Bilirubin oxidases in bioelectrochemistry: Features and recent findings

Cholate Adsorption Behavior at Carbon Electrode Interface and Its Promotional Effect in Laccase Direct Bioelectrocatalysis

Fast electron transfer between laccase (Lac) and single-walled carbon nanotubes (SWCNTs) can be achieved at a cholate-modified SWCNT interface. Furthermore, the catalytic reduction of O2 starts at a high potential, close to the equilibrium redox potential of the O2/H2O couple. A sodium cholate (SC)-modified electrode interface provides suitable conditions for Lac direct bioelectrocatalysis. In the present study, the SC promotional effect in Lac direct bioelectrocatalysis was investigated using various types of electrode materials. The fully hydrophilic surface of indium tin oxide and an Au electrode surface did not show a SC promotional effect, because SC did not bind to these surfaces. A carbon surface with a large number of defects was unsuitable for SC binding because of hydrophilic functional groups at the defect sites. Carbon surfaces with few defects, for example, basal-plane highly oriented pyrolytic graphite (HOPG), gave a SC promotional effect.

Enhancement of Electrochemical Performance of Bilirubin Oxidase Modified Gas Diffusion Biocathode by Porphyrin Precursor

Recent studies have focused on tailoring the catalytic currents of multicopper oxidase (MCO) enzymes-based biocathodes to enhance oxygen reduction. Biocathodes modified with natural substrates specific for MCO enzymes demonstrated drastic improvement for oxygen reduction. Performance of 1-pyrenebutanoic acid, succinimidyl ester (PBSE), and 2,5-dimethyl-1-phenyl-1H-pyrrole-3-carbaldehyde (Di-Carb) oriented bilirubin oxidase (BOx) modified gas diffusion biocathode has been highly improved by incorporating hematin, a porphyrin precursor as electron transfer enhancement moiety. Hematin modified electrodes demonstrated direct electron transfer reaction of BOx exhibiting larger O2 reduction in current density in phosphate buffer solution (pH 7.0) without the need of a mediator. A remarkable improvement in the catalytic currents with 2.5-fold increase compared to non-hematin modified oriented BOx electrodes was achieved. Moreover, a mediatorless and compartmentless glucose/O2 biofuel cell based on DET-type bioelectrocatalysis via the BOx cathode and the glucose dehydrogenase (GDH) anode demonstrated peak power densities of 1 mW/cm2 at pH 7.0 with 100 mM glucose/10 mM NAD fuel. The maximum current density of 1.6 mA/cm2 and the maximum power density of 0.4 mW/cm2 were achieved at 300 mV with nonmodified BOx cathode, while 3.5 mA/cm2 and 1.1 mW/cm2 of current and power density were achieved with hematin modified cathode. The performance improved 2.4 times which attributes to the hematin acting as a natural precursor and activator for BOx activity enhancement.

A self-powered biosensing device with an integrated hybrid biofuel cell for intermittent monitoring of analytes

In this work, we propose an integrated self-powered sensing system, driven by a hybrid biofuel cell (HBFC) with carbon paper discs coated with multiwalled carbon nanotubes. The sensing system has a biocathode made from laccase or bilirubin oxidase, and the anode is made from a zinc plate. The system includes a dedicated custom-built electronic control unit for the detection of oxygen and catechol analytes, which are central to medical and environmental applications. Both the HBFC and sensors, operate in a mediatorless direct electron transfer mode. The measured characteristics of the HBFC with externally applied resistance included the power-time dependencies under flow cell conditions, the sensors performance (evaluated by cyclic voltammetry), and chronoamperometry. The HBFC is integrated with analytical devices and operating in a pulse mode form long-run monitoring experiments. The HBFC generated sufficient power for wireless data transmission to a local computer.

