Thermal and pH inactivation of an immobilized thermostable β-galactosidase from Thermus sp. strain T2: Comparison to the free enzyme

Abstract

Kinetic modelling of the thermal and pH inactivation of the free and an immobilized form of the β-galactosidase from Thermus sp. strain T2 has been carried out. A lacteous buffer containing 50 g L −1 lactose and a commonly used phosphate buffer 50 mM pH 7.2 were employed for the thermal and the pH inactivation studies, respectively. Temperatures changed between 60 and 90 °C; acid pH range from 3 to 5 and basic pH from 10 to 13. Thermal inactivation kinetics depended on the form of the enzyme: from 60 to 90 °C the free enzyme followed a two first-order reactions in series pathway while the immobilized enzyme shows that behaviour from 80 to 90 °C; at lower temperatures, a one first-order reaction pathway suffices. In both cases, the final enzyme retained a certain activity, higher in the case of the immobilized enzyme from 60 to 70 °C. At acid pH, biphasic or simple exponential (one first-order reaction) trends, both leading to an inactive species, were observed. In basic conditions, a kinetic model chosen was based on a two first-order reactions in series scheme. Kinetic parameters values reflect the stabilization got by immobilization, specially in acid conditions, which could be of interest for the industrial treatment of acid whey. The trend of the kinetic parameters in the pH inactivation was studied: hyperbolic or sigmoid trends with concentration of protonic or hydroxide species were observed. Kinetic model selection was performed by statistically robust multivariable non-linear regression techniques.