LiP

Lignin is a kind of complex and stable biological macromolecular substance, and its yield is second only to cellulose. Lignin peroxidase (LiP) is the main enzyme in the biodegradation process of lignin, which can form free radicals in the lignin polymer, decrease the stability of the bond, and destroy the lignin macromolecule. Lignin peroxidase can hydrolyze lignin in straw and forage, improve the quality of feed ingredients, and thus improve their nutritional value.

Characteristics and Sources

In 1983, for the first time, LiP was found in the restricted medium of Phanerochaete chrysosporium. It is considered to be an oxidoreductase and is a glycosylated extracellular protein peroxidase of heme secreted by the fungus. Studies have shown that the glycosylated heme proteins of LiP are usually produced in a variety of complex forms, i.e. containing a variety of isozymes. The molecular mass of LiP is generally between 37 and 47 kDa. The molecular mass of the purified LiP is about 40 kDa, the isoelectric point is about 3.5, the optimum enzyme activity temperature is 35-55°C, and the optimum pH is 2-5. The LiP oxidation-reduction potential is 1.5 V. LiP can oxidize and decompose aromatic ring multimers in the presence of H₂O₂, which is considered to be one of the key enzymes for lignin degradation.

LiP is mainly produced by microbial secretion, such as Phanerochaete chrysosporium, Coriolus versicolor, Trametes versicolor, Pleurotus ostreatus and Dioscorea opposita. In 1989, it was found that the Gram-positive bacteria T7A can also produce Lip.

Structure

LiP contains about 343 amino acid residues and possesses four disulfide bonds. LiP folds to form a globular shape with a size of about 50 × 40 × 40 Å. It is segregated into proximal and distal domains by the heme which is completely fixed in the protein but made accessible through two small channels. The LiP folding motif contains eight major α-helices, eight minor helices, and three short antiparallel β-sheets.

Catalytic Mechanism

LiP is a series of isozymes containing Fe³⁺, porphyrin rings and heme prosthetic groups. In 1988, researchers discovered that LiP can catalyze a series of substrates using H₂O₂ and organic peroxides, and its catalysis is non-specific. In addition, studies have also found that LiP has a wide range of substrates, including phenolic and non-phenolic aromatic compounds, which can oxidize substrates such as lignin monomers, dimers, tris, and polycyclic aromatic hydrocarbons (such as benzopyrene). The catalytic cycle of lignin peroxidase involves three steps. The first reaction step is the oxidation of the resting ferric enzyme [Fe (III)] by hydrogen peroxide (H₂O₂) as an electron acceptor, resulting in the formation of compound I oxo-ferryl intermediate. In the second step, the oxo-ferryl intermediate (deficient of 2e⁻) is reduced by a molecule of substrate, such as non-phenolic aromatic substrate (S), which donates one electron (1e⁻) to compound I to form the second intermediate, compound II (deficient of 1e⁻). While the last step involves the subsequent donation of a second electron to compound II by the reduced substrate, thereby returning LiP to the resting ferric oxidation state which indicates the completion of the oxidation cycle.

Figure 2. Catalytic reaction of LiP. (Falade A O. et al. 2016)

Activity Assay

The enzyme activity of LiP is determined mainly by the resveratrol oxidation rate method. Since resveratrol is a direct substrate for the action of LiP, the enzyme activity of LiP is determined based on the absorbance value of resveratrol measured at 310 nm. The specific method is as follows: 1 mL of the reaction solution contains 0.2 mL of resveratrol solution, 0.4 mL of tartaric acid buffer (250 mmoL/L, pH 3.0), 0.4 mL of culture solution or diluent, and 20 μL of 20 mmoL/L H₂O₂ solution, and react at 30°C for 2 min. The absorbance at 310 nm was measured. In the control group, the H₂O₂ solution was replaced by distilled water, and the other reactants were unchanged.

Application

In the feed industry, LiP can be used to degrade lignin and cellulose in straw, which improves the palatability of straw and is easy to digest and absorb. In the pesticide industry, LiP produced by Phanerochaete chrysosporium can oxidize a variety of polyphenols, and some of these polyphenols are often precursors of broad-spectrum biocides or herbicides. In the pulping industry, the pulp is treated with LiP alone or in an appropriate combination with other enzymes, so that the produced paper has good tensile strength and smoothness, which not only improves the whiteness of the paper but also reduces the Kappa number of the paper. LiP can treat industrial wastewater such as 2,4-dibromophenol, forming dimers, trimers and tetramers during oxidation, thus eliminating the toxicity of wastewater.

References