Polyphenol oxidase

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Catechol oxidase
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EC no. 1.10.3.2
Alt. namesPolyphenol oxidase
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Polyphenol oxidase (PPO; also polyphenol oxidase i, chloroplastic), an enzyme involved in fruit browning, is a tetramer that contains four atoms of copper per molecule. [1]

Contents

PPO may accept monophenols and/or o-diphenols as substrates. [2] The enzyme works by catalyzing the o-hydroxylation of monophenol molecules in which the benzene ring contains a single hydroxyl substituent to o-diphenols (phenol molecules containing two hydroxyl substituents at the 1, 2 positions, with no carbon between). [3] It can also further catalyse the oxidation of o-diphenols to produce o-quinones. [4] PPO catalyses the rapid polymerization of o-quinones to produce black, brown or red pigments (polyphenols) that cause fruit browning.

The amino acid tyrosine contains a single phenolic ring that may be oxidised by the action of PPOs to form o-quinone. Hence, PPOs may also be referred to as tyrosinases. [5]

Common foods producing the enzyme include mushrooms ( Agaricus bisporus ), [6] [7] apples ( Malus domestica ), [8] [9] avocados ( Persea americana ), and lettuce ( Lactuca sativa ). [10]

Structure and function

PPO is listed as a morpheein, a protein that can form two or more different homo-oligomers (morpheein forms), but must come apart and change shape to convert between forms. It exists as a monomer, trimer, tetramer, octamer or dodecamer, [11] [12] creating multiple functions. [13]

In plants, PPO is a plastidic enzyme with unclear synthesis and function. In functional chloroplasts, it may be involved in oxygen chemistry like mediation of pseudocyclic photophosphorylation. [14]

Enzyme nomenclature differentiates between monophenol oxidase enzymes (tyrosinases) and o-diphenol:oxygen oxidoreductase enzymes (catechol oxidases). The substrate preference of tyrosinases and catechol oxidases is controlled by the amino acids around the two copper ions in the active site. [15]

Distribution and applications

A mixture of monophenol oxidase and catechol oxidase enzymes is present in nearly all plant tissues, and can also be found in bacteria, animals, and fungi. In insects, cuticular polyphenol oxidases are present [16] and their products are responsible for desiccation tolerance.

Grape reaction product (2-S glutathionyl caftaric acid) is an oxidation compound produced by action of PPO on caftaric acid and found in wine. This compound production is responsible for the lower level of browning in certain white wines.

Plants make use of polyphenol oxidase as one in a suite of chemical defences against parasites. [17]

Inhibitors

There are two types of inhibitor of PPO, those competitive to oxygen in the copper site of the enzyme and those competitive to phenolics. Tentoxin has also been used in recent research to eliminate the PPO activity from seedlings of higher plants. [18] Tropolone is a grape polyphenol oxidase inhibitor. [19] Another inhibitor of this enzyme is potassium metabisulfite. [20] Banana root PPO activity is strongly inhibited by dithiothreitol and sodium metabisulfite, [21] as is banana fruit PPO by similar sulfur-containing compounds including sodium dithionite and cysteine, in addition to ascorbic acid (vitamin C). [22]

Assays

Several assays were developed to monitor the activity of polyphenol oxidases and to evaluate the inhibition potency of polyphenol oxidase inhibitors. In particular, ultraviolet/visible (UV/Vis) spectrophotometry-based assays are widely applied. [23] The most common UV/Vis spectrophotometry assay involves the monitoring of the formation of o-quinones, which are the products of polyphenol oxidase-catalysed reactions, or the consumption of the substrate. [24] Alternative spectrophotometric method that involves the coupling of o-quinones with nucleophilic reagents such as 3-methyl-2-benzothiazolinonehydrazone hydrochloride (MBTH) was also used. [25] Other techniques, such as activity staining assays with the use of polyacrylamide gel electrophoresis, [26] tritium-based radioactive assays, [27] oxygen consumption assay, [28] and nuclear magnetic resonance (NMR)-based assay were also reported and used. [29]

Enzymatic browning

Polyphenol oxidase is an enzyme found throughout the plant and animal kingdoms, [30] including most fruits and vegetables. [31] PPO has importance to the food industry because it catalyzes enzymatic browning when tissue is damaged from bruising, compression or indentations, making the produce less marketable and causing economic loss. [30] [31] [32] Enzymatic browning due to PPO can also lead to loss of nutritional content in fruits and vegetables, further lowering their value. [10] [30] [31]

