Identification and characterization of thermostable glucose dehydrogenases from thermophilic filamentous fungi

Microgravity environment grown crystal structure information based engineering of direct electron transfer type glucose dehydrogenase

The heterotrimeric flavin adenine dinucleotide dependent glucose dehydrogenase is a promising enzyme for direct electron transfer (DET) principle-based glucose sensors within continuous glucose monitoring systems. We elucidate the structure of the subunit interface of this enzyme by preparing heterotrimer complex protein crystals grown under a space microgravity environment. Based on the proposed structure, we introduce inter-subunit disulfide bonds between the small and electron transfer subunits (5 pairs), as well as the catalytic and the electron transfer subunits (9 pairs). Without compromising the enzyme's catalytic efficiency, a mutant enzyme harboring Pro205Cys in the catalytic subunit, Asp383Cys and Tyr349Cys in the electron transfer subunit, and Lys155Cys in the small subunit, is determined to be the most stable of the variants. The developed engineered enzyme demonstrate a higher catalytic activity and DET ability than the wild type. This mutant retains its full activity below 70 °C as well as after incubation at 75 °C for 15 min - much higher temperatures than the current gold standard enzyme, glucose oxidase, is capable of withstanding.

The C2H2 Zinc Finger Protein MaNCP1 Contributes to Conidiation through Governing the Nitrate Assimilation Pathway in the Entomopathogenic Fungus Metarhizium acridum

Zinc finger proteins are an important class of multifunctional regulators. Here, the roles of a C2H2 zinc finger protein MaNCP1 (Metarhizium acridum nitrate-related conidiation pattern shift regulatory factor 1) in nitrogen utilization and conidiation were explored in the entomopathogenic fungus M. acridum. The results showed that MaNCP1-disruption mutant (ΔMaNCP1) impaired the ability to utilize nitrate, ammonium and glutamine and reduced the expression of nitrate assimilation-related genes, suggesting that MaNCP1 was involved in governing nitrogen utilization. In addition, the conidial yield of the ΔMaNCP1 strain, cultured on the microcycle conidiation medium (SYA), was significantly decreased, which could be restored or even enhanced than that of the WT strain through increasing the nitrate content in SYA medium. Further study showed that MaAreA, a core regulator in the nitrogen catabolism repression (NCR) pathway, was a downstream target gene of MaNCP1. Screening the differential expression genes between WT and ΔMaNCP1 strains revealed that the conidial yield of M. acridum regulated by nitrate might be related to NCR pathway on SYA medium. It could be concluded that MaNCP1 contributes to the nitrate assimilation and conidiation, which will provide further insights into the relationship between the nitrogen utilization and conidiation in fungi.

Comparison of the stability of Mucor-derived flavin adenine dinucleotide-dependent glucose dehydrogenase and glucose oxidase

Long-term stability at near-body temperature is important for continuous glucose monitoring (CGM) sensors. However, the stability of enzymes used in CGM sensors has often been evaluated by measuring their melting temperature (T_m) values and by short heat treatment but not at around 37 °C. Glucose oxidase (GOD) is used in current CGM sensors. In this study, we evaluated the stability of modified Mucor-derived flavin adenine dinucleotide-dependent glucose dehydrogenase (designated Mr144-297) with improved thermal stability at medium to high temperatures and compared it with that of GOD. The T_m value of Mr144-297 was 61.6 ± 0.3 °C and was similar to that of GOD (61.4 ± 1.2 °C). However, Mr144-297 was clearly more stable than GOD at 40 °C and 55 °C. At 37 °C, the stability of a carbon electrode with immobilized Mr144-297 was higher than that of an electrode with GOD. Our data indicate that Mr144-297 is a more suitable enzyme for CGM sensors than is GOD.

Bio-fabrication of thermozyme-based nano-biosensors: their components and present scenario

