Effect of pH, various divalent metal ion and different substrates on glucoamylase activity obtained from Aspergillus niger using amylopectin from tiger nut starch as carbon source

Abstract

Glucoamylases were obtained from Aspergillus niger using amylopectin from tiger nut starch as carbon source on the 5th (GluAgTN5) and 12th day (GluAgTN12) of fermentation. The optimal pH for GluAgTN5 at 55°C were 6.5, 7.0, 6.0 while that for GluAgTN12 were 8.5, 6.0, 7.5 at 50°C using cassava, guinea corn and tiger nut starch as substrates, respectively. The enzyme activity in GluAgTN5 was enhanced by Ca²⁺ and Fe²⁺ while Zn²⁺ and Co²⁺ had  inhibitory effects on the enzyme activity. Mn²⁺and Pb²⁺, however completely inactivated the enzyme. Enzyme activity in GluAgTN12 was enhanced by Ca²⁺ while Co²⁺and Zn²⁺ had inhibitory effects, Fe²⁺, Mn²⁺ and Pb²⁺ completely inactivated the enzyme. The Michealis-Menten constant, Km and maximum velocity, Vmax obtained from Line-Weaver-Burk plot of initial velocity data at different substrate concentrations were 222 mg/ml and 500 μmol/min, 291 mg/ml and 1000 μmol/min, 137.5 mg/ml and 500 μmol/min using cassava, guinea corn and tiger nut starch as substrate, respectively for GluAgTN5. While that for GluAgTN12 were 176.6 mg/ml and 100 μmol/min, 491 mg/ml and 1000 μmol/min, 131.5 mg/ml and 500 μmol/min using cassava, guinea corn and tiger nut starch as substrate, respectively.

Key words: Glucoamylase, pH, metal ions, Aspergillus niger, tiger nut starch, amylopectin.