A study of the optimal conditions for glucoamylases obtained from Aspergillus niger using amylopectin from cassava starch as carbon source

Abstract

A fourteen day pilot study carried out showed that high glucoamylase activities were obtained on the 4 and 11^th day of fermentation and the enzymes were harvested on the respective days giving the codes GluAgCSV4 and GluAgCSV11. The optimal pH and optimal temperatures for enzyme activities GluAgCSV4 and GluAgCSV11 were in a range of 6 to 7 and 50 to 55, using cassava, guinea corn and tiger nut starch as substrates, respectively. The enzyme activity (GluAgCSV4) was enhanced by Ca²⁺, Mn²⁺, Fe²⁺ and Zn²⁺. Co²⁺ had inhibitory effect on the enzyme while Pb²⁺ completely inactivated the enzyme. The enzyme activity (GluAgCSV11) was enhanced by Ca²⁺ and Co²⁺. Zn²⁺, Fe²⁺ Mn²⁺ and Pb²⁺ completely inactivated the enzyme. The Michaelis constant KM and maximum velocity Vmax obtained form Lineweaver-Burk plot of initial velocity data at different substrate concentrations were found to be 90.06 mg/ml and 188.67 μmol/min (using cassava starch as substrate), 173.70 mg/ml and 434.78 μmol/min (using guinea corn starch as substrate) and 28.57 mg/ml and 227.27 μmol/min (using tiger nut starch as substrate), respectively for GluAgCSV4. Also, 271.30 mg/ml and 1000 μmol/min (using cassava starch as substrate, 3093 mg/ml and 10000 μmol/min (using guinea corn starch as substrate) 2625 mg/ml and 10000 μmol/min (using tiger nut starch as substrate), respectively, were obtained for GluAgCSV4.

Keywords: Glucoamylases, Aspergillus niger, amylopectin and starch.