2 activity 3.4 times but did not affect exo-PG1. Analysis of the reaction products of (reduced) pentagalacturonate by high-performance anion-exchange chromatography revealed that both enzymes split the substrate from the non-reducing end in a multi-chain attack mode. Exo-PG1 had a broad specificity towards oligogalacturonates with different degrees of polymerisation, while digalacturonate was the most favorable substrate for exo-PG2. Both enzymes degraded xylogalacturonan from pea hull in an exo manner to produce galacturonic acid and Xyl-GalA disaccharide, as identified by electrospray ionization-ion trap mass spectrometry (ESI-ITMS). Moreover, exo-PGs split acetylated homogalacturonan in an exo manner, producing galacturonic acid and acetylated galacturonic acid, as shown by ESI-ITMS.