Kinetic and thermodynamic studies of a novel acid protease from Aspergillus foetidus

The kinetics of a thermostable extracellular acid protease produced by an Aspergillus foetidus strain was investigated at different pH, temperatures and substrate concentrations. The enzyme exhibited maximal activity at pH 5.0 and 55 °C, and its irreversible deactivation was well described by first-order kinetics. When temperature was raised from 55 to 70 °C, the deactivation rate constant increased from 0.018 to 5.06 h−1, while the half-life decreased from 37.6 to 0.13 h. The results of activity collected at different temperatures were then used to estimate, the activation energy of the hydrolysis reaction (E* = 19.03 kJ/mol) and the standard enthalpy variation of reversible enzyme unfolding (ΔH°U = 19.03 kJ/mol). The results of residual activity tests carried out in the temperature range 55-70 °C allowed estimating the activation energy (E*d = 314.12 kJ/mol), enthalpy (311.27 ≤ (ΔH°d ≤ 311.39 kJ/mol), entropy (599.59 ≤ ΔS*d ≤ 610.49 kJ/mol K) and Gibbs free energy (103.18 ≤ ΔG*d ≤ 113.87 kJ/mol) of the enzyme irreversible denaturation. These thermodynamic parameters suggest that this new protease is highly thermostable and could be important for industrial applications. To the best of our knowledge, this is the first report on thermodynamic parameters of an acid protease produced by A. foetidus.