36

3 Immobilized Enzymes for Bioconversion of Waste to Wealth

which will help during enzyme immobilization by forming hydrogen bonds.

Various modified methods are currently used to obtain a better immobilization.

Lipase from Yarrowia lipolytica has been immobilized on octadecyl-sepa beads

by physical adsorption [6]. Lipase of Candida rugosa has been adsorbed onto

poly(3-hydroxybutyrate-co-hydroxyvalerate) and it has a reusability till 12 years.

The eco-friendly supports of biological origin will help to cut down the cost and also

prevent the ethical issues. Biocompatible mesoporous silica nanoparticle supports

have been introduced in the biocatalysis of energy applications for long-term

efficiency and durability.

3.3.1.2

Covalent Bonding

Sustaining structural and the functional properties of enzymes during the immobi-

lization process is generally the key role played by the cross-linking agents. One such

bifunctional cross-linker which is popularly used is glutaraldehyde. They are soluble

in the aqueous solvents and form stable intra- or inter-subunits. The covalent bond-

ing to supports occurs through the side chains of the amino acids like aspartic acid,

arginine, and histidine present in the enzyme. The degree of reactivity during bond-

ing is based on several functional groups like phenol, hydroxyl, imidazole, indolyl,

etc. Peptide-modified surfaces were used to obtain greater specific activity and the

stability with a controlled protein orientation. The cross-linking of enzymes to elec-

trospun nanofibers has shown a greater residual activity due to an increase in the

surface area and porosity. Enzymes can be immobilized onto magnetic nanoclus-

ters through covalent bonding, and immobilized enzymes will have applications

in pharmaceutical sector due to their longevity, stability, reusability, and activity

enhancement [7].

3.3.1.3

Affinity Immobilization

It supports enzyme immobilization under different physiological conditions. Affin-

ity immobilizations can be achieved by two ways: (i) enzyme can be conjugated for

developing affinity toward the matrix and (ii) the matrix can be precoupled to the

affinity ligand which can target the enzyme. Affinity matrices are also used for the

purification of enzymes. Due to the existence of several non-covalent forces such as

hydrogen bonding, van der Waals force, etc., bioaffinity layering can also be consid-

ered as an efficient technique which will increase the enzyme binding capacity and

also reusability [8].

3.3.1.4

Entrapment

It is generally considered as the caging of the enzymes by non-covalent or covalent

bond within the gels or fibers. Hybrid material, alginate–gelatin–calcium, has

shown an efficient encapsulation which prevents enzyme leakage and provides an

increase in the mechanical stability. The enzyme immobilization has been revolu-

tionized with an effective entrapment by nanostructured supports like electrospun

nanofibers. This method has wide range of applications in the fields of chemistry,

biosensors, biomedical, and biofuel. Entrapment by mesoporous silica was rec-

ognized by its high surface area, high adsorption capacity, and uniform pore size