Monophenolase activity

Escribano, J. et al., Characterization of monophenolase activity of table beet polyphenol oxidase determination of kinetic parameters on the tyramine/dopamine pair, J. Agric. Food Ghem., 45, 4209, 1997. 

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... 

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 monophenolase activity of PPO is generally defined as the first step in the melaniza-tion pathway and consists of the o-hydroxylation of the monophenol to odiphenol. This activity distinguishes PPO from other phenol-oxidizing enzymes, such as laccase and peroxidase, and is characterized by the following facts ... 

Monophenolase activity shows a characteristic lag period before the maximum velocity of the hydroxylation step is reached. The time required to reach the steady-state rate depends on several factors enzyme source concentration of monophenol ... 

Gandia-Herrero F, Escribano J and Garcia-Carmona F. 2005. Characterization of the monophenolase activity of tyrosinase on betaxanthins the tyramine-betaxanthin/dopamine-betaxanthin pair. Planta... 

Kreis et al. (2000) characterized a PPO from the aerial part of E. purpurea as a diphenolase with a high affinity for caffeic acid. In addition, the PPO lacked monophenolase activity. The PPO was reported to be a 47-54 kDa protein having an optimal activity near pH 6.0 and reversibly inhibited by... 

While hemocyanins can only be found in two phyla, tyrosinases (EC 1.14.18.1) may be found in almost all types of organisms - from bacteria to mammals [251]. Tyrosinase, a monooxygenase, is responsible for the production of melanin and related pigments. It catalyses the o-hydroxylation of monophenols to o-diphenols (monophenolase activity), as well as the two-electron oxidation of o-diphenols to o-quinones (catecholase activity) [252,253]. 

The distinct difference between catechol oxidase and tyrosinase has not yet been explained. A lag phase in the monophenolase activity of tyrosinase has been found and studied and is proposed to be a result of temporary inhibition of the met state of tyrosinase by excess of the monophenol substrate (Figure 25). Monophenolase activity increases when the diphenol product displaces the monophenol from met tyrosinase and allows the continuation of the catalytic cycle. Catechol oxidase in its isolated form is present exclusively in the met state and is also inhibited by phenol. It was therefore suggested that lack of the oxy state is the reason catechol oxidase lacks cresolase activity. As oxy catechol oxidase also shows no monooxygenase activity, this explanation does not seem entirely satisfying. Another possible reason is that access to Cu, which has been proposed to be necessary for the oxygenation of monophenols, is blocked in the crystal structure of catechol oxidase from... 

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... 

Espin, J.C. et al. Monophenolase activity of polyphenol oxidase from verdedonceUa apple, J. Agric. Food Chem., 43, 2807,1995. 

Tyrosinase is an enzyme belonging to major families of polyphenol oxidases, which contain the spectroscopic properties of metal ions. In general, tyrosinases catalyze the hydroxylation of monophenols to o-diphenols (monophenolase activity). 

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]. 

Melanins are widely distributed in biopigments in bacteria, fungi, plants, and animals. Melanins are heterogeneous polyphenol-like biopolymers with a complex structure and color varying from yellow to black [210]. They are biosynthesized by a combination of enzymatic and chemical reactions which are initiated by the oxidation of tyrosine. Tyrosinase catalyzes the hydroxylation (monophenolase activity) of tyrosine to 3,4-dihydroxyphenylalanine (DOPA) and the oxidation (diphenolase or catecholase activity) of DOPA to the corresponding -quinone [211] as shown in eq. (28.)... 

Rodriguez-Lopez, JN Escribano, J Garcia-Canovas, FA. A Continuous Spectrophotometric Method for the Determination of Monophenolase Activity of Tyrosinase Using 3-methyl-2-benzothiazolinone hydrazone. Analytical Biochemistry, 1994 216, 205-212. 

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