Purification and Characterization of Two Thermostable Glucoamylases Produced from Aspergillus niger B-30

Two thermostable glucoamylases were produced from Aspergillus niger B-30 by submerged fermentation. The two glueoamylases GAM-1 and GAM-2 were purified by ammonium sulfate precipitation, diethylaminoethyl- cellulose fast flow（DEAE FF） and Superdex G-75 gel filtration columns. The molecular weights of GAM-1 and GAM-2 were determined as 9.72x 104 and 7.83x104 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis （SDS-PAGE）, while the molecular weights of GAM-1 and GAM-2 were determined to be 8.05x 104 and 7.04x 104 by matrix assisted laser desorption ionizationtime-of-flight（MALDI-TOF） mass spectrometry, respectively. Both the enzymes were glycosylated, with 10.4% and 11.4% carbohydrate content, respectively. The optimal pH and tempera- ture were 4.0--4.6 and 70 ℃ for both. The two glucoamylases were maintained 100% relative activity after incuba- tion at 60 ℃ for 120 min. After the hydrolysis of starch for 120 min, glucose was the only product, confirming that the two enzymes were of high efficiency towards starch. The GAM-2 exhibited higher catalytic activity towards oli- gosaccharides such as maltose than GAM-1, and the kinetic analysis shows that the affinity of GAM-2 to starch was lower than that of GAM-1. The high thermostability and effectiveness make the two glucoamylases potentially attrac- tive for biotechnological application.