Enzymes as industrial catalysts

The category Enzymes as industrial catalysts , including the starch-processing, antibiotics and the fine-chemicals industry, is covered in chapter 4. 

New enzymes for organic synthesis continue to represent a challenge both for those who prepare them as well as for those who finally apply them. The improved accessibility of enzymes as industrial catalysts has led to a large number of practical processes. The times are long past when it was believed that, at best, hydrolases would be suitable for industrial operations. Today, there are examples of practical applications for almost all enzyme classes. 

As the number of important catalyst families increased in the post World War II decades, researchers began to specialize in areas such as ammonia catalysts, reforming and cracking of petroleum fractions, Ziegler-Natta catalysts, zeolites, homogeneous catalysis, and use of enzymes as industrial catalysts. Creating a unified discipline of catalysis from all these fields continues to be challenge today as it was in the past. 

Recent progress in the production, isolation, and use of enzymes as industrial catalysts has been reported. ... 

The progress made in the use of immobilized enzymes as industrial catalysts has been discussed in detail. A number of natural and synthetic polymers have been selected for the immobilization of enzymes from a knowledge of the physical and chemical properties of the catalytically active protein. ... 

An intriguing influence of a cosolvent immiscible with water on the enantioselec-tivity of the enzyme-catalyzed hydrolysis was observed. It was proven that enzyme enantioselectivity is directly correlated with the cosolvent hydrophobicity. In the best example, for ethyl ether as cosolvent, the reaction proceeded with E = 55, and the target compound was obtained in 33% yield with 92.7% ee. This finding may be of great practical importance, particularly in industrial processes [24], since it will enable better optimization of enzyme-catalyzed processes. It is clear that, in future, immobilized enzymes, as heterogeneous catalysts, wiU be widely used in most industrial transformations, especially in the preparation of pharmaceuticals [25]. 

Archaea or Archaebacteria, which live in sulphurous waters around undersea volcanic vents. An extraordinarily stable enzyme which functions even at 135 °C and survives at pH 3.2-12.7 has been identified [142]. This enzyme has been termed STABLE (stalk-associated archaebacterial endoprotease). It is suggested that such exceptional stability may be attributable to unusually large Mr and tight folding of the protein chain. Suggested uses include washing powders and detergents, as well as industrial catalysts. It is even proposed that such remarkable properties may have contributed to the early evolution of life on earth [142]. 

Interest in immobilized enzyme derivatives stems primarily from our growing awareness of their potential as industrial catalysts and as a new type of model system for the investigation of isolated aspects of complex biological phenomena (l, 2, 3, y. 

Enzymes are used quite extensively as industrial catalysts. They offer the following advantages in comparison with chemical catalysts. 

The examples summarized here are just a first demonstration of the potential of single-molecule experiments for the analysis of enzyme-catalyzed reactions. However, they provide a clear perspective that single-molecule experiments will continue to contribute to our detailed understanding of enzymes. A more detailed understanding is not only of fundamental scientific importance, but will also provide the basis for the design of better enzymes and enzyme inhibitors for a broad range of biomedical and industrial applications. Furthermore, the concepts outlined here are generic and can be applied to other systems, such as industrial catalysts [47]. 

Enzymes are not widely used as catalysts in the fine chemicals industry. Among the reasons for this are their poor stability in aqueous environments and low efficiency as heterogeneous catalysts in liquids when compared with inorganic catalysts. However, the possibility of using enzymes as heterogeneous catalysts in supercritical media opens up new possibilities for chemical synthesis. 

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. 

Sweeney, M.D., Xu, F., 2012. Biomass converting enzymes as industrial biocatalysts for fuels and chemicals recent developments. Catalysts 2 (2), 244—263. 

Whole microbial cells as well as microbially derived enzymes have played a significant role in the production of novel antibiotics. The potential of microorganisms as chemical catalysts, however, was first fully realized in the synthesis of industrially important steroids. These reactions have assumed increasing importance following the discovery that certain steroids such as hydrocortisone have anti-inflammatory activity, whilst derivatives of the steroidal sex hormones are nsefiil as oral contraceptive agents. More recently, chiral inversion of non-steroidal anti-inflammatory dmgs (NS AIDs) has been demonstrated. 

Enzymes are generally classified into six groups. Table 1 shows typical polymers produced with catalysis by respective enzymes. The target macromolecules for the enzymatic polymerization have been polysaccharides, poly(amino acid)s, polyesters, polycarbonates, phenolic polymers, poly(aniline)s, vinyl polymers, etc. In the standpoint of potential industrial applications, this chapter deals with recent topics on enzymatic synthesis of polyesters and phenolic polymers by using enzymes as catalyst. 

An enzyme is a protein that speeds up a biochemical reaction without itself experiencing any overall change. In chemical language, such a compound is called a catalyst and is said to catalyze a reaction. Chemists employ a variety of compounds as laboratory catalysts, and many industrial chemical processes would be impracticably slow without catalysis. An automobile s catalytic converter makes use of a metal catalyst to accelerate conversion of toxic carbon monoxide in the exhaust to carbon dioxide. Similarly, our bodies biochemical machinery effects thousands of different reactions that would not proceed without enzymatic catalysis. Some enzymes are exquisitely specific, catalyzing only one particular reaction of a single compound. Many others have much less exacting requirements and consequently exhibit broader effects. Specific or nonspecific, enzymes can make reactions go many millions of times faster than they would without catalysis. 

From the practical viewpoint, enzyme-like synthetic catalysts, or syn-zymes, need not be specific for a given reactant structure. In nature enzymes distinguish among closely related molecules and transform only the substrate for which it is specific. Mixtures of molecules may not be involved in the industrial reaction to be catalyzed. Reaction specificity is, of course, a requirement. A synthetic hydrolase should not catalyze other reactions such as decarboxylation. Enzymes bring about rate enhancements of 10 -lO. A synzyme could be of great practical importance with far less efficiency than the natural enzyme if it is cheap and stable. In other words, a near miss in an attempt to mimic enzymes could be a fabulous success. 

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