Pyranose oxidase: A versatile sugar oxidoreductase for bioelectrochemical applications

Kaguacidine A: a novel spirohydantoin-containing cucurbitane glycoside from vines of Momordica charantia L

The spirohydantoin-containing cucurbitane-type triterpenoid, kaguacidine A (1), was isolated and purified from 95% ethanol extract of vines of Momordica charantia L. (Cucurbitaceae). Its unprecedented chemical structure, a spirohydantoin substituent at C-23 of cucurbitane, was elucidated by extensive spectroscopic analyses, including HRESIMS, IR, optical rotation, 1 D- and 2 D-NMR spectra. The possible biosynthetic pathway is deduced and may be attributed to the metabolic activity of microbial symbionts in M. charantia L. Compound 1 was evaluated for anti-inflammatory activity against LPS-induced NO production in RAW 264.7 cells and anti-proliferative activity against four cancer cell lines, including HEp-2, MCF-7, Hep-G2, and WiDr. Compound 1 showed moderate anti-inflammatory activity with an IC50 value of 18.5 ± 0.4 μg/mL and weak anti-proliferative activity against MCF-7, HEp-2, Hep-G2, and WiDr with IC50 values of >40, 33.8 ± 0.6, 31.0 ± 0.7, and 27.0 ± 0.7 μM, respectively.

Evolution and separation of actinobacterial pyranose and C-glycoside-3-oxidases

FAD-dependent pyranose oxidase (POx) and C-glycoside-3-oxidase (CGOx) are both members of the glucose-methanol-choline superfamily of oxidoreductases and belong to the same sequence space. Pyranose oxidases had been studied for their oxidation of monosaccharides such as D-glucose, but recently, a bacterial C-glycoside-3-oxidase that is phylogenetically related to POx and that reacts with C-glycosides such as carminic acid, mangiferin or puerarin has been described. Since these actinobacterial CGOx enzymes belong to the same sequence space as bacterial POx, they must have evolved from the same ancestor. Here, we performed a phylogenetic analysis of actinobacterial sequences and resurrected seven ancestral enzymes of the POx/CGOx sequence space to study the evolutionary trajectory of substrate preferences for monosaccharides and C-glycosides. Clade I, with its dimeric member POx from Kitasatospora aureofaciens, shows strict preference for monosaccharides (D-glucose and D-xylose) and does not react with any of the glycosides tested. No extant member of clade II has been studied to date. The two extant members of clades III and IV, monomeric POx/CGOx from Pseudoarthrobacter siccitolerans and Streptomyces canus, oxidized both monosaccharides as well as various C-glycosides (homoorientin, isovitexin, mangiferin, and puerarin). Steady-state kinetic parameters of several clades III and IV ancestral enzymes indicate that the generalist ancestor N35 slowly evolved to present-day enzymes with a much higher preference for C-glycosides than monosaccharides. Based on structural predictions of ancestors, we hypothesize that the strict specificity of bacterial clade I POx (and also fungal POx) is the result of oligomerization, which in turn results from the evolution of protein segments that were shown to be important for oligomerization, the arm, and the head domain.IMPORTANCEC-Glycosides often form active compounds in various plants. Breakage of the C-C bond in these glycosides to release the aglycone is challenging and proceeds via a two-step reaction, the oxidation of the sugar and subsequent cleavage of the C-C bond. Recently, an enzyme from a soil bacterium, FAD-dependent C-glycoside-3-oxidase (CGOx), was shown to catalyze the initial oxidation reaction. Here, we show that CGOx belongs to the same sequence space as pyranose oxidase (POx), and that an actinobacterial ancestor of the POx/CGOx family evolved into four clades, two of which show a high preference for C-glycosides.

