Structure and mechanism of galactose oxidase. The free radical site

Navigating Side Reactions for Robust Colorimetric Detection of Galactose Oxidase Activity

Colorimetric assays are a rapid, scalable technique well suited to enzyme activity screening. However, side reactions or chromogenic reagent instability can result in false positives or false negatives that compromise the accuracy of such assays. Here, we identify three classes of compounds incompatible with the 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) colorimetric assay for galactose oxidase activity. Dark green ABTS·+ cationic radicals indicating enzyme activity can get quenched to yield colorless solutions or couple with substrates to form differently colored adducts, thus preventing accurate colorimetric measurements. These side reactions limit the utility of the ABTS assay and introduce uncertainty in the substrate scope to which it is applicable. We have investigated the underlying mechanisms behind these side reactions to conclude that free radical scavengers, phenols with electron-donating substituents, and β,γ-unsaturated aryl ketones are incompatible with the ABTS colorimetric assay. In search of a viable alternative, we developed an assay using 2,4-dinitrophenylhydrazine under neutral conditions with isopropyl alcohol as a solubilizing agent. The use of neutral conditions was found to be critical to avoid hydrolysis of hydrazone adducts, ensuring reproducible measurements. Our assay is compatible with free radical scavengers (R2 = 0.98), phenols with electron-donating substituents (R2 = 0.97), and β,γ-unsaturated aryl ketones (R2 = 0.88). This modified assay enables galactose oxidase activity screening across a broader substrate scope, thus facilitating enzyme use for more practical applications.

Expression of the Fusarium graminearum galactose oxidase GaoA in Saccharomyces cerevisiae

Galactose oxidase, produced by fungi of the genus Fusarium, is an enzyme of great biotechnological importance. The gaoA gene has been recombinantly expressed in several hosts but has yet to be in Saccharomyces cerevisiae. This work aimed to express the Fusarium graminearum GaoA enzyme in S. cerevisiae. The full-length and the truncated F. graminearum gaoA gene were subcloned into a yeast expression vector. The GaoA enzyme expression level in S. cerevisiae was higher when the truncated gene, which codes for the mature form of the enzyme, was used. After purification of the expressed enzyme on a Sepharose® 6B column, the obtained yield of the pure and active enzyme was 16.7 mg/L. The purified protein showed a KM of 9.8 mM, lower than that of the wild-type enzyme, and a kcat/KM of 2.9 × 107 M-1s-1, higher than that of the wild-type enzyme. The expressed recombinant protein used several common substrates for galactose oxidase, such as galactose, raffinose, and 1,3-dihydroxyacetone dimer. In addition, it had increased activity on guar gum, lactose, and Arabic gum compared with the wild-type enzyme. The obtained enzyme's characteristics are compatible with the galactose oxidase biotechnological applications.

Stabilization of galactose oxidase by high hydrostatic pressure: Insights on the role of cavities size

High hydrostatic pressure stabilized galactose oxidase (GaOx) at 70.0-80.0°C against thermal inactivation. The pseudo-first-order rate constant of inactivation kinact decreased by a factor of 8 at 80°C and by a factor of 44 at 72.5°C. The most pronounced effect of pressure was at the lowest studied temperature of 70.0°C with an activation volume of inactivation ΔV‡ of 78.8 cm3 mol-1. The optimal pressure against thermal inactivation was between 200 and 300 MPa. Unlike other enzymes, as temperature increased the ΔV‡ of inactivation decreased, and as pressure increased the activation energy of inactivation Eai increased. Combining the results for GaOx with earlier research on the pressure-induced stabilization of other enzymes suggests that ΔV‡ of inactivation correlates with the total molar volume of cavities larger than ~100 Å3 in enzyme monomers for enzymes near the optimal pH and whose thermal unfolding is not accompanied by oligomer dissociation.

Diversity of oxidation state in copper complexes with phenolate ligands

The phenoxyl radical binding copper complexes have been widely developed and their detailed geometric and electronic structures have been clarified. While many one-electron oxidized CuII-phenolate complexes have been reported previously, recent studies of the Cu-phenolate complexes proceed toward elucidation of the complexes with other oxidation states, such as the phenoxyl radical binding CuI complexes and CuIV-phenolate complexes in the formal oxidation state. This Perspective focuses on new aspects of the properties and reactivities of various Cu-phenolate and Cu-phenoxyl radical complexes with emphasis on the relationship between geometric and electronic structures.

Development of a New Amperometric Biosensor for Measurement of Plasma Galactose Levels

Galactosemia is an inherited disease that occurs as a result of insufficient or no synthesis of some enzymes (GALT, GALK, and GALE) in galactose metabolism. Failure to make an early diagnosis, especially in newborns, can lead to severe clinical and even fatal consequences. The aim of this study is to develop a biosensor for measuring free galactose in plasma. The immobilization components of the developed free galactose biosensor are screen printed carbon electrode (SCPE), Prussian blue (PB), chitosan (CHIT), Nafion (NAF), gold nanoparticle (GNP), and galactose oxidase (GaOX). The CHIT/GaOX/NAF-GNP/GaOX/CHIT-GNP/SCPE-PB electrode showed a sensitive amperometric response to detect galactose. While the surface characterization of the biosensor was performed with cyclic voltammetry and scanning electron microscopy, the optimization and performance characterizations were made by applying an amperometry technique. The amperometric operating potential for the free galactose biosensor was determined as -0.05 V. The linear detection range for the free galactose biosensor is between 0.025 and 10 mM. This range includes galactose levels in plasma of both healthy and patients. The percent coefficient of variation values calculated for intraday and interday repeatability of the developed biosensor are below 10%. The practical use of the biosensor, for which optimization and characterization studies were carried out, was tested in 10 healthy 11 patients with galactosemia, and the results were compared with the colorimetric method. In conclusion, the unique analytical properties and effortless preparation of the new galactose biosensor developed in this study make them serious candidates for point-of-care diagnostic testing.

