Glucoamylase from a newly isolated Aspergillus niger FME: Detergent-Mediated production, purification, and characterization

Abstract

Glucoamylase (EC 3.2.1.3) is an important group of enzymes in starch processing, also referred to as amyloglucosidases, which are exo-acting amylases that release glucose from the nonreducing end of starch and related oligosaccharides. The glucoamylase newly isolated from the Aspergillus niger FME) was reported for the first time. This enzyme was produced by detergent-mediated release and purified to ∼9.11 fold using Sephadex-G 100 and ion-exchange chromatography. Molecular mass of the glucoamylase was ∼36 kDa as determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The product of starch hydrolysis, analysed by thin-layer chromatography, showed the presence of glucose. The optimum pH and temperature for glucoamylase activity was 5.0 and 45°C, respectively. The Km and Vmax values of the enzyme were also determined using soluble starch as substrate as 94 μg/mL and 39.02 U/mg, respectively. Moreover, glucoamylase was slightly activated by presence of Na and K ions and 10–20% inhibition was observed in presence of Zn2+, Sn2+, Mg2+, Ni2+, Mn2+, and almost 80% with Cu2+ ions, whereas the presence of ethylene diamine tetra acetic acid (EDTA) did not show significant inhibition. Glucoamylase, also assayed for surfactant property, shows significant surfactant tolerance at high concentrations of detergent and can retain 90% of its activity. Finally, secondary structure analysis of glucoamylase by circular dichroism spectroscopy showed the presence of 48% α-helix, 11% β-sheet, and 41% random structure.