Enzymes industrial catalysts

This has been a brief overview of a rich field. Details of enzyme stracture and catalytic activity are studied in laboratories worldwide. Moreover, genetic engineering makes it possible to manufacture key enzymes in large quantities, so enzymes may become industrial catalysts that accomplish reactions rapidly and selectively. 

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. 

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. 

X-ray absorption spectroscopy is an exciting new tool, ideally suited to probing the immediate environment of a specific atom type in a physical, chemical or biological system. The advent of synchrotron radiation has transformed this technique from a topic of relatively minor interest to one of major scientific importance and activity " . A major attraction of the technique is the possibility it provides of probing a reaction centre in a wide range of materials ranging from an industrial catalyst to an enzyme the technique is not limited by the physical state of the sample. In this review, suitability of this technique for biochemical systems is discussed. 

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

A difficult problem in utilizing enzymes as catalysts for reactions in a non-cellular environment is their instability. Most enzymes readily denature and become inactive on heating, exposure to air, or in organic solvents. An expensive catalyst that can be used only for one batch is not likely to be economical in an industrial process. Ideally, a catalyst, be it an enzyme or other, should be easily separable from the reaction mixtures and indefinitely reusable. A promising approach to the separation problem is to use the technique of enzyme immobilization. This means that the enzyme is modified by making it insoluble in the reaction medium. If the enzyme is insoluble and still able to manifest its catalytic activity, it can be separated from the reaction medium with minimum loss and reused. Immobilization can be achieved by linking the enzyme covalently to a polymer matrix in the same general manner as is used in solid-phase peptide synthesis (Section 25-7D). 

Food Applications. A number of features make enzymes ideal catalysts for the food industry. They are all natural, efficient, and specific work under mild conditions have a high degree of purity and are available as standardized preparations. Because enzymatic reactions can be conducted at moderate temperatures and pH values, simple equipment can be used, and only few by-products are formed. Furthermore, enzymatic reactions are easily controlled and can be stopped when the desired degree of conversion is reached. 

In the example of the asymmetric epoxidation of olefins, enzymes, synthetic catalysts, and catalytic antibodies have been compared side-by-side with respect to performance in chemical synthesis (Jacobsen, 1994). Epoxidation of olefins is a reaction of considerable industrial interest where, historically, enzymes have not performed extremely well. One reason is the dependence of the enantiomeric purity of the diol and epoxide products on the regiospecificity of the attack on the racemic epoxide by a water molecule (Figure 20.1). 

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. 

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

Hundreds of examples of the successful use of enzymes in asymmetric catalysis have been reported, including numerous industrial processes (Collins et al. 1997 Breuer et al. 2004 Drauz and Waldmann 2002 Liese et al. 2006). Nevertheless, the traditional limitations of enzymes as catalysts in synthetic organic chemistry revolve around limited substrate acceptance, poor enantioseletivity in many cases and/or insufficient thermostability under operating conditions. Various approaches to solving these problems have been suggested, and numerous successful examples are known which add to the power of enzyme cataly-... 

In real applications (such as in food industries, which are presently the major users of technical grade enzymes), the cost of the support is often higher than that of the enzyme. If the enzyme is inactivated during use, it can be replaced if it is reversibly immobilized. In such a case, the stability of the enzyme does not limit the time period of usefulness of the support of the industrial catalyst. 

Thio Mo centers are implicated in the turnover of industrial catalysts and enzymes, but mononuclear thiomolybdenyl complexes are rare. Examples include [MoSTp X2], where X = OPh, OC6H4X-2 (X = SEt, Pr) X2 = catecholate, tdt, bdt. These species have octahedral structures with short Mo=S bonds (c mo-s = 2.13 A). 

The relationship between the hydration shell and folding is of some importance for the use of enzymes as catalysts for syntheses, especially in industrial reactors (Wong, 1989). Nonaqueous media are preferred or are necessary for some reactions. Enzymes appear to be active when partially hydrated, at low water activity. In some cases there is activity in nearly dry nonaqueous solvents (Klibanov, 1986 Zaks and Klibanov, 1984). Thus, one should expect that, generally, it will be possible to find nonaqueous conditions for a particular enzyme-catalyzed process. Knowledge of the hydration shell is important, of course, for other aspects of the design of enzyme catalysts or drugs. 

A catalyst is a substance that speeds up a reaction without being consumed itself. Just as virtually all vital biological reactions are assisted by enzymes (biological catalysts), almost all industrial processes also benefit from the use of catalysts. For example, the production of sulfuric acid uses vana-dium(V) oxide, and the Haber process uses a mixture of iron and iron oxide. 

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