Biocatalysis advantages

In order to broaden the field of biocatalysis in ionic liquids, other enzyme classes have also been screened. Of special interest are oxidoreductases for the enan-tioselective reduction of prochiral ketones [40]. Formate dehydrogenase from Candida boidinii was found to be stable and active in mixtures of [MMIM][MeS04] with buffer (Entry 12) [41]. So far, however, we have not been able to find an alcohol dehydrogenase that is active in the presence of ionic liquids in order to make use of another advantage of ionic liquids that they increase the solubility of hydrophobic compounds in aqueous systems. On addition of 40 % v/v of [MMIM][MeS04] to water, for example, the solubility of acetophenone is increased from 20 mmol to 200 mmol L ... 

Another advantage of biocatalysis is that chemo-, regio-, and stereoselectivities are attainable that are difficult or impossible to achieve by chemical means. A pertinent example is the production of the artificial sweetener, aspartame, which has become somewhat of an industrial commodity. The enzymatic process (Fig. 2.31), operated by the Holland Sweetener Company (a joint venture of DSM and Tosoh), is completely regio- and enantiospecific (Oyama, 1992). 

There are many advantages of two-phase systems over aqueous systems when they are used as biocatalysis media [21,22,26,27] components (substrates and/or products) can be... 

As proven in this review and other papers, organic-aqueous biphasic media have been useful in many areas of biocatalysis applications. We summarize the potential advantages in carrying out biocatalysis in biphasic systems ... 

It was reported that PEGylated lipase entrapped in PVA cryogel could be conveniently used in organic solvent biocatalysis [279], This method for enzyme immobilization is more convenient in comparison to other types of immobilization that take advantage of enzyme covalent linkage to insoluble matrix, since the chemical step which is time consuming and harmful to enzyme activity is avoided. The application of this catalytic system to the hydrolysis of acetoxycoumarins demonstrated the feasibility of proposed method in the hydrolysis products of pharmaceutical interest and to obtain regioselective enrichment of one of the two monodeacetylated derivatives. 

The second general approach is to use whole cells that contain the enzyme or enzymes used in the biocatalytic process. The use of whole cells has the added advantage that coenzyme-dependent enzymes can be used because it is possible to regenerate the relevant coenzyme, through metabolism of the whole cells. This, of course, requires that the whole cells are not only physically intact but also meta-bolically active. Since coenzymes are often involved in building new molecules, industrial biocatalysis typically uses whole-cell systems. 

In summary, the combination of enzymes is advantageous from an enzymol-ogy and reachon engineering point of view. Reaction yields can be increased by avoiding product inhibition of single enzymatic reachons. Product decomposihon (e.g. by hydrolysis) can be overcome by further enzymatic transformahons. Tedious isolation of intermediate products is not necessary. However, both strategies - combinatorial biocatalysis and combinatorial biosynthesis - have their disadvantages. The in vitro approach needs every enzyme to be produced by recombinant techniques and purified in high amounts, which is in some cases difficult to achieve. On the other hand, product isolation from a biotransformation with permeabilized or whole host cells can be tedious and results in low yields. 

In summary, the synthesis and in situ regeneration of nucleotide sugars by combinatorial biocatalysis suffers from the main disadvantage that each enzyme has to be produced in sufficient amounts. This affords efficient recombinant protein produchon hosts being a bottleneck for some genes [25]. However, once a multi-enzyme system has been developed, the productivity can be improved by repetitive use of the biocatalysts as demonstrated for repetitive batch syntheses with soluble enzymes [25, 38] or with immobilized enzymes [48]. The advantage... 

An ionic liquid can be used as a pure solvent or as a co-solvent. An enzyme-ionic liquid system can be operated in a single phase or in multiple phases. Although most research has focused on enzymatic catalysis in ionic liquids, application to whole cell systems has also been reported (272). Besides searches for an alternative non-volatile and polar media with reduced water and orgamc solvents for biocatalysis, significant attention has been paid to the dispersion of enzymes and microorganisms in ionic liquids so that repeated use of the expensive biocatalysts can be realized. Another incentive for biocatalysis in ionic liquid media is to take advantage of the tunability of the solvent properties of the ionic liquids to achieve improved catalytic performance. Because biocatalysts are applied predominantly at lower temperatures (occasionally exceeding 100°C), thermal stability limitations of ionic liquids are typically not a concern. Instead, the solvent properties are most critical to the performance of biocatalysts. 

