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 Ca 2+ and Fe 2+ while Zn 2+ and Co 2+ had inhibitory effects on the enzyme activity. Mn 2+ and Pb 2+ , however completely inactivated the enzyme. Enzyme activity in GluAgTN12 was enhanced by Ca 2+ while Co 2+ and Zn 2+ had inhibitory effects, Fe 2+ , Mn 2+ and Pb 2+ 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.
Highlights
Glucoamylase (α-1, 4-glucan-glucohydrolases, EC 3.2.1.3) is an exoenzyme that hydrolyzes 1,4-alphaglycosidic bonds from the non-reducing ends of starch and 1,6-alpha-glucsidic linkages in polysaccharides yielding glucose as the end-product, which serves as a feedstock for biological fermentations (Kumari et al, 2013)
The MichealisMenten 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
A fourteen day pilot study was carried out to determine the day of highest protein production, α-amylase activity and glucoamylase activity in submerged fermentation using amylopectin obtained from tiger nut starch as carbon source
Summary
Glucoamylase (α-1, 4-glucan-glucohydrolases, EC 3.2.1.3) is an exoenzyme that hydrolyzes 1,4-alphaglycosidic bonds from the non-reducing ends of starch and 1,6-alpha-glucsidic linkages in polysaccharides yielding glucose as the end-product, which serves as a feedstock for biological fermentations (Kumari et al, 2013). Glucoamylase from Aspergillus oryzae showed maximum activity at pH 5.0 (Parbat and Singhal, 2011), unlike α-amylase which. The pH value at which an enzyme exhibits highest activity is called “optimum pH” (Devasena, 2010). The deprotonation of carboxyl termini could result in a potential loss of interaction with an adjacent subunit, changing the enzyme conformation which could cause a decrease in substrate affinity, or a complete loss of activity. Variations in pH can influence the following characteristics of an enzyme; the binding of the substrate to the enzyme, the ionization states of the amino acid residues at the catalytic site of the enzyme, the ionization state of the substrate and variation in protein structure or complete denaturation of the enzyme (which occurs at extreme pH values) (Berg, 2007)
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