An amalgamation of microbiology, biocatalysis, recombinant molecular biology, and nanotechnology is crucial for groundbreaking innovation in developing nano-biomedicines and sensoristics. Enzyme-based nano-biosensor finds prospective applications in various sectors (environmental, pharmaceutical, food, biorefineries). These applications demand reliable catalytic efficiency and functionality of the enzyme under an extreme operational environment for a prolonged period. Over the last few years, bio-fabrication of nano-biosensors in conjunction with thermozymes from thermophilic microbes is being sought after as a viable design. Thermozymes are known for their robustness, are chemically resistant toward organic solvents, possess higher durability for constant use, catalytic ability, and stability at elevated temperatures. Additionally, several other attributes of thermozymes like substrate specificity, selectivity, and sensitivity make them desirable in developing a customized biosensor. In this review, crucial designing aspects of enzyme-based nano-biosensors like enzyme immobilization on an electrode surface, new materials derived from microbial sources (biopolymers based nanocomposites), improvisation measures for sensitivity, and selectivity have been addressed. It also covers microbial biosynthesis of nanomaterials used to develop sensoristic devices and its numerous applications such as wastewater treatment, biorefineries, and diagnostics. The knowledge will pave the way toward creating consistent eco-friendly, economically viable nanostructured-based technologies with broad applicability and exploitation for industrial use in the near future.

Triggered Reversible Disassembly of an Engineered Protein Nanocage*

Protein nanocages play crucial roles in sub-cellular compartmentalization and spatial control in all domains of life and have been used as biomolecular tools for applications in biocatalysis, drug delivery, and bionanotechnology. The ability to control their assembly state under physiological conditions would further expand their practical utility. To gain such control, we introduced a peptide capable of triggering conformational change at a key structural position in the largest known encapsulin nanocompartment. We report the structure of the resulting engineered nanocage and demonstrate its ability to disassemble and reassemble on demand under physiological conditions. We demonstrate its capacity for in vivo encapsulation of proteins of choice while also demonstrating in vitro cargo loading capabilities. Our results represent a functionally robust addition to the nanocage toolbox and a novel approach for controlling protein nanocage disassembly and reassembly under mild conditions.

Characterization of Fungal FAD-Dependent AA3_2 Glucose Oxidoreductases from Hitherto Unexplored Phylogenetic Clades

The CAZy auxiliary activity family 3 (AA3) comprises FAD-dependent enzymes belonging to the superfamily of glucose-methanol-choline (GMC) oxidoreductases. Glucose oxidase (GOx; EC 1.1.3.4) and glucose dehydrogenase (GDH; EC 1.1.5.9) are part of subfamily AA3_2 and catalyze the oxidation of β-D-glucose at its anomeric carbon to D-glucono-1,5-lactone. Recent phylogenetic analysis showed that AA3_2 glucose oxidoreductases can be grouped into four major clades, GOx I and GDH I–III, and in minor clades such as GOx II or distinct subclades. This wide sequence space of AA3_2 glucose oxidoreductases has, however, not been studied in detail, with mainly members of GOx I and GDH I studied biochemically or structurally. Here, we report the biochemical characterization of four fungal glucose oxidoreductases from distinct, hitherto unexplored clades or subclades. The enzyme from Aureobasidium subglaciale, belonging to the minor GOx II clade, showed a typical preference for oxygen and glucose, confirming the correct annotation of this clade. The other three enzymes exhibited strict dehydrogenase activity with different substrate specificities. GDH II from Trichoderma virens showed an almost six-fold higher catalytic efficiency for maltose compared to glucose. The preferred substrate for the two GDH III enzymes from Rhizoctonia solani and Ustilago maydis was gentiobiose, a β(1→6) disaccharide, as judged from the catalytic efficiency. Overall, the newly studied AA3_2 glucose oxidoreductases showed a much broader substrate spectrum than the archetypal GOx from Aspergillus niger, which belongs to clade GOx I.

Design of an amperometric glucose oxidase biosensor with added protective and adhesion layers

Enzymes have demonstrated great potential in the development of advanced electroanalysis devices due to their unique recognition and catalytic properties. However, unsatisfactory stability and limited electron communication of traditional enzyme sensors seriously hinder their large-scale application. In this work, a simple and effective method is proposed to improve the stability of enzyme sensors by using sodium hyaluronate (SH) as a protective film, MXene-Ti₃C₂/Glucose oxidase (GOD) as the reaction layer, and chitosan (CS) /reduced graphene oxide (rGO) as the adhesion layer. Results demonstrate that the repeatability of the designed sensor increased by 73.3% after improving the adhesion between the reaction layer and the current collector and that its response ability was greatly enhanced. Moreover, the long-term stability of the electrode surface with SH protective film proved to be superior than that without protective film, which suggests that this design can effectively improve the overall performance of the enzyme biosensor. This work proposed a multi-tier synergistic approach for improving the reliability of enzyme sensors. Graphical abstract Our proposed protective and adhesion layer can greatly improve the stability of enzyme sensor and realize the rapid detection of glucose in serum sample.

