Streptomyces coelicolor oxidase (SCO2837p): A new free radical metalloenzyme secreted by Streptomyces coelicolor A3(2)

Expansion of Auxiliary Activity Family 5 sequence space via biochemical characterization of six new copper radical oxidases

Bacterial and fungal copper radical oxidases (CROs) from Auxiliary Activity Family 5 (AA5) are implicated in morphogenesis and pathogenesis. The unique catalytic properties of CROs also make these enzymes attractive biocatalysts for the transformation of small molecules and biopolymers. Despite a recent increase in the number of characterized AA5 members, especially from subfamily 2 (AA5_2), the catalytic diversity of the family as a whole remains underexplored. In the present study, phylogenetic analysis guided the selection of six AA5_2 members from diverse fungi for recombinant expression in Komagataella pfaffii (syn. Pichia pastoris) and biochemical characterization in vitro. Five of the targets displayed predominant galactose 6-oxidase activity (EC 1.1.3.9), and one was a broad-specificity aryl alcohol oxidase (EC 1.1.3.7) with maximum activity on the platform chemical 5-hydroxymethyl furfural (EC 1.1.3.47). Sequence alignment comparing previously characterized AA5_2 members to those from this study indicated various amino acid substitutions at active site positions implicated in the modulation of specificity.IMPORTANCEEnzyme discovery and characterization underpin advances in microbial biology and the application of biocatalysts in industrial processes. On one hand, oxidative processes are central to fungal saprotrophy and pathogenesis. On the other hand, controlled oxidation of small molecules and (bio)polymers valorizes these compounds and introduces versatile functional groups for further modification. The biochemical characterization of six new copper radical oxidases further illuminates the catalytic diversity of these enzymes, which will inform future biological studies and biotechnological applications.

Localization of Pyranose 2-Oxidase from Kitasatospora aureofaciens: A Step Closer to Elucidate a Biological Role

Lignin degradation in fungal systems is well characterized. Recently, a potential for lignin depolymerization and modification employing similar enzymatic activities by bacteria is increasingly recognized. The presence of genes annotated as peroxidases in Actinobacteria genomes suggests that these bacteria should contain auxiliary enzymes such as flavin-dependent carbohydrate oxidoreductases. The only auxiliary activity subfamily with significantly similar representatives in bacteria is pyranose oxidase (POx). A biological role of providing H2O2 for peroxidase activation and reduction of radical degradation products suggests an extracellular localization, which has not been established. Analysis of the genomic locus of POX from Kitasatospora aureofaciens (KaPOx), which is similar to fungal POx, revealed a start codon upstream of the originally annotated one, and the additional sequence was considered a putative Tat-signal peptide by computational analysis. We expressed KaPOx including this additional upstream sequence as well as fusion constructs consisting of the additional sequence, the KaPOx mature domain and the fluorescent protein mRFP1 in Streptomyces lividans. The putative signal peptide facilitated secretion of KaPOx and the fusion protein, suggesting a natural extracellular localization and supporting a potential role in providing H2O2 and reducing radical compounds derived from lignin degradation.

Copper radical oxidases: galactose oxidase, glyoxal oxidase, and beyond!

The copper radical oxidases (CROs) are an evolutionary and functionally diverse group of enzymes established by the historically significant galactose 6-oxidase and glyoxal oxidase from fungi. Inducted in 2013, CROs now constitute Auxiliary Activity Family 5 (AA5) in the Carbohydrate-Active Enzymes (CAZy) classification. CROs catalyse the two-electron oxidation of their substrates using oxygen as the final electron acceptor and are particularly distinguished by a cross-linked tyrosine-cysteine co-factor that is integral to radical stabilization. Recently, there has been a significant increase in the biochemically and structurally characterized CROs, which has revealed an expanded natural diversity of catalytic activities in the family. This review provides a brief historical introduction to CRO biochemistry and structural biology as a foundation for an update on current advances in CRO enzymology, biotechnology, and biology across kingdoms of life.

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.

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.

The DyP-type peroxidase DtpA is a Tat-substrate required for GlxA maturation and morphogenesis in Streptomyces

The filamentous bacterium Streptomyces lividans depends on the radical copper oxidase GlxA for the formation of reproductive aerial structures and, in liquid environments, for the formation of pellets. Incorporation of copper into the active site is essential for the formation of a cross-linked tyrosyl-cysteine cofactor, which is needed for enzymatic activity. In this study, we show a crucial link between GlxA maturation and a group of copper-related proteins including the chaperone Sco and a novel DyP-type peroxidase hereinafter called DtpA. Under copper-limiting conditions, the sco and dtpA deletion mutants are blocked in aerial growth and pellet formation, similarly to a glxA mutant. Western blot analysis showed that GlxA maturation is perturbed in the sco and dtpA mutants, but both maturation and morphology can by rescued by increasing the bioavailability of copper. DtpA acts as a peroxidase in the presence of GlxA and is a substrate for the twin-arginine translocation (Tat) translocation pathway. In agreement, the maturation status of GlxA is also perturbed in tat mutants, which can be compensated for by the addition of copper, thereby partially restoring their morphological defects. Our data support a model wherein a copper-trafficking pathway and Tat-dependent secretion of DtpA link to the GlxA-dependent morphogenesis pathway.