Because the substrates of these PPO reactions are located in the vacuoles of plant cells damaged mainly by improper harvesting, PPO initiates the chain of browning reactions. [32] [33] Exposure to oxygen when sliced or pureed also leads to enzymatic browning by PPO in fruits and vegetables. [31] Examples in which the browning reaction catalyzed by PPO may be desirable include avocados, prunes, sultana grapes, black tea, and green coffee beans. [10] [31]

In mango

In mangoes, PPO catalyzed enzymatic browning is mainly caused by sap burn which leads to skin browning.[ citation needed ] Catechol oxidase-type PPO is located in the chloroplasts of mango skin cells and its phenolic substrates in the vacuoles. Sap burn is therefore the initiating event of PPO in mango skin, as it breaks down cell compartments. [33] PPO is located in mango skin, sap and pulp, with highest activity levels in skin. [31]

In avocado

PPO in avocados causes rapid browning upon exposure to oxygen, [10] a multistep process involving oxidation reactions of both monophenols and polyphenols, resulting in o-quinone products subsequently converted irreversibly into brown polymeric pigments (melanins). [34]

In apple

Present in the chloroplasts and mitochondria of all parts of an apple, [31] PPO is the major enzyme responsible for enzymatic browning of apples. [35] Due to an increase in consumer demand for pre-prepared fruits and vegetables, a solution for enzymatic browning has been a targeted area of research and new product development. [36] As an example, pre-sliced apples are an appealing consumer product, but slicing apples induces PPO activity, leading to browning of the cut surfaces and lowering their esthetic quality. [36] Browning also occurs in apple juices and purees when poorly handled or processed. [37]

Arctic apples, an example of genetically modified fruit engineered to reduce PPO activity, are a suite of trademarked apples that contain a non-browning trait derived by gene silencing to suppress the expression of PPO, thus inhibiting fruit browning. [38]

In apricot

Apricot as a climacteric fruit undergoes fast post-harvest maturation. The latent PPO form can spontaneously activate during the first weeks of storage, generating the active enzyme with a molecular weight of 38 kDa. [39] Ascorbic acid/protease combinations constitute a promising practical anti-browning method as treated apricot purees preserved their color. [40]

In potato

Found in high concentrations in potato tuber peel and 1–2 mm of the outer cortex tissue, PPO is used in the potato as a defense against insect predation, leading to enzymatic browning from tissue damage.[ citation needed ] Damage in the skin tissue of potato tuber causes a disruption of cell compartmentation, resulting in browning. The brown or black pigments are produced from the reaction of PPO quinone products with amino acid groups in the tuber. [32] In potatoes, PPO genes are not only expressed in potato tubers, but also in leaves, petioles, flowers and roots. [32]

In walnut

In walnut ( Juglans regia ), two different genes (jr PPO1 and jr PPO2) encoding polyphenol oxidases have been identified. The two isoenzymes prefer different substrates, as jr PPO1 shows a higher activity towards monophenols, whereas jr PPO2 is more active towards diphenols. [41] [42]

In black poplar

A monomeric catechol oxidase from Populus nigra converts caffeic acid to quinone and melanine at injured cells. [43] [44]

Prophenoloxidase is a modified form of the complement response found in some invertebrates, including insects, crabs and worms. [45]

Hemocyanin is homologous to the phenol oxidases (e.g. tyrosinase) since both enzymes sharing type copper active site coordination. Hemocyanin also exhibits PPO activity, but with slowed kinetics from greater steric bulk at the active site. Partial denaturation actually improves hemocyanin's PPO activity by providing greater access to the active site. [46]

Aureusidin synthase is homologous to plant polyphenol oxidase, but contains certain significant modifications.

Aurone synthase [47] catalyzes the formation of aurones. Aurone synthase purified from Coreopsis grandiflora shows weak tyrosinase activity against isoliquiritigenin, but the enzyme does not react with the classic tyrosinase substrates L-tyrosine and tyramine and must therefore be classified as catechol oxidase. [48]

See also

Related Research Articles

Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. Autoxidation leads to degradation of organic compounds, including living matter. Antioxidants are frequently added to industrial products, such as polymers, fuels, and lubricants, to extend their usable lifetimes. Food are also treated with antioxidants to forestall spoilage, in particular the rancidification of oils and fats. In cells, antioxidants such as glutathione, mycothiol or bacillithiol, and enzyme systems like superoxide dismutase, can prevent damage from oxidative stress.

