Immobilization of fenugreek β-amylase onto functionalized graphene quantum dots (GQDs) using Box-Behnken design: Its biochemical, thermodynamic and kinetic studies

beta-Amylase was immobilized onto GQDs using 3-aminopropyltriethoxysilane and glutaraldehyde. Optimization was carried out by Box-Behnken design and binding was confirmed by SEM, AFM, FTIR and fluorescence microscopy. Predicted optimum immobilization efficiency (88.64%) was very close to actual (87.98%), which confirmed the success of the immobilization process. The immobilized enzyme showed maximum activity at pH 5.0 and 57 degrees C, whereas K-m and V-max were found to be 6.40 mg/mL and 714.28 mu mol/min/mg, respectively. The enzyme retained 75% activity after 12 uses at 30 degrees C. Increased values of Delta G degrees Delta H degrees, half-life and activation energy of the enzyme inactivation (Delta E-d) revealed that thereto-stability increases after immobilization and the process followed first-order kinetics (r(2) > 0.96). The activation energy of catalysis (Delta E-a and Delta E-d for immobilized enzyme were 22.58 and 158.99 +/- 1.10 kJ/mol, respectively which revealed that denaturation of the enzyme requires a higher amount of energy rather than catalysis. Thermodynamic and fluorescence spectroscopic studies revealed that the process is non-spontaneous (Delta G > 0) and endothermic (Delta H > 0) and occurred through protein unfolding rather than aggregation (Delta S > 0). Thus increase in thermo-stability of immobilized fenugreek beta-amylase and non-toxic nature of GQDs could be exploited for maltose production in beverage, food and pharmaceutical industries. (C) 2019 Elsevier B.V. All rights reserved.