Polyphenol substrate specificity

Moore NL, Mariam DH, Williams AL, Dashek WV. Substrate specificity, de novo synthesis and partial purification of polyphenol oxidase derived from the wood-decay fungus, Coriolus versicolor. J Indust Microbiol 1989 4 349-364. 

Laccases have very broad substrate specificities. Simple diphenols like hydroquinone and catechol, polyphenols, diamines, and aromatic amines are good substrates for most laccases. The fiill range of compounds oxidised by laccases is not yet known. Due to the discovery of the mediator concept in... 

Laccase. A polyphenol oxidase has been purified from the sap of the lac tree by Keilin and Mann. Laccase differs from the potato and mushroom enzyme in several respects. With regard to substrate specificity, it oxidizes p-phenylenediamine more rapidly than catechol. p-Phenylene-diamine is not a substrate for the other polyphenol oxidases described. Laccase apparently is inert with p-cresol. It is not inhibited by carbon monoxide. Unlike the other phenol oxidases, this enzyme is not a pale yellow, but is blue, as is ascorbic acid oxidase (see below). This enzyme, however, is not an ascorbic acid oxidase. 

Phenolic compounds are present separately from the substrates, predominantly located in the cell vacuoles. Particularly prominent substrates are caffeic acid and its esters, as well as some flavonoid substances, of which monomeric flavan-3-ols (catechins) are the most important. Other groups of phenolic compounds, such as condensed forms of flavan-3-ols and flavan-3,4-diols (tannins), flavonols, flavones, flavans, chalcones, dihydrochalcones and anthocyanins, are only partly oxidised. One of the reasons for this is probably the steric hindrance caused by the corresponding glycosides and substrate specificity of polyphenol oxidases. The content of phenohc compounds depends on genetic factors (on plant species and varieties), the degree of maturity and external (environmental) factors (hght, temperature, nutrients, use of pesticides and so on). The only substrate in animal tissues is the amino acid tyrosine. 

Miller A.R., Kelley,T.J., Mujer C.V. Anodic peroxidase isoenzymes and polyphenol oxidase activity from cucumber fruit tissue and substrate specifity. Phytochemistry, 29 705-709 (1990). 

The low specificity of electron-donating substrates is remarkable for laccases. These enzymes have high redox potential, making them able to oxidize a broad range of aromatic compounds (e.g. phenols, polyphenols, methoxy-substituted phenols, aromatic amines, benzenethiols) through the use of oxygen as electron acceptor. Other enzymatic reactions they catalyze include decarboxylations and demethylations [66]. 

The substrates of the polyphenol oxidase enzymes are phenolic compounds present in plant tissues, mainly flavonoids. These include catechins, anthocyanidins, leucoantho-cyanidins, flavonols, and cinnamic acid derivatives. Polyphenol oxidases from different sources show distinct differences in their activity for different substrates. Some specific examples of polyphenolase substrates are chlorogenic acid, caffeic acid, dicatechol, protocatechuic acid, tyrosine, catechol, di-hydroxyphenylalanine, pyrogallol, and catechins. 

Antioxidants, such as tea polyphenols, curcumin, and, camosol, may be expected to work by increasing intracellular GSH or total 011018, scavenging free radicals, or iron chelations. Many of the compounds classified as antioxidants that have been shown to inhibit NFkB activation are phytopolyphenols that are good peroxidase inhibitors or substrates. Many are effective at concentration that may be too low to be compatible with a radical-scavenging role, and their effects might be better explained if they were acting on a more specific enzymatic process. 

In Cu compounds of high covalence no precise decision can be reached regarding the oxidation state of the copper (53), while in ionic systems reference to the occupancy of the 3d subshell is sufficient. Thus, the oxidation states given in I—IY are of merely formal nature. The interesting phenomenon in this scheme is the redox mesomerism of Cu which implies that the chelated metal ion could have different biochemical actions. Type I would represent the reversible oxygenation as found in haemo-cyanin, type II would be the superoxide dismutation, provided OI-really is the substrate, and types III and IV are represented by the catalatic and oxidative action displayed by a considerable number of copper proteins (polyphenol oxidases, amine oxidases etc.). The biochemical specificity of each chelated copper is more of less given by the macromolecular ligands. 

One reservation must, however, be made, that is, as to whether the particular member is competent to act as a substrate for the polyphenol-oxidases present. We have already (p. 268) seen that the 3-glycosides of quercetin and myricetin are not attacked by the tea polyphenolase system, whereas the aglycones themselves are readily attacked. It may prove to be important, in any tissue, whether the specific glycosides present are attacked by the enzyme present, or introduced from other sources, as in products of mixed origin. Such considerations as these undoubtedly operate in the manufacture of such products as cider and perry. 

Catechol oxidases are widespread in nature. They are named according to their most important substrates as monophenol oxidases, polyphenol oxidases, phenolases, DOPA oxidases, cresolases, tyrosinases, etc. The specificity of most catechol oxidases is rather broad. 

A new dimeric plant tannin named woodfruticosin A, which is active as an inhibitor of DNA topoisomerase II, has been isolated from a Nepalese medicinal plant. Chelation with the ester groups at 0-1 and 0-6 of penta-O-galloyl- -D-glucose has been suggested as the mode of its complexation with various aromatic substrates (e.g., caffeine, daunomycin, methylene blue). -Pentagalloylglucose has also been reported to specifically inhibit two NADH reductases from E. coli. Two-dimensional H- and C-n.m.r. studies of natural polyphenols, with emphasis on the ester linkages between the phenolic adds and... 

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