Monooxygenases monophenol monooxygenase

The hemocyanlns which cooperatively bind dioxygen are found in two invertebrate phyla arthropod and mollusc. The mollusc hemocyanlns additionally exhibit catalase activity. Tyrosinase, which also reversibly binds dioxygen and dlsmutates peroxide, is a monooxygenase, using the dloxygen to hydroxylate monophenols to ortho-diphenols and to further oxidize this product to the quinone. Finally, the multicopper oxidases (laccase, ceruloplasmin and ascorbate oxidase) also contain coupled binuclear copper sites in combination with other copper centers and these catalyze the four electron reduction of dloxygen to water. 

These copper ion-dependent enzymes [EC 1.10.3.1] (also referred to as diphenol oxidases, O-diphenolase, phe-nolases, polyphenol oxidases, or tyrosinases) catalyze the reaction of two catechol molecules with dioxygen to produce two 1,2-benzoquinone and two water. A variety of substituted catechols can act as substrates. Many of the enzymes listed under this classification also catalyze a monophenol monooxygenase activity [/.c., EC 1.14.18.1]. See also Monophenol Monooxygenase Tyrosine Monooxygenase... 

MONOPHENOL MONOOXYGENASE NITRITE REDUCTASE PHOTOSYSTEM I QUERCETIN 2,3-DIOXYGENASE SUPEROXIDE DISMUTASES COPROPORPHYRINOGEN OXIDASE... 

ACTIN REGULATORY PROTEINS MONOMOLECULARITY Monomolecular surface coverage, BIOMINERALIZATION MONOPHENOL MONOOXYGENASE... 

MONOPHENOL MONOOXYGENASE NITRIC OXIDE SYNTHASE PEPTIDYL GLYCINE a-AMIDATING MONOOXYGENASE... 

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 alternative mechanism for the oxidation of phenolic compounds is enzyme-catalyzed oxidation. Several classes of enzymes can catalyze this reaction. According to the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (NC-IUBMB), these enzymes are part of the E C. 1 class of oxidoreductases (see the Internet web site http //www.chem.qmul.ac.uk/iubmb/enzyme/ECl). The three main classes of enzymes that catalyze the oxidation of phenolic compounds are the oxidoreductases that use oxygen as electron acceptor (E.C. 1.10.3), the peroxidases (E.C. 1.11.1), and monophenol monooxygenase (E.C. 

This class includes enzymes that use diphenols or related compounds as electron donors and oxygen as the acceptor, thereby forming the oxidized donor and water. Members include catechol oxidase (E.C. 1.10.3.1), laccase (E.C. 1.10.3.2), and o-aminophenol oxidase (E.C. 1.10.3.4). Laccase is also known as / -diphenoloxidase. whereas catechol oxidase is also known as diphenoloxidase, phenoloxidase, polyphenoloxidase, o-diphenolase, phenolase and tyrosinase. Many of these names are also used in reference to a different enzyme, monophenol monooxygenase (E.C. 1.14.18.1). This enzyme will be discussed further in Section 1.8.2.2. 

Tyrosinase inhibitors prevent browning in foodbecause they inhibit the oxidation caused by the enzyme tyrosinase. Cuminaldehyde is identified as a potent mushroom tyrosinase monophenol monooxygenase inhibitor from cumin seeds, ft inhibits the oxidation of L-3,4-dihydroxyphenylalanine (l-DOPA) by mushroom tyrosinase with an ID50 of 7.7g/ml (0.05 mM). Its oxidized analogue, cumic acid (p-isopropylbenzoic acid), also inhibits this oxidation with an 1D50 of 43g/ml (0.26mM). These two inhibitors affect mushroom tyrosinase activity in different ways (Kubo and Kinst-Hori, 1998). 

Tyrosinase catalyzes two reactions, the hydroxylation of phenolic compounds in ortho-position (cresolase activity) and subsequently the oxidation of the diphenolic products (cat-echolase activity).Tyrosinase as well as another enzyme that catalyzes only the oxidation reaction, catecholoxidase (EC 1.10.3.1), belongs to the group of phenoloxidases. The monooxygenase nature of Ty was established by Mason and coworkers in a pioneering study using 0-labeled oxygen. The two-electron donor required in the hydroxylation reaction is the o-diphenol, which is generated internally from the monophenol substrate. 

Tyrosinase (see Copper Proteins with Dinuclear Active Sites), a copper metalloenzyme with a very broad phylogenetic distribution, is responsible for the browning of fruits and mushrooms.Tyrosinase is a bifimctional phenol oxidase that is able to both hydroxylate monophenols like tyrosine (monooxygenase reaction, (equations)) and snbseqnently oxidize the diphenol product to the corresponding quinone (oxidase reaction, (equation 6)) at a single Type 3 binuclear copper active site. 

Phenoloxidase (monophenol monooxygenase, E.C. 1.14.18.1) introduces one atom of molecular oxygen into the substrate and was used in alginate-entrapped cells or in partially purified form. The pharmaceutical 7,8-dihydroxy-N-(di-n-propyl)-2-aminotetralin was produced continuously using a phenol oxidase suspension in dialysis tubing in an airlift fermenter coupled to an aluminium oxide column for selective product isolation (Figure 16.3-13)[6S1. A product concentration of 130 mg/L and a yield of 25 % were reached. 

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... 

Oxygenases. They catalyze substrate oxidation by molecular oxygen with a hydrogen donor that may be the substrate itself. Example monophenol monooxygenase (EC 1.14.18.1)... 

Monophenol monooxygenase, loccase (EC 1.14.18.1) see Oxygen metabolism. Lignin. 

Certain monooxygenases contain copper (copper-containing hydroxylases), e.g. dopamine- -hydroxy-lase this enzyme is associated with the chromaffin granules of the adrenal medulla, and is responsible for the oxidation of dopamine to noradrenalin the second substrate (electron donor) is ascorbic acid. Various phenolases are also Cu-containing monooxygenases. The mechanism of action is unclear, but it appears that a monophenol is hydroxylated to an o-diphenol, coupled to the oxidation of a diphenol (i.e. the second substrate) to an o-quinone. 

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