Modification of substrate specificity of L-arginine oxidase for detection of L-citrulline

Enzymatic detection of citrulline, a potential biomarker for various diseases, is beneficial. However, determining citrulline levels requires expensive instrumental analyses and complicated colorimetric assays. Although L-amino acid oxidase/dehydrogenase is widely used to detect L-amino acids, an L-citrulline-specific oxidase/dehydrogenase has not been reported. Therefore, in this study, we aimed to develop an L-citrulline-specific enzyme by introducing a mutation into L-arginine oxidase (ArgOX) derived from Pseudomonas sp. TPU 7192 to provide a simple enzymatic L-citrulline detection system. The ratio of the oxidase activity against L-arginine to that against L-citrulline (Cit/Arg) was 1.2%, indicating that ArgOX could recognize L-citrulline as a substrate. In the dehydrogenase assay, the specific dehydrogenase activity towards L-arginine was considerably lower than the specific oxidase activity. However, the specific dehydrogenase activity towards L-citrulline was only slightly lower than the oxidase activity, resulting in improved substrate specificity with a Cit/Arg ratio of 49.5%. To enhance the substrate specificity of ArgOX, we performed site-directed mutagenesis using structure-based engineering. The 3D model structure indicated that E486 interacted with the L-arginine side chain. By introducing the E486 mutation, the specific dehydrogenase activity of ArgOX/E486Q for L-citrulline was 3.25 ± 0.50 U/mg, which was 3.8-fold higher than that of ArgOX. The Cit/Arg ratio of ArgOX/E486Q was 150%, which was higher than that of ArgOX. Using ArgOX/E486Q, linear relationships were observed within the range of 10-500 μM L-citrulline, demonstrating its suitability for detecting citrulline in human blood. Consequently, ArgOX/E486Q can be adapted as an enzymatic sensor in the dehydrogenase system.

Mechanistic insights into glycoside 3-oxidases involved in C-glycoside metabolism in soil microorganisms

C-glycosides are natural products with important biological activities but are recalcitrant to degradation. Glycoside 3-oxidases (G3Oxs) are recently identified bacterial flavo-oxidases from the glucose-methanol-coline (GMC) superfamily that catalyze the oxidation of C-glycosides with the concomitant reduction of O2 to H2O2. This oxidation is followed by C-C acid/base-assisted bond cleavage in two-step C-deglycosylation pathways. Soil and gut microorganisms have different oxidative enzymes, but the details of their catalytic mechanisms are largely unknown. Here, we report that PsG3Ox oxidizes at 50,000-fold higher specificity (kcat/Km) the glucose moiety of mangiferin to 3-keto-mangiferin than free D-glucose to 2-keto-glucose. Analysis of PsG3Ox X-ray crystal structures and PsG3Ox in complex with glucose and mangiferin, combined with mutagenesis and molecular dynamics simulations, reveal distinctive features in the topology surrounding the active site that favor catalytically competent conformational states suitable for recognition, stabilization, and oxidation of the glucose moiety of mangiferin. Furthermore, their distinction to pyranose 2-oxidases (P2Oxs) involved in wood decay and recycling is discussed from an evolutionary, structural, and functional viewpoint.

Preparation of a novel glucose oxidase-N-succinyl chitosan nanospheres and its antifungal mechanism of action against Colletotrichum gloeosporioides

In this work, a new glucose oxidase-N-succinyl chitosan (GOD-NSCS) nanospheres was prepared through the immobilization of glucose oxidase (GOD) on N-succinyl chitosan (NSCS) nanospheres. Compared to the free GOD, GOD-NSCS nanospheres demonstrated the excellent anti-Colletotrichum gloeosporioides activity with the EC₅₀ values of 211.2 and 10.7 μg/mL against mycelial growth and spores germination. The computational biology analysis demonstrated that the substrate presented the similar binding free energy with GOD-NSCS nanospheres (-27.64 kcal/mol) compared with the free GOD (-24.04 kcal/mol), indicating that GOD-NSCS nanospheres had the same oxidation efficiency and produced more H₂O₂. Moreover, the enzyme activity stability of GOD-NSCS nanospheres could be prolonged to 10 d. The cell membrane was destructed by the treatment of H₂O₂ produced by GOD, leading to the cell death. In vivo test, GOD-NSCS nanospheres treatment significantly prolonged the preservation period of mangoes 2-fold. Collectively, these results suggested that GOD-NSCS nanospheres suppresses anthracnose in postharvest mangoes by inhibiting the growth of C. gloeosporioides and might become a potential natural preservative for fruits and vegetables.

