Functional properties of a manganese-activated exo-polygalacturonase produced by a thermotolerant fungus Aspergillus niveus

Abstract

A thermotolerant fungus identified as Aspergillus niveus was isolated from decomposing materials and it has produced excellent levels of hydrolytic enzymes that degrade plant cell walls. A. niveus germinated faster at 40°C, presenting protein levels almost twofold higher than at 25°C. The crude extract of the A. niveus culture was purified by diethylaminoethyl (DEAE)-cellulose, followed by Biogel P-100 column. Polygalacturonase (PG) is a glycoprotein with 37.7% carbohydrate, molecular mass of 102.6kDa, and isoelectric point of 5.4. The optimum temperature and pH were 50°C and 4.0-6.5, respectively. The enzyme was stable at pH3.0 to 9.0 for 24h. The DEAE-cellulose derivative was about sixfold more stable at 60°C than the free enzyme. Moreover, the monoaminoethyl-N-aminoethyl-agarose derivative was tenfold more stable than the free enzyme. PG was 232% activated by Mn(2+). The hydrolysis product of sodium polypectate corresponded at monogalacturonic acid, which classifies the enzyme as an exo-PG. The K m, V max, K cat, and K cat/K m values were 6.7mg/ml, 230U/mg, 393.3/s, and 58.7mg/ml/s, respectively. The N-terminal amino acid sequence presented 80% identity with PglB1, PglA2, and PglA3 putative exo-PG of Aspergillus fumigatus and an exo-PG Neosartorya fischeri.