Utilization of FAD-Glucose Dehydrogenase from T. emersonii for Amperometric Biosensing and Biofuel Cell Devices

Flavin-dependent glucose dehydrogenases (FAD-GDH) are oxygen-independent enzymes with high potential to be used as biocatalysts in glucose biosensing applications. Here, we present the construction of an amperometric biosensor and a biofuel cell device, which are based on a thermophilic variant of the enzyme originated from Talaromyces emersonii. The enzyme overexpression in Escherichia coli and its isolation and performance in terms of maximal bioelectrocatalytic currents were evaluated. We examined the biosensor's bioelectrocatalytic activity in 2,6-dichlorophenolindophenol-, thionine-, and dichloro-naphthoquinone-mediated electron transfer configurations or in a direct electron transfer one. We showed a negligible interference effect and good stability for at least 20 h for the dichloro-naphthoquinone configuration. The constructed biosensor was also tested in interstitial fluid-like solutions to show high bioelectrocatalytic current responses. The bioanode was coupled with a bilirubin oxidase-based biocathode to generate 270 μW/cm2 in a biofuel cell device.

Single Amino Acid Mutation of Pyranose 2-Oxidase Results in Increased Specificity for Diabetes Biomarker 1,5-Anhydro-D-Glucitol

Pyranose 2-oxidases catalyze the oxidation of various pyranose sugars at the C2 position. However, their potential application for detecting sugars other than glucose in blood is hindered by relatively high activity towards glucose. In this study, in order to find a mutant enzyme with enhanced specificity for 1,5-anhydro-D-glucitol (1,5-AG), which is a biomarker for diabetes mellitus, we conducted site-directed mutagenesis of pyranose 2-oxidase from the basidiomycete Phanerochaete chrysosporium ( Pc POX). Considering the three-dimensional structure of the substrate-binding site of Pc POX and the structural difference between glucose and 1,5-AG, we selected alanine 551 of Pc POX as a target residue for mutation. Kinetic studies of the 19 mutants of Pc POX expressed as recombinant proteins in E. coli revealed that the ratio of k _cat / K _m for 1,5-AG to k _cat / K _m for glucose was three times higher for the A551L mutant than for wild-type Pc POX. Although the A551L mutant has lower specific activity towards each substrate than the wild-type enzyme, its increased specificity for 1,5-AG makes it a promising lead for the development of POX-based 1,5-AG detection systems.

FAD dependent glucose dehydrogenases - Discovery and engineering of representative glucose sensing enzymes

The history of the development of glucose sensors goes hand-in-hand with the history of the discovery and the engineering of glucose-sensing enzymes. Glucose oxidase (GOx) has been used for glucose sensing since the development of the first electrochemical glucose sensor. The principle utilizing oxygen as the electron acceptor is designated as the first-generation electrochemical enzyme sensors. With increasing demand for hand-held and cost-effective devices for the "self-monitoring of blood glucose (SMBG)", second-generation electrochemical sensor strips employing electron mediators have become the most popular platform. To overcome the inherent drawback of GOx, namely, the use of oxygen as the electron acceptor, various glucose dehydrogenases (GDHs) have been utilized in second-generation principle-based sensors. Among the various enzymes employed in glucose sensors, GDHs harboring FAD as the redox cofactor, FADGDHs, especially those derived from fungi, fFADGDHs, are currently the most popular enzymes in the sensor strips of second-generation SMBG sensors. In addition, the third-generation principle, employing direct electron transfer (DET), is considered the most elegant approach and is ideal for use in electrochemical enzyme sensors. However, glucose oxidoreductases capable of DET are limited. One of the most prominent GDHs capable of DET is a bacteria-derived FADGDH complex (bFADGDH). bFADGDH has three distinct subunits; the FAD harboring the catalytic subunit, the small subunit, and the electron-transfer subunit, which makes bFADGDH capable of DET. In this review, we focused on the two representative glucose sensing enzymes, fFADGDHs and bFADGDHs, by presenting their discovery, sources, and protein and enzyme properties, and the current engineering strategies to improve their potential in sensor applications.

