Direct energy conversion from xylose using xylose dehydrogenase surface displayed bacteria based enzymatic biofuel cell

Sensitive biosensors based on topological insulator Bi2Se3 and peptide

In this work, we designed a facile and label-free electrochemical biosensor based on intrinsic topological insulator (TI) Bi₂Se₃ and peptide for the detection of immune checkpoint molecules. With topological protection, Bi₂Se₃ could have robust surface states with low electronic noise, which was beneficial for the stable and sensitive electron transport between electrode and electrolyte interface. The peptides are easily synthesized and chemically modified, and have good biocompatibility and bioavailability, which is a suitable candidate as the recognition units for immune checkpoint molecules. Therefore, the peptide/Bi₂Se₃ was developed as a suitable working electrode for the electrochemical biosensor. The basic performance of the designed peptide/Bi₂Se₃ biosensor was investigated to determine the Anti-HA Tag Antibody and PD-L1 molecules. The linear detection range was from 3.6 × 10^-10 mg mL^-1 to 3.6 × 10^-5 mg mL^-1, and the detection limit was 1.07 × 10^-11 mg mL^-1. Moreover, the biosensor also displayed good selectivity and stability.

A membraneless starch/O2 biofuel cell based on bacterial surface regulable displayed sequential enzymes of glucoamylase and glucose dehydrogenase

Enzymatic biofuel cells (EBFCs) provide a new strategy to enable direct biomass-to-electricity conversion, posing considerable demand on sequential enzymes. However, artificial blend of multi-enzyme systems often suffer biocatalytic inefficiency due to the rambling mixture of catalytic units. In an attempt to construct a high-performance starch/O₂ EBFC, herein we prepared a starch-oxidizing bioanode based on displaying a sequential enzyme system of glucoamylase (GA) and glucose dehydrogenase (GDH) on E.coli cell surfaces in a precise way using cohesin-dockerin interactions. The enzyme stoichiometry was optimized, with GA&GDH (3:1)-E.coli exhibiting the highest catalytic reaction rate. The bioanode employed polymerized methylene blue (polyMB) to collect electrons from the oxidation of NADH into NAD⁺, which jointly oxidized starch together with co-displayed GA and GDH. The bioanode was oxygen-insensitive, which can be combined with a laccase based biocathode, resulting in a membranless starch/O₂ EBFC in a non-compartmentalized configuration. The optimal EBFC exhibited an open-circuit voltage (OCV) of 0.74 V, a maximum power density of 30.1 ± 2.8 μW cm^-2, and good operational stability.

The Role of Peptides in the Design of Electrochemical Biosensors for Clinical Diagnostics

Peptides represent a promising class of biorecognition elements that can be coupled to electrochemical transducers. The benefits lie mainly in their stability and selectivity toward a target analyte. Furthermore, they can be synthesized rather easily and modified with specific functional groups, thus making them suitable for the development of novel architectures for biosensing platforms, as well as alternative labelling tools. Peptides have also been proposed as antibiofouling agents. Indeed, biofouling caused by the accumulation of biomolecules on electrode surfaces is one of the major issues and challenges to be addressed in the practical application of electrochemical biosensors. In this review, we summarise trends from the last three years in the design and development of electrochemical biosensors using synthetic peptides. The different roles of peptides in the design of electrochemical biosensors are described. The main procedures of selection and synthesis are discussed. Selected applications in clinical diagnostics are also described.

