Covalent linkage, immobilized enzymes

It was reported that PEGylated lipase entrapped in PVA cryogel could be conveniently used in organic solvent biocatalysis [279], This method for enzyme immobilization is more convenient in comparison to other types of immobilization that take advantage of enzyme covalent linkage to insoluble matrix, since the chemical step which is time consuming and harmful to enzyme activity is avoided. The application of this catalytic system to the hydrolysis of acetoxycoumarins demonstrated the feasibility of proposed method in the hydrolysis products of pharmaceutical interest and to obtain regioselective enrichment of one of the two monodeacetylated derivatives. 

As schematically depicted in Figure 5, two different routes are available for immobilizing biotin-labeled enzymes on the support through avidin-biotin complexation. The first procedure employs the biotin-modified surface on which biotin-labeled enzymes are immobilized through avidin as binder protein. For this procedure, the covalent linkage of biotin onto the surface of a carbon electrode and the preparation of biotin-labeled lipid bilayer on electrode have been studied. An alternative way involves the direct modification of an electrode surface with avidin. If avidin could be immobilized directly without loss of the binding activity to biotin, biotin-labeled enzymes could be loaded more easily on the electrode surface. 

The approaches that have been proposed to immobilize artificial mediators include the adsorption of the redox mediator (9), the immobilization in carbon paste (75), the covalent linkage on electroinactive (75) or conducting polymer backbone (10), the covalent attachement to the enzyme structures (3) and... 

Figure 25 Immobilization of enzymes by covalent linkage with poly [ (organo)phosphazenes].

Enzymes have been widely used as biorecognition elements in biosensors as they are able to catalyze reactions that produce electrochemical, optical, and thermal sig-nals." " Most popularly, glucose oxidase is widely used in electrochemical glucose biosensors that have found wide usage in clinical and medical settings. Enzymes offer ease of immobilization on transducers by physical adsorption, covalent linkage, and... 

Conditions for achieving efficient DET via enzyme immobilization are dictated partly by materials architecture. Enzyme immobilization techniques may include nonspecific adsorption, covalent linkage, entrapment in conductive polymeric films, association with metal colloids, and encapsulation within porous matrices (see Chapter 11). The simplest method is nonspecific adsorption, but control is limited various noncovalent interactions will yield different orientations of the redox center with respect to the electrode interface and, as a result, inefficient DET. 

In some cases, nanosupports innately possess functional groups suitable for enzyme immobilization in others, their surfaces must first be modified. Enzymes can be immobilized on nanostructured supports either by adsorption or by covalent linkage. For covalent linkage, epoxy groups or amines (combined with glutaraldehyde) are most common. Enzymes immobilized on nonporous nanoparticles are attached on the surface of such particles. In this case, they interact more easily with external interfaces, enzyme molecules immobilized on other particles, and molecules from the bulk of substrate. 

Enzyme immobilization on magnetic nanoparticles by covalent linkage... 

Non-covalent immobilization includes aU kind of interactions between the enzyme and the support not involving covalent bonds. Physical adsorption relies on non-specific physical interaction between the enzyme protein and the surface of the matrix. In this method, only weak bonds are involved, for example, hydrogen bonds, multiple salt linkages, van der Waals forces, which on one hand is less disruptive towards enzyme, while on the other hand leads to desorption from the surfaces once there are changes in temperature, pH, ionic strength, etc. Physical forces are non-specific and further adsorption of other proteins or other substances can occur as the immobilized enzyme is used, which may alter the properties of the immobiHzed enzyme. 

Traditional techniques such as physical adsorption and covalent linkage onto solid supports, entrapment in polymer matrices, and microencapsulation have long been used for immobilizing such enzymes as lipases, proteases, hydantoinases, acylases, amidases, oxidases, isomerases, lyases, and transferases [12-18]. Encapsulation and adsorption have also proved their utility in the immobilization of bacterial, fungal, animal, and plant cells [12-21]. However, as biocatalysis applications have grown, so the drawbacks and limitations of traditional approaches have become increasingly evident. The forefront issues now facing bioimmobilization are indicated in Table 1. 

Creation of covalent bonds between surface amino acids of the enzyme and an insoluble matrix is perhaps the most frequendy exploited method of immobilization. Polar amino acids, which are likely to be present on the protein surface, have struaures that lend themselves to the chemical manipulation necessary for immobilization. e-Amino groups of lysine residues are the most frequendy employed points of linkages, though cysteine (via -SH), tyrosine, histidine, aspartic and glutamic acids, tryptophan, and arginine can also be used. 

---