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.

Biochemical Characterization of Pyranose Oxidase from Streptomyces canus—Towards a Better Understanding of Pyranose Oxidase Homologues in Bacteria

Pyranose oxidase (POx, glucose 2-oxidase; EC 1.1.3.10, pyranose:oxygen 2-oxidoreductase) is an FAD-dependent oxidoreductase and a member of the auxiliary activity (AA) enzymes (subfamily AA3_4) in the CAZy database. Despite the general interest in fungal POxs, only a few bacterial POxs have been studied so far. Here, we report the biochemical characterization of a POx from Streptomyces canus (ScPOx), the sequence of which is positioned in a separate, hitherto unexplored clade of the POx phylogenetic tree. Kinetic analyses revealed that ScPOx uses monosaccharide sugars (such as d-glucose, d-xylose, d-galactose) as its electron-donor substrates, albeit with low catalytic efficiencies. Interestingly, various C- and O-glycosides (such as puerarin) were oxidized by ScPOx as well. Some of these glycosides are characteristic substrates for the recently described FAD-dependent C-glycoside 3-oxidase from Microbacterium trichothecenolyticum. Here, we show that FAD-dependent C-glycoside 3-oxidases and pyranose oxidases are enzymes belonging to the same sequence space.

Advances in Metal-Organic Framework (MOFs) based biosensors for diagnosis: An update

Metal-organic frameworks (MOFs) have significant advantages over other candidate classes of chemo-sensory materials owing to their extraordinary structural tunability and characteristics. MOF-based biosensing is a simple, and convenient method for identifying various species. Biomarkers are molecular or cellular processes that link environmental exposure to a health outcome. Biomarkers are important in understanding the links between environmental chemical exposure and the development of chronic diseases, as well as in identifying disease-prone subgroups. Until now, several species, including nanoparticles (NPs) and their nanocomposites, small molecules, and unique complex systems, have been used for the chemical sensing of biomarkers. Following the overview of the field, we discussed the various fabrication methods for MOFs development in this review. We provide a thorough overview of the previous five years of progress to broaden the scope of analytes for future research. Several enzymatic and non-enzymatic sensors are offered, together with a mandatory measuring method that includes detection range and dynamic range. In addition, we reviewed the comparison of enzymatic and non-enzymatic biosensors, inventive edges, and the difficulties that need to be solved. This work might open up new possibilities for material production, sensor development, medical diagnostics, and other sensing fields.

Nanozyme-mediated cascade reaction system for electrochemical detection of 1,5-anhydroglucitol

Diabetes is one of metabolic diseases affecting major human health. The early diagnosis and treatment of diabetes have significant benefits. 1,5-anhydroglucitol (1,5-AG) accurately reflects a patient's average blood glucose level for the past 3-7 days and becomes a promising marker for real-time detection of diabetes. In this study, a novel biosensor for determination 1,5-AG is constructed using reduce graphene oxide-carboxymethylated chitosan-hemin@platinum nanocomposites (rGO-CMC-H@Pt NCs) nanozyme and pyranose oxidase (PROD) enzyme as the electrochemical biosensing platform. The rGO-CMC-H@Pt NCs nanozyme has good electro-conductibility, high specific surface area, and admirable peroxide-like catalysis effect to enhance the electrochemical response. 1,5-AG is catalyzed by PROD and produces hydrogen peroxide (H₂O₂), which in turn can be decomposed by rGO-CMC-H@Pt NCs and produce a current signal recorded by differential pulse voltammetry (DPV) technique. Under optimal conditions, the response currents have a linear relationship in the 1,5-AG concentration of 0.1-2.0 mg/mL with R² of 0.9869. The sensitivity is 2.1895 μA/μg·mL^-1 and the limit of detection (LOD) is 38.2 μg/mL (S/N = 3). In addition, the specificity, reproducibility, stability and recovery (94.5-107.6%) of 1,5-AG biosensors all exhibit good performance. Therefore, the designed 1,5-AG biosensor has a good effect and can be used for the diagnosis of diabetes.

