Industrial Processes with Biocatalysts

Biocatalytic processes and technologies are penetrating increasingly in all branches of the chemical process industries. In basic chemicals, nitrile hydratase and nitri-lases have been most successful. For example, acrylamide from acrylonitrile is now a 30,000 t/a process. In fine chemicals, enantiomerically pure amino acids are produces by several different companies. 

The food industry is also a large area for biocatalysis applications high-fructose com syrap (HFCS) from glucose with glucose isomerase, the thermolysin-catalyzed synthesis of the artificial sweetener Aspartame , and synthesis of nutraeeutieals such as L-carnitine can serve as examples. 

Enzymatic processes are important in the areas of erop protection and pharma intermediates too. Technical improvements can result directly from immobilization (e. g. increased product purity and/or yield, reduced waste production) but also indirectly. Immobilization of cells or enzymes enables the use of eontinuous rather than batch operation, thus simplifying process control and reducing labor costs. Of course, this is only advantageous for large-scale processes, whereas most bioproducts are only produced on a small scale. 

Immobilized enzymes are mainly used in the production of fine chemicals and pharmaceuticals, because currently they cannot compete economically with conventional catalysts in the bulk chemical industry. Here, we will focus only on some examples from the following areas basic chemicals, fine chemicals, food industry and crop protection intermediates. 

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Numerous other amino acid decarboxylases have been isolated and characterized, and much interest has been shown as a result of the irreversible nature of the reaction with the release of C02 as the thermodynamic driving force. Although these enzymes have narrow substrate-specificity profiles, their utility has been widely demonstrated. Additional industrial processes will continue to be developed once other decarboxylases become available. Such biocatalysts would include the aromatic amino acid (E.C. 4.1.1.28), phenylalanine (E.C. 4.1.1.53) and tyrosine (E.C. 4.1.1.25) decarboxylases, which likely could be used to produce derivatives of their respective substrates. These derivatives are finding increased use in the development of peptidomimetic drugs and as possible positron emission tomography imaging agents.267-268... 

Although the interest of scientists in peroxidase enzymes has increased tremendously during the past decades, the application of these enzymes as biocatalysts in industrial processes is still negligible. Often the low activity and the fragile nature of these enzymes make their use challenging and sometimes results in poor productivities. Different aspects including heme deactivation (Chap. 12), redox potential modulation (Chap. 4), protein denaturation, and substrate availability have to be dealt with. 

In other words, biocatalysis should be not considered by itself but integrated in the whole bio-process (Figure 2.16a). Substantially increased emphasis on biocatalyst development is an important goal for chemistry-related industries, even if biocatalysts carmot reach their potential without a concerted eflfort on the parallel development of other components of bioprocessing, as well as an integration with... 

The two-stage biocatalytic reaction can be performed in a single reactor [14], but the separation of the two reactions is preferred because of different reaction parameters (e.g., pH value, temperature, oxygen) and stability of the enzymes used. With water as the solvent and enzymes fixed on a carrier, the process runs in a repeated batch mode at room temperature (20-30°C). Higher temperatures lead to increased reaction rates, but also to higher byproduct formation and reduced stability of the biocatalysts. A pH value between 7.0 and 8.5 is recommended with respect to thermodynamics, enzyme activities and stability and formation of byproducts. The use of cells is not recommended with respect to operational stability and possible product contamination. Therefore purified enzymes covalently immobilized on a polymeric carrier are chosen for the industrial process for both steps. The particle diameter of the spherical biocatalyst is about 100-300 pm, to allow for acceptable mass transfer and filtration times. 

Many of today s large-scale applications use immobilized enzymes. In particular, the Japanese industry has for a long time pioneered this sector of biotechnology. In the multi-author publication edited by Tanaka, Tosa and Kobayashi [90] and in a more recent review [89], a number of well-established continuous production processes with immobilized biocatalysts are described in detail (for a selection, see below). Complementary information on some of these Japanese bioprocesses as well as additional case studies for other bioprocesses has been compiled by Cheetham [170],... 

Enzymes have been naturally tailored to perform under physiological conditions. However, biocatalysis refers to the use of enzymes as process catalysts under artificial conditions (in vitro), so that a major challenge in biocatalysis is to transform these physiological catalysts into process catalysts able to perform under the usually tough reaction conditions of an industrial process. Enzyme catalysts (biocatalysts), as any catalyst, act by reducing the energy barrier of the biochemical reactions, without being altered as a consequence of the reaction they promote. However, enzymes display quite distinct properties when compared with chemical catalysts most of these properties are a consequence of their complex molecular stracture and will be analyzed in section 1.2. Potentials and drawbacks of enzymes as process catalysts are summarized in Table 1.1. 

Most industrial enzymatic processes refer to reactions conducted by hydrolases in aqueous medium for the degradation of complex molecules (often polymers) into simpler molecules in conventional processes with limited added value (Neidelman 1991). Reasons underlying are clear since hydrolases are robust, usually extracellular and have no coenzyme requirements, which makes them ideal process biocatalysts. Enzyme immobilization widened the scope of application allowing less stable, intracellular and non-hydrolytic enzymes to be developed as process biocatalysts (Poulsen 1984 D Souza 1999), as illustrated by the paradigmatic case of glucose isomerase for the production of HFS (Carasik and Carroll 1983) and the production of acrylamide from acrylonitrile by nitrile hydratase (Yamada and Kobayashi 1996). 

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