Mention some of the advantages and disadvantages of using isolated enzymes rather than whole cells in biocatalysis. Is it always needed to purify enzymes for a biocatalytic application To what degree should one purify ... 

Bioreactions. The use of supercritical fluids, and in particular C02, as a reaction media for enzymatic catalysis is growing. High diffusivities, low surface tensions, solubility control, low toxicity, and minimal problems with solvent residues all make SCFs attractive. In addition, other advantages for using enzymes in SCFs instead of water include reactions where water is a product, which can be driven to completion increased solubilities of hydrophobic materials increased biomolecular thermostability and the potential to integrate both the reaction and separation bioprocesses into one step (98). There have been a number of biocatalysis reactions in SCFs reported (99—101). The use of lipases shows perhaps the most commercial promise, but there are a number of issues remaining unresolved, such as solvent—enzyme interactions and the influence of the reaction environment. A potential area for increased research is the synthesis of monodisperse biopolymers in supercritical fluids (102). 

Current Advantages and Drawbacks of Biocatalysis 1.1.2.1 Advantages of Biocatalysts... 

Whereas enzymes often feature great advantages in terms of selectivity, their stability is often insufficient. Additionally, long development times of new biocatalysts owing to an insufficient knowledge base of biocatalysis and biotechnology remain a problem and a challenge. 

Dehydrogenases are very valuable en2ymes in biocatalysis, although one of their challenges is the use of an additional cofactor, such as NAD(P)(H). There are several advantages to dealing only with dehydrogenases that are dependent on NAD(H) but not on NADP(H) ... 

There are many factors that influence the outcome of enzymatic reactions in carbon dioxide. These include enzyme activity, enzyme stability, temperature, pH, pressure, diffusional limitations of a two-phase heterogeneous mixture, solubility of enzyme and/or substrates, water content of the reaction system, and flow rate of carbon dioxide (continuous and semibatch reactions). It is important to understand the aspects that control and limit biocatalysis in carbon dioxide if one wants to improve upon the process. This chapter serves as a brief introduction to enzyme chemistry in carbon dioxide. The advantages and disadvantages of running reactions in this medium, as well as the factors that influence reactions, are all presented. Many of the reactions studied in this area are summarized in a manner that is easy to read and referenced in Table 6.1. 

Biocatalysis is a rather special case, somewhere between homogeneous and heterogeneous catalysis. In most cases, the biocatalyst is an enzyme - a complex protein that catalyzes the reactions in living cells. Enzymes are extremely effident catalysts. An enzyme typically completes 1000 catalytic cycles in one second. Compared to this, conventional homogeneous and heterogeneous catalysts are slow and inefficient (100-10000 cydes per hour). Speed, however, is not the only advantage enzymes specialize in converting one specific reactant into another... 

Further advantages of biocatalysis over chemical catalysis include shorter synthesis routes and milder reaction conditions. Enzymatic reactions are not confined to in vivo systems - many enzymes are also available as isolated compounds which catalyze reactions in water and even in organic solvents [28]. Despite these advantages, the activity and stability of most wild-type enzymes do not meet the demands of industrial processes. Fortunately, modern protein engineering methods can be used to change enzyme properties and optimize desired characteristics. In Chapter 5 we will outline these optimization methods, including site-directed mutagenesis and directed evolution. 

For most enzymes, the CLEC is much more robust than the simple isolated enzyme. CLECs can withstand higher temperatures, they denature more slowly in organic solvents, and they are less susceptible to proteolysis [71]. Moreover, since there is no external support involved, CLECs exhibit a high volumetric productivity. These advantages, together with the tunable particle size (typically 1-100 pm), make CLECs attractive for industrial biocatalysis applications. 

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