Biochemical characterization of Dimocarpus longan polyphenol oxidase provides insights into its catalytic efficiency

Tyrosinases: a family of copper-containing metalloenzymes

Tyrosinases (TYRs) are a family of copper-containing metalloenzymes that are present in all domains of life. TYRs catalyze the reactions that start the biosynthesis of melanin, the main pigment of the animal kingdom, and are also involved in the formation of the bright colors seen on the caps of mushrooms and in the petals of flowers. TYRs catalyze the ortho-hydroxylation and oxidation of phenols and the oxidation of catechols to the respective o-quinones. They only need molecular oxygen to do that, and the products of TYRs-o-quinones-are highly reactive and will usually react with the next available nucleophile. This reactivity can be harnessed for pharmaceutical applications as well as in environmental and food biotechnology. The majority of both basic and applied research on TYRs utilizes "mushroom tyrosinase", a crude enzyme preparation derived from button mushroom (Agaricus bisporus) fruiting bodies. Access to pure TYR preparations comes almost exclusively from the production of recombinant TYRs as the purification of these enzymes from the natural source is usually very laborious and plagued by low yields. In this text an introduction into the biochemistry of the enzyme TYR will be given, followed by an overview of available structural data of TYRs, the current model for the catalytic mechanism, a survey of reports on the recombinant production of this important metalloenzyme family, and a review of the applications of TYRs for the synthesis of catechols, as biosensors, in bioremediation, for the cross-linking of proteins and medical hydrogels as well as for melanoma treatment.

Graphical Abstract

Polyphenol Oxidase Activity on Guaiacyl and Syringyl Lignin Units

The natural heterogeneity of guaiacyl (G) and syringyl (S) compounds resulting from lignin processing hampers their direct use as plant-based chemicals and materials. Herein, we explore six short polyphenol oxidases (PPOs) from lignocellulose-degrading ascomycetes for their capacity to react with G-type and S-type phenolic compounds. All six PPOs catalyze the ortho-hydroxylation of G-type compounds (guaiacol, vanillic acid, and ferulic acid), forming the corresponding methoxy-ortho-diphenols. Remarkably, a subset of these PPOs is also active towards S-compounds (syringol, syringic acid, and sinapic acid) resulting in identical methoxy-ortho-diphenols. Assays with 18O2 demonstrate that these PPOs in particular catalyze ortho-hydroxylation and ortho-demethoxylation of S-compounds and generate methanol as a co-product. Oxidative (ortho-) demethoxylation of S-compounds is a novel reaction for PPOs, which we propose occurs by a distinct reaction mechanism as compared to aryl-O-demethylases. We further show that addition of a reducing agent can steer the PPO reaction to form methoxy-ortho-diphenols from both G- and S-type substrates rather than reactive quinones that lead to unfavorable polymerization. Application of PPOs opens for new routes to reduce the heterogeneity and methoxylation degree of mixtures of G and S lignin-derived compounds.

Overview on tyrosinases: Genetics, molecular biology, phylogenetic relationship

Tyrosinases (TYRs) are enzymes found in various organisms that are crucial for melanin biosynthesis, coloration, and UV protection. They play vital roles in insect cuticle sclerotization, mollusk shell formation, fungal and bacterial pigmentation, biofilm formation, and virulence. Structurally, TYRs feature copper-binding sites that are essential for catalytic activity, facilitating substrate oxidation via interactions with conserved histidine residues. TYRs exhibit diversity across animals, plants, fungi, mollusks, and bacteria, reflecting their roles and function. Eukaryotic TYRs undergo post-translational modifications, such as glycosylation, which affect protein folding and activity. Bacterial TYRs are categorized into five types based on their structural variation, domain organization and enzymatic properties, showing versatility across bacterial species. Moreover, bacterial TYRs, akin to fungal TYRs, have been implicated in the synthesis of secondary metabolites with antimicrobial properties. TYRs share significant sequence homology with hemocyanins, oxygen-carrier proteins in mollusks and arthropods, highlighting their evolutionary relationships. The evolution of TYRs underscores the dynamic nature of these enzymes and reflects adaptive strategies across diverse taxa.

Extraction, Purification, and Characterization of Olive (Olea europaea L., cv. Chemlal) Polyphenol Oxidase

Among fruits susceptible to enzymatic browning, olive polyphenol oxidase (OePPO) stood out as being unisolated from a natural source until this study, wherein we successfully purified and characterized the enzyme. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of heated and nonheated OePPO revealed distinct molecular weights of 35 and 54 kDa, respectively, indicative of its oligomeric nature comprising active and C-terminal subunits. OePPO displayed latency, fully activating with 5 mM SDS under optimal conditions of pH 7.5 and 15 °C. The enzyme demonstrated monophenolase activity and showcased the highest efficiency toward hydroxytyrosol. Despite its low optimal temperature, OePPO exhibited high thermal resistance, maintaining stability up to 90 °C. However, beyond this threshold, the oligomeric enzyme disassociated, yielding a denatured main subunit and C-terminal fragments. Six OePPO genes were found in the fruits. Tryptic digestion identified the enzyme as mature OePPO1 (INSDC OY733096), while mass spectrometry detected the active form mass alongside several C-terminal fragments, revealing potential cleavage sites (Gly407, Tyr408).

Immobilization of Agaricus bisporus Polyphenol Oxidase 4 on mesoporous silica: Towards mimicking key enzymatic processes in peat soils

HYPOTHESIS

The use of immobilized enzyme-type biocatalysts to mimic specific processes in soil can be considered one of the most promising alternatives to overcome the difficulties behind the structural elucidation of riverine humic-derived iron-complexes. Herein, we propose that the immobilization of the functional mushroom tyrosinase, Agaricus bisporus Polyphenol Oxidase 4 (AbPPO4) on mesoporous SBA-15-type silica could contribute to the study of small aquatic humic ligands such as phenols.

EXPERIMENTS

The silica support was functionalized with amino-groups in order to investigate the impact of surface charge on the tyrosinase loading efficiency as well as on the catalytic performance of adsorbed AbPPO4. The oxidation of various phenols was catalyzed by the AbPPO4-loaded bioconjugates, yielding high levels of conversion and confirming the retention of enzyme activity after immobilization. The structures of the oxidized products were elucidated by integrating chromatographic and spectroscopic techniques. We also evaluated the stability of the immobilized enzyme over a wide range of pH values, temperatures, storage-times and sequential catalytic cycles.

FINDINGS

This is the first report where the latent AbPPO4 is confined within silica mesopores. The improved catalytic performance of the adsorbed AbPPO4 shows the potential use of these silica-based mesoporous biocatalysts for the preparation of a column-type bioreactor for in situ identification of soil samples.