Introduction

The use of enzymes and microorganisms, which nature has developed, is undoubtedly an ideal choice toward "greening" chemical reactions. Biotransformations have been known since the early stages of human civilization and have been used since then to make fermented foods and beverages. However, biotransformations planted firm roots in industrial processes only when an industrial interest in steroid modifications emerged in the 1950s. The industrial use of enzymes and microorganisms in food processing and in the production of fine chemicals, drugs, and detergents has been undergoing a drastic development in the last decades (Stinson, 1999; Thayer, 2001). Stereospecific synthesis the main stay of biotransformations is of immense importance in the drug industry, with an estimated market of about US$100 billion worldwide (Stinson, 1999; Krishna, 2002). Moreover, the homochiral drug market is expected to grow even bigger, increasing from having 25% to accounting for 70% of the total fine chemical market during the 21st century (Schulze and Wubbolts, 1999). These numbers suggest the important role of biocatalysis to the chemical industry. In addition to stereospecific synthesis, biocatalysis is unique in that a drastic difference between the molecular weights of the catalyst and reaction products could become a basis for large-scale, cost-efficient separation of those compounds. Chemical reactions performed by microorganisms or catalyzed by enzymes are essentially the same as those carried out in conventional inorganic and organic chemistry. The most striking differences between enzymes and chemical catalysts are summarized in Table 4.1.