Production and characterisation of AoSOX2 from Aspergillus oryzae, a novel flavin-dependent sulfhydryl oxidase with good pH and temperature stability

Characterization of wheat endoplasmic reticulum oxidoreductin 1 and its application in Chinese steamed bread

This study investigated characteristics of recombinant wheat Endoplasmic Reticulum Oxidoreductin 1 (wEro1) and its influence on Chinese steamed bread (CSB) qualities. The purified wEro1 monomer, which contained two conserved redox active motif sites, bound to flavin adenine dinucleotide (FAD) cofactor with a molecular weight of ∼47 kDa. wEro1 catalyzed the reduction of both bound and free FAD, and its reduction activity of free FAD reached 7.8 U/mg. Moreover, wEro1 catalyzed the oxidation of dithiothreitol and wheat protein disulfide isomerase (wPDI). Both glutathione and the reduced ribonuclease could work as electron donors for wEro1 in catalyzing the oxidation of wPDI. Additionally, wEro1 supplementation improved the CSB qualities with an increased specific volume of CSB and decreased crumb hardness, which was attributed to water-insoluble wheat proteins increasing and gluten network strengthening. The results give an understanding of the properties and function of wEro1 to facilitate its application especially in the flour-processing industry.

Characterization of sulfhydryl oxidase from Aspergillus tubingensis

Background

Despite of the presence of sulfhydryl oxidases (SOXs) in the secretomes of industrially relevant organisms and their many potential applications, only few of these enzymes have been biochemically characterized. In addition, basic functions of most of the SOX enzymes reported so far are not fully understood. In particular, the physiological role of secreted fungal SOXs is unclear.

Results

The recently identified SOX from Aspergillus tubingensis (AtSOX) was produced, purified and characterized in the present work. AtSOX had a pH optimum of 6.5, and showed a good pH stability retaining more than 80% of the initial activity in a pH range 4-8.5 within 20 h. More than 70% of the initial activity was retained after incubation at 50 °C for 20 h. AtSOX contains a non-covalently bound flavin cofactor. The enzyme oxidised a sulfhydryl group of glutathione to form a disulfide bond, as verified by nuclear magnetic resonance spectroscopy. AtSOX preferred glutathione as a substrate over cysteine and dithiothreitol. The activity of the enzyme was totally inhibited by 10 mM zinc sulphate. Peptide- and protein-bound sulfhydryl groups in bikunin, gliotoxin, holomycin, insulin B chain, and ribonuclease A, were not oxidised by the enzyme. Based on the analysis of 33 fungal genomes, SOX enzyme encoding genes were found close to nonribosomal peptide synthetases (NRPS) but not with polyketide synthases (PKS). In the phylogenetic tree, constructed from 25 SOX and thioredoxin reductase sequences from IPR000103 InterPro family, AtSOX was evolutionary closely related to other Aspergillus SOXs. Oxidoreductases involved in the maturation of nonribosomal peptides of fungal and bacterial origin, namely GliT, HlmI and DepH, were also evolutionary closely related to AtSOX whereas fungal thioreductases were more distant.

Conclusions

AtSOX (55 kDa) is a fungal secreted flavin-dependent enzyme with good stability to both pH and temperature. A Michaelis-Menten behaviour was observed with reduced glutathione as a substrate. Based on the location of SOX enzyme encoding genes close to NRPSs, SOXs could be involved in the secondary metabolism and act as an accessory enzyme in the production of nonribosomal peptides.

Electronic supplementary material

The online version of this article (10.1186/s12858-017-0090-4) contains supplementary material, which is available to authorized users.

Discovery of novel secreted fungal sulfhydryl oxidases with a plate test screen

Sulfhydryl oxidases (SOX) are FAD-dependent enzymes capable of oxidising free thiol groups and forming disulphide bonds. Although the quantity of scientific papers and suggested applications for SOX is constantly increasing, only a limited number of microbial SOX have been reported and are commercially available. Hence, the aim of this study was to develop a fast and reliable qualitative plate test for screening novel secreted fungal SOX. The screening was based on the Ellman's reagent, i.e. 5,5'-dithiobis[2-nitrobenzoic acid]. Altogether, 32 fungal strains from an in-house culture collection were screened. A total of 13 SOX-producing strains were found positive in the plate test screen. The novel SOX producers were Aspergillus tubingensis, Chaetomium globusum, Melanocarpus albomyces, Penicillium aurantiogriseum, Penicillium funiculosum, Coniophora puteana and Trametes hirsuta. Six of the discovered SOX were partially characterised by determination of isoelectric point, pH optimum and substrate specificity. A. tubingensis was identified as the most efficient novel SOX producer.