Advantages of Biocatalysts

Typically the rates of enzyme-mediated processes are faster by a factor of 10 -10 ° than those of the corresponding noncatalyzed reactions, - in some cases even exceeding a factor of lO, and are thus far above the values that chemical catalysts are capable of achieving [12-14], As a consequence, chemical catalysts are generally employed in concentrations of a mole percentage of 0.1-1%, whereas most enzymatic reactions can be performed at reasonable rates with a mole percentage of 10 -10 % of catalyst, which clearly makes them more effective by some orders of magnitude (Table 1.1). 

Unlike heavy metals, for instance, biocatalysts are environmentally benign reagents since they are completely biodegradable. 

Enzymes act within a range of about pH 5-8 (typically around pH 7) and in a temperature range of 20 0°C (preferably at around 30°C). This minimizes problems of undesired side-reactions such as decomposition, isomerization, racemization, and rearrangement, which often plague traditional methodology. 

Since enzymes generally function under the same or similar conditions, several biocatalytic reactions can be carried out in a reaction cascade in a single flask. Thus, sequential reactions are feasible by using multienzyme systems in order to simplify reaction processes, in particular if the isolation of an unstable intermediate can be omitted. Furthermore, an unfavorable equilibrium can be shifted towards the desired product by linking consecutive enzymatic steps. This unique potential of enzymes is increasingly being recognized as documented by the development of multienzyme systems, also denoted as artificial metabolism [15]. 

They exhibit a high substrate tolerance by accepting a large variety of man-made nonnatural substances and often they are not required to work in water. If advantageous for a process, the aqueous medium can often be replaced by an 

Current Advantages and Drawbacks of Biocatalysis 1.1.2.1 Advantages of Biocatalysts 

Biocatalysts are able to catalyze an increasing breadth of reactions (see Table 1.4). This breadth translates into an increasing number of applications of biocatalysts on [and] industrial scale (Liese, 1999, 2000 Zaks, 2001 Straathof, 2002). Increasingly, biocatalysts are combined with chemical catalysts (Chapter 18) or utilized in a network of reactions in the cell ( metabolic engineering , Chapters 15 and 20). 

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Evidence for the Perceived Advantages of Biocatalysts in Organic Media 341... 

While an [S]/[C] ratio serves as a good dimension of merit for the productivity of one batch, it does not yield any information on the stability of the catalyst over its lifetime. As already discussed in Chapter 19, Section 19.3.4, the homogeneous catalysis community typically does not feel the need to recycle catalysts (Blaser, 2001), so the turnover number (TON) equals the total turnover number (TTN). The true utility and productivity advantage of biocatalysts is captured upon reuse of the catalyst, achieving catalyst lifetime productivities far in excess of catalysts used only once. 

One of the most important advantages of biocatalysts is their stereospedficity. An example of a stereospedfic biotransformation is given in Figure 2.3. In 1992 the Food and Drug Administration (FDA) in the United States addressed the issue of whether it mattered, for drug approval purposes, that a preparation of a synthetic chiral molecule (one or more as)rmmetric centers) contains not one but two compounds (racemates) -the two mirror-image stereoisomers (individual enantiomers). 

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