Four cucurbitane glycosides taimordisins A–D with novel furopyranone skeletons isolated from the fruits of Momordica charantia

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Four new cucurbitane-type triterpenoids glycosides were isolated from the fresh fruit of Momordica charantia and determined by NMR, HRESIMS, and biosynthesis.

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Taimordisins A and B possess rare bicyclic-fused and trifuso-centro-fused ring systems at side chain of the cucurbitane-type triterpenoids at the first time.

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Taimordisins A-D showed the inhibition of NO production by LPS-stimulated in RAW264.7 macrophage cells.

Four novel triterpene glycosides, taimordisins A–D (1–4), were discovered from fresh fruits of Taiwanese Momordica charantia. The chemical framework and relative stereochemistry of these four natural products were isolated, purified, and determined by using various separation and spectroscopy techniques. Each of them features a unique bicyclic-fused or trifuso-centro-fused ring system. Notably, 1 and 2 are cucurbitane-based compounds possessing a new C-24 and C-2″ carbon–carbon linkage with 5-hydroxy-2-(hydroxymethyl)tetrahydro-4H-pyran-4-one and 6-(hydroxymethyl)tetrahydro-4H-pyran-3,4,4-triol units, respectively, and represented an unprecedented molecular skeleton. In terms of biosynthesis, they all originate from a common precursor 3-hydroxycucurbita-5,24-dien-19-al-7,23-di-O-β-glucopyranoside. Of two sugar moieties, the one at 23-O-β-glucopyranoside grants each individual congener uniqueness likely through microbial symbiont-mediated intramolecular transformation into two major types of furo[2,3-b]pyranone and furo[3,2-c]pyranone derivatives. These new products possess desirable anti-inflammatory biological activities in addition to being generally regarded as safe.

Comparative Proteomic Analysis of Rhizoctonia solani Isolates Identifies the Differentially Expressed Proteins with Roles in Virulence

Sheath blight of rice is a destructive disease that could be calamitous to rice cultivation. The significant objective of this study is to contemplate the proteomic analysis of the high virulent and less virulent isolate of Rhizoctonia solani using a quantitative LC-MS/MS-based proteomic approach to identify the differentially expressed proteins promoting higher virulence. Across several rice-growing regions in Odisha, Eastern India, 58 Rhizoctonia isolates were obtained. All the isolates varied in their pathogenicity. The isolate RS15 was found to be the most virulent and RS22 was identified as the least virulent. The PCR amplification confirmed that the RS15 and RS22 belonged to the Rhizoctonia subgroup of AG1-IA with a specific primer. The proteomic information generated has been deposited in the PRIDE database with PXD023430. The virulent isolate consisted of 48 differentially abundant proteins, out of which 27 proteins had higher abundance, while 21 proteins had lower abundance. The analyzed proteins acquired functionality in fungal development, sporulation, morphology, pathogenicity, detoxification, antifungal activity, essential metabolism and transcriptional activities, protein biosynthesis, glycolysis, phosphorylation and catalytic activities in fungi. A Quantitative Real-Time PCR (qRT-PCR) was used to validate changes in differentially expressed proteins at the mRNA level for selected genes. The abundances of proteins and transcripts were positively correlated. This study provides the role of the proteome in the pathogenicity of R. solani AG1-IA in rice and underpins the mechanism behind the pathogen’s virulence in causing sheath blight disease.

