The use of natural enzymes to catalyse chemical reactions in industry is one method to achieve a greener reaction process, since enzymes are targeted and can shorten pathways for chemical synthesis, allowing for reduced use of toxic reagents and the production of side products.
However, one problem is that enzymes are a homogeneous catalyst, that means they are dissolved within the solvent. Although enzymes are theoretically reusable, extracting the enzymes after the reaction to be reused is a difficult task.
Therefore, there has been research interest in immobilized enzymes, which are enzymes affixed onto a porous structure, allowing them to be in contact with the reaction mixture, but can be removed together with the matrix it is affixed onto. Examples of such matrices which can be used include alginate-based hydrogels. Another benefit of immobilized enzymes is that they tend to be more stable than the free enzyme.
There are various methods of affixing enzymes to the matrix. The simplest way is to simply trap them within the pores in the matrix. Other ways of enzyme immobilization include adsorption to the matrix, encapsulation within a membrane, and forming cross-links between enzyme monomers. Though entrapment is an easy method, the random arrangement of the enzymes could lead to the active sites of the enzymes being blocked and thus inaccessible for catalysing the reaction.
Site-directed immobilization can instead be done by forming a bond between the enzyme and the matrix, directed in such a way that the active site of the enzyme is pointed away from the matrix and towards the reaction mixture. To be able to form such bonds, usually the enzyme must be artificially modified to contain a functional group which can bond to the matrix.