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.

Pivotal roles for Streptomyces cell surface polymers in morphological differentiation, attachment and mycelial architecture

Cells that are part of a multicellular structure are typically embedded in an extracellular matrix, which is produced by the community members. These matrices, the composition of which is highly diverse between different species, are typically composed of large amounts of extracellular polymeric substances, including polysaccharides, proteins, and nucleic acids. The functions of all these matrices are diverse: they provide protection, mechanical stability, mediate adhesion to surfaces, regulate motility, and form a cohesive network in which cells are transiently immobilized. In this review we discuss the role of matrix components produced by streptomycetes during growth, development and attachment. Compared to other bacteria it appears that streptomycetes can form morphologically and functionally distinct matrices using a core set of building blocks.

High-level overproduction of Thermobifida enzyme in Streptomyces lividans using a novel expression vector

In this study, we constructed a novel Streptomyces-E.coli shuttle vector pZRJ362 combining the xylose isomerase promoter and amylase terminator. A gene encoding the endoglucanase Cel6A in Thermobifida fusca was amplified by PCR, cloned into Streptomyces lividans host strain using the novel expression vector and Pichia pastoris GS115 host strain using the vector pPICZα-C, respectively. Afterwards, the expression pattern and the maximum expression level were comparatively studied in both expression systems. The maximum enzyme activity of Cel6A-(His)₆ secreted in S. lividans supernatant after 84-h of cultivation amounted to 5.56 U/mL, which was dramatically higher than that secreted in P. pastoris about 1.4 U/mL after 96-h of cultivation. The maximum expression level of Cel6A-(His)₆ in S. lividans supernatant reached up to 173 mg/L after 84-h of cultivation. The endoglucanase activity staining SDS-PAGE showed that there were some minor proteins in S. lividans supernatant which may be the Cel6A derivant by proteolytic degradation, while there was no proteolytic product detected in supernatant of P. pastoris.

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.

The electronic structure of the Cys-Tyr(*) free radical in galactose oxidase determined by EPR spectroscopy

Galactose oxidase is a metalloenzyme containing a novel metalloradical complex in its active site, comprised of a mononuclear copper ion associated with a protein free radical. The free radical has been shown to be localized on an intrinsic redox cofactor, 3'-(S-cysteinyl)tyrosine (Cys-Tyr), formed by a posttranslational covalent coupling of tyrosine and cysteine side chains in a self-processing reaction. The role of the thioether linkage in the function of the Cys-Tyr cofactor is unresolved, and some computational studies have suggested that the thioether substituent has a negligible effect on the properties of the tyrosyl free radical. In order to address this question experimentally, we have incorporated site-selectively labeled tyrosine ((2)H, (13)C, (17)O) into galactose oxidase using an engineered tyrosine auxotroph strain of Pichia pastoris . (33)S was also incorporated into the protein. EPR spectra for the Cys-Tyr(*) free radical in each of these isotopic variants were analyzed to extract nuclear hyperfine parameters for comparison with theoretical predictions, and the unpaired spin distribution in the free radical was reconstructed from the hyperfine data. These labeling studies allow the first comprehensive experimental evaluation of the effect of the thioether linkage on the properties of Cys-Tyr(*) and indicate that previous calculations significantly underestimated the contribution of this feature to the electronic ground state of the free radical.

A cellulose synthase-like protein involved in hyphal tip growth and morphological differentiation in streptomyces

Cellulose synthase and cellulose synthase-like proteins, responsible for synthesizing beta-glucan-containing polysaccharides, play a fundamental role in cellular architectures, such as plant cell and tissue morphogenesis, bacterial biofilm formation, and fruiting-body development. However, the roles of the proteins involved in the developmental process are not well understood. Here, we report that a cellulose synthase-like protein (CslA(Sc)) in Streptomyces has a function in hyphal tip growth and morphological differentiation. The cslA(Sc) replacement mutant showed pleiotropic defects, including the severe delay of aerial-hyphal formation and altered cell wall morphology. Calcofluor white fluorescence analysis demonstrated that polysaccharide synthesis at hyphal tips was dependent on CslA(Sc). cslA(Sc) was constitutively transcribed, and an enhanced green fluorescent protein-CslA(Sc) fusion protein was mostly located at the hyphal tips. An extract enriched in morphogenetic chaplin proteins promoted formation of aerial hyphae by the mutant. Furthermore, a two-hybrid experiment indicated that the glycosyltransferase domain of CslA(Sc) interacted with the tropomyosin-like polarity-determining DivIVA protein, suggesting that the tip-located DivIVA governed tip recruitment of the CslA(Sc) membrane protein. These results imply that the cellulose synthase-like protein couples extracellular and cytoskeletal components functioning in tip growth and cell development.