Enzymes are promising biocatalysts for the production or degradation of chemical compounds, but low stabilities of free enzymes restrict their industrial applications. Therefore, development of effective immobilization methods to maintain or increase enzyme activity and stability remains a challenge. In this work, a novel support made of zwitterionic polymer-grafted silica nanoparticles (p-SNPs) was fabricated and Candida rugosa lipase (CRL) was covalently attached onto the p-SNPs. The zwitterionic polymer was a product of the reaction between poly(maleic anhydride-alt-1-octadecene) and N,N-dimethylenediamine and contained a cetane side chain. The hydrolytic activity, reaction kinetics, thermal stability, pH tolerance, storage stability and reusability of the immobilized CRL (SNPs-CRL) were investigated. It revealed that the specific activity of SNPs-CRL was two to four times higher than the free CRL in the temperature range of 25-60 °C. It is considered mainly due to the interfacial activation effect regulated by the cetane side chains of the zwitterionic polymer. Kinetic studies revealed remarkable improvement of the enzymatic reaction efficiency by the immobilized enzyme as demonstrated by the significant increases of the reaction rate constant and the decreases of Michaelis constant (i.e., increase of enzyme-substrate affinity) determined with two different substrates (p-nitrophenyl acetate and p-nitrophenyl palmitate). Moreover, the immobilization improved the enzyme stabilities and SNPs-CRL displayed good reusability. Finally, the SNPs-CRL was proven to catalyze the hydrolysis of methyl mandelate to produce mandelic acid at an activity three times higher than the free enzyme. The results indicate that zwitterionic polymers deserved further development for enzyme immobilization.
Keywords: Enzymatic hydrolysis; Immobilization; Interfacial activation; Lipase; Silica nanoparticles; Zwitterionic polymer.
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