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Laccases () are multicopper oxidases found in plants, fungi, and bacteria. Laccases oxidize a variety of
phenolic
phenolic
substrates, performing one-electron
oxidation Redox (reduction–oxidation, pronunciation: or ) is a type of chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter ...

oxidation
s, leading to
crosslinking Cross-linking may refer to *Cross-link, a chemical bond of one polymer chain to another *Corneal collagen cross-linking, a parasurgical treatment for corneal ectasia and keratoconus {{Disambiguation ...
. For example, laccases play a role in the formation of
lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and Bark (botany), bark, because they l ...

lignin
by promoting the oxidative coupling of
monolignol Monolignols, also called lignols, are the source materials for biosynthesis of both lignans and lignin. The starting material for production of monolignols is the amino acid phenylalanine. Via the phenylpropanoid pathway, phenylalanine is first co ...
s, a family of naturally occurring phenols. Other laccases, such as those produced by the fungus ''
Pleurotus ostreatus ''Pleurotus ostreatus'', the oyster mushroom or oyster fungus, is a common edible mushroom. It was first cultivated in Germany as a subsistence measure during World War I World War I or the First World War, often abbreviated as WWI or W ...

Pleurotus ostreatus
'', play a role in the degradation of lignin, and can therefore be classed as lignin-modifying enzymes. Other laccases produced by fungi can facilitate the biosynthesis of
melanin Melanin (; from el, μέλας ''melas'', "black, dark") is a broad term for a group of natural pigments found in most organisms. Melanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the ami ...

melanin
pigments. Laccases catalyze ring cleavage of aromatic compounds. Laccase was first studied by Hikorokuro Yoshida in 1883 and then by Gabriel Bertrand in 1894 in the sap of the Japanese lacquer tree, where it helps to form
lacquer Lacquer is a type of hard and potentially shiny coating A coating is a covering that is applied to the surface of an object, usually referred to as the substrate Substrate may refer to: Physical layers *Substrate (biology), the natural e ...

lacquer
, hence the name laccase.


Active site

The active site consists of four copper centers, which adopt structures classified as type I, type II, and type III. A tricopper ensemble contains types II and III copper (see figure). It is this center that binds O2 and reduces it to water. Each Cu(I,II) couple delivers one electron required for this conversion. The type 1 copper does not bind O2, but functions solely as an electron transfer site. The type I copper center consists of a single copper atom that is ligated to a minimum of two
histidine Histidine (symbol His or H) is an α-amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bon ...

histidine
residues and a single
cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the chemical formula, formula HOOC-CH-(NH2)-CH2-SH. The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. The thiol is suscepti ...

cysteine
residue, but in some laccases produced by certain plants and bacteria, the type I copper center contains an additional methionine ligand. The type III copper center consists of two copper atoms that each possess three histidine ligands and are linked to one another via a hydroxide bridging ligand. The final copper center is the type II copper center, which has two histidine ligands and a hydroxide ligand. The type II together with the type III copper center forms the tricopper ensemble, which is where dioxygen reduction takes place. The type III copper can be replaced by Hg(II), which causes a decrease in laccase activity. Cyanide removes all copper from the enzyme, and re-embedding with type I and type II copper has been shown to be impossible. Type III copper, however, can be re-embedded back into the enzyme. A variety of other anions inhibit laccase. Laccases affects the oxygen reduction reaction at low overpotentials. The enzyme has been examined as the cathode in Enzymatic Biofuel Cells, enzymatic biofuel cells. They can be paired with an electron mediator to facilitate electron transfer to a solid electrode wire. Laccases are some of the few oxidoreductases commercialized as industrial catalysts.


Activity in wheat dough

Laccases have the potential to cross link food polymers such as proteins and nonstarch polysaccharides in dough. In non starch polysaccharides, such as arabinoxylans (AX), laccase catalyzes the oxidative gelation of feruloylated arabinoxylans by dimerization of their ferulic esters. These cross links have been found to greatly increased the maximum resistance and decreased extensibility of the dough. The resistance was increased due to the crosslinking of AX via ferulic acid and resulting in a strong AX and gluten network. Although laccase is known to cross link AX, under the microscope it was found that the laccase also acted on the flour proteins. Oxidation of the ferulic acid on AX to form ferulic acid radicals increased the oxidation rate of free SH groups on the gluten proteins and thus influenced the formation of S-S bonds between gluten polymers. Laccase is also able to oxidize peptide bound tyrosine, but very poorly. Because of the increased strength of the dough, it showed irregular bubble formation during proofing. This was a result of the gas (carbon dioxide) becoming trapped within the crust and could not diffuse out (like it would have normally) and causing abnormal pore size. Resistance and extensibility was a function of dosage, but at very high dosage the dough showed contradictory results: maximum resistance was reduced drastically. The high dosage may have caused extreme changes in structure of dough, resulting in incomplete gluten formation. Another reason is that it may mimic overmixing, causing negative effects on gluten structure. Laccase treated dough had low stability over prolonged storage. The dough became softer and this is related to laccase mediation. The laccase mediated radical mechanism creates secondary reactions of FA-dervived radicals that result in breaking of covalent linkages in AX and weakening of the AX gel.


Biotechnology

The ability of laccases to degrade various aromatic polymers has led to research into their potential for bioremediation and other industrial applications. Laccases have been applied in the production of wines as well as in the food industry. Studies utilizing fungal and bacterial laccases to degrade Contaminants of emerging concern , emerging pollutants have also been conducted. In particular, it has been shown that laccases can be applied to catalyze the degradation and detoxification a large range of Aromatic compound , aromatic contaminants, including azo dyes, bisphenol A and pharmaceuticals.


See also

*


References


Citations


General sources

* * *


External links


BRENDA
* {{Enzymes Copper enzymes EC 1.10.3 Natural phenols metabolism Proteins