Tackling the Challenges of Enzymatic (Bio)Fuel Cells

The ever-increasing demands for clean and sustainable energy sources combined with rapid advances in biointegrated portable or implantable electronic devices have stimulated intensive research activities in enzymatic (bio)fuel cells (EFCs). The use of renewable biocatalysts, the utilization of abundant green, safe, and high energy density fuels, together with the capability of working at modest and biocompatible conditions make EFCs promising as next generation alternative power sources. However, the main challenges (low energy density, relatively low power density, poor operational stability, and limited voltage output) hinder future applications of EFCs. This review aims at exploring the underlying mechanism of EFCs and providing possible practical strategies, methodologies and insights to tackle these issues. First, this review summarizes approaches in achieving high energy densities in EFCs, particularly, employing enzyme cascades for the deep/complete oxidation of fuels. Second, strategies for increasing power densities in EFCs, including increasing enzyme activities, facilitating electron transfers, employing nanomaterials, and designing more efficient enzyme-electrode interfaces, are described. The potential of EFCs/(super)capacitor combination is discussed. Third, the review evaluates a range of strategies for improving the stability of EFCs, including the use of different enzyme immobilization approaches, tuning enzyme properties, designing protective matrixes, and using microbial surface displaying enzymes. Fourth, approaches for the improvement of the cell voltage of EFCs are highlighted. Finally, future developments and a prospective on EFCs are envisioned.

Wearable biofuel cells based on the classification of enzyme for high power outputs and lifetimes

Wearable enzymatic biofuel cells would be the most prospective fuel cells for wearable devices because of their low cost, compactness and flexibility. As the high specificity and catalytic properties of enzymes, enzymatic biofuel cells (EBFCs) catalyze the fuel associated with the redox reaction and get electrical energy. Available biofuels such as glucose, lactate and pyruvate can be harvested from biofluids of sweat, tears and blood, which afford cells a favorable use in implantable and wearable devices. However, the development of wearable enzymatic biofuel cells requires significant improvements on the power density and enzymes lifetime. In this paper, some new advances in improving the performance of wearable enzymatic biofuel cells are reviewed based on the bioanode and biocathode by classifying single-enzyme and multi-enzyme catalysis system. Thereinto, the bioanode usually contains oxidases and dehydrogenases as catalyst, and the biocathode utilizes the catalysis of multi-copper oxidases (MCOs) in the single system. For further enhancing the power density, efforts to develop multi-enzyme catalysis strategies are discussed in bioanode and biocathode respectively. Moreover, some potential technologies in recent years, such as carbon nanodots, CNT sponges and mixed operational/storage electrode are summarized owing to notable efficiency and the capability of enhancing electron transfer on the electrode. Finally, major challenges and future prospects are discussed for the high power output, stable and practical wearable enzymatic biofuel cells.

Co-immobilization of glucose oxidase and catalase for enhancing the performance of a membraneless glucose biofuel cell operated under physiological conditions

Glucose oxidase (GOx)-catalase co-immobilized catalyst (CNT/PEI/(GOx-Cat)) was synthesized, and its catalytic activity and electrical performance were investigated and compared, whereas the amount of immobilized catalase was optochemically inspected by chemiluminescence (CL) assay. With the characterizations, it was confirmed that the catalase was well immobilized on the CNT/PEI surface, whereas both the GOx and catalase play their roles well in the catalyst. According to the measurements of the current density peak of the flavin adenine dinucleotide (FAD) redox reaction, electron transfer rate, Michaelis-Menten constants and sensitivity, CNT/PEI/(GOx-Cat) shows the best values, and this is attributed to the excellent catalytic activity of GOx and the H₂O₂ decomposition capability of the catalase. To evaluate the electrical performance, a membraneless glucose biofuel cell (GBFC) adopting the catalyst was operated under physiological conditions and produced a maximum power density (MPD) of 180.8 ± 22.3 μW cm^-2, which is the highest value compared to MPDs obtained by adoption of other catalysts. With such results, it was clarified that the CNT/PEI/(GOx-Cat) manufactured by co-immobilization of GOx and catalase leads to enhancements in the catalytic activity and GBFC performance due to the synergetic effects of (i) effective removal of harmful H₂O₂ moiety by catalase and (ii) superior activation of desirable reactions by GOx.

