Immobilization of Enzymes or Whole Cells
In recent years, the technique of enzyme or whole cell immobilization has revolutionized the prospects of enzyme application in industry. Immobilization is defined as the imprisonment of a biocatalyst in distinct phase that allows exchange with, but is separated from, bulk phase in which substrate, effector or inhibitor molecules are dispersed and monitored'. In other words, an immobilized enzyme is physically entrapped or covalently bonded by chemical means to an inert and usually insoluble matrix, where it can act upon its natural substrate. The matrix is usually a high molecular' weight polymer such as polyacrylamide, cellulose, starch, glass, beads, etc.

Some of the advantages of using immobilized enzyme over free enzyme include the following: (i) Because of its binding with a matrix, the immobilized enzyme has better stability in many cases. (ii) Efficiency of immobilized enzyme is better. (iii) The enzyme can be recovered at the end of reaction and can be used repeatedly; also the processed product is not contaminated with enzyme. (iv) Some manipulations of enzyme catalysed reactions are better and easier, when the enzyme is used in the immobilized form. For example, the reaction can be stopped instantly by removing the immobilized enzyme from the reaction solution.

The immobilization of enzyme is performed under mild and controlled condition, so that the enzymes retain their tertiary and quaternary structures', which are necessary for their activity. Immobilized enzymes are preferred over immobilized cells or tissues, since they have a high specificity and yield pure products.