Poly enzyme catalysts

Polyketide and fatty acid biosyntheses begin with condensation of the coenzyme A thioester of a short-chain carboxylic acid starter unit such as acetate or propionate with the coenzyme A thioester of a dicarboxylic acid extender unit such as malonate or methyl malonate. The driving force for the condensation is provided by the decarboxylation of the extender unit. In the case of fetty acid synthesis, the resulting -carbonyl is completely reduced to a methylene however, during the synthesis of complex poly-ketides, the -carbonyl may be left untouched or variably reduced to alcohol, olefinic, or methylene functionalities depending on the position that the extender unit will occupy in the final product. This cycle is repeated, and the number of elongation cycles is a characteristic of the enzyme catalyst. In polyketide biosynthesis, the full-length polyketide chain cyclizes in a specific manner, and is tailored by the action of additional enzymes in the pathway. 

Poly (p-nitrophenyl acrylate)-coated wide-pore glass (WPG) was also used as an activated carrier for the immobilization of biospecific ligands and enzymes, A detailed description of properties of these sorbents and catalysts as well as some specific features of their functioning is given in Sect. 6. 

Enzymes are generally classified into six groups. Table 1 shows typical polymers produced with catalysis by respective enzymes. The target macromolecules for the enzymatic polymerization have been polysaccharides, poly(amino acid)s, polyesters, polycarbonates, phenolic polymers, poly(aniline)s, vinyl polymers, etc. In the standpoint of potential industrial applications, this chapter deals with recent topics on enzymatic synthesis of polyesters and phenolic polymers by using enzymes as catalyst. 

Mutans streptococci are the major pathogenic organisms of dental caries in humans. The pathogenicity is closely related to production of extracellular, water-insoluble glucans from sucrose by glucosyltransferase and acid release from various fermentable sugars. Poly(catechin) obtained by HRP catalyst in a phosphate buffer (pH 6) markedly inhibited glucosyltransferase from Streptococcus sorbrinus 6715, whereas the inhibitory effect of catechin for this enzyme was very low. 

The conjugation of catechin on poly(allylamine) using ML as catalyst was examined under air. During the conjugation, the reaction mixture turned brown and a new peak at 430 nm was observed in the UV-vis spectrum. At pH 7, the reaction rate was the highest. The conjugation hardly occurred in the absence of laccase, indicating that the reaction proceeded via enzyme catalysis. 

Vorlop et al. described a novel cross-linked and subsequently poly(vinyl alcohol-entrapped PaHNL for synthesis of (//(-cyanohydrins. These immobilized lens-shaped biocatalysts have a well-defined macroscopic size in the millimeter range, show no catalyst leaching, and can be recycled efficiently. Furthermore, this immobilization method is cheap and the entrapped (/ )-oxynitrilases gave similar good results compared with those of free enzymes. The (//(-cyanohydrin was obtained in good yields and with high enantioselectivity of up to >99% ee [55],... 

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... 

Since the oxidative polymerization of phenols is the industrial process used to produce poly(phenyleneoxide)s (Scheme 4), the application of polymer catalysts may well be of interest. Furthermore, enzymic, oxidative polymerization of phenols is an important pathway in biosynthesis. For example, black pigment of animal kingdom "melanin" is the polymeric product of 2,6-dihydroxyindole which is the oxidative product of tyrosine, catalyzed by copper enzyme "tyrosinase". In plants "lignin" is the natural polymer of phenols, such as coniferyl alcohol 2 and sinapyl alcohol 3. Tyrosinase contains four Cu ions in cataly-tically active site which are considered to act cooperatively. These Cu ions are presumed to be surrounded by the non-polar apoprotein, and their reactivities in substitution and redox reactions are controlled by the environmental protein. 

Kirsh et al. 42) prepared apolar derivatives of poly(4-vinylpyridine) by benzylation. With nitrophenyl acetate as the substrate the benzylated catalyst is 100 times more effective than 4-ethylpyridine. A double-displacement mechanism was observed. The rate constants for deacylation of the acylpoly(vinylpyridine) derivatives were about 4 x 10" /sec. The comparable value for a-chymotrypsin is 8 x 10 /sec. The factor of 20 seems small, but it should be kept in mind that deacetylation of a-chymotrypsin is very slow compared with the deacylation reactions involving the natural substrates of the enzyme. 

