Figure

Basic biosensor design. (Reproduced from Walsh, G., Proteins: Biochemistry and Biotechnology, John Wiley & Sons, Chichester, UK, 2001., With permission from J. Wiley & Sons.)

a device that senses one form of energy and converts it to another. The transducer can, for example, be an electrode that converts the biochemical signal generated by the enzyme reaction (e.g., changes in concentration of substrate or product, absorbance, etc.) into a quantifiable electrical signal. The electrical signal generated by the transducer is related to the concentration of the analyte. A standard curve can be constructed from readings obtained when the electrode is immersed in standard solutions containing known concentrations of the analyte.

Most enzyme biosensors use either membrane-based enzyme entrapment, or the enzyme is covalently bound to the inside of a nylon tube. The stability of the electrode is dependent on the stability of the enzyme, which is partially dependent on the method of immobilization. Many enzyme electrodes are available to detect, for example, glucose, urea, creatine, and pyruvate in clinical samples. However, only the glucose biosensor has been widely commercialized.

There are many advantages associated with the use of biosensors as analyte detection and quantitation systems: (1) they are highly specific; (2) they are cost-effective (mainly because they are reusable); (3) no additional reagents are required to make measurements; (4) the assay sample need not be pretreated (e.g., removal of particulate matter etc. is not necessary); (5) they are flexibe (portable biosensors can be used in the field for multiple samples); and (6) they are not complicated to operate, (i.e. technical training is not essential).

1.4 Enzymes Used for Industrial Purposes

By the turn of the 20th century, the annual worldwide sales value of industrial enzymes stood at US$1.5 billion. Protein-degrading (proteolytic) enzymes (proteases) represent the most significant single group of industrial enzymes currently in use and were one of the first enzyme groups to find applied use [7]. Currently, the annual sales value of proteases represents over half of the total sales revenue generated by all industrial enzyme sales combined. Carbohydrate-degrading enzymes (carbohydrases) account for much of the remainder. A comprehensive review of all industrial enzymes and their uses is well beyond the scope of this chapter. Instead, we focus upon industrial proteases as a representative group, followed by a summary of carbohydrases and other enzymes of applied use.

1.4.1 Detergent Proteases

Proteases first found applied use in the food and detergent industries. These continue to be important areas. Currently, proteases also find numerous applications in leather processing and as therapeutic agents. Table 1.5 provides an overview of their industrial uses.

Economically, the most significant single use of proteases is their incorporation into detergents [7]. They facilitate the removal of "biological" dirt, which is mainly protein-based. Obviously, the characteristics of the proteases must be compatible with standard washing conditions. Accordingly, the most successful detergent proteases are stable at alkaline pH values; at relatively high temperatures; and in the presence of bleach, surfactants, and sequestering agents. Screening of proteases from a wide range of sources has identified members of the bacterial subtilisin subfamily as the most appropriate for detergent application [15,16]. Typically, they are optimally active at temperatures between 45 to 65oC and in the pH range of 9 to 12. Many such subtilisins have been modified by genetic engineering with the general aims of increasing their thermal stability as well as improving their resistance to oxidation.

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