Magnetic Nanoparticle-Reduced Graphene Oxide Nanocomposite as a Novel Bioelectrode for Mediatorless-Membraneless Glucose Enzymatic Biofuel Cells

In this work, an enzymatic biofuel cell (EBC) based on a membraneless and mediatorless glucose enzymatic fuel cell system was constructed for operation in physiological conditions (pH 7.0 and temperature 37 °C). The new platform EBC made of nanocomposite, including magnetic nanoparticles (Fe₃O₄ NPs) and reduced graphene oxide (RGO), was used for the immobilization of glucose oxidase (GOD) as bioanode and bilirubin oxidase (BOD) as biocathode. The EBC bioelectrodes were fabricated without binder or adhesive agents for immobilized enzyme and the first EBC using superparamagnetic properties with Fe₃O₄ NPs has been reported. The performance of the EBC was evaluated with promising results. In EBC tests, the maximum power density of the EBC was 73.7 μW cm⁻² and an open circuit voltage (OCV) as +0.63 V with 5 mM of glucose concentration for the physiological condition of humans. The Fe₃O₄-RGO nanocomposite offers remarkable enhancement in large surface areas, is a favorable environment for enzyme immobilization, and facilitates electron transfer between enzymes and electrode surfaces. Fe₃O₄ and RGO have been implied as new promising composite nanomaterials for immobilizing enzymes and efficient platforms due to their superparamagnetism properties. Thus, glucose EBCs could potentially be used as self-powered biosensors or electric power sources for biomedical device applications.

Switchable aerobic/anaerobic multi-substrate biofuel cell operating on anodic and cathodic enzymatic cascade assemblies

Enzymatic fuel cells may become more accessible for applications powering portable electronic devices by broadening the range of potentially usable fuels and oxidizers. In this work we demonstrate the operation of an integrated, yet versatile multi-substrate biofuel cell utilizing either glucose, fructose, sucrose or combinations of thereof as biofuels, and molecular oxygen originating from solution phase and/or an internal chemical source, as the oxidizer. In order to achieve this goal we designed an enzymatic cascade-functionalized anode consisting of invertase (INV), mutarotase (MUT), glucose oxidase (GOX), and fructose dehydrogenase (FDH), deposited on top of a mesoporous carbon nanoparticle matrix, in which electron relay molecules had been entrapped. The anode was then conjugated to a compatible enzymatic cathode that employs a cascade of catalase (CAT) and bilirubin oxidase (BOD), allowing the cell to operate in an aerobic environment and/or to utilize, under anaerobic conditions for instance, hydrogen peroxide as a source for the oxygen oxidizer. While operated in the presence of the sugar mixture and hydrogen peroxide, the power output of the dually cascaded biofuel cell reaches a peak power density of 0.25 mW cm^-2 and demonstrates an open circuit potential of 0.65 V. To our knowledge this is the first reported biofuel cell that discharges with both anodic and cathodic enzymatic cascade architectures and the first biofuel cell that is repeatedly switched between aerobic and anaerobic conditions without any significant decrease in the discharge performance.

Advances in biotechnological synthetic applications of carbon nanostructured systems

In the last few years carbon nanostructures have been applied for the immobilization of enzymes and biomimetic organo-metallic species useful for biotechnological applications. The nature of the support and the method of immobilization are responsible for the stability, reactivity and selectivity of the system. In this review, we focus on the recent advances in the use of carbon nanostructures, carbon nanotubes, carbon nanorods, fullerene and graphene for the preparation of biocatalytic and biomimetic systems and for their application in the development of green chemical processes.

Mechanism of chloride inhibition of bilirubin oxidases and its dependence on potential and pH

Bilirubin oxidases (BODs) belong to the multi-copper oxidase (MCO) family and efficiently reduce O2 at neutral pH and in physiological conditions where chloride concentrations are over 100 mM. BODs were consequently considered to be Cl- resistant contrary to laccases. However, there has not been a detailed study on the related effect of chloride and pH on the redox state of immobilized BODs. Here, we investigate by electrochemistry the catalytic mechanism of O2 reduction by the thermostable Bacillus pumilus BOD immobilized on carbon nanofibers in the presence of NaCl. The addition of chloride results in the formation of a redox state of the enzyme, previously observed for different BODs and laccases, which is only active after a reductive step. This behavior has not been previously investigated. We show for the first time that the kinetics of formation of this state is strongly dependent on pH, temperature, Cl- concentration and on the applied redox potential. UV-visible spectroscopy allows us to correlate the inhibition process by chloride with the formation of the alternative resting form of the enzyme. We demonstrate that O2 is not required for its formation and show that the application of an oxidative potential is sufficient. In addition, our results suggest that the reactivation may proceed thought the T3 β.