<span class="mw-page-title-main">Xanthine oxidase</span> Class of enzymes

Xanthine oxidase is a form of xanthine oxidoreductase, a type of enzyme that generates reactive oxygen species. These enzymes catalyze the oxidation of hypoxanthine to xanthine and can further catalyze the oxidation of xanthine to uric acid. These enzymes play an important role in the catabolism of purines in some species, including humans.

<span class="mw-page-title-main">Hemocyanin</span> Proteins that transport oxygen throughout the bodies of some invertebrate animals

Hemocyanins (also spelled haemocyanins and abbreviated Hc) are proteins that transport oxygen throughout the bodies of some invertebrate animals. These metalloproteins contain two copper atoms that reversibly bind a single oxygen molecule (O2). They are second only to hemoglobin in frequency of use as an oxygen transport molecule. Unlike the hemoglobin in red blood cells found in vertebrates, hemocyanins are not confined in blood cells but are instead suspended directly in the hemolymph. Oxygenation causes a color change between the colorless Cu(I) deoxygenated form and the blue Cu(II) oxygenated form.

<span class="mw-page-title-main">Polyphenol</span> Class of chemical compounds

Polyphenols are a large family of naturally occurring phenols. They are abundant in plants and structurally diverse. Polyphenols include flavonoids, tannic acid, and ellagitannin, some of which have been used historically as dyes and for tanning garments.

<span class="mw-page-title-main">Food browning</span> Food process

Browning is the process of food turning brown due to the chemical reactions that take place within. The process of browning is one of the chemical reactions that take place in food chemistry and represents an interesting research topic regarding health, nutrition, and food technology. Though there are many different ways food chemically changes over time, browning in particular falls into two main categories: enzymatic versus non-enzymatic browning processes.

<span class="mw-page-title-main">Catechol</span> Organic compound (C6H4(OH)2); benzene with two adjacent –OH groups

Catechol, also known as pyrocatechol or 1,2-dihydroxybenzene, is an organic compound with the molecular formula C6H4(OH)2. It is the ortho isomer of the three isomeric benzenediols. This colorless compound occurs naturally in trace amounts. It was first discovered by destructive distillation of the plant extract catechin. About 20,000 tonnes of catechol are now synthetically produced annually as a commodity organic chemical, mainly as a precursor to pesticides, flavors, and fragrances.

<span class="mw-page-title-main">Tyrosinase</span> Enzyme for controlling the production of melanin

Tyrosinase is an oxidase that is the rate-limiting enzyme for controlling the production of melanin. The enzyme is mainly involved in two distinct reactions of melanin synthesis otherwise known as the Raper Mason pathway. Firstly, the hydroxylation of a monophenol and secondly, the conversion of an o-diphenol to the corresponding o-quinone. o-Quinone undergoes several reactions to eventually form melanin. Tyrosinase is a copper-containing enzyme present in plant and animal tissues that catalyzes the production of melanin and other pigments from tyrosine by oxidation. It is found inside melanosomes which are synthesized in the skin melanocytes. In humans, the tyrosinase enzyme is encoded by the TYR gene.

<span class="mw-page-title-main">Caffeic acid</span> Chemical compound

Caffeic acid is an organic compound that is classified as a hydroxycinnamic acid. This yellow solid consists of both phenolic and acrylic functional groups. It is found in all plants because it is an intermediate in the biosynthesis of lignin, one of the principal components of woody plant biomass and its residues.

<span class="mw-page-title-main">ABTS</span> Chemical compound

In biochemistry, ABTS is a chemical compound used to observe the reaction kinetics of specific enzymes. A common use for it is in the enzyme-linked immunosorbent assay (ELISA) to detect the binding of molecules to each other.

Catechol oxidase is a copper oxidase that contains a type 3 di-copper cofactor and catalyzes the oxidation of ortho-diphenols into ortho-quinones coupled with the reduction of molecular oxygen to water. It is present in a variety of species of plants and fungi including Ipomoea batatas and Camellia sinensis. Metalloenzymes with type 3 copper centers are characterized by their ability to reversibly bind dioxygen at ambient conditions. In plants, catechol oxidase plays a key role in enzymatic browning by catalyzing the oxidation of catechol to o-quinone in the presence of oxygen, which can rapidly polymerize to form the melanin that grants damaged fruits their dark brown coloration.