Crystal structure of a phenoxyl radical complex relevant to the metal site of the galactose oxidase enzyme: A facile one-pot synthesis, evidence for hydrogen atom transfer and DNA cleavage via self-activation

Copper complexes [Cu(L1H)ClO4] (1) and [Cu(L2)NO3] (2), which are relevant to the metal site of the galactose oxidase enzyme, were synthesized and characterized by different spectroscopic methods. L1H2 and L2H2 [where L1H2 stands for 2,2'-((1E,1'E)(2,2'-(pyridine-2,6-diyl)bis(2-phenylhydrazin-2-yl-1-ylidene))bis(methanylylidene))diphenol and L2H2 stands for 6,6'-((1E,1'E)-(2,2'-(pyridine-2,6-diyl)bis(2-phenylhydrazin-2-yl-1-ylidene))bis(methanylylidene))bis(2,4-di-tert-butylphenol), H stands for dissociable proton] are pentadentate ligands. These ligands provide pyridyl N, two imine N, and two non-innocent phenoxyl and phenolato O donors, forming complex 1 as a non-radical complex, while complex 2 is a phenoxyl radical complex. The molecular structures of complexes 1 and 2 were authenticated by X-ray crystallography. Benzyl alcohol oxidation was investigated, and the conversion of 9,10-dihydroanthracene to anthracene was examined to scrutinize the H-atom abstraction reaction. Nuclease activity with complexes 1 and 2 was investigated by self-activated plasmid DNA (pBR322) cleavage. Non-innocent properties of the ligand-containing phenolato function were investigated by DFT calculations.

Co-immobilization of galactose oxidase, catalase, and Mn-superoxide dismutase for efficient conversion of 5-hydroxymethylfurfural to 2,5-diformylfuran in water

The three enzymes galactose oxidase (GO), catalase (CAT), and Mn-superoxide dismutase (SOD) were simultaneously immobilized by coordinating to CuII in phosphate buffer saline. The biocatalyst GO&CAT&SOD@CuII was used for the conversion of 5-hydroxymethylfurfural (HMF). The immobilized GO catalyzes the oxidation of HMF to 2,5-diformylfuran (DFF), concomitantly the co-substrate O2 is reduced to hydrogen peroxide (H2O2). A portion of the byproduct H2O2 is broken down to O2 and H2O by the co-immobilized CAT, and the evolved O2 can be recycled and used as the co-substrate. A portion of the byproduct H2O2 is broken down to produce hydroxyl radicals •OH under the synergistic catalysis of the immobilized SOD and coordinated CuII, and the produced •OH can reactivate the immobilized galactose oxidase. Two aspects contribute to the high catalytic efficiency by GO&CAT&SOD@CuII: the reactivation of the immobilized galactose oxidase by producing •OH and the enrichment of the co-substate O2 by recycling the produced O2. For the conversion of 10 mM HMF, GO&CAT&SOD@CuII (with encapsulated GO 0.2 mg/mL) achieved 97% HMF conversion within 2 h reaction. In contrast, free galactose oxidase M3-5 variant (ACS Catalysis 2018, 8, 4025) (0.2 mg/mL) achieved 25.3% HMF conversion within 2 h reaction. All the reactions were carried out in pure water, not in PBS.

Comparative Transcriptome Analysis Reveals Novel Candidate Resistance Genes Involved in Defence against Phytophthora cactorum in Strawberry

Crown rot, caused by Phytophthora cactorum, is a devastating disease of strawberry. While most commercial octoploid strawberry cultivars (Fragaria × ananassa Duch) are generally susceptible, the diploid species Fragaria vesca is a potential source of resistance genes to P. cactorum. We previously reported several F. vesca genotypes with varying degrees of resistance to P. cactorum. To gain insights into the strawberry defence mechanisms, comparative transcriptome profiles of two resistant genotypes (NCGR1603 and Bukammen) and a susceptible genotype (NCGR1218) of F. vesca were analysed by RNA-Seq after wounding and subsequent inoculation with P. cactorum. Differential gene expression analysis identified several defence-related genes that are highly expressed in the resistant genotypes relative to the susceptible genotype in response to P. cactorum after wounding. These included putative disease resistance (R) genes encoding receptor-like proteins, receptor-like kinases, nucleotide-binding sites, leucine-rich repeat proteins, RPW8-type disease resistance proteins, and 'pathogenesis-related protein 1'. Seven of these R-genes were expressed only in the resistant genotypes and not in the susceptible genotype, and these appeared to be present only in the genomes of the resistant genotypes, as confirmed by PCR analysis. We previously reported a single major gene locus RPc-1 (Resistance to Phytophthora cactorum 1) in F. vesca that contributed resistance to P. cactorum. Here, we report that 4-5% of the genes (35-38 of ca 800 genes) in the RPc-1 locus are differentially expressed in the resistant genotypes compared to the susceptible genotype after inoculation with P. cactorum. In particular, we identified three defence-related genes encoding wall-associated receptor-like kinase 3, receptor-like protein 12, and non-specific lipid-transfer protein 1-like that were highly expressed in the resistant genotypes compared to the susceptible one. The present study reports several novel candidate disease resistance genes that warrant further investigation for their role in plant defence against P. cactorum.

Characterization of a Galactose Oxidase from Fusarium odoratissimum and Its Application in the Modification of Agarose

A galactose oxidase gene, gao-5f, was cloned from Fusarium odoratissimum and successfully expressed in E. coli. The galactose oxidase GAO-5F belongs to the AA5 family and consists of 681 amino acids, with an estimated molecular weight of 72 kDa. GAO-5F exhibited maximum activity at 40 °C and pH 7.0 and showed no change in activity after 24 h incubation at 30 °C. Moreover, GAO-5F exhibited 40% of its maximum activity after 24 h incubation at 50 °C and 60% after 40 h incubation at pH 7.0. The measured thermostability of GAO-5F is superior to galactose oxidase's reported thermostability. The enzyme exhibited strict substrate specificity toward D-galactose and oligosaccharides/polysaccharides containing D-galactose. Further analysis demonstrated that GAO-5F specifically oxidized agarose to a polyaldehyde-based polymer, which could be used as a polyaldehyde to crosslink with gelatin to form edible packaging films. To our knowledge, this is the first report about the modification of agarose by galactose oxidase, and this result has laid a foundation for the further development of edible membranes using agarose.

Characterization of a novel AA3_1 xylooligosaccharide dehydrogenase from Thermothelomyces myriococcoides CBS 398.93

BACKGROUND

The Carbohydrate-Active enZymes (CAZy) auxiliary activity family 3 (AA3) comprises flavin adenine dinucleotide-dependent (FAD) oxidoreductases from the glucose-methanol-choline (GMC) family, which play auxiliary roles in lignocellulose conversion. The AA3 subfamily 1 predominantly consists of cellobiose dehydrogenases (CDHs) that typically comprise a dehydrogenase domain, a cytochrome domain, and a carbohydrate-binding module from family 1 (CBM1).