Direct enzymatic bioelectrocatalysis: differentiating between myth and reality

Enzymatic bioelectrocatalysis is being increasingly exploited to better understand oxidoreductase enzymes, to develop minimalistic yet specific biosensor platforms, and to develop alternative energy conversion devices and bioelectrosynthetic devices for the production of energy and/or important chemical commodities. In some cases, these enzymes are able to electronically communicate with an appropriately designed electrode surface without the requirement of an electron mediator to shuttle electrons between the enzyme and electrode. This phenomenon has been termed direct electron transfer or direct bioelectrocatalysis. While many thorough studies have extensively investigated this fascinating feat, it is sometimes difficult to differentiate desirable enzymatic bioelectrocatalysis from electrocatalysis deriving from inactivated enzyme that may have also released its catalytic cofactor. This article will review direct bioelectrocatalysis of several oxidoreductases, with an emphasis on experiments that provide support for direct bioelectrocatalysis versus denatured enzyme or dissociated cofactor. Finally, this review will conclude with a series of proposed control experiments that could be adopted to discern successful direct electronic communication of an enzyme from its denatured counterpart.

Biosurfactant functionalized single-walled carbon nanotubes to promote laccase bioelectrocatalysis

The type and coverage of biosurfactants adsorbed on single-walled carbon nanotubes strongly influence the direct electron transfer reaction of laccase.

Layer-by-Layer Assembly of Glucose Oxidase on Carbon Nanotube Modified Electrodes

The use of enzymatically modified electrodes for the detection of glucose or other non-electrochemically active analytes is becoming increasingly common. Direct heterogeneous electron transfer to glucose oxidase has been shown to be kinetically difficult, which is why electron transfer mediators or indirect detection is usually used for monitoring glucose with electrochemical sensors. It has been found, however, that electrodes modified with single or multi-walled carbon nanotubes (CNTs) demonstrate fast heterogeneous electron transfer kinetics as compared to that found for traditional electrodes. Incorporating CNTs into the assembly of electrochemical glucose sensors, therefore, affords the possibility of facile electron transfer to glucose oxidase, and a more direct determination of glucose. This chapter describes the methods used to use CNTs in a layer-by-layer structure along with glucose oxidase to produce an enzymatically modified electrode with high turnover rates, increased stability and shelf-life.

How the Intricate Interactions between Carbon Nanotubes and Two Bilirubin Oxidases Control Direct and Mediated O2 Reduction

Due to the lack of a valid approach in the design of electrochemical interfaces modified with enzymes for efficient catalysis, many oxidoreductases are still not addressed by electrochemistry. We report in this work an in-depth study of the interactions between two different bilirubin oxidases, (from the fungus Myrothecium verrucaria and from the bacterium Bacillus pumilus), catalysts of oxygen reduction, and carbon nanotubes bearing various surface charges (pristine, carboxylic-, and pyrene-methylamine-functionalized). The surface charges and dipole moment of the enzymes as well as the surface state of the nanomaterials are characterized as a function of pH. An original electrochemical approach allows determination of the best interface for direct or mediated electron transfer processes as a function of enzyme, nanomaterial type, and adsorption conditions. We correlate these experimental results to theoric voltammetric curves. Such an integrative study suggests strategies for designing efficient bioelectrochemical interfaces toward the elaboration of biodevices such as enzymatic fuel cells for sustainable electricity production.

Immobilization of bilirubin oxidase on graphene oxide flakes with different negative charge density for oxygen reduction. The effect of GO charge density on enzyme coverage, electron transfer rate and current density

Previously we showed that an effective bilirubin oxidase (BOD)-based biocathode using graphene oxide (GO) could be prepared in 2 steps: 1. electrostatic adsorption of BOD on GO; 2. electrochemical reduction of the BOD-GO composite to form a BOD-ErGO (electrochemically reduced GO) film on the electrode. In order to identify an optimal charge density of GO for BOD-ErGO composite preparation, several GO fractions differing in an average flake size and ζ-potential were prepared using centrifugation and consequently employed for BOD-ErGO biocathode preparation. A simple way to express surface charge density of these particular GO nanosheets was developed. The values obtained were then correlated with biocatalytic and electrochemical parameters of the prepared biocathodes, i.e. electrocatalytically active BOD surface coverage (Γ), heterogeneous electron transfer rate (k_S) and a maximum biocatalytic current density. The highest bioelectrocatalytic current density of (597±25)μAcm^-2 and the highest Γ of (23.6±0.9)pmolcm^-2 were obtained on BOD-GO composite having the same moderate negative charge density, but the highest k_S of (79.4±4.6)s^-1 was observed on BOD-GO composite having different negative charge density. This study is a solid foundation for others to consider the influence of a charge density of GO on direct bioelectrochemistry/bioelectrocatalysis of other redox enzymes applicable for construction of biosensors, bioanodes, biocathodes or biofuel cells.

