Biocatalysis covers a broad range of scientific and technical disciplines with the aim to develop biocatalysts and biocatalytic processes for practical purposes. Biocatalysts are based on diverse natural sources, they include whole cells of microbial, plant or animal origin, as well as cell-free extracts and enzymes. Currently, only a very small fraction of the known biocatalysts are being applied on a commercial scale. For example, of the approximately 4000 known enzymes, about 400 are available commercially, but only about 40 are actually used for industrial processes. Biocatalysts are generally much more efficient than chemical catalysts. Typically, enzymes can show turnover numbers of >100 000 s -1 compared with the values of 0.01-1 s -1 usually observed for heterogeneous and homogeneous catalysts.
Enzymes can operate under relatively mild conditions and usually exhibit a very high degree of substrate-, chemo-, regio-, and enantioselectivity. In this chapter we cannot cover the whole area of biocatalysis. We will give preference to biocatalysts used in industrial production processes.
Subsequently, selected applications of biocatalysts will be examined, used as either isolated enzymes or enzymes that operate in immobilized or permeabilized cells. Synthesis routes in which one or all of the steps are biocatalytic have advanced dramatically in recent years. Increasingly, biocatalysts are combined with chemical catalysts or utilized in a network of reactions in a whole cell. It can be pointed out, that biocatalysts do not operate by different scientific principles from usual catalysts. All enzyme actions can be explained by rational chemical and physical principles. However, enzymes can create unusual and superior reaction conditions such as extremely low pK a values or a high positive potential for a redox metal ion.
Biocatalysis is an interdisciplinary area. For a successful practice of biocatalysis three disciplines are needed: biochemistry and organic chemistry from chemistry, molecular biology, enzymology, and protein chemistry from biology, and catalysis, transport phenomena, and reaction engineering from chemical engineering. The most important areas of application are the pharmaceuticals, food, fine chemicals, basic chemicals, pulp and paper, agriculture, medicine, energy production, and mining industries (Fig. 4-1).