Other Enzymatic Transformations

A different mechanism was adopted in the biosynthesis of cyclic polyethers such as monensin. These PK-derived polycycles are formed in a cascade fashion with other enzymatic transformations. For example, in the biosynthesis of monensin (Equation 8.2), the cascade polyether formation is triggered by epoxidation of a polyene template [40]. Similar mechanisms can probably be used to make other polyether antibiotics containing tetrahydrofurans and tetrahydropyrans [41]. 

Drugs and other chemicals such as food additives or insecticides foreign to the body undergo enzymatic transformations that result in loss of pharmacological activity detoxification), or lead to the formation of metabolites with therapeutic or toxic effects bioactivation). 

The complete degradation of sulfamethoxazole was also reported within 14 days with P. chrysosporium, Bjerkandera sp. R1 and B. adusta [4], although, contrary to the reports of enzymatic transformation, metabolites were not identified. Partial removal (from 30% to 55%) of sulfamethoxazole from activated-sludge-mixed liquor and the effluent of a WWTP was demonstrated at bench scale within 5 days with P. chrysosporium propagules entrapped in a granular bioplastic formulation [25]. This approach was also successful in the partial elimination of other kinds of antibiotics, eg., ciprofloxacin (see below) and the macrolide erythromycin. 

RME shows particular promise in the recovery of proteins/enzymes [12-14]. In the past two decades, the potential of RME in the separation of biological macromolecules has been demonstrated [15-20]. RMs have also been used as media for hosting enzymatic reactions [21-23]. Martinek et al. [24] were the first to demonstrate the catalytic activity of a-chymotrypsin in RMs of bis (2-ethyl-hexyl) sodium sulfosuccinate (Aerosol-OT or AOT) in octane. Since then, many enzymes have been solubilized and studied for their activity in RMs. Other important applications of RME include tertiary oil recovery [25], extraction of metals from raw ores [26], and in drug delivery [27]. Application of RMs/mi-croemulsions/surfactant emulsions were recognized as a simple and highly effective method for enzyme immobilization for carrying out several enzymatic transformations [28-31]. Recently, Scheper and coworkers have provided a detailed account on the emulsion immobiUzed enzymes in an exhaustive review [32]. 

From these inventories and data, it is clear that society is facing an enormous problem of contamination. Many of the polluting compounds that are continuously dispersed are products of industrial activities such as phenols and halogenated phenols, polycyclic aromatic hydrocarbons (PAH s), endocrine disruptive chemicals (EDC), pesticides, dioxins, polychlorinated biphenyls (PCB s), industrial dyes, and other xenobiotics. In this chapter, we critically review the literature information on the enzymatic transformation of these polluting xenobiotics. This work is focused on peroxidases as enzymes able to transform a variety of pollutant compounds with the aim to reduce their toxicity and their environmental impact. 

A second example of an activated chemical defense concerns the Indo Pacific sponge Aplysinella rhax, in which tissue damage results in the rapid enzymatic transformation of psammaplin A sulfate 63 into psammaplin A 64 exposure of 63 to tissue from other sponges does not result in any conversion. Compound 63 deters feeding by reef fish, but when offered a choice between psammaplin A and its sulfate, both foods were avoided. In aquarium assays with C. solandri, extracts of damaged tissue were more deterrent than extracts from intact tissue, but both treatments were less palatable than control foods. In choice experiments, C. solandri preferred food treated with 63 over 64.104... 

Reactions proceed faster and more smoothly when the reactants are dissolved, because of diffusion. Although reactions in the solid state are known [1] they are often condensations in which a molecule of water is formed and reaction takes place in a thin film of water at the boundary of the two solid surfaces. Other examples include the formation of a liquid product from two solids, e.g. dimethylimidazolium chloride reacts with aluminum chloride to produce the ionic liquid, dimethylimidazolium tetrachloroaluminate [2]. It is worth noting, however, that not all of the reactant(s) have to be dissolved and reactions can often be readily performed with suspensions. Indeed, so-called sol-id-to-solid conversions, whereby a reactant is suspended in a solvent and the product precipitates, replacing the reactant, have become popular in enzymatic transformations [3]. In some cases, the solvent may be an excess of one of the reactants. In this case the reaction is often referred to as a solvolysis, or, when the reactant is water, hydrolysis. 

The disaccharide 6 is derived from sucrose by an enzymatic transformation catalyzed by Protaminobacter rubrum as well as other organisms [14,31],... 

Deprotection using enzymes can be quite useful. An added benefit is that a racemic or meso substrate can often be resolved with excellent enantioselec-tivity. Numerous examples of this process are described in the literature. Although acetates are the most common substrates in enzymatic reactions, other aliphatic esters have been examined with good success. Enzymatic transformations in nucleoside chemistry have been reviewed. ... 

A key feature of CuZnSOD is the maintenance of high dismutase activity while shielding the active site copper from other redox transformations [55]. However, it has been repeatedly suggested that apart from its main activity, CuZnSOD shows other enzymatic activities. One reason for these suggestions involved the role of CuZnSOD mutations in the pathogenesis of amyotrophic lateral sclerosis (see Sect. 1.9). 

In addition lipase-catalyzed ROP of lactones was successfully used to synthesize macromers by using hydroxyl moieties of carbohydrates as sites for initiation [68, 69, 88], Specifically, ethylglucopyranoside (EGP) was used as a multifunctional initiator and e-CL/trimethylene carbonate (TMC) as monomers for lipase-catalyzed ROPs. Initiation of ROP occurred selectively from the 6-hydroxyl position forming macromers with a carbohydrate head group with three remaining hydroxyl groups that remained available for other enzymatic or chemical transformations (Scheme 4.13). 

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