Properties and structures of commercial polygalacturonase with ultrasound treatment: role of ultrasound in enzyme activation

Abstract

Polygalacturonase (PG) is one of the most commonly used enzymes during fruit and vegetable processing in the food industry. Ultrasound has the potential to enhance enzyme activity, modify the PG enzyme and enlarge its application range. This study investigated the enzymatic properties of commercial PG under ultrasound treatment, including enzyme activity, kinetic and thermodynamic properties and temperature stability. These properties were investigated with the aid of a chemical reaction kinetics model, Michaelis–Menten equation, Arrhenius equation, Eyring transition state theory and biphasic inactivation kinetics model. PG structures were also studied using fluorescence spectroscopy and circular dichroism (CD) spectroscopy. The maximum activity of PG was observed at 4.5 W ml⁻¹ intensity and ultrasound duration of 15 min, under which the enzyme activity increased by 20.98% over the control. Results of degradation kinetics and thermodynamics of hydrolysis reactions catalysed by PG certified that ultrasound treatment could make PG exhibit higher reaction ability, which was evidenced from the increased rate constants and reduced thermodynamic parameters. Meanwhile, after ultrasound treatment, the value of V_max in the enzymatic reaction increased, whereas K_m decreased as compared with the control. These results demonstrated that the substrate was converted into the product at a higher rate and efficiency, and the enzyme displayed better affinity to the substrate. Ultrasound improved the temperature stability of PG and prolonged its lifetime without affecting its optimum temperature. Fluorescence spectra and far-UV CD spectra revealed that ultrasound treatment irreversibly decreased the amount of tryptophan on the PG surface but increased the β-sheet in PG secondary conformation, possibly by the exposure of more active sites.