Fungal laccases

Laccases are multi-copper oxidase enzymes found in fungi, plants, insects and other natural sources.

“Laccases are multi-copper oxidase enzymes found in fungi, plants, insects and other natural sources [20].”

Laccases catalyze the oxidation of a wide variety of organic-aromatic compounds.

“Laccases catalyze the oxidation of a wide variety of organic-aromatic compounds [20], is amplified by the addition of redox mediators [23,29,30].”

Laccases have been applied in the transformation of dyes and the treatment of wastewater.

“Laccases (EC 1.10.3.2) are enzymes with high catalytic efficiency [19,20] and have been applied in the delignification of lignocellulosic compounds, in bio-pulping and bio-bleaching, in the transformation of dyes, in the treatment of wastewater and the degradation of recalcitrant compounds, among others [11,20,21,22,23,24,25,26,27].”

The production of heterologous laccases using yeast reduce production costs and improves productivity.

“In this order, the production of heterologous laccases using yeast reduce production costs and improves productivity [20,38,39].”

The active site of laccase has four Cu2+ distributed in three sites: Type I (CuT1), Type II (CuT2) and Type III (CuT3), where CuT2 and a CuT3 copper pair form the trinuclear copper centre (TNC).

“The active site of laccase has four Cu+2, distributed in three sites [176], identified as Type I (CuT1), Type II (CuT2) and Type III (CuT3) copper, where CuT2 coppers and a CuT3 copper pair forms the trinuclear copper centre (TNC), [20,177,178].”

CuT2 is a mononuclear copper site coordinated by two histidine residues and an oxygen atom as a hydroxide ligand, located near the T3 copper.

“CuT2 is a mononuclear copper site [20] that does not exhibit characteristic absorbance but has a hyperfine parallel coupling constant, similar to that of a typical tetragonal copper core [183]. This copper coordinated by two histidine residues and an oxygen atom as a hydroxide (OH-) ligand and is strategically located near the T3 copper, forming a coplanar trigonal configuration [176].”

Fungal laccases are involved in lignin degradation and mineralization, pigment synthesis, fruiting body formation, fungal morphogenesis, detoxification, sporulation and some pathogenesis processes.

“Fungal laccases are involved in lignin degradation and mineralization, pigment synthesis, fruiting body formation, fungal morphogenesis, detoxification, sporulation and some pathogenesis processes [20,171,172].”

Commonly, laccases have the same structural architecture, consisting of three cupredoxin-type domains arranged sequentially, where CuT1 is in Domain 1 and the copper that makes up TNC join Domains 1 and 3.

“Commonly, laccases have the same structural architecture, consisting of three cupredoxin-type domains, arranged sequentially (Domains 1, 2 and 3), where CuT1 is in Domain 1 and the copper that makes up TNC (CuT2 and two CuT3) join Domains 1 and 3 [20,177,179].”

The CuT3 copper site is binuclear, consists of CuT3α and CuT3β, has weak UV absorbance near 330 nm and no EPR signal due to antiferromagnetic coupling from a hydroxide bridge.

“The CuT3 copper site is binuclear and consists of a pair of copper atoms identified as CuT3α and CuT3β [20,176]. CuT3 site have a weak UV absorbance (close to 330 nm) and no electronic paramagnetic resonance (EPR) signal, due to the antiferromagnetic (AF) coupling, that results from a ligand forming a hydroxide bridge between the coppers [183].”

The catalytic cycle of laccases begins with oxidation of the substrate; CuT1 sequesters electrons and transfers them to the trinuclear copper centre via His-Cys-His; after oxidation of four electrons, the fully reduced form is produced; O2 is reduced to H2O.

“The catalytic cycle in laccases begins with the oxidation of the substrate, CuT1 (Cu++) is responsible for sequestering electrons and transferring them to the trinuclear copper centre via His-Cys-His [20,178,179]. After oxidation of four electrons, is produced the fully reduced form (Cu+). Oxidation and intramolecular electron transfer are simultaneous to proton transfer, allowing the O2 reduction”

Mediators are low molecular weight compounds that upon oxidation generate cation radicals that oxidize more complex compounds through a redox cycle by mechanisms such as hydrogen atom transfer, electron transfer or ionic mechanisms.

“Mediators are low molecular weight compounds that oxidation, generate cation radicals that oxidize more complex compounds through a redox cycle by mechanisms such as hydrogen atom transfer, electron transfer or ionic mechanisms [20,267].”