Thermally controlled biotransformation of glycyrrhizic acid via an asymmetric temperature-responsive polyurethane membrane

Abstract

Separating a target product from a relatively complex bioreaction system is often difficult. In this work, a “smart” bioreaction system was developed by using the special characteristic of temperature-responsive polyurethane (TRPU). By combining solvent evaporation with a wet phase inversion technique, an asymmetric membrane consisting of an integral and dense skin layer supported by a porous sublayer was prepared from a thermally responsive polyurethane that experiences a sudden free volume increase upon heating through a phase transition temperature of 56 °C. Subsequently, the asymmetric TRPU membrane served as the carrier of an immobilized enzyme, wherein β-glucuronidase was multipoint-conjugated by using biotin and streptavidin on the porous sublayer. Then, the material-asymmetric TRPU membrane served jointly as the antennae as well as the actuator, which reversibly responds to temperature to switch (on–off) the access of the reactant glycyrrhizic acid (GL). Under the optimal temperature (40 °C) and pH (7.0) conditions, the immobilized β-glucuronidase contributed to almost 33% yield of glycyrrhetinic acid 3-O-mono-β-D-glucuronide (GAMG) of the isolated counterpart for the same concentration of substrate (250 mg L⁻¹) reaction for 24 h, while costing 1% of that of the isolated β-glucuronidase. Kinetic results showed that V_max and K_m values were 8.89 × 10³ mg L⁻¹ and 2.30 × 10³ mg L⁻¹ h⁻¹, respectively. The specific functional polymer-immobilized β-glucuronidase design serves as a bioreactor of GL into GAMG, as well as a separator deliberately irritated and controlled by temperature. This “smart” support material presents a potential facilitator for the separation of complex biotransformation reactions.