RESULTS

In this work, an AA3_1 gene from T. myriococcoides CBS 398.93 encoding only a GMC dehydrogenase domain was expressed in Aspergillus niger. Like previously characterized CDHs, this enzyme (TmXdhA) predominantly accepts linear saccharides with β-(1 → 4) linkage and targets the hydroxyl on the reducing anomeric carbon. TmXdhA was distinguished, however, by its preferential activity towards xylooligosaccharides over cellooligosaccharides. Amino acid sequence analysis showed that TmXdhA possesses a glutamine at the substrate-binding site rather than a threonine or serine that occupies this position in previously characterized CDHs, and structural models suggest the glutamine in TmXdhA could facilitate binding to pentose sugars.

CONCLUSIONS

The biochemical analysis of TmXdhA revealed a catalytic preference for xylooligosaccharide substrates. The modeled structure of TmXdhA provides a reference for the screening of oxidoreductases targeting xylooligosaccharides. We anticipate TmXdhA to be a good candidate for the conversion of xylooligosaccharides to added-value chemicals by its exceptional catalytic ability.

Copper-radical oxidases: A diverse group of biocatalysts with distinct properties and a broad range of biotechnological applications

Copper-radical oxidases (CROs) catalyze the two-electron oxidation of a large number of primary alcohols including carbohydrates, polyols and benzylic alcohols as well as aldehydes and α-hydroxy-carbonyl compounds while reducing molecular oxygen to hydrogen peroxide. Initially, CROs like galactose oxidase and glyoxal oxidase were identified only in fungal secretomes. Since the last decade, their representatives have also been identified in some bacteria. CROs are grouped in the AA5 family of "auxiliary activities" in the database of Carbohydrate-Active enzymes. Despite low overall sequence similarity and different substrate specificities, sequence alignments and the solved crystal structures revealed a conserved architecture of the active sites in all CROs, with a mononuclear copper ion coordinated to an axial tyrosine, two histidines, and a cross-linked cysteine-tyrosyl radical cofactor. This unique post-translationally modified protein cofactor has attracted much attention in the past, which resulted in a large number of reports that shed light on key steps of the catalytic cycle and physico-chemical properties of CROs. Thanks to their broad substrate spectrum accompanied by the only need for molecular oxygen for catalysis, CROs since recently experience a renaissance and have been applied in various biocatalytic processes. This review provides an overview of the structural features, catalytic mechanism and substrates of CROs, presents an update on the engineering of these enzymes to improve their expression in recombinant hosts and to enhance their activity, and describes their potential fields of biotechnological application.

A simple and direct ionic chromatography method to monitor galactose oxidase activity

Galactose oxidase (GalOx, EC.1.1.3.9) is one of the most extensively studied copper radical oxidases (CROs). The reaction catalyzed by GalOx leads to the oxidation of the C-6 hydroxyl group of galactose and galactosides (including galactosylated polysaccharides and glycoproteins) to the corresponding aldehydes, coupled to the reduction of dioxygen to hydrogen peroxide. Despite more than 60 years of research including mechanistic studies, enzyme engineering and application development, GalOx activity remains primarily monitored by indirect measurement of the co-product hydrogen peroxide. Here, we describe a simple direct method to measure GalOx activity through the identification of galactosylated oxidized products using high-performance anion-exchange chromatography coupled to pulsed amperometric detection (HPAEC-PAD). Using galactose and lactose as representative substrates, we were able to separate and detect the C-6 oxidized products, which were confirmed by LC-MS and NMR analyses to exist in their hydrated (geminal-diol) forms. We show that the HPAEC-PAD method is superior to other methods in terms of sensitivity as we could detect down to 0.08 μM of Lac^OX (eq. 30 μg L⁻¹). We believe the method will prove useful for qualitative detection of galactose oxidase activity in biological samples or for quantitative purposes to analyze enzyme kinetics or to compare enzyme variants in directed evolution programs.

Galactose oxidase (GalOx, EC.1.1.3.9) is one of the most extensively studied copper radical oxidases. Here, we show it can be monitored through the release of oxidized galactosylated products using a simple, direct and sensitive HPAEC-PAD method.

A molecular dynamics-guided mutagenesis identifies two aspartic acid residues involved in the pH-dependent activity of OG-OXIDASE 1

During the infection, plant cells secrete different OG-oxidase (OGOX) paralogs, defense flavoproteins that oxidize the oligogalacturonides (OGs), homogalacturonan fragments released from the plant cell wall that act as Damage Associated Molecular Patterns. OGOX-mediated oxidation inactivates their elicitor nature, but on the other hand makes OGs less hydrolysable by microbial endo-polygalacturonases (PGs). Among the different plant defense responses, apoplastic alkalinization can further reduce the degrading potential of PGs by boosting the oxidizing activity of OGOXs. Accordingly, the different OGOXs so far characterized showed an optimal activity at pH values greater than 8. Here, an approach of molecular dynamics (MD)-guided mutagenesis succeeded in identifying the amino acids responsible for the pH dependent activity of OGOX1 from Arabidopsis thaliana. MD simulations indicated that in alkaline conditions (pH 8.5), the residues Asp325 and Asp344 are engaged in the formation of two salt bridges with Arg327 and Lys415, respectively, at the rim of enzyme active site. According to MD analysis, the presence of such ionic bonds modulates the size and flexibility of the cavity used to accommodate the OGs, in turn affecting the activity of OGOX1. Based on functional properties of the site-directed mutants OGOX1.D325A and OGOX.D344A, we demonstrated that Asp325 and Asp344 are major determinants of the alkaline-dependent activity of OGOX1.

A Putative Lignin Copper Oxidase from Trichoderma reesei

The ability of Trichoderma reesei, a fungus widely used for the commercial production of hemicellulases and cellulases, to grow and modify technical soda lignin was investigated. By quantifying fungal genomic DNA, T. reesei showed growth and sporulation in solid and liquid cultures containing lignin alone. The analysis of released soluble lignin and residual insoluble lignin was indicative of enzymatic oxidative conversion of phenolic lignin side chains and the modification of lignin structure by cleaving the β-O-4 linkages. The results also showed that polymerization reactions were taking place. A proteomic analysis conducted to investigate secreted proteins at days 3, 7, and 14 of growth revealed the presence of five auxiliary activity (AA) enzymes in the secretome: AA6, AA9, two AA3 enzymes), and the only copper radical oxidase encoded in the genome of T. reesei. This enzyme was heterologously produced and characterized, and its activity on lignin-derived molecules was investigated. Phylogenetic characterization demonstrated that this enzyme belonged to the AA5_1 family, which includes characterized glyoxal oxidases. However, the enzyme displayed overlapping physicochemical and catalytic properties across the AA5 family. The enzyme was remarkably stable at high pH and oxidized both, alcohols and aldehydes with preference to the alcohol group. It was also active on lignin-derived phenolic molecules as well as simple carbohydrates. HPSEC and LC-MS analyses on the reactions of the produced protein on lignin dimers (SS ββ, SS βO4 and GG β5) uncovered the polymerizing activity of this enzyme, which was accordingly named lignin copper oxidase (TrLOx). Polymers of up 10 units were formed by hydroxy group oxidation and radical formation. The activations of lignin molecules by TrLOx along with the co-secretion of this enzyme with reductases and FAD flavoproteins oxidoreductases during growth on lignin suggest a synergistic mechanism for lignin breakdown.

