An acetylglucomannan esterase of Aspergillus oryzae; purification, characterization and role in the hydrolysis of O-acetyl-galactoglucomannan.

An acetyl glucomannan esterase (AGME) was purified to electrophoretic homogeneity from the culture supernatant of Aspergillus oryzae. This new enzyme had a molecular mass of 36 kDa and an isoelectric point of 4.6. It was most active in the pH range 5.0-5.5 and was stable for 24 h at 40 degrees C at pH 5.0-6.0. The purified esterase liberated acetic acid from O-acetyl-galactoglucomannan, O-acetyl-4-O- methylglucuronoxylan and alpha-naphtyl acetate. The specific activity was 10-times higher for acetylated mannan than for acetylated xylan. The enzyme was able to act on polymeric substrate but activity was clearly enhanced by addition of mannanase from Trichoderma reesei and alpha-galactosidase from guar seeds. Presence of mannanase also increased the liberation of acetic acid in long-term hydrolysis (24 h), while the addition of alpha-galactosidase had no effect. No significant synergism between these two glycanases and the previously characterized esterase of A. oryzae (FE), which is also able to deacetylate galactoglucomannan, was observed. Even though the AGME had 8-times higher specific galactomannan deacetylating activity than the FE, the maximum amount of acetic acid liberated from the polymeric galactoglucomannan by AGME was only 80% of that of FE. Both esterases clearly enhanced the action of mannanase and alpha-galactosidase in the degradation of O-acetyl-galactoglucomannan isolated from Norway spruce.