pH and heat-dependent behaviour of glucose oxidase down to single molecule level by combined fluorescence spectroscopy and molecular modelling

Imaging metabolic mechanisms and the binding behavior of nutrients/transporters of edible Matricaria flowers VOCs

To promote the consumption of flowers and to utilize the nutritional value of proteins, the efficacy of the beneficial components of flowers has been intensively studied. Anthemis nobilis was used as the study object, and all its volatile components (VOCs) were fingerprinted using headspace solid-phase micro-extraction gas-mass spectrometry (HS-SPME/GC-MS). GC-MS fingerprints of five parts of Anthemis nobilis were established using three proteins, bovine lactoferrin (BLF), bovine lactoglobulin (β-Lg), and human serum albumin (HSA), as nutrient transporters. The interactions between the volatile components from different parts of the mother chrysanthemum plant and the nutrient/transport proteins were investigated. The results of fingerprinting showed that the flavor components were dominated by alkenes. In addition, this study revealed that among the three nutrient transporters, the strongest binding to the adsorbed volatile components was HSA, followed by BLF, and β-Lg was second. In addition, a characteristic molecule, camphene, was screened. Integrated molecular simulation using fluorescence spectroscopy was used to validate the results of the interaction of the nutrient/transport proteins systems with characteristic molecule. The properties of the characteristic molecules such as absorption, distribution, metabolism, excretion and toxicity in vivo were analyzed using ADMET to provide a theoretical basis for the preparation of flower-flavored dairy products.

The Impact of the Functional Layer Composition of Glucose Test-Strips on the Stability of Electrochemical Response

Herein, the impact of the chemical stability of RedOx mediator ferricyanide, K3[Fe(CN)6] (FC), a type of buffer solution used for bioreceptor preparation, gel composition (carboxymethylcellulose, CMC, Aerosile, AS, and alginate, ALG) on the long term stability of glucose test-strips and their analytical performance was examined. By simple addition of ALG to the functional gel aiming to improve its viscosity, we managed to enhance the sensitivity of conventional CMC-containing amperometric glucose test-strips from 3.3 µA/mM to 3.9 µA/mM and extend their shelf life from 8 months to 1.7 years. Moreover, during the course of investigations, it was revealed that the activity of enzyme in dependence with the used buffer did not linearly correlate with its activity in a dried functional layer, and the entire long-term electrochemical signal of glucose test-strips was determined by RedOx mediator FC chemical stability. The most stable and sensitive test-strips were obtained by the screen-printing approach from a gel containing 24 mg/mL GOx prepared in citrate buffer with pH 6, 200 mg/mL of FC and 10 mg/mL of CMC supplemented with 25 mg/mL of ALG.

Mechanism of enhancing the water-solubility and stability of curcumin by using self-assembled cod protein nanoparticles at an alkaline pH

Curcumin (Cur) is a bioactive phytochemical which is claimed to have several health-promoting benefits, whose applications are challenging due to its poor water-solubility, chemical instability, and low bioavailability. In this research, Cur was encapsulated in the cod protein (CP) using a pH-driven method to enhance its solubility and stability. The physicochemical and structural properties of cod protein-curcumin nanoparticles (CP-Cur) formed were characterized. Fluorescence spectroscopy (FL), ultraviolet spectroscopy (UV), circular dichroism (CD), and dynamic light scattering (DLS) results collectively suggest that the protein originally with a molten-globule state refolded into a more ordered structure after neutralization, during which Cur was incorporated. Fluorescence quenching and isothermal titration calorimetry (ITC) further showed that the CP/Cur binding was mainly driven by hydrophobic interactions, resulting in static fluorescence quenching and energy release. Up to 99.50% of Cur was loaded in the CP delivery system. Furthermore, the thermal stability and photostability of Cur were greatly improved due to the protection of the protein. The present study proved that cod protein could be a great potential edible carrier for encapsulating curcumin.

