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| Polyphenol oxidase | |
|---|---|
| Name | Polyphenol oxidase |
| Ec number | 1.14.18.1 (varies) |
| Cofactors | Copper |
| Substrates | Phenolic compounds |
| Products | Quinones |
Polyphenol oxidase is a class of copper-containing oxidoreductases found across taxa that catalyze the oxidation of phenolic substrates to quinones, driving pigment formation and defense reactions. These enzymes are studied in contexts ranging from plant physiology to food science, with implications for agriculture, biotechnology, and medicine. Research on polyphenol oxidases involves molecular biology, structural biochemistry, and applied enzymology, intersecting with work from laboratories at institutions such as Max Planck Society, University of Cambridge, Harvard University, Wageningen University & Research, and USDA.
The overall fold of many polyphenol oxidases resembles dinuclear copper enzymes characterized by coordination motifs seen in models from Stanford University and crystallographic studies at facilities like European Synchrotron Radiation Facility, Diamond Light Source, and Argonne National Laboratory. The active site contains two copper ions (CuA and CuB) held by histidine ligands in a conserved three-histidine motif, an arrangement analogous to that observed in structures resolved by teams at Massachusetts Institute of Technology, University of Oxford, and ETH Zurich. Crystal structures from investigations involving researchers affiliated with Max Planck Institute for Biophysical Chemistry and Scripps Research reveal substrate channels and surface residues that modulate accessibility, paralleling insights from structural studies of enzymes at Cold Spring Harbor Laboratory and Johns Hopkins University.
Catalytic mechanisms proposed by groups at California Institute of Technology, Princeton University, and University of Tokyo invoke oxygen activation at the dicopper center followed by sequential electron transfer steps similar to mechanisms modeled for hemocyanin and tyrosinase analogs studied at University of California, Berkeley and University of Geneva. Kinetic and spectroscopic experiments conducted in laboratories at University of Chicago and Imperial College London support a cycle involving oxy, met, and deoxy states, with intermediates detected using methods pioneered at Lawrence Berkeley National Laboratory and Rutherford Appleton Laboratory. Computational chemists from ETH Zurich and University of Pennsylvania have applied quantum mechanics/molecular mechanics (QM/MM) to delineate transition states and energy barriers for mono- and diphenol oxidations.
Polyphenol oxidases are widespread in Kingdom Plantae, present in members studied at University of California, Davis and Cornell University, and occur in fungi researched at University of Copenhagen and University of Melbourne and some bacteria investigated by teams at MIT and University of British Columbia. In plants, PPOs contribute to wound sealing, pathogen defense, and secondary metabolism, phenomena documented in model organisms at John Innes Centre and Sainsbury Laboratory. Fungal laccases related to PPO families have roles in lignin degradation examined by researchers at Yale University and industrial projects from National Renewable Energy Laboratory. Animal and microbial PPO-like enzymes feature in pigmentation and immune responses explored by groups at University of California, San Diego and Max Planck Institute for Chemical Ecology.
Enzymes commonly classified within the polyphenol oxidase umbrella include tyrosinases, catechol oxidases, and laccases, each with distinct substrate scopes characterized in comparative studies at University of Strasbourg, University of Freiburg, and University of Helsinki. Tyrosinases, investigated by teams at University of São Paulo and University of Porto, catalyze both monophenol hydroxylation and o-diphenol oxidation; catechol oxidases, analyzed at University of Barcelona and University of Granada, preferentially oxidize o-diphenols; laccases, extensively engineered at Aalto University and Chalmers University of Technology, oxidize a broad range of phenolics and non-phenolic substrates via radical mechanisms. Comparative genomics work from Broad Institute and Wellcome Sanger Institute has mapped gene families and sequence motifs that underlie specificity differences.
Gene expression patterns for PPOs are regulated developmentally and by stress signals, as shown in transcriptomic studies at Michigan State University, University of Illinois, and University of Melbourne. Post-translational processing, glycosylation, and targeting to plastids or secretory pathways have been characterized by proteomics teams at EMBL and Max Planck Institute for Molecular Plant Physiology. Inhibitors such as ascorbic acid, citric acid, and synthetic chelators have been evaluated in applied studies at Nestlé Research Center, PepsiCo Research & Development, and academic labs at University of Minnesota; clinical and pharmacological inhibitor development has engaged researchers at Novartis and Pfizer for potential cosmetic and therapeutic uses.
PPO-mediated oxidation of phenolics to quinones leads to enzymatic browning in fruits and vegetables, a process extensively documented by food science groups at University of California, Davis, University of Reading, and University of Florida. Browning affects commodity value in crops studied by breeders at International Potato Center, CIMMYT, and IRRI, prompting strategies including genetic modification exemplified by work at Donald Danforth Plant Science Center and post-harvest interventions developed at USDA ARS. Economic impacts have driven collaborations between academe and industry, involving partners like PepsiCo and Kellogg Company to reduce quality loss in processing.
PPOs and related oxidases are exploited in bioremediation, biosensors, biocatalysis, and textile processing, with applied projects at Lawrence Livermore National Laboratory, Dow Chemical Company, and startups spun out of University of Cambridge. Laccases are employed for pulp bleaching and wastewater treatment in industrial demonstrations coordinated with EPA guidelines and engineering teams at ABB and Siemens. Enzyme engineering, immobilization, and process integration efforts have been led by groups at ETH Zurich, Technical University of Munich, and Delft University of Technology to enhance stability and substrate scope for commercial applications.
Category:Enzymes