Multicopper oxidases with laccase-ferroxidase activity: Classification and study of ferroxidase activity determinants in a member from Heterobasidion annosum s. l.

Multi-copper oxidases (MCO) share a common molecular architecture and the use of copper ions as cofactors to reduce O2 to H2O, but show high sequence heterogeneity and functional diversity. Many new emerging MCO genes are wrongly annotated as laccases, the largest group of MCOs, with the widest range of biotechnological applications (particularly those from basidiomycete fungi) due to their ability to oxidise aromatic compounds and lignin. Thus, comprehensive studies for a better classification and structure-function characterisation of MCO families are required. Laccase-ferroxidases (LAC-FOXs) constitute a separate and unexplored group of MCOs with proposed dual features between laccases and ferroxidases. We aim to better define this cluster and the structural determinants underlying putative hybrid activity. We performed a phylogenetic analysis of the LAC-FOXs from basidiomycete fungi, that resulted in two subgroups. This division seemed to correlate with the presence or absence of some of the three acidic residues responsible for ferroxidase activity in Fet3p from Saccharomyces cerevisiae. One of these LAC-FOXs (with only one of these residues) from the fungus Heterobasidion annosum s. l. (HaLF) was synthesised, heterologously expressed and characterised to evaluate its catalytic activity. HaLF oxidised typical laccase substrates (phenols, aryl amines and N-heterocycles), but no Fe (II). The enzyme was subjected to site-directed mutagenesis to determine the key residues that confer ferroxidase activity. The mutated HaLF variant with full restoration of the three acidic residues exhibited efficient ferroxidase activity, while it partially retained the wide-range oxidative activity of the native enzyme associated to laccases sensu stricto.