Development of a new method for d-xylose detection and quantification in urine, based on the use of recombinant xylose dehydrogenase from Caulobacter crescentus

The gene xylB from Caulobacter crescentus has been cloned and expressed in Escherichia coli providing a high yield of xylose dehydrogenase (XylB) production and excellent purity (97%). Purified recombinant XylB showed an absolute dependence on the cofactor NAD(+) and a strong preference for d-xylose against other assayed mono and disaccharides. Additionally, XylB showed strong stability when stored as freeze-dried powder at least 250days both at 4°C and room temperature. In addition, more than 80% of the initial activity of rehydrated freeze-dried enzyme remained after 150days of incubation at 4°C. Based on these characteristics, the capability of XylB in d-xylose detection and quantification was studied. The linearity of the method was maintained up to concentrations of d-xylose of 10mg/dL and the calculated limits of detection (LoD) and quantification (LoQ) of xylose in buffer were 0.568mg/dL and 1.89mg/dL respectively. Thus, enzymatic detection was found to be an excellent method for quantification of d-xylose in both buffer and urine samples. This method can easily be incorporated in a new test for the diagnosis of hypolactasia through the measurement of intestinal lactase activity.

Fabrication of Mediatorless/Membraneless Glucose/Oxygen Based Biofuel Cell using Biocatalysts Including Glucose Oxidase and Laccase Enzymes

Mediatorless and membraneless enzymatic biofuel cells (EBCs) employing new catalytic structure are fabricated. Regarding anodic catalyst, structure consisting of glucose oxidase (GOx), poly(ethylenimine) (PEI) and carbon nanotube (CNT) is considered, while three cathodic catalysts consist of glutaraldehyde (GA), laccase (Lac), PEI and CNT that are stacked together in different ways. Catalytic activities of the catalysts for glucose oxidation and oxygen reduction reactions (GOR and ORR) are evaluated. As a result, it is confirmed that the catalysts work well for promotion of GOR and ORR. In EBC tests, performances of EBCs including 150 μm-thick membrane are measured as references, while those of membraneless EBCs are measured depending on parameters like glucose flow rate, glucose concentration, distance between two electrodes and electrolyte pH. With the measurements, how the parameters affect EBC performance and their optimal conditions are determined. Based on that, best maximum power density (MPD) of membraneless EBC is 102 ± 5.1 μW · cm(-2) with values of 0.5 cc · min(-1) (glucose flow rate), 40 mM (glucose concentration), 1 mm (distance between electrodes) and pH 3. When membrane and membraneless EBCs are compared, MPD of the membraneless EBC that is run at the similar operating condition to EBC including membrane is speculated as about 134 μW · cm(-2).

Study on an improved bio-electrode made with glucose oxidase immobilized mesoporous carbon in biofuel cells

Response surface methodology (RSM) was used for process optimization to immobilize glucose oxidase (GOx) on ordered mesoporous carbon (OMC).

Electrochemical Glucose Biosensor Based on Glucose Oxidase Displayed on Yeast Surface

The conventional enzyme-based biosensor requires chemical or physical immobilization of purified enzymes on electrode surface, which often results in loss of enzyme activity and/or fractions immobilized over time. It is also costly. A major advantage of yeast surface display is that it enables the direct utilization of whole cell catalysts with eukaryote-produced proteins being displayed on the cell surface, providing an economic alternative to traditional production of purified enzymes. Herein, we describe the details of the display of glucose oxidase (GOx) on yeast cell surface and its application in the development of electrochemical glucose sensor. In order to achieve a direct electrochemistry of GOx, the entire cell catalyst (yeast-GOx) was immobilized together with multiwalled carbon nanotubes on the electrode, which allowed sensitive and selective glucose detection.

Layer-by-layer assembly of a redox enzyme displayed on the surface of elongated bacteria into a hierarchical artificial biofilm based anode

A highly efficient artificial biofilm based anode was constructed by an assembly of an elongatedE. coliwith surface displayed redox-enzyme.