Biocatalytic oxidation of fatty alcohols into aldehydes for the flavors and fragrances industry

From Egyptian mummies to the Chanel n°5 perfume, fatty aldehydes have long been used and keep impacting our senses in a wide range of foods, beverages and perfumes. Natural sources of fatty aldehydes are threatened by qualitative and quantitative variability while traditional chemical routes are insufficient to answer the society shift toward more sustainable and natural products. The production of fatty aldehydes using biotechnologies is therefore the most promising alternative for the flavors and fragrances industry. In this review, after drawing the portrait of the origin and characteristics of fragrant fatty aldehydes, we present the three main classes of enzymes that catalyze the reaction of fatty alcohols oxidation into aldehydes, namely alcohol dehydrogenases, flavin-dependent alcohol oxidases and copper radical alcohol oxidases. The constraints, challenges and opportunities to implement these oxidative enzymes in the flavors and fragrances industry are then discussed. By setting the scene on the biocatalytic production of fatty aldehydes, and providing a critical assessment of its potential, we expect this review to contribute to the development of biotechnology-based solutions in the flavors and fragrances industry.

Formation Mechanism of Cofactor Cys-Tyr in the Cysteine Dioxygenases (CDO and F2-CDO) and Its Influence on Catalysis: A QM/MM Study

Cysteine dioxygenase (CDO) is a nonheme mononuclear iron enzyme, which catalyzes the oxidation of cysteine to cysteine sulfinic acid. Crystal structure studies of mammalian CDO showed that there is a cross-linked cysteine-tyrosine (Cys-Tyr) cofactor in its active site. Moreover, the formation of the Cys-Tyr cofactor requires the metal cofactor (Fe²⁺) and O₂, and it was previously considered to substantially enhance the catalytic efficiency and half-life of CDO. Recently, a pure human CDO (F₂-CDO) without including the Cys-Tyr cofactor was crystalized by the site-directed mutagenesis approach in the anaerobic condition. In this work, to gain insights into the formation mechanism of the Cys-Tyr cofactor and whether it can really promote the catalytic reactivity of CDO, a series of computational models have been constructed, and quantum mechanical/molecular mechanical (QM/MM) calculations have been performed. Our calculation results reveal that WT-CDO and F₂-CDO follow different mechanisms for the formation of the Cys-Tyr cofactor. In F₂-CDO, the cofactor formation contains the H-abstraction, C-S bond formation, intramolecular F migration, and aromatization of the residue F₂Y157, in which the Fe-coordinate dioxygen can be recovered after the formation cofactor; however, in the WT-CDO, the cofactor formation shows some differences. During the reaction, hydrogen peroxide is generated, and the final aromatization requires the assistance of one water molecule. Furthermore, the overall barriers of cofactor formation are always higher than l-cysteine oxidation for both WT-CDO and F₂-CDO irrespective of the absence or presence of the cofactor. Thus, we can theoretically confirm that the Cys-Tyr cofactor is not essential for the oxidation activity of CDO, and cofactor formation is just an accompanying reaction but not a prerequisite for the oxidation reaction. These results may provide useful information for understanding the catalysis of CDO.

Recent Advances in One-Electron-Oxidized CuII -Diphenoxide Complexes as Models of Galactose Oxidase: Importance of the Structural Flexibility in the Active Site

The phenoxyl radical plays important roles in biological systems as cofactors in some metalloenzymes, such as galactose oxidase (GO) catalyzing oxidation of primary alcohols to give the corresponding aldehydes. Many metal(II)-phenoxyl radical complexes have hitherto been studied for understanding the detailed properties and reactivities of GO, and thus the nature of GO has gradually become clearer. However, the effects of the subtle geometric and electronic structural changes at the active site of GO, especially the structural change in the catalytic cycle and the effect of the second coordination sphere, have not been fully discussed yet. In this Review, we focus on further details of the model studies of GO and discuss the importance of the structural change at the active site of GO.

A High-Throughput Screening System Based on Droplet Microfluidics for Glucose Oxidase Gene Libraries

Glucose oxidase (GOx) is an important industrial enzyme that can be optimized for specific applications by mutagenesis and activity-based screening. To increase the efficiency of this approach, we have developed a new ultrahigh-throughput screening platform based on a microfluidic lab-on-chip device that allows the sorting of GOx mutants from a saturation mutagenesis library expressed on the surface of yeast cells. GOx activity was measured by monitoring the fluorescence of water microdroplets dispersed in perfluorinated oil. The signal was generated via a series of coupled enzyme reactions leading to the formation of fluorescein. Using this new method, we were able to enrich the yeast cell population by more than 35-fold for GOx mutants with higher than wild-type activity after two rounds of sorting, almost double the efficiency of our previously described flow cytometry platform. We identified and characterized novel GOx mutants, the most promising of which (M6) contained a combination of six point mutations that increased the catalytic constant kcat by 2.1-fold compared to wild-type GOx and by 1.4-fold compared to a parental GOx variant. The new microfluidic platform for GOx was therefore more sensitive than flow cytometry and supports comprehensive screens of gene libraries containing multiple mutations per gene.

