Quinones formation, enzymatic

SCHEME 10.2 Common pathways of QM formation in biological systems, (a) Stepwise two-electron oxidation by cytochrome P450 or a peroxidase, (b) Enzymatic oxidation of a catechol followed by spontaneous isomerization of the resulting n-quinone. (c) Enzymatic hydrolysis of a phosphate ester followed by base-catalyzed elimination of a leaving group from the benzylic position. 

The catalytic cycle of laccase includes several one-electron transfers between a suitable substrate and the copper atoms, with the concomitant reduction of an oxygen molecule to water during the sequential oxidation of four substrate molecules [66]. With this mechanism, laccases generate phenoxy radicals that undergo non-enzymatic reactions [65]. Multiple reactions lead finally to polymerization, alkyl-aryl cleavage, quinone formation, C> -oxidation or demethoxylation of the phenolic reductant [67]. 

If this theory of the biogenesis of the morphine alkaloids is correct, the cell must be capable of preventing formation of an o-quinone on enzymatic oxidation of [xxxvi], and it seems probable that such an oxidation will only be achieved by the use of an enzyme system. 

Bolton, J. L. Valerio, L. G. Thompson, J. A. The enzymatic formation and chemical reactivity of quinone methides correlate with alkylphenol-induced toxicity in rat hepato-cytes. Chem. Res. Toxicol. 1992, 5, 816-822. 

Organic acids are added to some foods in order to prevent sedimentation or darkening thus, citric, malic, phosphoric, and ascorbic acid are used to reduce or delay enzymatic browning. Melanin formation following production of a quinone is a pH-dependent process (the rate of the former process increases with increasing pH) (8). Polyhydroxyacids such as citric and malic acid... 

Mitomycin C, 1, is a potent antitumor antibiotic discovered by Japanese scientists in fermentation cultures of Streptomyces caespitosus. It has been described as "small, fast and deadly (but very selective)" and has an extraordinary ability to crosslink the complementary strands of the DNA double helix with high efficiency and absolute specificity. It is so lethal that one crosslink per genome is sufficient to cause death of a bacterial cell. Mitomycin C, which is widely used clinically as an antitumor drug, does not react with DNA, but enzymatic reduction of the quinone induces a cascade of transformations which results, ultimately, in formation of the DNA crosslink 2. 

Substituted 2,6-dimethylphenols (325-327) underwent enzymatic oxidation with mushroom tyrosinase in 50% MeCN-phosphate buffer (pH 6.8) (room temp., 48-72 h) resulting in the formation of the corresponding optically active compounds (328-330) in 50-60% yields. It is noted that the intramolecular cyclization of the initially formed quinone methide will take place in the hole of the enzyme or very close to the surface of... 

Quinone ketals, fonnation of 1170, 1171 /j-Quinone methide radical 1158 Quinone methides 1217, 1282 dimerization of 1255, 1256 enzymatic formation of 1223 from biphenyl derivatives 1035-1038 from fluorenols 1038 intramolecular cyclization of 1220 reactions of 1308 synthesis of 1253, 1255 m-Quinone methides 1024... 

In some situations substrate-derived free radicals occur by non-enzymatic reactions following product formation. This phenomenon seems to be common in enzyme reactions involving either two-electron oxidation of a hydroquinone or two-electron reduction of a quinone. In each case a mixture of quinone and hydroquinone is produced which is in chemical equilibrium with semiquinone radicals. 

The rest of the malvidin-3-glucoside in Remy et al. could not be recovered by thiolysis, probably because it is in a flavene-type structure or there are additional linkages to the pigment such as vinyl condensation (9) (10). It has been reported that enzymatic oxidation of caffeic yields an o-quinone that will condense with malvidin-3-glucoside (11) and overall 520 nm absorbance decreases. Similarly, fIavan-3-ols could also react with anthocyanins through an oxidized catechol ring (non-enzymatic formation). Such products would have bonds between the tannin and anthocyanins that cannot be cleaved by mild acid. 

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