Graphene Oxide Nanosheets Shielding of Lipase Immobilized on Magnetic Composites for the Improvement of Enzyme Stability

In this study, a novel enzyme immobilization method was developed to enhance the catalytic stability of enzymes. In this strategy, ionic liquid (IL) modified magnetic chitosan (MCS) composites were used as supports for lipase adsorption and graphene oxide (GO) was employed as shell coating for the first time. The modifier used was imidazolium-based IL with a side alkyl chain which was composed of 8 -CH2 and a terminal hydroxyl group. The prepared supports IL-MCS, immobilized lipase PPL-IL-MCS, and GO/PPL-IL-MCS were well characterized. The GO shielding lipase GO/PPL-IL-MCS maintained high activity (2468 U/g), which was 6.72-fold of free lipase. In addition, the pH and temperature effect on lipase activity were investigated. The thermal stability, denaturants stability, storage stibility, and reusing stability were also studied. Compared to PPL-IL-MCS, the stabilities of GO/PPL-IL-MCS were all enhanced while keeping high activity. For example, after 10 cycles of reuse, the residual activity of GO/PPL-IL-MCS was 92.1%, which was higher than that of 88.4% for PPL-IL-MCS. Furthermore, the apparent K-m of PPL-IL-MCS and GO/PPL-IL-MCS was 5.7 and 8.8 mg/mL, respectively, which were both lower than that of PPL-MCS (17.1 mg/mL). Circular dichroism (CD) was used to analyze the secondary structure of lipase to explain the mechanism of stable enhancement of immobilized enzyme. This work demonstrated that GO was used as a shell coating for the first time to improve the lipase stability. This immobilization method provides a reference for the immobilization of other kinds of enzymes.