A novel thermostable D-amino acid oxidase of the thermophilic fungus Rasamsonia emersonii strain YA

d-Amino acid oxidase (DAAO) is a valuable flavoenzyme capable of being used in various practical applications, such as in determining d-amino acids and producing a material for semisynthetic cephalosporins, requiring higher thermal stability, higher catalytic activity, and broad substrate specificity. In this study, we isolated the thermophilic fungus Rasamsonia emersonii strain YA, which can grow on several d-amino acids as the sole nitrogen source, from a compost and characterized DAAO (ReDAAO) of the fungus. ReDAAO expressed in Escherichia coli exhibited significant oxidase activity against various neutral and basic d-amino acids, in particular hydrophobic d-amino acids. In addition, the enzyme also significantly acted on cephalosporin C, a starting material for semisynthetic antibiotics, and d-Glu, a general substrate for d-aspartate oxidase but not for DAAO, showing its unique and practically useful substrate specificity. The apparent k_cat and K_m values of the enzyme toward good substrates were comparable to those of higher catalytic fungal DAAOs, and the thermal stability (T₅₀ value of ~60 °C) was comparable to that of a thermophilic bacterial DAAO and significantly higher than that of other eukaryotic DAAOs. These results highlight the great potential of ReDAAO for use in practical applications.

Improvement in the thermal stability of Mucor prainii-derived FAD-dependent glucose dehydrogenase via protein chimerization

FAD-dependent glucose dehydrogenase (FAD-GDH, EC 1.1.5.9) is an enzyme utilized industrially in glucose sensors. Previously, FAD-GDH isolated from Mucor prainii (MpGDH) was demonstrated to have high substrate specificity for glucose. However, MpGDH displays poor thermostability and is inactivated after incubation at 45 °C for only 15 min, which prevents its use in industrial applications, especially in continuous glucose monitoring (CGM) systems. Therefore, in this study, a chimeric MpGDH (Mr144-297) was engineered from the glucose-specific MpGDH and the highly thermostable FAD-GDH obtained from Mucor sp. RD056860 (MrdGDH). Mr144-297 demonstrated significantly higher heat resistance, with stability at even 55 °C. In addition, Mr144-297 maintained both high affinity and accurate substrate specificity for D-glucose. Furthermore, eight mutation sites that contributed to improved thermal stability and increased productivity in Escherichia coli were identified. Collectively, chimerization of FAD-GDHs can be an effective method for the construction of an FAD-GDH with greater stability, and the chimeric FAD-GDH described herein could be adapted for use in continuous glucose monitoring sensors.

Comparison of Direct and Mediated Electron Transfer in Electrodes with Novel Fungal Flavin Adenine Dinucleotide Glucose Dehydrogenase

Direct and mediated electron transfer (DET and MET) in enzyme electrodes with a novel flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) from fungi are compared for the first time. DET is achieved by placing a single-walled carbon nanotube (CNT) between GDH and a flat gold electrode where the CNT is close to FAD within the distance for DET. MET is induced by using a free electron transfer mediator, potassium hexacyanoferrate, and shuttles electrons from FAD to the gold electrode. Cyclic voltammetry shows that the onset potential for glucose response current in DET is smaller than in MET, and that the distinct redox current peak pairs in MET are observed whereas no peaks are found in DET. The chronoamperometry with respect to a glucose biosensor shows that (i) the response in DET is more rapid than in MET; (ii) the current at more than +0.45V in DET is larger than the current at the current-peak potential in MET; (iii) a DET electrode covers the glucose concentration range for clinical requirements and is not susceptible to interfering agents at +0.45 V; and (iv) a DET electrode with the novel fungal FAD-GDH does not affect sensing accuracy in the presence of up to 5 mM xylose, while it often shows a similar response level to glucose with other conventionally used fungus-derived FAD-GDHs. It is concluded that our DET system overcomes the disadvantage of MET.

Designer fungus FAD glucose dehydrogenase capable of direct electron transfer

Fungi-derived flavin adenine dinucleotide glucose dehydrogenases (FADGDHs) are currently the most popular and advanced enzymes for self-monitoring of blood glucose sensors; however, the achievement of direct electron transfer (DET) with FADGDHs is difficult. In this study, a designer FADGDH was constructed by fusing Aspergillus flavus derived FADGDH (AfGDH) and a Phanerochaete chrisosporium CDH (PcCDH)-derived heme b-binding cytochrome domain to develop a novel FADGDH that is capable of direct electron transfer with an electrode. A structural prediction suggested that the heme in the CDH may exist in proximity to the FAD of AfGDH if the heme b-binding cytochrome domain is fused to the AfGDH N-terminal region. Spectroscopic observations of recombinantly produced designer FADGDH confirmed the intramolecular electron transfer between FAD and the heme. A decrease in pH and the presence of a divalent cation improved the intramolecular electron transfer. An enzyme electrode with the immobilized designer FADGDH showed an increase in current immediately after the addition of glucose in a glucose concentration-dependent manner, whereas those with wild-type AfGDH did not show an increase in current. Therefore, the designer FADGDH was confirmed to be a novel GDH that possesses electrode DET ability. The difference in the surface electrostatic potentials of AfGDH and the catalytic domain of PcCDH might be why their intramolecular electron transfer ability is inferior to that of CDH. These relevant and consistent findings provide us with a novel strategic approach for the improvement of the DET properties of designer FADGDH. (241 words).