Production of Galactose Oxidase Inside the Fusarium fujikuroi Species Complex and Recombinant Expression and Characterization of the Galactose Oxidase GaoA Protein from Fusarium subglutinans

Galactose oxidase catalyzes a two-electron oxidation, mainly from the C6 hydroxyl group of D-galactose, with the concomitant reduction of water to hydrogen peroxide. This enzyme is secreted by Fusarium species and has several biotechnological applications. In this study, a screening of galactose oxidase production among species of the Fusarium fujikuroi species complex demonstrated Fusarium subglutinans to be the main producer. The truncated F. subglutinans gaoA gene coding for the mature galactose oxidase was expressed from the prokaryotic vector pTrcHis2B in the E. coli Rosetta™ (DE3) strain. The purified recombinant enzyme presented temperature and pH optima of 30 °C and 7.0, respectively, K_M of 132.6 ± 18.18 mM, V_max of 3.2 ± 0.18 µmol of H₂O₂/min, k_cat of 12,243 s^-1, and a catalytic efficiency (k_cat/K_M) of 9.2 × 10⁴ M^-1 s^-1. In the presence of 50% glycerol, the enzyme showed a T₅₀ of 59.77 °C and was stable for several hours at pH 8.0 and 4 °C. Besides D-(+)-galactose, the purified enzyme also acted against D-(+)-raffinose, α-D-(+)-melibiose, and methyl-α-D-galactopyranoside, and was strongly inhibited by SDS. Although the F. subglutinans gaoA gene was successfully expressed in E. coli, its endogenous transcription was not confirmed by RT-PCR.

A family AA5_2 carbohydrate oxidase from Penicillium rubens displays functional overlap across the AA5 family

Copper radical alcohol oxidases belonging to auxiliary activity family 5, subfamily 2 (AA5_2) catalyze the oxidation of galactose and galactosides, as well as aliphatic alcohols. Despite their broad applied potential, so far very few AA5_2 members have been biochemically characterized. We report the recombinant production and biochemical characterization of an AA5_2 oxidase from Penicillium rubens Wisconsin 54–1255 (PruAA5_2A), which groups within an unmapped clade phylogenetically distant from those comprising AA5_2 members characterized to date. PruAA5_2 preferentially oxidized raffinose over galactose; however, its catalytic efficiency was 6.5 times higher on glycolaldehyde dimer compared to raffinose. Deep sequence analysis of characterized AA5_2 members highlighted amino acid pairs correlated to substrate range and conserved within the family. Moreover, PruAA5_2 activity spans substrate preferences previously reported for AA5 subfamily 1 and 2 members, identifying possible functional overlap across the AA5 family.

Probing the Cys-Tyr Cofactor Biogenesis in Cysteine Dioxygenase by the Genetic Incorporation of Fluorotyrosine

Cysteine dioxygenase (CDO) is a nonheme iron enzyme that adds two oxygen atoms from dioxygen to the sulfur atom of l-cysteine. Adjacent to the iron site of mammalian CDO, there is a post-translationally generated Cys-Tyr cofactor, whose presence substantially enhances the oxygenase activity. The formation of the Cys-Tyr cofactor in CDO is an autocatalytic process, and it is challenging to study by traditional techniques because the cross-linking reaction is a side, uncoupled, single-turnover oxidation buried among multiple turnovers of l-cysteine oxygenation. Here, we take advantage of our recent success in obtaining a purely uncross-linked human CDO due to site-specific incorporation of 3,5-difluoro-l-tyrosine (F₂-Tyr) at the cross-linking site through the genetic code expansion strategy. Using EPR spectroscopy, we show that nitric oxide (^•NO), an oxygen surrogate, similarly binds to uncross-linked F₂-Tyr157 CDO as in wild-type human CDO. We determined X-ray crystal structures of uncross-linked F₂-Tyr157 CDO and mature wild-type CDO in complex with both l-cysteine and ^•NO. These structural data reveal that the active site cysteine (Cys93 in the human enzyme), rather than the generally expected tyrosine (i.e., Tyr157), is well-aligned to be oxidized should the normal oxidation reaction uncouple. This structure-based understanding is further supported by a computational study with models built on the uncross-linked ternary complex structure. Together, these results strongly suggest that the first target to oxidize during the iron-assisted Cys-Tyr cofactor biogenesis is Cys93. Based on these data, a plausible reaction mechanism implementing a cysteine radical involved in the cross-link formation is proposed.

Biocatalytic Production of Amino Carbohydrates through Oxidoreductase and Transaminase Cascades

Plant-derived carbohydrates are an abundant renewable resource. Transformation of carbohydrates into new products, including amine-functionalized building blocks for biomaterials applications, can lower reliance on fossil resources. Herein, biocatalytic production routes to amino carbohydrates, including oligosaccharides, are demonstrated. In each case, two-step biocatalysis was performed to functionalize d-galactose-containing carbohydrates by employing the galactose oxidase from Fusarium graminearum or a pyranose dehydrogenase from Agaricus bisporus followed by the ω-transaminase from Chromobacterium violaceum (Cvi-ω-TA). Formation of 6-amino-6-deoxy-d-galactose, 2-amino-2-deoxy-d-galactose, and 2-amino-2-deoxy-6-aldo-d-galactose was confirmed by mass spectrometry. The activity of Cvi-ω-TA was highest towards 6-aldo-d-galactose, for which the highest yield of 6-amino-6-deoxy-d-galactose (67 %) was achieved in reactions permitting simultaneous oxidation of d-galactose and transamination of the resulting 6-aldo-d-galactose.

Characterization of a New Glyoxal Oxidase from the Thermophilic Fungus Myceliophthora thermophila M77: Hydrogen Peroxide Production Retained in 5-Hydroxymethylfurfural Oxidation

Myceliophthora thermophyla is a thermophilic industrially relevant fungus that secretes an assortment of hydrolytic and oxidative enzymes for lignocellulose degradation. Among them is glyoxal oxidase (MtGLOx), an extracellular oxidoreductase that oxidizes several aldehydes and α-hydroxy carbonyl substrates coupled to the reduction of O2 to H2O2. This copper metalloprotein belongs to a class of enzymes called radical copper oxidases (CRO) and to the “auxiliary activities” subfamily AA5_1 that is based on the Carbohydrate-Active enZYmes (CAZy) database. Only a few members of this family have been characterized to date. Here, we report the recombinant production, characterization, and structure-function analysis of MtGLOx. Electron Paramagnetic Resonance (EPR) spectroscopy confirmed MtGLOx to be a radical-coupled copper complex and small angle X-ray scattering (SAXS) revealed an extended spatial arrangement of the catalytic and four N-terminal WSC domains. Furthermore, we demonstrate that methylglyoxal and 5-hydroxymethylfurfural (HMF), a fermentation inhibitor, are substrates for the enzyme.

