Biocatalysts

In this thiamine pyrophosphate-mediated process, ben2aldehyde (29), added to fermenting yeast, reacts with acetaldehyde (qv) (30), generated from glucose by the biocatalyst, to yield (R)-l-phen5l-l-hydroxy-2-propanone (31). The en2ymatically induced chiral center of (31) helps in the asymmetric reductive (chemical) condensation with methylamine to yield (lR,23)-ephedrine [299-42-3] (32). Substituted ben2aldehyde derivatives react in the same manner (80). 

Deamination, Transamination. Two kiads of deamination that have been observed are hydrolytic, eg, the conversion of L-tyrosiae to 4-hydroxyphenyUactic acid ia 90% yield (86), and oxidative (12,87,88), eg, isoguanine to xanthine and formycia A to formycia B. Transaminases have been developed as biocatalysts for the synthetic production of chiral amines and the resolution of racemic amines (89). The reaction possibiUties are illustrated for the stereospecific synthesis of (T)-a-phenylethylamine [98-84-0] (ee of 99%) (40) from (41) by an (5)-aminotransferase or by the resolution of the racemic amine (42) by an (R)-aminotransferase. 

The abihty of iron to exist in two stable oxidation states, ie, the ferrous, Fe ", and ferric, Fe ", states in aqueous solutions, is important to the role of iron as a biocatalyst (79) (see Iron compounds). Although the cytochromes of the electron-transport chain contain porphyrins like hemoglobin and myoglobin, the iron ions therein are involved in oxidation—reduction reactions (78). Catalase is a tetramer containing four atoms of iron peroxidase is a monomer having one atom of iron. The iron in these enzymes also undergoes oxidation and reduction (80). 

End Point vs Kinetic Methods. Samples may be assayed for enzymes, ie, biocatalysts, and for other substances, all of which are referred to as substrates. The assay reactions for substrates and enzymes differ in that substrates themselves are converted into some detectable product, whereas enzymes are detected indirectly through their conversion of a starting reagent A into a product B. The corresponding reaction curves, or plots of detector response vs time, differ for these two reaction systems, as shown in Eigure 2. Eigure 2a illustrates a typical substrate reaction curve Eigure 2b shows a typical enzyme reaction curve (see Enzyme applications). 

Impressive developments in the area of immunology have culminated in the development of catalytic antibodies (186,187). These synthetic biocatalysts that have the potential to catalyze virtually any type of reaction with unsurpassed selectivity have great promise in the future. 

It is apparent that the use of enzymatic catalysis continues to grow Greater availabiUty of enzymes, development of new methodologies for thek utilization, investigation of enzymatic behavior in nonconventional environments, and the design and synthesis of new biocatalysts with altered selectivity and increased stabiUty are essential for the successhil development of this field. As more is learned about selectivity of enzymes toward unnatural substrates, the choice of an enzyme for a particular transformation will become easier to predict. It should simplify a search for an appropriate catalyst and help to estabhsh biocatalytic procedures as a usehil supplement to classical organic synthesis. 

Biocatalyst An enzyme tliat plays a fundamental role in living organisms or in industry by activating or accelerating a bioprocess. 

Bioconversion Chemical conversion of a naturally occurring biodegradable substance using a biocatalyst. 

Biooxidation Oxidation (loss of electrons) process accelerated by a biocatalyst. 

The use of biocatalysts for the selective introduction and cleavage of esters is vast and has been extensively reviewed." Therefore only a few examples of the types of transformations that are encountered in this area of protective group chemistry will be illustrated to show some of the basic transformations that have appeared in the literature. The selective... 

Pathways and biocatalysts of bacterial degradation quinolines 98AG(E)577. 

In this chapter, we try to summarize the work so far reported in this field. We first give a short introduction into the different forms of biocatalytic reactions, highlighting some special properties of biocatalysts. 

Biocatalysts in nature tend to be optimized to perform best in aqueous environments, at neutral pH, temperatures below 40 °C, and at low osmotic pressure. These conditions are sometimes in conflict with the need of the chemist or process engineer to optimize a reaction with respect to space-time yield or high product concentration in order to facilitate downstream processing. Furthermore, enzymes and whole cells are often inhibited by products or substrates. This might be overcome by the use of continuously operated stirred tank reactors, fed-batch reactors, or reactors with in situ product removal [14, 15]. The addition of organic solvents to increase the solubility of substrates and/or products is a common practice [16]. 

When starting our first experiments with available ionic liquids, in screening programs to identify suitable systems, we encountered several difficulties such as pH shifts or precipitation. More generally, the following aspects should be taken into account when ionic liquids are used with biocatalysts ... 

For kinetic investigations and for activity measurements, either photometric assays or - because of the higher complexity of the reactants converted by biocatalysts - HPEC methods can often be used. Here the ionic liquid itself or impurities may interfere with the analytical method. 

When ionic liquids are used as replacements for organic solvents in processes with nonvolatile products, downstream processing may become complicated. This may apply to many biotransformations in which the better selectivity of the biocatalyst is used to transform more complex molecules. In such cases, product isolation can be achieved by, for example, extraction with supercritical CO2 [50]. Recently, membrane processes such as pervaporation and nanofiltration have been used. The use of pervaporation for less volatile compounds such as phenylethanol has been reported by Crespo and co-workers [51]. We have developed a separation process based on nanofiltration [52, 53] which is especially well suited for isolation of nonvolatile compounds such as carbohydrates or charged compounds. It may also be used for easy recovery and/or purification of ionic liquids. 

Identify which of the following statements are true for immobilised biocatalysts, when compared to free enzyme or free cell systems. 

Biotechnology has attracted enormous interest and high expectations over the past decade. However, the implementation of new technologies into industrial processes has been slower than initially predicted. Although biocatalytic methods hold great industrial potential, there are relatively few commercial applications of biocatalysts in organic chemical synthesis. The main factors that limit the application of biocatalysts are ... 

The disadvantages of biocatalysts will now be considered, followed by their advantages. You should note that the pros and cons of biotechnology versus chemical synthesis are very general and that exceptions may exist. 

Apart from economic considerations, the inherent disadvantages of biocatalysts have also limited the transformation of new technologies into industrial processes. Table 2.3 lists the major drawbacks of bioprocesses. 

Need for purified substrates and water to avoid poisoning of biocatalysts. 

When compared to traditional chemical synthesis, processes based on biocatalysts are generally less reliable. This is due, in part, to the fact that biological systems are inherently complex. In bioprocesses involving whole cells, it is essential to use the same strain from the same culture collection to minimise problems of reproducibility. If cell free enzymes are used the reliability can depend on the purity of the enzyme preparation, for example iso-enzyme composition or the presence of other proteins. It is, therefore, important to consider the commercial source of the enzyme and the precise specifications of the biocatalyst employed. 

The specificity of biocatalysts also extends to site specificity (regiospecificity). This means that if several functional groups of one type are present on the molecule, only one specific position will be affected. An example of this is the microbial oxidation of D-soibitol to L-soibose, a key step in the synthesis of vitamin C (Figure 2.4). 

The high specifity of biocatalysts also has the advantage of reducing disposal costs (pollution control costs) because relatively few useless and potential harmful byproducts are generated. In addition, waste products that might be produced are, by their very nature, likely to be biodegradable and, therefore, less environmentally damaging compared to those produced in purely chemical synthetic processes. 

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