The economic argument for immobilisation

An important factor determining the use of enzymes in a technological process is their expense. Several hundred enzymes are commercially available at prices of about £1 mg⁻¹, although some are much cheaper and many are much more expensive. As enzymes are catalytic molecules, they are not directly used up by the processes in which they are used. Their high initial cost, therefore, should only be incidental to their use. However due to denaturation, they do lose activity with time. If possible, they should be stabilised against denaturation and utilised in an efficient manner. When they are used in a soluble form, they retain some activity after the reaction which cannot be economically recovered for re-use and is generally wasted. This activity residue remains to contaminate the product and its removal may involve extra purification costs. In order to eliminate this wastage, and give an improved productivity, simple and economic methods must be used which enable the separation of the enzyme from the reaction product. The easiest way of achieving this is by separating the enzyme and product during the reaction using a two-phase system; one phase containing the enzyme and the other phase containing the product. The enzyme is imprisoned within its phase allowing its re-use or continuous use but preventing it from contaminating the product; other molecules, including the reactants, are able to move freely between the two phases. This is known as immobilisation and may be achieved by fixing the enzyme to, or within, some other material. The term 'immobilisation' does not necessarily mean that the enzyme cannot move freely within its particular phase, although this is often the case. A wide variety of insoluble materials, also known as substrates (not to be confused with the enzymes' reactants), may be used to immobilise the enzymes by making them insoluble. These are usually inert polymeric or inorganic matrices.

Immobilisation of enzymes often incurs an additional expense and is only undertaken if there is a sound economic or process advantage in the use of the immobilised, rather than free (soluble), enzymes. The most important benefit derived from immobilisation is the easy separation of the enzyme from the products of the catalysed reaction. This prevents the enzyme contaminating the product, minimising downstream processing costs and possible effluent handling problems, particularly if the enzyme is noticeably toxic or antigenic. It also allows continuous processes to be practicable, with a considerable saving in enzyme, labour and overhead costs. Immobilisation often affects the stability and activity of the enzyme, but conditions are usually available where these properties are little changed or even enhanced. The productivity of an enzyme, so immobilised, is greatly increased as it may be more fully used at higher substrate concentrations for longer periods than the free enzyme. Insoluble immobilised enzymes are of little use, however, where any of the reactants are also insoluble, due to steric difficulties.