Alternatively, the transformation of organic compounds into all kind of industrial products can also be performed by biocatalysis. Biocatalysis can be defined as the use of biolo­gical systems (including whole cells or isolated components thereof, natural and modi­fied enzymes and catalytic antibodies) to perform chemical transformations on organic compounds [31-33]. Millions of years of evolution have created thousands of microorgan­isms containing enzymes known to catalyze almost every chemical reaction. Biocatalysis is increasingly used in the chemical industry and has developed into a main contributor for sustainable chemistry. Biocatalytic reactions are typically performed at normal tempe­ratures and pressures, whereby no dangerous intermediate products are needed, nor are dangerous waste products generated. Biocatalysts, substrates, intermediates, by-products and the product itself are biodegradable. Water is usually used as a solvent. The use of enzymes as biocatalysts can have significant performance benefits compared to conven­tional chemical technology; for example, a high reaction selectivity, higher reaction rate, increased conversion efficiency, improved product purity, lowered energy consumption and a significant decrease in chemical waste generation. There are also frequent disadvantages, however, including difficult enzyme recovery, low product concentration, low productivity due to substrate and/or product inhibition and, hence, high recovery costs [34]. An impor­tant route to improving the performance of enzymes in non-natural environments and their ability to work in continuous processes is to immobilize them by either adsorption, covalent attachment or by incorporation in hydrophobic organic-inorganic hybrid materials [35-38].

As they are typically very selective (contrary to most conventional chemical catalysts), enzymes are particularly useful to produce, for example, chiral molecules or enantiomeri — cally pure compounds [39]. Biocatalysts can be used to initiate major chemical reactions, such as the direct polymerization of phenols, the direct oxidation of propylene, or highly selective transformations with polyfunctional substrates, such as sugars [34]. A wide vari­ety of chemical substances are already industrially produced through the use of enzymes [40]. Numerous syntheses are conducted exclusively using enzymes (lipases, amylases, pro­teases, and, also, increasingly, cellulases [34]). As now all the molecular and biological tools to make enzymes more stable and even to discover more stable enzymes are available, more bulk chemical products from enzymatic processes can be expected in the coming years [41].