Cloning of a thermostable ascorbate oxidase gene from Acremonium sp. HI-25 and modification of the azide sensitivity of the enzyme by site-directed mutagenesis

A Fungal Ascorbate Oxidase with Unexpected Laccase Activity

Ascorbate oxidases are an enzyme group that has not been explored to a large extent. So far, mainly ascorbate oxidases from plants and only a few from fungi have been described. Although ascorbate oxidases belong to the well-studied enzyme family of multi-copper oxidases, their function is still unclear. In this study, Af_AO1, an enzyme from the fungus Aspergillus flavus, was characterized. Sequence analyses and copper content determination demonstrated Af_AO1 to belong to the multi-copper oxidase family. Biochemical characterization and 3D-modeling revealed a similarity to ascorbate oxidases, but also to laccases. Af_AO1 had a 10-fold higher affinity to ascorbic acid (K_M = 0.16 ± 0.03 mM) than to ABTS (K_M = 1.89 ± 0.12 mM). Furthermore, the best fitting 3D-model was based on the ascorbate oxidase from Cucurbita pepo var. melopepo. The laccase-like activity of Af_AO1 on ABTS (V_max = 11.56 ± 0.15 µM/min/mg) was, however, not negligible. On the other hand, other typical laccase substrates, such as syringaldezine and guaiacol, were not oxidized by Af_AO1. According to the biochemical and structural characterization, Af_AO1 was classified as ascorbate oxidase with unusual, laccase-like activity.

Characterization of three multicopper oxidases in the filamentous fungus Podospora anserina: A new role of an ABR1-like protein in fungal development?

The Podospora anserina genome contains a large family of 15 multicopper oxidases (MCOs), including three genes encoding a FET3-like protein, an ABR1-like protein and an ascorbate oxidase (AO)-like protein. FET3, ABR1 and AO1 are involved in global laccase-like activity since deletion of the relevant genes led to a decrease of activity when laccase substrate (ABTS) was used as substrate. However, contrary to the P. anserina MCO proteins previously characterized, none of these three MCOs seemed to be involved in lignocellulose degradation and in resistance to phenolic compounds and oxidative stress. We showed that the bulk of ferroxidase activity was clearly due to ABR1, and only in minor part to FET3, although ABR1 does not contain all the residues typical of FET3 proteins. Moreover, we showed that ABR1, related to the Aspergillus fumigatus ABR1 protein, was clearly and specifically involved in pigmentation of ascospores. Surprisingly, phenotypes were more severe in mutants lacking both abr1 and ao1. Deletion of the ao1 gene led to an almost total loss of AO activity. No direct involvement of AO1 in fungal developmental process in P. anserina was evidenced, except in a abr1^Δ background. Overall, unlike other previously characterized MCOs, we thus evidence a clear involvement of ABR1 protein in fungal development.

Multiple multi-copper oxidase gene families in basidiomycetes - what for?

Genome analyses revealed in various basidiomycetes the existence of multiple genes for blue multi-copper oxidases (MCOs). Whole genomes are now available from saprotrophs, white rot and brown rot species, plant and animal pathogens and ectomycorrhizal species. Total numbers (from 1 to 17) and types of mco genes differ between analyzed species with no easy to recognize connection of gene distribution to fungal life styles. Types of mco genes might be present in one and absent in another fungus. Distinct types of genes have been multiplied at speciation in different organisms. Phylogenetic analysis defined different subfamilies of laccases sensu stricto (specific to Agaricomycetes), classical Fe2+-oxidizing Fet3-like ferroxidases, potential ferroxidases/laccases exhibiting either one or both of these enzymatic functions, enzymes clustering with pigment MCOs and putative ascorbate oxidases. Biochemically best described are laccases sensu stricto due to their proposed roles in degradation of wood, straw and plant litter and due to the large interest in these enzymes in biotechnology. However, biological functions of laccases and other MCOs are generally little addressed. Functions in substrate degradation, symbiontic and pathogenic intercations, development, pigmentation and copper homeostasis have been put forward. Evidences for biological functions are in most instances rather circumstantial by correlations of expression. Multiple factors impede research on biological functions such as difficulties of defining suitable biological systems for molecular research, the broad and overlapping substrate spectrum multi-copper oxidases usually possess, the low existent knowledge on their natural substrates, difficulties imposed by low expression or expression of multiple enzymes, and difficulties in expressing enzymes heterologously.