Interestingly, enzymes are chiral catalysts and their potential for enantio-selective polymerization has been investigated [93]. Several examples are reported where a racemic mixture of lactones is polymerized by enzymatic polymerization to afford the corresponding optically active polyester [93]. For instance, lipase CA (Novozym 435) catalyses the ROP of racemic 4-methyl-s-caprolactone and 4-ethyl-s-caprolactone in bulk at 45 °C and 60 °C to afford (S )-eiuiched poly(4-methyl-e-caprolactone) and poly(4-ethyl- -caprolactone) with an enantiomeric purity higher than 95% [153]. 

By taking advantage of the simultaneous enzyme inhibition by nickel, the nickel-catalyzed ATRP, and the stereoselectivity of the enzyme, Peters et al. obtained chiral block copolymers by this method from 4-methyl-e-caprolactone (4-MeCL) by [27], The polymerization of racemic 4-MeCL showed good enantioselectivity and produced a chiral macroinitiator with ATRP endgroup by selectively polymerizing only the (5 )-4-MeCL. Macroinitiation was then started by adding the nickel catalyst and methyl methacrylate (MMA) to the reaction mixture, which simultaneously inhibited the enzyme and activated the ATRP process. Chiral poly[MMA-fe-(5 )-4-MeCL] was successfully obtained in this synthesis. 

This at once raises the question Could RNA ever have arisen without the help of enzymes, without replicases Experiments by Leslie Orgel and his co-workers12 suggest that this was possible. It was found that zinc ions, found today as cofactors in all replicases, are excellent catalysts for the 3 -5 union of nucleotides, thus allowing the template-instructed synthesis of polymers. This was first demonstrated with poly-C as template. If activated G and A nucleotides are offered, in equal concentration, then G is preferentially incorporated into the product by a factor, depending on reaction conditions, between 30 and 200. 

Copper-catalyzed oxidations of phenols by dioxygen have attracted considerable interest owing to their relevance to enzymic tyrosinases (which transform phenols into o-quinones equation 24) and laccases (which dimerize or polymerize diphenols),67 and owing to their importance for the synthesis of specialty polymers [poly(phenylene oxides)]599 and fine chemicals (p-benzoquinones, muconic acid). A wide variety of oxidative transformations of phenols can be accomplished in the presence of copper complexes, depending on the reaction conditions, the phenol substituents and the copper catalyst.56... 

Poly(amino acids) are attractive enzyme models because of their structural similarity. In fact, characteristic pH dependences of the cataljrtic rate were found and this was considered to reflect the conformational peculiarity of poly(amino acids). Unfortur nately, rate enhancements are only moderate and characterization of the catal3rtic site is difficult. Interesting results were obtained in the catalysis of oligopeptides, which supposedly mimic the active site of some hydrolytic enzymes. The stereoselectivity seems to be realized with oligopeptides more easily than with vinyl polymer catalysts. 

In relation to enzymic cytochrome P-450 oxidations, catalysis by iron porphyrins has inspired many recent studies. The use of CgFslO as oxidant and Fe(TDCPP)Cl as catalyst has resulted in a major improvement in both the yields and the turnover numbers of the epoxidation of alkenes. The Michaelis-Menten kinetic rate, the higher reactivity of alkyl-substituted alkenes compared to that of aryl-substituted alkenes, and the strong inhibition by norbornene in competitive epoxidations suggested that the mechanism shown in Scheme 13 is heterolytic and presumably involves the reversible formation of a four-mernbered Fe -oxametallacyclobutane intermediate. Picket-fence porphyrin (TPiVPP)FeCl-imidazole, O2 and [H2+colloidal Pt supported on poly(vinylpyrrolidone)] act as an artificial P-450 system in the epoxidation of alkenes. ... 

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