<span class="mw-page-title-main">Tropolone</span> Chemical compound

Tropolone is an organic compound with the chemical formula C7H5(OH)O. It is a pale yellow solid that is soluble in organic solvents. The compound has been of interest to research chemists because of its unusual electronic structure and its role as a ligand precursor. Although not usually prepared from tropone, it can be viewed as its derivative with a hydroxyl group in the 2-position.

<span class="mw-page-title-main">Protocatechuic acid</span> Chemical compound

Protocatechuic acid (PCA) is a dihydroxybenzoic acid, a type of phenolic acid. It is a major metabolite of antioxidant polyphenols found in green tea. It has mixed effects on normal and cancer cells in in vitro and in vivo studies.

<span class="mw-page-title-main">Resazurin</span> Chemical compound

Resazurin is a phenoxazine dye that is weakly fluorescent, nontoxic, cell-permeable, and redox‐sensitive. Resazurin has a blue to purple color and is used in microbiological, cellular, and enzymatic assays because it can be irreversibly reduced to the pink-colored and highly fluorescent resorufin (7-Hydroxy-3H-phenoxazin-3-one). At circum-neutral pH, resorufin can be detected by visual observation of its pink color or by fluorimetry, with an excitation maximum at 530-570 nm and an emission maximum at 580-590 nm.

<span class="mw-page-title-main">Caftaric acid</span> Chemical compound

Caftaric acid is a non-flavonoid phenolic compound.

<span class="mw-page-title-main">Naturally occurring phenols</span> Group of chemical compounds

In biochemistry, naturally occurring phenols are natural products containing at least one phenol functional group. Phenolic compounds are produced by plants and microorganisms. Organisms sometimes synthesize phenolic compounds in response to ecological pressures such as pathogen and insect attack, UV radiation and wounding. As they are present in food consumed in human diets and in plants used in traditional medicine of several cultures, their role in human health and disease is a subject of research. Some phenols are germicidal and are used in formulating disinfectants.

<span class="mw-page-title-main">Grape reaction product</span> Chemical compound

The grape reaction product is a phenolic compound explaining the disappearance of caftaric acid from grape must during processing. It is also found in aged red wines. Its enzymatic production by polyphenol oxidase is important in limiting the browning of musts, especially in white wine production. The product can be recreated in model solutions.

Aureusidin synthase is an enzyme with systematic name 2',4,4',6'-tetrahydroxychalcone 4'-O-beta-D-glucoside:oxygen oxidoreductase.

<span class="mw-page-title-main">Fungal extracellular enzyme activity</span> Enzymes produced by fungi and secreted outside their cells

Extracellular enzymes or exoenzymes are synthesized inside the cell and then secreted outside the cell, where their function is to break down complex macromolecules into smaller units to be taken up by the cell for growth and assimilation. These enzymes degrade complex organic matter such as cellulose and hemicellulose into simple sugars that enzyme-producing organisms use as a source of carbon, energy, and nutrients. Grouped as hydrolases, lyases, oxidoreductases and transferases, these extracellular enzymes control soil enzyme activity through efficient degradation of biopolymers.

<span class="mw-page-title-main">Rhododendrol</span> Chemical compound

Rhododendrol (RD) also called 4-[(3R)-3-hydroxybutyl]phenol (systemic name), is an organic compound with the formula C10H14O2. It is a naturally occurring ingredient present in many plants, such as the Rhododendron. The phenolic compound was first developed in 2010 as a tyrosinase inhibitor for skin-lightening cosmetics. In 2013, after rhododendrol reportedly caused skin depigmentation in consumers using RD-containing skin-brightening cosmetics, the cosmetics were withdrawn from the market. The skin condition, caused by RD, is called RD-induced leukoderma. Rhododendrol exerts melanocyte cytotoxicity via a tyrosinase-dependent mechanism. It has been shown to impair the normal proliferation of melanocytes through reactive oxygen species-dependent activation of GADD45. It is now well established that rhododendrol is a potent tyrosinase inhibitor.

References

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