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Raw Starch Degrading, Acidic-Thermostable Glucoamylase from Aspergillus fumigatus CFU-01: Purification and Characterization for Biotechnological Application

Adeyemi O Ayodeji, Frank A Ogundolie, Olufemi S Bamidele, Ayodele O Kolawole and Joshua O Ajele

Improvements in the industrial process involved in starch degradation have led to the search for thermostable, salt-loving amylolytic enzymes especially glucoamylase, an enzyme that can completely hydrolyze starch to glucose. In this research, glucoamylase was optimally produced in liquid culture from Aspergillus fumigatus and purified to homogeneity by ammonium sulphate precipitation, ion-exchange chromatography and gel filtration, giving a yield of 8.19% with 20-fold purification. The 50 kDa glucoamylase, molecular weight estimated by Sodium Dodecyl Sulphate Polyacrylamide Gel Electrophoresis (SDS-PAGE) was highly stable over a wide pH range in the acidic region and also thermostable; retaining about 70% of initial activity after 60 min of incubation at 60ºC. The purified enzyme was active on amylose, dextran and different starches with K_m and V_max values of 1.59 mg/mL and 14.33 U/mg respectively, when raw cassava starch was used as substrate. Remarkably, this glucoamylase possesses high salt-tolerant property, with about 50% residual activity after 24 h incubation in 3.0 M NaCl solution. Some metal ions including K⁺, Ca²⁺ and Mg²⁺ were activators of the enzyme while Cu²⁺, Pb²⁺ and Hg²⁺ inhibited its activity. The unique biochemical characteristics of this glucoamylase qualify it for biotechnological use especially in food, pharmaceuticals and biofuel industries.