Characterization of the one-electron oxidized Cu(II)-salen complexes with a side chain aromatic ring: the effect of the indole ring on the Cu(II)-phenoxyl radical species

To gain insights into the role of the proximal indole ring in the redox-active metal center as seen in galactose oxidase, we prepared the Cu(II)-salen-type complexes having a pendent indol-3-ylmethyl (1), methyl (2) or benzyl (3) group substituted on the ethylenediamine moiety and investigated the structures and redox properties by various physicochemical methods and theoretical calculations. Neutral complexes 1, 2, and 3 showed no significant difference in the UV-Vis-NIR and EPR spectra. One-electron oxidation of 1, 2, and 3 by addition of 1 equiv. of thianthrenyl radical gave [1]SbCl ₆ , [2]SbCl ₆ , and [3]SbCl ₆ , respectively, which could be assigned to relatively localized phenoxyl radical species. The cyclic and differential pulse voltammograms of [1]SbCl ₆ showed two redox waves with a large separation between the first and second redox potentials compared with the separations observed for [2]SbCl ₆ and [3]SbCl ₆ . This suggests that [1]SbCl ₆ is more stabilized than [2]SbCl ₆ and [3]SbCl ₆ . The NIR band of [1]SbCl ₆ showed a larger blue shift than that of [2]SbCl ₆ and [3]SbCl ₆ . The EPR spectrum of [2]SbCl ₆ exhibited an intense signal at the g value of 2 due to partial disproportionation to form the EPR active two-electron oxidized complex [2] ²⁺ , while the EPR intensity of [1]SbCl ₆ was much weaker than that of [2]SbCl ₆ . These results indicate that the pendent indole moiety stabilizes the Cu(II)-phenoxyl radical in [1]SbCl ₆ most probably by stacking with the phenoxyl moiety, which is further supported by DFT calculations.

A Novel Colletotrichum graminicola Raffinose Oxidase in the AA5 Family

We describe here the identification and characterization of a copper radical oxidase from auxiliary activities family 5 (AA5_2) that was distinguished by showing preferential activity toward raffinose. Despite the biotechnological potential of carbohydrate oxidases from family AA5, very few members have been characterized. The gene encoding raffinose oxidase from Colletotrichum graminicola (CgRaOx; EC 1.1.3.-) was identified utilizing a bioinformatics approach based on the known modular structure of a characterized AA5_2 galactose oxidase. CgRaOx was expressed in Pichia pastoris, and the purified enzyme displayed the highest activity on the trisaccharide raffinose, whereas the activity on the disaccharide melibiose was three times lower and more than ten times lower activity was detected on d-galactose at a 300 mM substrate concentration. Thus, the substrate preference of CgRaOx was distinguished clearly from the substrate preferences of the known galactose oxidases. The site of oxidation for raffinose was studied by ¹H nuclear magnetic resonance and mass spectrometry, and we confirmed that the hydroxyl group at the C-6 position was oxidized to an aldehyde and that in addition uronic acid was produced as a side product. A new electrospray ionization mass spectrometry method for the identification of C-6 oxidized products was developed, and the formation mechanism of the uronic acid was studied. CgRaOx presented a novel activity pattern in the AA5 family.IMPORTANCE Currently, there are only a few characterized members of the CAZy AA5 protein family. These enzymes are interesting from an application point of view because of their ability to utilize the cheap and abundant oxidant O₂ without the requirement of complex cofactors such as FAD or NAD(P). Here, we present the identification and characterization of a novel AA5 member from Colletotrichum graminicola As discussed in the present study, the bioinformatics approach using the modular structure of galactose oxidase was successful in finding a C-6 hydroxyl carbohydrate oxidase having substrate preference for the trisaccharide raffinose. By the discovery of this activity, the diversity of the CAZy AA5 family is increasing.

Enzyme-based amperometric galactose biosensors: a review

This review (with 35 references) summarizes the various strategies used in biosensors for galactose, and their analytical performance. A brief comparison of the enzyme immobilization methods employed and the analytical performance characteristics of a range of galactose biosensors are first summarized in tabular form and then described in detail. Selected examples have been included to demonstrate the various applications of these biosensors to real samples. Following an introduction into the field that covers the significance of sensing galactose in various fields, the review covers biosensors based on the use of galactose oxidase, with a discussion of methods for their immobilization (via cross-linking, adsorption, covalent bonding and entrapment). This is followed by a short section on biosensors based on the use of galactose dehydrogenase. The conclusion section summarizes the state of the art and addresses current challenges. Graphical abstractFabrication of a disposable screen-printed (a) electrochemical galactose biosensor (b) for real sample analysis and a dummy biosensor

FvSNF1, the sucrose non-fermenting protein kinase gene of Fusarium virguliforme, is required for cell-wall-degrading enzymes expression and sudden death syndrome development in soybean

Fusarium virguliforme is a soil-borne pathogenic fungus that causes sudden death syndrome (SDS) in soybean. Its pathogenicity is believed to require the activity of cell-wall-degrading enzymes (CWDEs). The sucrose non-fermenting protein kinase 1 gene (SNF1) is a key component of the glucose de-repression pathway in yeast, and a regulator of gene expression for CWDEs in some plant pathogenic fungi. To elucidate the functional role of the SNF1 homolog in F. virguliforme, FvSNF1 was disrupted using a split-marker strategy. Disruption of FvSNF1 in F. virguliforme abolishes galactose utilization and causes poor growth on xylose, arabinose and sucrose. However, the resulting Fvsnf1 mutant grew similar to wild-type and ectopic transformants on glucose, fructose, maltose, or pectin as the main source of carbon. The Fvsnf1 mutant displayed no expression of the gene-encoding galactose oxidase (GAO), a secretory enzyme that catalyzes oxidation of D-galactose. It also exhibited a significant reduction in the expression of several CWDE-coding genes in contrast to the wild-type strain. Greenhouse pathogenicity assays revealed that the Fvsnf1 mutant was severely impaired in its ability to cause SDS on challenged soybean plants. Microscopy and microtome studies on infected roots showed that the Fvsnf1 mutant was defective in colonizing vascular tissue of infected plants. Cross and longitudinal sections of infected roots stained with fluorescein-labeled wheat germ agglutinin and Congo red showed that the Fvsnf1 mutant failed to colonize the xylem vessels and phloem tissue at later stages of infection. Quantification of the fungal biomass in inoculated roots further confirmed a reduced colonization of roots by the Fvsnf1 mutant when compared to the wild type. These findings suggest that FvSNF1 regulates the expression of CWDEs in F. virguliforme, thus affecting the virulence of the fungus on soybean.

