Enzyme-mediated chemical transformations

Scheme 9 Enzyme-mediated chemical transformations. (A) Enatioselective enzymatic oxidation and lactonization (B) enzyme reduction with baker s yeast and enantioselective rule and (C) enzymatic hydrolytic desymmetrization.

Pesticides are susceptible to a variety of transformations in the environment, including both chemical degradation and microbial metaboHsm. Microbial transformations are catalyzed exclusively by enzymes, whereas chemical transformations are mediated by a variety of organic and inorganic compounds. Many pesticide transformations can occur either chemically or biologically. Consequentiy, most pesticide dissipation studies include sterile treatments to... 

Another possible explanation for the limitations of catalytic antibodies raised against TSA can be found in the different accessibility of the active site. In the case of natural enzymes, it is that their catalytic machinery and bound substrates are often buried. This feature isolates from the solvent the reactive functionalities that mediate chemical transformations. On the contrary, in antibody catalysis, the moieties of the bound haptens that mimic the TS are often positioned near the entrance of the antibody-combining site. This disparity in the overall architecture of natural enzymes and catalytic antibodies is undoubtedly a factor in the lower catalytic... 

In the following sections we will explain some applications of enzymes (and cells) in the transformation of sterols and steroids. You should realise, however, that for each process a decision has to be made whether to use an enzyme-mediated transformation or to use a chemical reaction. In many instances the biotransformation process is foe most attractive but, as we will see later, this is not always the case. 

Several approaches have been undertaken to construct redox active polymermodified electrodes containing such rhodium complexes as mediators. Beley [70] and Cosnier [71] used the electropolymerization of pyrrole-linked rhodium complexes for their fixation at the electrode surface. An effective system for the formation of 1,4-NADH from NAD+ applied a poly-Rh(terpy-py)2 + (terpy = terpyridine py = pyrrole) modified reticulated vitreous carbon electrode [70]. In the presence of liver alcohol dehydrogenase as production enzyme, cyclohexanone was transformed to cyclohexanol with a turnover number of 113 in 31 h. However, the current efficiency was rather small. The films which are obtained by electropolymerization of the pyrrole-linked rhodium complexes do not swell. Therefore, the reaction between the substrate, for example NAD+, and the reduced redox catalyst mostly takes place at the film/solution interface. To obtain a water-swellable film, which allows the easy penetration of the substrate into the film and thus renders the reaction layer larger, we used a different approach. Water-soluble copolymers of substituted vinylbipyridine rhodium complexes with N-vinylpyrrolidone, like 11 and 12, were synthesized chemically and then fixed to the surface of a graphite electrode by /-irradiation. The polymer films obtained swell very well in aqueous... 

Now we consider situations in which transformation of the organic compound of interest does not cause growth of the microbial population. This may apply in many engineered laboratory and field situations (e.g., Semprini, 1997 Kim and Hao, 1999 Rittmann and McCarty, 2001). The rate of chemical removal in such cases may be controlled by the speed with which an enzyme catalyzes the chemical s structural change (e.g., steps 2, 3 and 4 in Fig. 17.1). This situation has been referred to as co-metabolism, when the relevant enzyme, intended to catalyze transformations of natural substances, also catalyzes the degradation of xenobiotic compounds due to its imperfect substrate specificity (Horvath, 1972 Alexander, 1981). Although the term, co-metabolism, may be used too broadly (Wackett, 1996), in this section we only consider instances in which enzyme-compound interactions limit the overall substrate s removal. Since enzyme-mediated kinetics were characterized long ago by Michaelis and Menten (Nelson and Cox, 2000), we will refer to such situations as Michaelis-Menten cases. 

Now we can see the types of biochemical factors that determine the rate constant, fcbio for Michaelis-Menten cases the ability of the enzyme to catalyze the transformation as reflected by the quotient, kE/KiMM, and the presence of enzyme in the microorganism population involved, as quantified by [Enz]tot/[B], In the following section, we develop some detailed kinetic expressions for one case of enzyme-mediated transformations. Examination of these results will help us to see how structural features of xenobiotic compounds may affect rates. Finally, we will improve our ability to understand the relative rates for structurally related chemicals that are transformed by the same mechanism and are limited at the same biodegradation step. 

Whether a toxin is naturally reactive to biological macromolecules or receptors, or requires metabolic activation to produce such a species, e.g. the enzyme-mediated transformation of tremorine (9) to the active parasympathomimetic agent oxotremorine (10), it will usually be subject to chemical or enzymic inactivation in vivo. Interruption of the latter process via appropriate substitution may thus lead to an increase in biological activity or toxicity over that of the parent compound. Perhaps the most striking example of this is provided by the extreme metabolic stability and toxicity of TCDD and the nontoxicity of its de- chloro analogue dibenzodioxin (Table 3). 

Enzymes, the catalysts of biological systems, are remarkable molecular devices that determine the patterns of chemical transformations. They also mediate the transformation of one form of energy into another. The most striking characteristics of enzymes are their catalytic power and specificity. Catalysis takes place at a particular site on the enzyme called the active site. Nearly all known enzymes are proteins. However, proteins do not have an absolute monopoly on catalysis the discovery of catalytically active RNA molecules provides compelling evidence that RNA was an early biocatalyst (Section 2.2.2). 

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