Analytical Parameters of a Novel Glucose Biosensor Based on Grafted PFM as a Covalent Immobilization Technique

Bioanalytical methods, in particular electrochemical biosensors, are increasingly used in different industrial sectors due to their simplicity, low cost, and fast response. However, to be able to reliably use this type of device, it is necessary to undertake in-depth evaluation of their fundamental analytical parameters. In this work, analytical parameters of an amperometric biosensor based on covalent immobilization of glucose oxidase (GOx) were evaluated. GOx was immobilized using plasma-grafted pentafluorophenyl methacrylate (pgPFM) as an anchor onto a tailored HEMA-co-EGDA hydrogel that coats a titanium dioxide nanotubes array (TiO2NTAs). Finally, chitosan was used to protect the enzyme molecules. The biosensor offered outstanding analytical parameters: repeatability (RSD = 1.7%), reproducibility (RSD = 1.3%), accuracy (deviation = 4.8%), and robustness (RSD = 2.4%). In addition, the Ti/TiO2NTAs/ppHEMA-co-EGDA/pgPFM/GOx/Chitosan biosensor showed good long-term stability; after 20 days, it retained 89% of its initial sensitivity. Finally, glucose concentrations of different food samples were measured and compared using an official standard method (HPLC). Deviation was lower than 10% in all measured samples. Therefore, the developed biosensor can be considered to be a reliable analytical tool for quantification measurements.

Effect of pH on pulsed light inactivation of polyphenol oxidase

The inactivation of diverse food enzymes by pulsed light (PL) has been described before, including the inactivation of polyphenol oxidase (PPO) (at pH 6.5). Since the pH affects the conformation of enzymes, it may influence the inactivation of enzymes by PL. The aim of this work was to evaluate the effect of pH on the kinetics of the PL-inactivation and associated structural changes of a case enzyme. To this, PPO was treated by PL at different pHs (4.0-6.5) and its inactivation kinetics and changes in its structure were evaluated by spectrophotometric and spectrofluorometric methods. The inactivation proceeded faster at low pH and was highly correlated with the decrease in peak intrinsic fluorescence intensity. Phase diagrams and parameter A evolution indicated the absence of intermediate unfolded states during the course of the inactivation. No protein aggregation was detected by turbidimetry. Results indicate that although a low pH favors the PL-inactivation of PPO, the mechanism of inactivation is pH-independent. Beyond the specific outcome for PPO, the results are evidence of a general pH-independence in the mechanism of enzyme inactivation by PL in the pH range 4.0-6.5 and acidification can be a strategy to decrease treatment times during PL processing.

The Change Mechanism of Structural Characterization and Thermodynamic Properties of Tannase from Aspergillus niger NL112 Under High Temperature

Tannase from Aspergillus niger NL112 was purified 5.1-fold with a yield of 50.44% via ultrafiltration, DEAE-Sepharose Fast Flow column chromatography, and Sephadex G-100 column chromatography. The molecular weight of the purified tannase was estimated as 45 kDa. The optimum temperature and pH for its activity were 45 °C and 5.0, respectively. The results of circular dichroism, FT-IR (Fourier transform infrared) spectroscopy, and fluorescence spectra indicated that high temperature could lead to the change of tannase secondary and tertiary structures. Tannase had a greater affinity for tannic acid at 40 °C with a K_m value of 2.12 mM and the greatest efficiency hydrolysis (K_cat/K_m) at 45 °C. The rate of inactivation (k) increased with the increase of temperature and the half-life (t_1/2) gradually decreased. It was found to be 1.0 of the temperature quotient (Q₁₀) value for tannic acid hydrolysis by tannase. The thermodynamic parameters of the interaction system were calculated at various temperatures. The positive enthalpy (ΔH) values and decreasing ΔH values with the increase of temperature indicated that the hydrolysis of tannase was an endothermic process. Our results indicated that elevated temperature could change the tertiary structure of tannase and reduce its thermostability, which caused a gradual decrease of tannase activity with an increase in temperature.

Time-resolved fluorescence spectroscopy based evaluation of stability of glucose oxidase

Glucose oxidase (GOx) is one of the most frequently used enzymes in a design of enzymatic biosensors and biofuel cells, which are novel electrical energy generation systems. Therefore, a better understanding of the mode of action of this enzyme is very important for further development of GOx-based sensors. In this research fluorescence properties of GOx in different acidic media have been estimated by the evaluation of redox states of active center that is flavine adenine dinucleotide (FAD). Steady-state fluorescence spectroscopy was applied to monitor the activity of GOx. A variation of pH has been invoked to gain a better understanding in the variations of GOx activity. The tendency of GOx activity to decrease over the time was determined, while increased intensity of the fluorescence band of GOx at 530 nm was associated with a decreased activity of the enzyme. The changes in fluorescence intensity of this band are caused by the dissociation of FAD from the enzyme. This process is not reversible, therefore, the decrease in the fluorescence intensity can be also associated with structural changes of the FAD during its reduction.