Active-site maturation and activity of the copper-radical oxidase GlxA are governed by a tryptophan residue

GlxA from Streptomyces lividans is a mononuclear copper-radical oxidase and a member of the auxiliary activity family 5 (AA5). Its domain organisation and low sequence homology make it a distinct member of the AA5 family in which the fungal galactose 6-oxidase (Gox) is the best characterised. GlxA is a key cuproenzyme in the copper-dependent morphological development of S. lividans with a function that is linked to the processing of an extracytoplasmic glycan. The catalytic sites in GlxA and Gox contain two distinct one-electron acceptors comprising the copper ion and a 3′-(S-cysteinyl) tyrosine. The latter is formed post-translationally through a covalent bond between a cysteine and a copper-co-ordinating tyrosine ligand and houses a radical. In GlxA and Gox, a second co-ordination sphere tryptophan residue (Trp288 in GlxA) is present, but the orientation of the indole ring differs between the two enzymes, creating a marked difference in the π–π stacking interaction of the benzyl ring with the 3′-(S-cysteinyl) tyrosine. Differences in the spectroscopic and enzymatic activity have been reported between GlxA and Gox with the indole orientation suggested as a reason. Here, we report a series of in vivo and in vitro studies using the W288F and W288A variants of GlxA to assess the role of Trp288 on the morphology, maturation, spectroscopic and enzymatic properties. Our findings point towards a salient role for Trp288 in the kinetics of copper loading and maturation of GlxA, with its presence essential for stabilising the metalloradical site required for coupling catalytic activity and morphological development.

Influence of a family 29 carbohydrate binding module on the recombinant production of galactose oxidase in Pichia pastoris

Herein, we report the extracellular expression of carbohydrate active fusion enzymes in Pichia pastoris. Particularly, CBM29-1-2 from Piromyces equi was separately fused to the N- and C-terminus of galactose 6-oxidase (GaO, D-galactose: oxygen 6-oxidoreductase, EC 1.1.13.9, CAZy family AA5) from Fusarium graminearum, generating CBM29-GaO and GaO-CBM29, respectively. P. pastoris was transformed with expression vectors encoding GaO, CBM29-GaO and GaO-CBM29, and the fusion proteins were expressed in both shake-flask and 2L bioreactor systems. Volumetric production yields and specific GaO activity increased when expression was performed in a bioreactor system compared to shake-flask cultivation. This was observed for both CBM29-GaO and GaO-CBM29, and is consistent with previous reports of GaO expression in P. pastoris (Spadiut et al., 2010; Anasontzis et al., 2014) [1], [2]. Fusion of CBM29 to the C-terminal of GaO (GaO-CBM29) resulted in a stable uniform protein at the expected calculated size (107 kDa) when analyzed with SDS-PAGE. By comparison, the expression of the N-terminal fusion protein (CBM29-GaO) was low, and two truncated versions of CBM29-GaO were coexpressed with the full-sized protein. Despite differences in protein yield, the specific GaO activity on galactose was not affected by CBM29 fusion to either the N- or C-terminus of the enzyme. A detailed description of the catalytic and physiochemical properties of CBM29-GaO and GaO-CBM29 is available in the parent publication (Mollerup et al., 2015) [3].

Influence of a family 29 carbohydrate binding module on the activity of galactose oxidase from Fusarium graminearum

BACKGROUND

Galactose oxidase (GaO) selectively oxidizes the primary hydroxyl of galactose to a carbonyl, facilitating targeted chemical derivatization of galactose-containing polysaccharides, leading to renewable polymers with tailored physical and chemical properties. Here we investigate the impact of a family 29 glucomannan binding module on the activity and binding of GaO towards various polysaccharides. Specifically, CBM29-1-2 from Piromyces equi was separately linked to the N- and C-termini of GaO.

RESULTS

Both GaO-CBM29 and CBM29-GaO were successfully expressed in Pichia pastoris, and demonstrated enhanced binding to galactomannan, galactoglucomannan and galactoxyloglucan. The position of the CBM29 fusion affected the enzyme function. Particularly, C-terminal fusion led to greatest increases in galactomannan binding and catalytic efficiency, where relative to wild-type GaO, kcat/Km values increased by 7.5 and 19.8 times on guar galactomannan and locust bean galactomannan, respectively. The fusion of CBM29 also induced oligomerization of GaO-CBM29.

MAJOR CONCLUSIONS

Similar to impacts of cellulose-binding modules associated with cellulolytic enzymes, increased substrate binding impeded the action of GaO fusions on more concentrated preparations of galactomannan, galactoglucomannan and galactoxyloglucan; this was especially true for GaO-CBM29. Given the N-terminal positioning of the native galactose-binding CBM32 in GaO, the varying impacts of N-terminal versus C-terminal fusion of CBM29-1-2 may reflect competing action of neighboring CBMs.

GENERAL SIGNIFICANCE

This study thoroughly examines and discusses the effects of CBM fusion to non-lignocellulytic enzymes on soluble polysaccharides. Herein kinetics of GaO on galactose containing polysaccharides is presented for the first time.

The substrate tolerance of alcohol oxidases

Alcohols are a rich source of compounds from renewable sources, but they have to be activated in order to allow the modification of their carbon backbone. The latter can be achieved via oxidation to the corresponding aldehydes or ketones. As an alternative to (thermodynamically disfavoured) nicotinamide-dependent alcohol dehydrogenases, alcohol oxidases make use of molecular oxygen but their application is under-represented in synthetic biotransformations. In this review, the mechanism of copper-containing and flavoprotein alcohol oxidases is discussed in view of their ability to accept electronically activated or non-activated alcohols and their propensity towards over-oxidation of aldehydes yielding carboxylic acids. In order to facilitate the selection of the optimal enzyme for a given biocatalytic application, the substrate tolerance of alcohol oxidases is compiled and discussed: Substrates are classified into groups (non-activated prim- and sec-alcohols; activated allylic, cinnamic and benzylic alcohols; hydroxy acids; sugar alcohols; nucleotide alcohols; sterols) together with suitable alcohol oxidases, their microbial source, relative activities and (stereo)selectivities.

