IMMOBILIZATION METHODS in .NET

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TABLE 4.1. Representative Support Materials for Enzyme Immobilizations Functional Group Si(OH)3 CONH2 COOH Support Materials Used Controlled-pore glass Polyacrylamide Polyacrylic acid Polyaspartic acid Polyglutamic acid Carboxymethylcellulose Maleic anhydride ethylene copolymer Polystyrene (nitrated and reduced) Nylon (after cleavage in HCl) Cellulose Sephadex Agarose Sepharose Carboxymethylcellulose
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R COO R0 CO 2 NH OH
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and the stability of the enzyme-support bonds. Carriers may be activated under harsh conditions, to convert relatively unreactive functional groups into activated species that will then readily react with proteins under mild conditions of T, ionic strength, and pH. Table 4.1 gives a representative listing of support materials onto which enzymes have been immobilized, and shows the reactive functional groups present on the support that may be used for coupling. The amino acid residues constituting the polypeptide component of enzymes provide sites at which covalent attachment to the support material may occur. Functional groups present on the side chains include primary amines (lysine), phenol hydroxyls (tyrosine), carboxylic acids (aspartate, glutamate), thiols (cysteine), hydroxyls (serine, threonine), and imidazole nitrogen (histidine). Of these, primary amines, carboxylic acids, and hydroxyl groups are most commonly used for protein immobilizations, because their hydrophilic character suggests that they will occur in an accessible area of the protein, at the aqueous interface, and will therefore be readily available for chemical reactions. Mild coupling conditions are essential in order to avoid the chemical modi cation of amino acid residues near the active site, as well as any change in tertiary structure that may affect the activity. It is sometimes possible to include a substrate analogue, or competitive inhibitor, in the enzyme immobilization reaction to protect the enzyme s active site from coupling reagents. 4.2.1.1. Controlled-Pore Glass. Silanol groups on the glass surface provide sites at which silanization may be performed. Silanization may be effectively performed with a 5% aqueous solution of aminopropyltri(ethoxy)silane (APTES),1
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IMMOBILIZED ENZYMES
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which results in the provision of more reactive primary amino groups for further derivatization: ! Si OH 2 OSi OH 2 CH2 3 NH2 Si OH 3 CH3 CH2 O 3 Si CH2 3 NH2 3 CH3 CH2 OH 4:1 Since primary amine groups are reasonably reactive as nucleophiles, a number of reactions may follow this initial activation step. Dehydrating reagents, such as water-soluble carbodiimides, may be used to activate protein carboxylic acids to O-acylisourea intermediates under mild (pH 7) conditions, to allow amide bond formation with the APTES-derivatized glass.2 If carried out in the presence of sulfonated N-hydroxysuccinimide, this reaction is highly selective for primary amine groups, but may still result in signi cant protein cross-linking. For more selective nonpolymerizing immobilization, however, further derivatization of the surface is necessary. Exposure to p-nitrobenzoyl chloride results in the addition of a nitrophenyl group, which is then reduced to the aminophenyl group (Eq. 4.2):
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RNH2 ClCO C6 H4 NO2 ! RNHCO C6 H4 NO2 ! RNHCO C6 H4 NH2 4:2
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One nal activation step is required, and is the conversion of the aminophenyl group to the diazonium salt: ! RNHCO C6 H4 N RNHCO C6 H4 NH2 2
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HONO
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4:3
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The diazonium salt is readily prepared from the aminophenyl group, but does not result from treatment of an aliphatic amine (such as the APTES-derivatized surface) with nitrous acid. The activated surface is now ready for enzyme coupling, since diazonium salts are very reactive toward protein tyrosine residues (Eq. 4.4):
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4:4 Although four activation steps are required prior to enzyme immobilization, this method possesses the advantages of not only providing selective covalent immobilization through tyrosines, but also of avoiding enzyme enzyme crosslinks that can result in activity losses.4 4.2.1.2. Polysaccharides. Polysaccharides possess readily accessible hydroxyl groups that may be derivatized for selective protein immobilization. We will consider the two most commonly used immobilization reactions, the triazine and the cyanogen bromide reactions.
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