Diphenolase activity

Polyphenoloxidase (PPO, EC 1.14.18.1) is one of the most studied oxidative enzymes because it is involved in the biosynthesis of melanins in animals and in the browning of plants. The enzyme seems to be almost universally distributed in animals, plants, fungi, and bacteria (Sanchez-Ferrer and others 1995) and catalyzes two different reactions in which molecular oxygen is involved the o-hydroxylation of monophenols to o-diphenols (monophenolase activity) and the subsequent oxidation of 0-diphenols to o-quinones (diphenolase activity). Several studies have reported that this enzyme is involved in the degradation of natural phenols with complex structures, such as anthocyanins in strawberries and flavanols present in tea leaves. Several polyphenols... 

Figure 4.2. Monophenolase and diphenolase activities of polyphenoloxidase and stabilization of the formed quinones. (A) Quinone reduction. (B) Descarboxylation. (C) Modification by an exomolecular nucleophile. (D) Modification by an endomolecular nucleophile.

As mentioned previously, PPO shows two catalytic activities the conversion of monophenols into o-diphenols (monophenolase activity) and the oxidation to the corresponding o-quinones (diphenolase activity) (Fig. 4.2) (Sanchez-Ferrer and others, 1995). 

The inactivation reaction is similar to that presented by diphenolase activity, only slower. 

The diphenolase activity involves the oxidation of two o-diphenols to two o-quinones... 

Polyphenol Oxidases. Some plant polyphenol oxidases possess both mono- and diphenolase activity (102.103), and thus are able to hydroxy late tyrosinyl residues in protein leading to the formation of the o-quinone of dihyroxyphenylalanine (108). This may lead to cross-linking between dietary proteins, digestive enzymes and/or proteins of the gut wall with ensuing reduction of digestive capabilties. 

These two processes are also referred as cresolase activity or monophenolase activity and diphenolase activity, respectively. Such reactions represent the initial steps of vertebrate pigmentation (melanin biosynthesis) and the browning of fruits and vegetables.Catechol oxidases (EC 1.10.3.1) are ubiquitous plant enzymes, which also catalyze the oxidation of a broad... 

Tyrosinase (E.C. 1.14.18.1, monophenol monooxygenase) is a copper monooxygenases enzyme that catalyzes two different oxygen-dependent reactions, namely the oxidations of both monophenols (cresolase or monophenolase activity) and o-diphenols (catecholase or diphenolase activity) into reactive o-quinones [28, 89]. 

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