Glucose Oxidase, an Enzyme "Ferrari": Its Structure, Function, Production and Properties in the Light of Various Industrial and Biotechnological Applications

Glucose oxidase (GOx) is an important oxidoreductase enzyme with many important roles in biological processes. It is considered an "ideal enzyme" and is often called an oxidase "Ferrari" because of its fast mechanism of action, high stability and specificity. Glucose oxidase catalyzes the oxidation of β-d-glucose to d-glucono-δ-lactone and hydrogen peroxide in the presence of molecular oxygen. d-glucono-δ-lactone is sequentially hydrolyzed by lactonase to d-gluconic acid, and the resulting hydrogen peroxide is hydrolyzed by catalase to oxygen and water. GOx is presently known to be produced only by fungi and insects. The current main industrial producers of glucose oxidase are Aspergillus and Penicillium. An important property of GOx is its antimicrobial effect against various pathogens and its use in many industrial and medical areas. The aim of this review is to summarize the structure, function, production strains and biophysical and biochemical properties of GOx in light of its various industrial, biotechnological and medical applications.

1,5-anhydroglucitol biosensor based on light-addressable potentiometric sensor with RGO-CS-Fc/Au NPs nanohybrids

In this paper, a novel silicon-based light-addressable potentiometric sensor (LAPS) has been designed for the detection of 1,5-anhydroglucitol (1,5-AG) in human serum. Reduced graphene oxide-chitosan-ferrocene (RGO-CS-Fc)/AuNPs nanohybrids and pyranose oxidase (PROD) enzyme is used to fabricate biological sensitive membrane unit by layer-by-layer assembly technology. When a bias voltage is provided to the LAPS system, the catalytic oxidation reaction between 1,5-AG and PROD to produce H₂O₂. The by-product H₂O₂ can oxidize Fc(Fe²⁺) ions in RGO-CS-Fc nanohybrids into Fc(Fe³⁺) ions, which cause the potential of the sensitive membrane surface to change and the potential shift of I-V curve will generate a corresponding offset response. Under the optimal conditions, the potential shift of the LAPS is linearly related to the concentration of 1,5-AG at 10 µg·mL^-1 -350 µg·mL^-1 with the correlation coefficient of 0.97414. The sensitivity is 0.44273 mV/µg·mL^-1 and the lowest detection limit is 10 µg·mL^-1. In addition, the biosensor showed good specificity, acceptable stability and satisfactory recovery rates (91.28%-107.66%), which would be a potential testing methods in actual clinical samples.

Recent Advances in Enzymatic and Non-Enzymatic Electrochemical Glucose Sensing

The detection of glucose is crucial in the management of diabetes and other medical conditions but also crucial in a wide range of industries such as food and beverages. The development of glucose sensors in the past century has allowed diabetic patients to effectively manage their disease and has saved lives. First-generation glucose sensors have considerable limitations in sensitivity and selectivity which has spurred the development of more advanced approaches for both the medical and industrial sectors. The wide range of application areas has resulted in a range of materials and fabrication techniques to produce novel glucose sensors that have higher sensitivity and selectivity, lower cost, and are simpler to use. A major focus has been on the development of enzymatic electrochemical sensors, typically using glucose oxidase. However, non-enzymatic approaches using direct electrochemistry of glucose on noble metals are now a viable approach in glucose biosensor design. This review discusses the mechanisms of electrochemical glucose sensing with a focus on the different generations of enzymatic-based sensors, their recent advances, and provides an overview of the next generation of non-enzymatic sensors. Advancements in manufacturing techniques and materials are key in propelling the field of glucose sensing, however, significant limitations remain which are highlighted in this review and requires addressing to obtain a more stable, sensitive, selective, cost efficient, and real-time glucose sensor.

Chemical and physical Chitosan modification for designing enzymatic industrial biocatalysts: How to choose the best strategy?