In situ and real-time insight into Rhizopus chinensis lipase under high pressure and temperature: Conformational traits and biobehavioural analysis

An in situ and real-time investigation was performed using an optical cell system and in-silico analysis to reveal the impacts of pressure and temperature on the conformational state and behaviours of Rhizopus chinensis lipase (RCL). The fluorescence intensity (FI) of RCL increased remarkably under high pressure, and part of this increase was recovered after depressurization. This result displayed the partially reversible conformational change of RCL, which may be associated with the local change of Trp224 near the catalytic centre. High temperature caused a significant loss of secondary structure, whereas the α-helical segments including the lid were preserved by high pressure even at temperatures over 60 °C. The parameters of enzymatic reaction monitored by UV showed that the hydrolysis rate was remarkably enhanced by the pressure of 200 MPa. In the pressure range of 0.1-200 MPa, the active volume measured by the in situ system decreased from -2.85 to -6.73 mL/mol with the temperature increasing from 20 °C to 40 °C. The high catalytic capacity of the lipase under high pressure and high temperature was primarily attributed to pressure protection on RCL.

An insight into pH-induced changes in FAD conformational structure by means of time-resolved fluorescence and circular dichroism

Optical properties of flavin adenine dinucleotide (FAD) moiety are widely used nowadays for biotechnological applications. Given the fundamental role played by FAD, additional structural information about this enzymatic cofactor can be extremely useful in order to obtain a greater insight into its functional role in proteins. For this purpose, we have investigated FAD behaviour in aqueous solutions at different pH values by a novel approach based on the combined use of time-resolved fluorescence and circular dichroism spectroscopies. The results showed that pH strongly affects time-resolved fluorescence emission and the analysis allowed us to detect a three-component decay for FAD in aqueous solution with pH-depending lifetimes and relative amplitudes. Circular dichroism data were analyzed by a multi-Gaussian fitting procedure and the trends of properly chosen parameters confirmed pH-depending changes. The comparison between the results obtained by these two optical techniques allowed us to improve the significance of the outcome of circular dichroism. This combined approach may provide a useful tool for biotechnological investigation.

Novel grafted electrochemical interface for covalent glucose oxidase immobilization using reactive pentafluorophenyl methacrylate

One of the most important factors for the proper functioning of enzymatic electrochemical biosensors is the enzyme immobilization strategy. In this work, glucose oxidase was covalently immobilized using pentafluorophenyl methacrylate (PFM) by applying two different surface modification techniques (plasma polymerization and plasma-grafting). The grafted surface was specifically designed to covalently anchor enzyme molecules. It was observed using QCM-D measurements the PFM plasma-grafted surfaces were able to retain a higher number of active enzyme molecules than the PFM polymerized surfaces. An amperometric glucose biosensor using titanium dioxide nanotubes array (TiO₂NTAs) modified by PFM plasma-grafted surface was prepared. The resulting biosensor exhibited a fast response and short analysis time (approximately eight minutes per sample). Moreover, this biosensor achieved high sensitivity (9.76 μA mM^-1) with a linear range from 0.25 to 1.49 mM and a limit of detection (LOD) equal to 0.10 mM of glucose. In addition, the glucose content of 16 different food samples was successfully measured using the developed biosensor. The obtained results were compared with the respective HPLC value and a deviation smaller than 10% was obtained in all the cases. Therefore, the biosensor was able to overcome all possible interferences in the selected samples/matrices.

Interaction between quinoline yellow and human serum albumin: spectroscopic, chemometric and molecular docking studies

BACKGROUND

Quinoline yellow (QY), a synthetic colourant widely used in the food industry, has caused extensive concerns because of its potentially harmful effects on human health. In the present work, the interactions between QY and human serum albumin (HSA) were characterized by multiple spectroscopic methods, a chemometric algorithm, and molecular modelling studies.

RESULTS

The concentration profiles and pure spectra obtained for the components (QY, HSA and QY-HSA complex) from analyses of the expanded UV-visible absorption data matrices by multivariate curve resolution alternating least squares confirmed the QY-HSA interaction process. QY quenched the fluorescence of HSA through formation of a QY-HSA complex that was stabilized by moderate affinity. Hydrophobic forces and hydrogen bonding play major roles in the binding of QY to HSA. Site-specific marker-induced displacement results suggest that QY binds to subdomain IIA of HSA. This was corroborated by the molecular docking results. Decreases in HSA surface hydrophobicity and free sulfhydryl group content indicate that QY causes a contraction of the peptide strand in HSA, hiding the hydrophobic patches of the protein. Analyses by UV-visible absorption, circular dichroism, and three-dimensional fluorescence spectroscopy found that QY causes microenvironmental perturbations around the fluorophores and secondary structure changes in HSA.

