Kinetics and thermodynamics of high level β-glucosidase production by mutant derivative of Aspergillus niger under submerged growth conditions

Abstract

Gamma rays mediated mutagenesis and subsequent selection on 2-deoxy-D-glucose developed a highly catabolite de-repressed and stable β-glucosidase (BGL) hyper-producer mutant of local Aspergillus niger NIBGE, which has high potential for industrial applications. The M6 mutant showed highest constitutive production of BGL (14,900 IU/L) on glucose as compared to control (5,900 IU/L). Wheat bran was the best carbon source and BGL production by parent and mutant strain was 16,200 and 29,500 IU/L, respectively. Effect of inoculum density, pH, temperature and wheat bran concentration on the BGL production was determined in terms of kinetic growth parameters, that is, specific growth rate (m), biomass doubling time (td), product yield coefficient with respect to cell mass (Yp/x) and specific rate of product formation (qp). Optimal conditions for BGL production by parent and mutant strain were same: inoculum level= 0.184 mg cells/mL (10% v/v); optimum temperature= 30°C and pH= 5.0; wheat bran= 3% (w/v). The apparent subunit molecular mass of BGL from both strains was also same (130 kDa). Determination of thermodynamic parameters led to consider that the hyper-production of BGL by the mutant was due to decrease in: activation energy (Ea(p)), change in enthalpy (ΔH*(p)) and Gibbs free energy (ΔG*(p)) for product (BGL) formation. Key words: β-glucosidase, Aspergillus niger, activation energy, enthalpy, Gibbs free energy.