Bacterial cell-surface displaying of thermo-tolerant glutamate dehydrogenase and its application in L-glutamate assay

In this paper, glutamate dehydrogenase (Gldh) is reported to efficiently display on Escherichia coli cell surface by using N-terminal region of ice the nucleation protein as an anchoring motif. The presence of Gldh was confirmed by SDS-PAGE and enzyme activity assay. Gldh was detected mainly in the outer membrane fraction, suggesting that the Gldh was displayed on the bacterial cell surface. The optimal temperature and pH for the bacteria cell-surface displayed Gldh (bacteria-Gldh) were 70°C and 9.0, respectively. Additionally, the fusion protein retained almost 100% of its initial enzymatic activity after 1 month incubation at 4°C. Transition metal ions could inhibit the enzyme activity to different extents, while common anions had little adverse effect on enzyme activity. Importantly, the displayed Gldh is most specific to l-glutamate reported so far. The bacterial Gldh was enabled to catalyze oxidization of l-glutamate with NADP(+) as cofactor, and the resultant NADPH can be detected spectrometrically at 340nm. The bacterial-Gldh based l-glutamate assay was established, where the absorbance at 340nm increased linearly with the increasing l-glutamate concentration within the range of 10-400μM. Further, the proposed approach was successfully applied to measure l-glutamate in real samples.

Simultaneously improving stability and specificity of cell surface displayed glucose dehydrogenase mutants to construct whole-cell biocatalyst for glucose biosensor application

The improved stability and substrate specificity of cell surface displayed glucose dehydrogenase (GDH) mutants by replacing four amino acids from Bacillus subtilis by using site-directed mutagenesis was systematically investigated. A series of mutated GDHs including E170R/Q252L, V149K/E170R/Q252L, E170R/Q252L/G259A and V149K/E170R/Q252L/G259A, were fused to the ice nucleation protein for displaying on cell surface of Eschericia coli. Q252L/E170R/V149K, Q252L/E170R/G259A and Q252L/E170R/V149K/G259A variants were found stable at a wide pH range and shown excellent thermostability. Especially, the Q252L/E170R/V149K/G259A mutant showed half-life of ~3.8days at 70 °C. Q252L/E170R/V149K/G259A variant exhibited the narrowest substrate specificity for d-glucose. The whole cell displayed GDH mutant could be cultured in a large scale with excellent enzyme activity and productivity. In addition, a sensitive and stable electrochemical glucose biosensor can be prepared using the GDH-mutant bacteria modified electrode. Thus, the whole cell biocatalysts are promising candidates for exploitation in a wide range of industrial applications.

Cell surface display of organophosphorus hydrolase for sensitive spectrophotometric detection of p-nitrophenol substituted organophosphates

Organophosphates (OPs) widely exist in ecosystem as toxic substances, for which sensitive and rapid analytical methods are highly requested. In the present work, by using N-terminal of ice nucleation protein (INP) as anchoring motif, a genetically engineered Escherichia coli (E. coli) strain surface displayed mutant organophosphorus hydrolase (OPH) (S5) with improved enzyme activity was successfully constructed. The surface location of INP-OPH fusion was confirmed by SDS-PAGE analysis and enzyme activity assays. The OPH-displayed bacteria facilitate the hydrolysis of p-nitrophenol (PNP) substituted organophosphates to generate PNP, which can be detected spectrometrically at 410 nm. Over 90% of the recombinant protein present on the surface of microbes demonstrated enhanced enzyme activity and long-term stability. The OPH activity of whole cells was 2.16 U/OD₆₀₀ using paraoxon as its substrate, which is the highest value reported so far. The optimal temperature for OPH activity was around 55 °C, and suspended cultures retained almost 100% of its activity over a period of one month at room temperature, exhibiting the better stability than free OPH. The recombinant E. coli strain could be employed as a whole-cell biocatalyst for detecting PNP substituted OPs at wider ranges and lower detection limits. Specifically, the linear ranges of the calibration curves were 0.5-150 μM paraoxon, 1-200 μM parathion and 2.5-200 μM methyl parathion, and limits of detection were 0.2 μM, 0.4 μM and 1 μM for paraoxon, parathion and methyl parathion, respectively (S/N=3). These results indicate that the engineered OPH strain is a promising multifunctional bacterium that could be used for further large-scale industrial and environmental applications.