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2-Methylbenzimidazole-copper nanozyme with high laccase activity for colorimetric differentiation and detection of aminophenol isomers

Laccase is well-known for its eco-friendly applications in environmental remediation and biotechnology, but its high cost and low stability have limited its practical use. Therefore, there is an urgent need to develop efficient laccase mimetics. In this study, a novel laccase-mimicking nanozyme (MBI-Cu) was successfully synthesized using 2-methylbenzimidazole (MBI) coordinated with Cu2+ by mimicking the copper active site and electron transfer pathway of natural laccase. MBI-Cu nanozyme exhibited excellent catalytic activity and higher stability than laccase, and was utilized to oxidize a series of phenolic compounds. Environmental pollutant aminophenol isomers were found to display different color in solution when catalytically oxidized by MBI-Cu, which provided a simple and feasible method to identify them by the naked eye. Based on the distinct absorption spectra of the oxidized aminophenol isomers, a colorimetric method for quantitatively detecting o-AP, m-AP, and p-AP was established, with detection limits of 0.06 μM, 0.27 μM, and 0.18 μM, respectively. Furthermore, by integrating MBI-Cu-based cotton pad colorimetric strips with smartphone and utilizing color recognition software to identify and analyze the RGB values of the images, a portable colorimetric sensing platform was designed for rapid detection of aminophenol isomers without the need for any analytical instrument. This work provides an effective reference for the design of laccase nanozymes and holds significant potential for applications in the field of environmental pollutant monitoring.

Fungal Laccases: Fundamentals, Engineering and Classification Update

Multicopper oxidases (MCOs) share a common catalytic mechanism of activation by oxygen and cupredoxin-like folding, along with some common structural determinants. Laccases constitute the largest group of MCOs, with fungal laccases having the greatest biotechnological applicability due to their superior ability to oxidize a wide range of aromatic compounds and lignin, which is enhanced in the presence of redox mediators. The adaptation of these versatile enzymes to specific application processes can be achieved through the directed evolution of the recombinant enzymes. On the other hand, their substrate versatility and the low sequence homology among laccases make their exact classification difficult. Many of the ever-increasing amounts of MCO entries from fungal genomes are automatically (and often wrongly) annotated as laccases. In a recent comparative genomic study of 52 basidiomycete fungi, MCO classification was revised based on their phylogeny. The enzymes clustered according to common structural motifs and theoretical activities, revealing three novel groups of laccase-like enzymes. This review provides an overview of the structure, catalytic activity, and oxidative mechanism of fungal laccases and how their biotechnological potential as biocatalysts in industry can be greatly enhanced by protein engineering. Finally, recent information on newly identified MCOs with laccase-like activity is included.

Role and structure of the small subunit forming heterodimers with laccase-like enzymes

Unlike laccases sensu stricto, which are usually monomeric enzymes, laccase-like enzymes recently re-classified as Novel Laccases (NLACs) are characterized by the formation of heterodimers with small proteins (subunits) of unknown function. Here the NLAC from Pleurotus eryngii (PeNL) and a small protein selected from the fungal genome, that is homologous to reported POXA3 from Pleurotus ostreatus, were produced in Aspergillus oryzae separately or together. The two proteins interacted regardless of whether the small subunit was co-expressed or exogenously added to the enzyme. The stability and catalytic activity of PeNL was significantly enhanced in the presence of the small subunit. Size exclusion chromatography-multi angle light scattering (SEC-MALS) analysis confirmed that the complex PeNL-ss is a heterodimer of 77.4 kDa. The crystallographic structure of the small protein expressed in Escherichia coli was solved at 1.6 Å resolution. This is the first structure elucidated of a small subunit of a NLAC. The helix bundle structure of the small subunit accommodates well with the enzyme model structure, including interactions with specific regions of NLACs and some amino acid residues of the substrate-binding loops.