Catalysis is frequently a prerequisite for efficiency in organic reactions (Woltinger et al. 2001). As biocatalysts, enzymes exhibit several features that make their use advantageous compared to conventional chemical catalysts. The specific features presented by enzymes are the high level of catalytic efficiency and high degree of specificity including substrate specificity, region specificity and stereospecificity. However, conventional catalytic processes are often carried out in a homogeneous way, which frequently renders the separation of the catalyst from the reaction medium a cumbersome nuisance (Annis and Jacobsen 1999; Garber et al. 2000). Moreover, the loss of the precious enzymes in these catalysis processes becomes a great obstacle for commercial application. Therefore, efforts have been made to bind enzymes to an insoluble support/carrier.
In recent years, artificial membranes have been applied in biotechnology because of their interesting properties of high specific surface area and the possibility of combining separation with the chemical reaction (Gekas 1986). Among these membranes, the polypropylene membrane is particularly interesting due to its well-controlled porosity, chemical and thermal inertness, and high potential for comprehensive applications. However, PP-MMs are pressed for polar functional groups, which are necessary for the covalent immobilization of proteins. Moreover, the poor biocompatibility of this membrane may cause nonbiospecific interactions, protein denatu-ration, and loss of enzyme activity (Kasemo 2002). Thus, one can envisage that it is possible to introduce a reactive and biofriendly interface on the PPMM surface for enzyme immobilization through surface modification technologies, which may reduce some of the nonspecific enzyme-support interactions, create a specific microenvironment for the enzyme, and benefit the enzyme activity (Deng 2004b). The introduction of polymer chains carrying polar and reactive groups to the PPMM surface may solve this problem easily. Tethering of polymers to the membrane surface can offer relatively high functional group density and depress the hindrance effect, which may cumber the immobilization of the enzyme. In general, the polymer carboxylic, amino, or thio groups, grafted to the membrane surface, are used for the immobilization of enzymes (Sano et al. 1993). Hydrophilic monomers, such as acrylic acid, HEMA, glycidyl methacrylate (GMA), are often used to modify the membrane surface, enhancing the hydrophilicity and biocompatibility of the membranes. However, for some enzymes, hy-drophilic polymers are not always a suitable choice. In the case of lipase, the enzyme is activated in the presence of aqueous-hydrophobic interfaces. In addition, polymers that exhibit semiconductivity are preferred for the immobilization of enzymes that catalyze reactions via electron trans fer. Anyway, the purpose of modification is to provide the enzyme with conditions similar to those that it requires in nature.
There are three methods for immobilizing enzymes: physical adsorption/entrapment, direct grafting, and site-specific immobilization. In general, physical adsorption/entrapment offers better enzyme activity but relatively low stability. On the contrary, covalent immobilization is much more stable but often causes the enzyme to denature.
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