Integrated kinetic and thermo-catalytic analysis of β-Xylosidase production by Aspergillus sp. under variable carbon substrates.

The production, kinetics, thermodynamics, and catalytic properties of β-xylosidase synthesized by Aspergillus niger were systematically investigated using different carbon sources under batch fermentation conditions. Among the substrates evaluated, disaccharides exhibited the strongest inductive effect on enzyme synthesis, with lactose yielding the highest specific enzyme productivity (Yₚ/ₓ = 480 IU g⁻¹ cells), followed by galactose (434 IU g⁻¹ cells), sucrose (391 IU g⁻¹ cells), and cellobiose (377 IU g⁻¹ cells). Xylose also acted as an effective inducer, producing a significant enzyme yield of 333 IU g⁻¹ cells. In contrast, glucose-supported cultures showed only basal enzyme production (2-5 IU g⁻¹ cells), confirming strong carbon catabolite repression. The induction ratio for xylose relative to glucose was approximately 66-67 fold, indicating a regulatory mechanism governed by both substrate induction and growth-dependent repression. Kinetic analysis demonstrated a close association between cell growth and enzyme formation, with Luedeking-Piret constants of α = 560 ± 90 IU g⁻¹ cells and β = 4.5 ± 0.5 IU g⁻¹ h⁻¹, indicating a mixed growth-associated and non-growth-associated production pattern. The maximum specific productivity (qP) was achieved at an optimal fermentation temperature of 35-37 °C. Thermodynamic analysis revealed an activation enthalpy (ΔH‡) of 45 kJ mol⁻¹ for enzyme production, while the activation enthalpy for thermal inactivation was lower (28 kJ mol⁻¹), indicating that enzyme denaturation requires less energy and is more temperature-sensitive than its synthesis. The partially purified β-xylosidase exhibited optimal catalytic activity at 55 °C and demonstrated considerable thermostability, retaining 50% of its activity for approximately 231 h at 50 °C, 165 h at 55 °C, and 63 h at 60 °C. The midpoint inactivation temperature (Tₘ) was determined to be 65 °C. The activation energy for substrate hydrolysis was 57 kJ mol⁻¹, increasing to 89 kJ mol⁻¹ upon enzyme denaturation. Additionally, the enzyme remained stable over a broad pH range of 5.0-7.0. Overall, the high inducibility, well-defined kinetic behavior, and substantial thermal stability of β-xylosidase produced by Aspergillus niger highlight its strong potential for industrial applications in lignocellulosic biomass conversion, biofuel production, and food-processing biotechnologies.