Multifunctional Epoxy Supports: A New Tool To Improve the Covalent Immobilization of Proteins. The Promotion of Physical Adsorptions of Proteins on the Supports before Their Covalent Linkage

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Abstract
Multifunctional supports containing epoxy groups are here proposed as a second generation of activated supports for covalent immobilization of enzymes following the epoxy chemistry on any type of support (hydrophobic or hydrophilic ones) under very mild experimental conditions (e.g., low ionic strength, neutral pH values, and low temperatures). These multifunctional supports have been easily prepared by modifying a small fraction (10−20%) of the epoxy groups contained in commercial epoxy supports. In this way, additional groups that were able to physically adsorb proteins (e.g., cationic or anionic groups, metal chelate, phenyl boronate) are generated on the support surface. The covalent immobilization of proteins on these supports proceeds via their initial physical adsorption on the supports (via different structural features). Then, “intramolecular” covalent linkages between some nucleophilic groups of the adsorbed enzyme (e.g., amino, thiol, or hydroxy groups) and the dense layer of nearby epoxy groups on the support are established. This two-step covalent immobilization dramatically improves the very low reactivity of epoxy groups toward nonadsorbed proteins. In this way, all other relevant practical advantages of epoxy groups for protein immobilization (their high stability and their ability to form very strong linkages with several nucleophilic enzyme residues with minimal chemical modification) can be an object of universal exploitation. The use of these new multifunctional supports exhibits important advantages regarding immobilization of enzymes previously adsorbed on hydrophobic homofunctional epoxy supports: (i) hydrophilic supports can also be used for immobilization of industrial enzymes; (ii) immobilization can also be carried out at low ionic strength; (iii) every protein contained in crude extracts from Escherichia coli and Acetobacter turbidans can be immobilized by sequentially using a set of different supports; (iv) in most cases, each enzyme has been immobilized on different supports, orientated through different structural features and very likely involving different areas of its surface. For example, three industrial enzymes (penicillin G acylase, lipase, and β-galactosidase) could be immobilized through different strategies yielding immobilized derivatives with very different activities. The best derivatives preserved 75−100% of activity corresponding to the soluble enzymes used for immobilization, while in some cases a particular immobilization protocol promoted the full inactivation of the enzyme.