Engineered fungus derived FAD-dependent glucose dehydrogenase with acquired ability to utilize hexaammineruthenium(III) as an electron acceptor

Fungal FAD-dependent glucose dehydrogenases (FADGDHs) are considered to be superior enzymes for glucose sensor strips because of their insensitivity to oxygen and maltose. One highly desirable mediator for enzyme sensor strips is hexaammineruthenium(III) chloride because of its low redox potential and high storage stability. However, in contrast to glucose oxidase (GOx), fungal FADGDH cannot utilize hexaammineruthenium(III) as electron acceptor. Based on strategic structure comparison between FADGDH and GOx, we constructed a mutant of Aspergillus flavus-derived FADGDH, capable of utilizing hexaammineruthenium(III) as electron acceptor: AfGDH-H403D. In AfGDH-H403D, a negative charge introduced at the pathway-entrance leading to the FAD attracts the positively charged hexaammineruthenium(III) and guides it into the pathway. The corresponding amino acid in wild-type GOx is negatively charged, which explains the ability of GOx to utilize hexaammineruthenium(III) as electron acceptor. Electrochemical measurements showed a response current of 46.0 μA for 10 mM glucose with AfGDH-H403D and hexaammineruthenium(III), similar to that with wild-type AfGDH and ferricyanide (47.8 μA). Therefore, AfGDH-H403D is suitable for constructing enzyme electrode strips with hexaammineruthenium(III) chloride as sole mediator. Utilization of this new, improved fungal FADGDH should lead to the development of sensor strips for blood glucose monitoring with increased accuracy and less stringent packing requirements.

Thermophilic Talaromyces emersonii Flavin Adenine Dinucleotide-Dependent Glucose Dehydrogenase Bioanode for Biosensor and Biofuel Cell Applications

Flavin adenine dinucleotide (FAD)-dependent glucose dehydrogenase (GDH) was identified and cloned from thermophilic filamentous fungi Talaromyces emersonii using the homology cloning method. A direct electron transfer bioanode composed of T. emersonii FAD-GDH and a single-walled carbon nanotube was produced. Enzymes from thermophilic microorganisms generally have low activity at ambient temperature; however, the T. emersonii FAD-GDH bioanode exhibits a large anodic current due to the enzymatic reaction (1 mA cm^-2) at ambient temperature. Furthermore, the T. emersonii FAD-GDH bioanode worked at 70 °C for 12 h. This is the first report of a bioanode with a glucose-catalyzing enzyme from a thermophilic microorganism that has potential for biosensor and biofuel cell applications. In addition, we demonstrate how the glycoforms of T. emersonii FAD-GDHs expressed by various hosts influence the electrochemical properties of the bioanode.

Bimolecular Rate Constants for FAD-Dependent Glucose Dehydrogenase from Aspergillus terreus and Organic Electron Acceptors

The flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) from Aspergillus species require suitable redox mediators to transfer electrons from the enzyme to the electrode surface for the application of bioelectrical devices. Although several mediators for FAD-GDH are already in use, they are still far from optimum in view of potential, kinetics, sustainability, and cost-effectiveness. Herein, we investigated the efficiency of various phenothiazines and quinones in the electrochemical oxidation of FAD-GDH from Aspergillus terreus. At pH 7.0, the logarithm of the bimolecular oxidation rate constants appeared to depend on the redox potentials of all the mediators tested. Notably, the rate constant of each molecule for FAD-GDH was approximately 2.5 orders of magnitude higher than that for glucose oxidase from Aspergillus sp. The results suggest that the electron transfer kinetics is mainly determined by the formal potential of the mediator, the driving force of electron transfer, and the electron transfer distance between the redox active site of the mediator and the FAD, affected by the steric or chemical interactions. Higher k₂ values were found for ortho-quinones than for para-quinones in the reactions with FAD-GDH and glucose oxidase, which was likely due to less steric hindrance in the active site in the case of the ortho-quinones.