Fungal Laccases: The Forefront of Enzymes for Sustainability

Abstract

Enzymatic catalysis is one of the main pillars of sustainability for industrial production. Enzyme application allows minimization of the use of toxic solvents and to valorize the agro-industrial residues through reuse. In addition, they are safe and energy efficient. Nonetheless, their use in biotechnological processes is still hindered by the cost, stability, and low rate of recycling and reuse. Among the many industrial enzymes, fungal laccases (LCs) are perfect candidates to serve as a biotechnological tool as they are outstanding, versatile catalytic oxidants, only requiring molecular oxygen to function. LCs are able to degrade phenolic components of lignin, allowing them to efficiently reuse the lignocellulosic biomass for the production of enzymes, bioactive compounds, or clean energy, while minimizing the use of chemicals. Therefore, this review aims to give an overview of fungal LC, a promising green and sustainable enzyme, its mechanism of action, advantages, disadvantages, and solutions for its use as a tool to reduce the environmental and economic impact of industrial processes with a particular insight on the reuse of agro-wastes.

Keywords: agro-wastes; catalysis; enzymes; fungal laccase; immobilization; solid state fermentation; sustainability.

Figure 1

Oxidation of substrates by laccase. Panel (A) shows the direct and mediated oxidation mechanisms. The route of “false” mediators is also depicted. Panel (B) shows three examples of coupling reactions. In particular homomolecular coupling of 2,6-dimethoxyphenol (i) and resveratrol (ii), and the heteromolecular coupling of aminopenicillin and cathecol (iii).

Figure 2

Amino acid sequence of T. hirsuta LacA laccase (gb | KP027478.1). The L1-L4 regions are shown in purple, the copper ion Cu1 ligands are shown in orange, Cu2—in green, and Cu3—in blue. Cysteine residues that are involved in the formation of disulfide bonds are shown in yellow.

Figure 3

The overall structure of Coriolopsis caperata laccase (PDB 3JHV, [60]). The first domain is shown in gold, the second in green, and the third in blue color. Copper atoms are shown with purple spheres. Sugars are shown with stick models, atoms are colored by type (C—orange, O—red, N—blue). Disulfide bridges are shown in yellow.

Figure 4

Active center of T. hirsuta laccase (PDB: 3FPX, [61]). Copper ions are shown in purple, oxygen atoms in red, and nitrogen atoms in blue. The carbon atoms of histidine residues coordinating copper ions Cu1, Cu2, Cu3_α, and Cu3_β are shown in coral, green, purple, and blue colors, respectively. Carbon atoms of non-coordinating amino acid residues from the nearest surrounding of the copper ion Cu1 are shown in gray. T2 and T3 water channels are shown with red cylinders and water molecules inside them.

Figure 5

Substrate-binding pocket of T. versicolor laccase (structure of the laccase complex with 2,5-xylidine, PDB 1KYA [68].

Figure 6

Mechanism of molecular oxygen reduction by laccases to water: O₂ + 4e⁻ + 4H⁺ → 2H₂O. Intermediate states are shown in Figure (A–F). Panel F shows the mechanism of the release of a water molecule from the TNC. Coordination and covalent bonds are shown by solid lines. Ion-dipole electrostatic interactions are shown with dotted lines.