Polyphenols, oxidoreductases

Polyphenol oxidase (PPO) (EC 1.14.18.1 monophenol monooxygenase [tyrosinase] or EC 1.10.3.2 0-diphenol 02-oxidoreductase) is one of the more important enzymes involved in the formation of black tea polyphenols. The enzyme is a metallo-protein thought to contain a binudear copper active site. The substance PPO is an oligomeric particulate protein thought to be bound to the plant membranes. The bound form of the enzyme is latent and activation is likely to be dependent upon solubilization of the protein (35). PPO is distributed throughout the plant (35) and is localized within in the mitochondria (36), the cholorplasts (37), and the peroxisomes (38). Using antibody techniques, polyphenol oxidase activity has also been localized in the epidermis palisade cells (39). Reviews on the subject of PPO are available (40—42). 

An example of a pharmaceutical synthesis reaction involving an oxidoreductase is the synthesis of 3,4-dihydroxylphenyl alanine (DOPA). 3,4-Dihy-droxylphenyl alanine is a chemical used in the treatment of Parkinson s disease. The industrial process that synthesizes DOPA utilizes the oxidoreductase polyphenol oxidase. As shown in Fig. 3, the monohydroxy compound is oxidized by the regio-specific addition of a hydroxyl group. It is worth mentioning that epinephrine (adrenaline) can also be synthesized by a similar reaction path using the same enzyme. 

So far, several oxidoreductases, peroxidase, laccase, polyphenol oxidase (tyrosinase), and so on have been reported to catalyze oxidative polymerization of phenol derivatives among which peroxidase is most often used [165,166]. Peroxidase is an enzyme that catalyzes the oxidation of a donor to an oxidized donor by the action of hydrogen peroxide, liberating two water molecules. Horseradish peroxidase (HRP) is a single-chain p-type hemoprotein that catalyzes the decomposition of hydrogen peroxide at the expense of aromatic proton donors. 

In living cells, various oxidoreductases play an important role in maintaining the metabolism of living systems. Most of oxidoreductases contain low valent metals as their catalytic center. In vitro enzymatic oxidoreductions have afforded functional organic materials. Some oxidoreductases such as peroxidase, laccase, and polyphenol oxidase have received much attention as catalysts for oxidative polymerizations of phenol derivatives to produce novel polyaromatics [1-10], This chapter deals with enzymatic oxidative polymerization of phenolic compounds. 

Oxidoreductases Transferases Polyphenols, polyanilines, vinyl polymers Polysaccharides, cyclic oligosaccharides, polyesters Polysaccharides, polyesters, polycarbonates, polyamides, polyphosphates, polythioesters... 

Polymer Modification by Oxidoreductases. Tyrosinase (polyphenol oxidase, a copper-containing monooxygenation enzyme) was used as catalyst for modification of chitosan. The enzymatic treatment of chitosan film in the presence of tyrosinase and phenol derivatives produced a new material of chitosan derivative (309). During the reaction, imstable o-quinones were formed, followed by the reaction with chitosan to give the modified chitosan. In the enzymatic treatment of p-cresol with a low concentration of chitosan (<1%), the reaction solution was converted into a gel (310). 

Polyphenol oxidase (monophenol, dihydroxyphenylalanine oxygen oxidoreductase, E.C. 1.14.18.1), often called tyrosinase, catalyzes two distinctly different chemical reactions. One reaction is the orthohydroxylation of monophenols the second reaction is deliy-drogenation of orthodihydroxyphenols. These reactions are shown in Equations 1 and 2. 

Thus far, oxidoreductases are mostly engaged in polymer synthesis in two major areas. First, they are employed for the polymerization of phenols and anilines. In the former case, the products include polyphenols (39) and polyCphenylene oxide) (40). In the latter case, wata -soIuble polyaniline polymers can be made (41). Secondly, oxidoreductases are employed for the free-radical polymerization of vinyl monomers (42). 

Polyphenol oxidase catalyzes two reactions first the hydroxylation of a monophenol to o-diphenol (EC 1.14.18.1, monophenol monooxygenase) followed by an oxidation to o-quinone (EC 1.10.3.1, o-diphenol oxygen oxidoreductase). Both activities are also known as cresolase and catecholase activity. At its active site, polyphenol oxidase contains two Cu ions with two histidine residues each in the ligand field. In an ordered mechanism (cf. 2.5.1.2.1) the enzyme first binds oxygen and later monophenol with participation of the intermediates shown in Fig. 2.8. The Cu ions change their valency (Cu Cu ). The newly formed complex ([] in Fig. 2.8) has a strongly polarized... 

Phenolic compounds known as polyphenols (such as phenohc acids and flavonoids) are easily oxidised by atmospheric oxygen in reactions catalysed by oxidoreductases (o-diphenokO, oxidoreduc-tases). These reactions are known as enzymatic browning reactions (see Section 9.12). Autoxidation of plant phenols catalysed by heavy metal ions (mainly cupric and ferric ions) and oxidation by lipid hydroperoxides lead to similar products. The products of oxidation of the so-called o-diphenok (1,2-dihydroxybenzenes) are o-quinones (1,2-benzoquinones), highly reactive compounds that react with proteins (amino acids) and other food components with the formation of dark-coloured polymeric products resistant to proteolysis. 

Enzymes from the class of oxidoreductases catalysing enzymatic browning reactions are most often known trivially as polyphenol oxidases. Two groups of enzymes can be identified ... 

Oxidoreductases Peroxidase, laccase, tyrosinase, glucose oxidase Polyphenols, polyanilines, vinyl polymers... 

---