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 differenttemperatures
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 (H◦
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 ≤ (H◦
d ≤ 311.39 kJ/mol), entropy
(599.59 ≤ S*
d ≤ 610.49 kJ/mol K) and Gibbs free energy (103.18 ≤ G*
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.
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