|PDB structures||RCSB PDB PDBe PDBsum|
|Gene Ontology||AmiGO / QuickGO|
Laccases (EC 126.96.36.199) are copper-containing oxidase enzymes found in many plants, fungi, and microorganisms. Laccases act on phenols and similar substrates, performing one-electron oxidations, leading to crosslinking. For example laccases play a role in the formation of lignin by promoting the oxidative coupling of monolignols, a family of naturally occurring phenols. Other laccases, such as those produced by the fungus Pleurotus ostreatus, play a role in the degradation of lignin, and can therefore be classed as lignin-modifying enzymes. Laccases catalyze ring cleavage of aromatic compounds.
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 a electron transfer site. 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 effects the oxygen reduction reaction at low overpotentials. The enzyme has been examined as the cathode in 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.
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.