CONCLUSION

This work shows that QY binds to HSA, affecting its structural and functional properties, and provides new insights into the binding mechanism and a comprehensive understanding of the toxicity of QY to biological processes. © 2018 Society of Chemical Industry.

Biocatalytic "Oxygen-Fueled" Atom Transfer Radical Polymerization

Atom transfer radical polymerization (ATRP) can be carried out in a flask completely open to air using a biocatalytic system composed of glucose oxidase (GOx) and horseradish peroxidase (HRP) with an active copper catalyst complex. Nanomolar concentrations of the enzymes and ppm amounts of Cu provided excellent control over the polymerization of oligo(ethylene oxide) methyl ether methacrylate (OEOMA₅₀₀ ), generating polymers with high molecular weight (M_n >70 000) and low dispersities (1.13≤Đ≤1.27) in less than an hour. The continuous oxygen supply was necessary for the generation of radicals and polymer chain growth as demonstrated by temporal control and by inducing hypoxic conditions. In addition, the enzymatic cascade polymerization triggered by oxygen was used for a protein and DNA functionalized with initiators to form protein-b-POEOMA and DNA-b-POEOMA bioconjugates, respectively.

The Glycosyltransferases of LPS Core: A Review of Four Heptosyltransferase Enzymes in Context

Bacterial antibiotic resistance is a rapidly expanding problem in the world today. Functionalization of the outer membrane of Gram-negative bacteria provides protection from extracellular antimicrobials, and serves as an innate resistance mechanism. Lipopolysaccharides (LPS) are a major cell-surface component of Gram-negative bacteria that contribute to protecting the bacterium from extracellular threats. LPS is biosynthesized by the sequential addition of sugar moieties by a number of glycosyltransferases (GTs). Heptosyltransferases catalyze the addition of multiple heptose sugars to form the core region of LPS; there are at most four heptosyltransferases found in all Gram-negative bacteria. The most studied of the four is HepI. Cells deficient in HepI display a truncated LPS on their cell surface, causing them to be more susceptible to hydrophobic antibiotics. HepI-IV are all structurally similar members of the GT-B structural family, a class of enzymes that have been found to be highly dynamic. Understanding conformational changes of heptosyltransferases are important to efficiently inhibiting them, but also contributing to the understanding of all GT-B enzymes. Finding new and smarter methods to inhibit bacterial growth is crucial, and the Heptosyltransferases may provide an important model for how to inhibit many GT-B enzymes.

The Stories Tryptophans Tell: Exploring Protein Dynamics of Heptosyltransferase I from Escherichia coli

Heptosyltransferase I (HepI) catalyzes the addition of l-glycero-β-d-manno-heptose to Kdo₂-Lipid A, as part of the biosynthesis of the core region of lipopolysaccharide (LPS). Gram-negative bacteria with gene knockouts of HepI have reduced virulence and enhanced susceptibility to hydrophobic antibiotics, making the design of inhibitors of HepI of interest. Because HepI protein dynamics are partially rate-limiting, disruption of protein dynamics might provide a new strategy for inhibiting HepI. Discerning the global mechanism of HepI is anticipated to aid development of inhibitors of LPS biosynthesis. Herein, dynamic protein rearrangements involved in the HepI catalytic cycle were probed by combining mutagenesis with intrinsic tryptophan fluorescence and circular dichroism analyses. Using wild-type and mutant forms of HepI, multiple dynamic regions were identified via changes in Trp fluorescence. Interestingly, Trp residues (Trp199 and Trp217) in the C-terminal domain (which binds ADP-heptose) are in a more hydrophobic environment upon binding of ODLA to the N-terminal domain. These residues are adjacent to the ADP-heptose binding site (with Trp217 in van der Waals contact with the adenine ring of ADP-heptose), suggesting that the two binding sites interact to report on the occupancy state of the enzyme. ODLA binding was also accompanied by a significant stabilization of HepI (heating to 95 °C fails to denature the protein when it is in the presence of ODLA). These results suggest that conformational rearrangements, from an induced fit model of substrate binding to HepI, are important for catalysis, and the disruption of these conformational dynamics may serve as a novel mechanism for inhibiting this and other glycosyltransferase enzymes.