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Enzyme-catalyzed electrophilic aromatic

In Nature, methyl groups are selectively introduced into reactive aromatic rings by methyltransferases (Mtases), in particular with SAM as the cofactor. Furthermore, enzyme-catalyzed reactions are important for access to isoprenoids. SAM can act as an electrophile that transfers a methyl group to a specific nucleophilic atom. [Pg.13]

The high-valent iron-oxo sites of nonheme iron enzymes catalyze a variety of reactions (halogenation and hydroxylation of alkanes, desaturation and cyclization, electrophilic aromatic substitution, and cis-dihydroxylation of olefins) [lb]. Most of these (and other) reactions have also been achieved and studied with model systems [Ic, 2a-c]. With the bispidine complexes, we have primarily concentrated on olefin epoxidation and dihydroxylation, alkane hydroxylation and halogenation, and sulfoxidation and demethylation processes. The focus in these studies so far has been on a thorough analysis of the reaction mechanisms rather than the substrate scope and catalyst optimization. [Pg.132]

The second part of lanosterol biosynthesis is catalyzed by oxidosqualene lanosterol cyclase and occurs as shown in Figure 27.14. Squalene is folded by the enzyme into a conformation that aligns the various double bonds for undergoing a cascade of successive intramolecular electrophilic additions, followed by a series of hydride and methyl migrations. Except for the initial epoxide protonation/cyclization, the process is probably stepwise and appears to involve discrete carbocation intermediates that are stabilized by electrostatic interactions with electron-rich aromatic amino acids in the enzyme. [Pg.1085]

Covalent binding of chemical carcinogens to cellular macromolecules, DNA, RNA and protein, is wel1-accepted to be the first step in the tumor initiation process ( 1, 2). Most carcinogens, including polycyclic aromatic hydrocarbons (PAH), require metabolic activation to produce the ultimate electrophilic species which react with cellular macromolecules. Understanding the mechanisms of activation and the enzymes which catalyze them is critical to elucidating the tumor initiation process. [Pg.293]

Enzymatic halogenation catalyzed by haloperoxidases and perhydrolases involves the oxidation of halide ions to a halonium ion species which leads to the formation of hypohalous acids (Fig. 16.9-1). The products obtained by enzymatic halogenation with these enzymes are the same as the products obtained by chemical electrophilic halogenation with hypohalous acids. The differences in the para ortho ratios in the halogenation of some aromatic compounds could be due to a mixture of halogenation at or near the active site and in solution. [Pg.1277]

Polycyclic aromatic hydrocarbons (PAH) are mainly products of incomplete combustion and can be found at high concentrations in PM. PAHs require metabolic activation to electrophiles, catalyzed by various enzymes through free radical mechanisms, to exert their carcinogenic effects. Research found that there are... [Pg.412]

In nature, the alkylation of aromatic rings most commonly proceeds using 5-adenosyhnethionine ( SAM, 31a) as the source of an electrophilic methyl group. Such alkylations are catalyzed by C-methyltransferase enzymes and are analogous to Friedel-Crafts alkylations. Two C-methyl-transferases, NovO and CouO (from Streptomyces spheroides and Streptomyces rishiriensis, respectively), have been expressed in recombinant E. coli and have been shown to have a synthetically useful substrate scope. Not only are they able to catalyze the regioselective alkylation of various coumarins 32 (X=0), 2-quinolones 32 (X=NH), and naphthalenediols 34-36, but in addition, they are able to accept nonnatural cofactors 31b-f in place of SAM and so can transfer various alkyl groups other than methyl to the substrates (Scheme 32.4) [26]. [Pg.919]

Synthetic peptide dendrimers, catalytic antibodies, RNA catalysts, peptide foldamers as well as other native or modified enzymes with completely different fxmctions were discovered to catalyze carbon-carbon bond formation [15]. 4-Oxalocrotonate tau-tomerase (4-OT) catalyzes in vivo the conversion of 2-hydroxy-2,4-hexadienedioate (136) to 2-oxo-3-hexenedioate (137) (Scheme 10.33a), and it belongs to the catabolic pathway for aromatic hydrocarbons in P. putida mt-2 [200]. This enzyme carries a catalytic amino-terminal proline, which could act as catalyst in the same fashion as the proline mediated by organocatalytic reactions. Initial studies demonstrate that this enzyme was able to catalyze aldol condensations of acetaldehyde to a variety of electrophiles 138 (Scheme 10.33b) [200]. This enzyme was also examined as a potential catalyst for carbon-carbon bond forming Michael-type reactions of acetaldehyde to nitroolefins 139 (Scheme 10.33c) [201,202]. [Pg.293]


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Enzyme-catalyzed

Enzyme-catalyzed electrophilic aromatic substitution

Enzymes catalyze

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