Fungal Peroxygenases: A Phylogenetically Old Superfamily of Heme Enzymes with Promiscuity for Oxygen Transfer Reactions

Fungal unspecific peroxygenases (UPOs, EC 1.11.2.1) form a superfamily of heme proteins and possess promiscuity for oxygen transfer reactions. The first UPO was discovered in the agaric fungus Agrocybe aegerita in 2004, and later, well-known fungal chloroperoxidase (CPO) turned out to be a phylogenetic relative (but outlier) of this enzyme type. Functionally, UPOs are extracellular monoperoxygenases that transfer one oxygen atom from a peroxide to diverse organic substrates including alkanes/alkyls, cycloalkanes/cycloalkyls, aromatics/phenyls, heterocycles, halides, etc. These target structures are subject of hydroxylation, epoxidation, O- and N-dealkylation, sulfoxidation, N-oxidation and deacylation. In many cases, the product patterns of UPOs resemble those of cytochrome P450 monooxygenases (P450), and in fact, their catalytic cycle combines characteristics of heme peroxidases and P450s (‘peroxide shunt’). Meanwhile, several thousand putative UPO sequences that form two large clusters—‘short’ and ‘long’ peroxygenases—have been found in genetic databases and fungal genomes, indicating their widespread occurrence in the whole fungal kingdom. This includes all phyla of true fungi (Eumycota) and certain pseudofungal stramenopiles (formerly known as ‘Oomycota’). Despite their ubiquitous occurrence in the fungal kingdom, only a handful of UPOs have been characterized so far, and almost nothing substantial is known on their natural function(s). Their reliable heterologous expression remains tricky, despite all recent progress in this field.