Chitosan is one of the most abundant natural polymer worldwide, and due to its inherent characteristics, its use in industrial processes has been extensively explored. Because it is biodegradable, biocompatible, non-toxic, hydrophilic, cheap, and has good physical-chemical stability, it is seen as an excellent alternative for the replacement of synthetic materials in the search for more sustainable production methodologies. Thus being, a possible biotechnological application of Chitosan is as a direct support for enzyme immobilization. However, its applicability is quite specific, and to overcome this issue, alternative pretreatments are required, such as chemical and physical modifications to its structure, enabling its use in a wider array of applications. This review aims to present the topic in detail, by exploring and discussing methods of employment of Chitosan in enzymatic immobilization processes with various enzymes, presenting its advantages and disadvantages, as well as listing possible chemical modifications and combinations with other compounds for formulating an ideal support for this purpose. First, we will present Chitosan emphasizing its characteristics that allow its use as enzyme support. Furthermore, we will discuss possible physicochemical modifications that can be made to Chitosan, mentioning the improvements obtained in each process. These discussions will enable a comprehensive comparison between, and an informed choice of, the best technologies concerning enzyme immobilization and the application conditions of the biocatalyst.

The vast repertoire of carbohydrate oxidases: An overview

Carbohydrates are widely abundant molecules present in a variety of forms. For their biosynthesis and modification, nature has evolved a plethora of carbohydrate-acting enzymes. Many of these enzymes are of particular interest for biotechnological applications, where they can be used as biocatalysts or biosensors. Among the enzymes catalysing conversions of carbohydrates are the carbohydrate oxidases. These oxidative enzymes belong to different structural families and use different cofactors to perform the oxidation reaction of CH-OH bonds in carbohydrates. The variety of carbohydrate oxidases available in nature reflects their specificity towards different sugars and selectivity of the oxidation site. Thanks to their properties, carbohydrate oxidases have received a lot of attention in basic and applied research, such that nowadays their role in biotechnological processes is of paramount importance. In this review we provide an overview of the available knowledge concerning the known carbohydrate oxidases. The oxidases are first classified according to their structural features. After a description on their mechanism of action, substrate acceptance and characterisation, we report on the engineering of the different carbohydrate oxidases to enhance their employment in biocatalysis and biotechnology. In the last part of the review we highlight some practical applications for which such enzymes have been exploited.

Overview of flavin-dependent enzymes

Flavin-dependent enzymes catalyze a wide variety of biological reactions that are important for all types of living organisms. Knowledge gained from studying the chemistry and biological functions of flavins and flavin-dependent enzymes has continuously made significant contributions to the development of the fields of enzymology and metabolism from the 1970s until now. The enzymes have been applied in various applications such as use as biocatalysts in synthetic processes for the chemical and pharmaceutical industries or in the biodetoxification and bioremediation of toxic or unwanted compounds, and as biosensors or biodetection tools for quantifying various agents of interest. Many flavin-dependent enzymes are also prime targets for drug development. Based on their reaction mechanisms, they can be classified into five categories: oxidase, dehydrogenase, monooxygenase, reductase, and redox neutral flavin-dependent enzymes. In this chapter, the general properties of flavin-dependent enzymes and the nature of their chemical reactions are discussed, along with their practical applications.

Structure and function relationships of sugar oxidases and their potential use in biocatalysis

Several sugar oxidases that catalyze the oxidation of sugars have been isolated and characterized. These enzymes can be classified as flavoenzyme due to the presence of flavin adenine dinucleotide (FAD) as a cofactor. Sugar oxidases have been proposed to be the key biocatalyst in biotransformation of carbohydrates which can potentially convert sugars to provide a pool of intermediates for synthesis of rare sugars, fine chemicals and drugs. Moreover, sugar oxidases have been applied in biosensing of various biomolecules in food industries, diagnosis of diseases and environmental pollutant detection. This review provides the discussions on general properties, current mechanistic understanding, structural determination, biocatalytic application, and biosensor integration of representative sugar oxidase enzymes, namely pyranose 2-oxidase (P2O), glucose oxidase (GO), hexose oxidase (HO), and oligosaccharide oxidase. The information regarding the relationship between structure and function of these sugar oxidases points out the key properties of this particular group of enzymes that can be modified by engineering, which had resulted in a remarkable economic importance.