GlxA is a new structural member of the radical copper oxidase family and is required for glycan deposition at hyphal tips and morphogenesis of Streptomyces lividans

Streptomyces lividans displays a distinct dependence on copper to fully initiate morphological development. Evidence has accumulated to implicate the participation of an extracytoplasmic cuproenzyme in morphogenesis. In the present study, we show that GlxA fulfils all criteria to be that cuproenzyme. GlxA is membrane associated and has an active site consisting of a mononuclear copper and a cross-linked Y-C cofactor. The domain organization of the tertiary structure defines GlxA as a new structural member of the mono-copper oxidase family, with copper co-ordination geometry similar to, but spectroscopically distinct from fungal galactose oxidase (Gox). EPR spectroscopy reveals that the oxidation of cupric GlxA generates a protein radical residing on the Y-C cross-link. A variety of canonical Gox substrates (including D-galactose) were tested but none were readily turned over by GlxA. A glxA null-mutant leads to loss of glycan accumulation at hyphal tips and consequently a drastically changed morphology both on solid substrates and in liquid-grown environments, a scenario similarly observed in the absence of the neighbouring glycan synthase CslA (cellulase synthase-like protein). In addition the glxA mutant has lost the stimulation of development by copper, supporting a model whereby the enzymatic action of GlxA on the glycan is required for development and morphology. From a biotechnology perspective, the open mycelium morphology observed with the glxA mutant in submerged culture has implications for use as an enzyme production host.

Expression, purification, and characterization of galactose oxidase of Fusarium sambucinum in E. coli

A gene encoding a galactose oxidase (GalOx) was isolated from Fusarium sambucinum cultures and overexpressed in Escherichia coli yielding 4.4mg enzyme per L of growth culture with a specific activity of 159Umg(-1). By adding a C-terminal His-tag the enzyme could be easily purified with a single affinity chromatography step with high recovery rate (90%). The enzyme showed a single band on SDS-PAGE with an apparent molecular mass of 68.5kDa. The pH optimum for the oxidation of galactose was in the range of pH 6-7.5. Optimum temperature for the enzyme activity was 35°C, with a half-life of 11.2min, 5.3min, and 2.7min for incubation at 40°C, 50°C, and 60°C, respectively. From all tested substrates, the highest relative activity was found for 1-methyl-β-galactopyranoside (226Umg(-1)) and the highest catalytic efficiency (kcat/Km) for melibiose (2700mM(-1)s(-1)). The enzyme was highly specific for molecular oxygen as an electron acceptor, and showed no appreciable activity with a range of alternative acceptors investigated. Different chemicals were tested for their effect on GalOx activity. The activity was significantly reduced by EDTA, NaN3, and KCN.

Galactose oxidase from Fusarium oxysporum--expression in E. coli and P. pastoris and biochemical characterization

A gene coding for galactose 6-oxidase from Fusarium oxysporum G12 was cloned together with its native preprosequence and a C-terminal His-tag, and successfully expressed both in Escherichia coli and Pichia pastoris. The enzyme was subsequently purified and characterized. Among all tested substrates, the highest catalytic efficiency (k_cat/K_m) was found with 1-methyl-β-D-galactopyranoside (2.2 mM⁻¹ s⁻¹). The Michaelis constant (K_m) for D-galactose was determined to be 47 mM. Optimal pH and temperature for the enzyme activity were 7.0 and 40°C, respectively, and the enzyme was thermoinactivated at temperatures above 50°C. GalOx contains a unique metalloradical complex consisting of a copper atom and a tyrosine residue covalently attached to the sulphur of a cysteine. The correct formation of this thioether bond during the heterologous expression in E. coli and P. pastoris could be unequivocally confirmed by MALDI mass spectrometry, which offers a convenient alternative to prove this Tyr-Cys crosslink, which is essential for the catalytic activity of GalOx.

Ultra-high-throughput screening method for the directed evolution of glucose oxidase

Glucose oxidase (GOx) is used in many industrial processes that could benefit from improved versions of the enzyme. Some improvements like higher activity under physiological conditions and thermal stability could be useful for GOx applications in biosensors and biofuel cells. Directed evolution is one of the currently available methods to engineer improved GOx variants. Here, we describe an ultra-high-throughput screening system for sorting the best enzyme variants generated by directed evolution that incorporates several methodological refinements: flow cytometry, in vitro compartmentalization, yeast surface display, fluorescent labeling of the expressed enzyme, delivery of glucose substrate to the reaction mixture through the oil phase, and covalent labeling of the cells with fluorescein-tyramide. The method enables quantitative screening of gene libraries to identify clones with improved activity and it also allows cells to be selected based not only on the overall activity but also on the specific activity of the enzyme.

Effects of temperature and glycerol and methanol-feeding profiles on the production of recombinant galactose oxidase in Pichia pastoris

Optimization of protein production from methanol-induced Pichia pastoris cultures is necessary to ensure high productivity rates and high yields of recombinant proteins. We investigated the effects of temperature and different linear or exponential methanol-feeding rates on the production of recombinant Fusarium graminearum galactose oxidase (EC 1.1.3.9) in a P. pastoris Mut⁺ strain, under regulation of the AOX1 promoter. We found that low exponential methanol feeding led to 1.5-fold higher volumetric productivity compared to high exponential feeding rates. The duration of glycerol feeding did not affect the subsequent product yield, but longer glycerol feeding led to higher initial biomass concentration, which would reduce the oxygen demand and generate less heat during induction. A linear and a low exponential feeding profile led to productivities in the same range, but the latter was characterized by intense fluctuations in the titers of galactose oxidase and total protein. An exponential feeding profile that has been adapted to the apparent biomass concentration results in more stable cultures, but the concentration of recombinant protein is in the same range as when constant methanol feeding is employed. © 2014 The Authors Biotechnology Progress published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers Biotechnol. Prog., 30:728–735, 2014

Assembly, postsynthetic modification and hepatocyte targeting by multiantennary, galactosylated soft structures

Enzyme modifiable, hollow self-assembled structures offer an excellent scope for multiantennary delivery vectors.

A laterally acquired galactose oxidase-like gene is required for aerial development during osmotic stress in Streptomyces coelicolor

Phylogenetic reconstruction revealed that most Actinobacterial orthologs of S. coelicolor SCO2837, encoding a metal-dependent galactose oxidase-like protein, are found within Streptomyces and were probably acquired by horizontal gene transfer from fungi. Disruption of SCO2837 (glxA) caused a conditional bld phenotype that could not be reversed by extracellular complementation. Studies aimed at characterising the regulation of expression of glxA showed that it is not a target for other bld genes. We provide evidence that glxA is required for osmotic adaptation, although independently from the known osmotic stress response element SigB. glxA has been predicted to be part of an operon with the transcription unit comprising the upstream cslA gene and glxA. However, both phenotypic and expression studies indicate that it is also expressed from an independent promoter region internal to cslA. GlxA displays an in situ localisation pattern similar to that one observed for CslA at hyphal tips, but localisation of the former is independent of the latter. The functional role of GlxA in relation to CslA is discussed.