Phylogenetic comparison and classification of laccase and related multicopper oxidase protein sequences

A phylogenetic analysis of more than 350 multicopper oxidases (MCOs) from fungi, insects, plants, and bacteria provided the basis for a refined classification of this enzyme family into laccases sensu stricto (basidiomycetous and ascomycetous), insect laccases, fungal pigment MCOs, fungal ferroxidases, ascorbate oxidases, plant laccase-like MCOs, and bilirubin oxidases. Within the largest group of enzymes, formed by the 125 basidiomycetous laccases, the gene phylogeny does not strictly follow the species phylogeny. The enzymes seem to group at least partially according to the lifestyle of the corresponding species. Analyses of the completely sequenced fungal genomes showed that the composition of MCOs in the different species can be very variable. Some species seem to encode only ferroxidases, whereas others have proteins which are distributed over up to four different functional clusters in the phylogenetic tree.

Genes for multicopper proteins and laccase activity: common features in plant-associatedFusariumisolates

Eight Fusarium isolates from diverse plant habitats produced laccase activities when cultured on low-carbon medium. Three sequences with high homology to laccase genes were detected in the genome of Fusarium proliferatum, NRRL 31071, an opportunistic pathogen of wheat (Triticum spp.). The sequences aligned with laccase genes from Ascomycetes, whereas genes from other fungal taxonomic groups and from plants grouped separately. Sequences for related multicopper oxidases with ascorbate oxidase activity were also separated discretely. A fourth sequence from NRRL 31071 had highest homology to genes for multicopper proteins associated with ferrooxidase activities in yeasts. Each of the four sequences had high homology to sequences within the genome of a ligninolytic F. proliferatum from forest soil. RFLP analysis of the genomes of three other F. proliferatum isolates and a phylogenetically closely related isolate, Fusarium fujikuroi, showed that fragments hybridizing with the four multicopper oxidase genes were similar in size to those from the NRRL 31071 genome. Weaker hybridization was observed with the phylogenetically more distant Fusarium anthophilum. No hybridization was observed with an isolate of Fusarium culmorum, a wheat pathogen, from a different section of the genus Fusarium, although this isolate produced laccases.Key words: ferrooxidase, Fusarium proliferatum, laccase, multicopper oxidase.

Upward shift of the pH optimum of Acremonium ascorbate oxidase

A gene encoding a thermostable Acremonium ascorbate oxidase (ASOM) was randomly mutated to generate mutant enzymes with altered pH optima. One of the mutants, which exhibited a significantly higher activity in the pH range 4.5-7 compared to ASOM, had a Gln183Arg substitution in the region corresponding to SBR1, one of the substrate binding regions of the zucchini enzyme. The other mutant with almost the same pH profile as Gln183Arg had a Thr527Ala substitution near the type 3 copper center and became more sensitive to azide than ASOM. Site-directed mutagenesis in the substrate binding regions with reference to the amino acid sequences of plant enzymes led to isolation of mutants shifted upward in the pH optimum; Val193Pro and Val193Pro/Pro190Ile increased the pH optimum by 1 and 0.5 units, respectively, while retaining the near-wild-type thermostability and azide sensitivity. The homology model of ASOM constructed from the zucchini enzyme coordinates suggested that replacement of Val193 by Pro could disturb the ion pair networks among Arg309, Glu192, Arg194 and Glu311. This perturbation could affect either the molecular recognition between the substrate and ASOM or the electron transfer from the substrate to the type 1 copper center, leading to the alkaline shift of the catalytic activity of the mutant enzyme. The other mutations, Val193Pro/Pro190Ile, could also induce similar structural perturbations involving the ion pair networks.