Increasing the catalytic activity of Bilirubin oxidase from Bacillus pumilus: Importance of host strain and chaperones proteins

Aggregation of recombinant proteins into inclusion bodies (IBs) is the main problem of the expression of multicopper oxidase in Escherichia coli. It is usually attributed to inefficient folding of proteins due to the lack of copper and/or unavailability of chaperone proteins. The general strategies reported to overcome this issue have been focused on increasing the intracellular copper concentration. Here we report a complementary method to optimize the expression in E. coli of a promising Bilirubin oxidase (BOD) isolated from Bacillus pumilus. First, as this BOD has a disulfide bridge, we switched E.coli strain from BL21 (DE3) to Origami B (DE3), known to promote the formation of disulfide bridges in the bacterial cytoplasm. In a second step, we investigate the effect of co-expression of chaperone proteins on the protein production and specific activity. Our strategy allowed increasing the final amount of enzyme by 858% and its catalytic rate constant by 83%.

Optimizing the Mass-Specific Activity of Bilirubin Oxidase Adlayers through Combined Electrochemical Quartz Crystal Microbalance and Dual Polarization Interferometry Analyses

Two surface analysis techniques, dual polarization interferometry (DPI) and analysis by an electrochemical quartz crystal microbalance with dissipation capability (E-QCM-D), were paired to find the deposition conditions that give the highest and most stable electrocatalytic activity per adsorbed mass of enzyme. Layers were formed by adsorption from buffered solutions of bilirubin oxidase from Myrothecium verrucaria at pH 6.0 to planar surfaces, under high enzyme loading (≥1 mg mL(-1)) for contact periods of up to 2 min. Both unmodified and carboxylate-functionalized gold-coated sensors showed that a deposition solution concentration of 10-25 mg mL(-1) gave the highest activity per mass of adsorbed enzyme with an effective catalytic rate constant (k(cat)) of about 60 s(-1). The densification of adsorbed layers observed by DPI correlated with reduced bioactivity observed by parallel E-QCM-D measurements. Postadsorption changes in thickness and density observed by DPI were incorporated into Kelvin-Voigt models of the QCM-D response. The modeled response matched experimental observations when the adlayer viscosity tripled after adsorption.

Graphene oxide as a protein matrix: influence on protein biophysical properties

Background

This study provides fundamental information on the influence of graphene oxide (GO) nanosheets and glycans on protein catalytic activity, dynamics, and thermal stability. We provide evidence of protein stabilization by glycans and how this strategy could be implemented when GO nanosheets is used as protein immobilization matrix. A series of bioconjugates was constructed using two different strategies: adsorbing or covalently attaching native and glycosylated bilirubin oxidase (BOD) to GO.

Results

Bioconjugate formation was followed by FT-IR, zeta-potential, and X-ray photoelectron spectroscopy measurements. Enzyme kinetic parameters (k_m and k_cat) revealed that the substrate binding affinity was not affected by glycosylation and immobilization on GO, but the rate of enzyme catalysis was reduced. Structural analysis by circular dichroism showed that glycosylation did not affect the tertiary or the secondary structure of BOD. However, GO produced slight changes in the secondary structure. To shed light into the biophysical consequence of protein glycosylation and protein immobilization on GO nanosheets, we studied structural protein dynamical changes by FT-IR H/D exchange and thermal inactivation.

Conclusions

It was found that glycosylation caused a reduction in structural dynamics that resulted in an increase in thermostability and a decrease in the catalytic activity for both, glycoconjugate and immobilized enzyme. These results establish the usefulness of chemical glycosylation to modulate protein structural dynamics and stability to develop a more stable GO-protein matrix.

Electronic supplementary material

The online version of this article (doi:10.1186/s12951-015-0134-0) contains supplementary material, which is available to authorized users.

Rational design of quinones for high power density biofuel cells

Enzymatic fuel cells (EFCs) are devices that can produce electrical energy by enzymatic oxidation of energy-dense fuels (such as glucose). When considering bioanode construction for EFCs, it is desirable to use a system with a low onset potential and high catalytic current density. While these two properties are typically mutually exclusive, merging these two properties will significantly enhance EFC performance. We present the rational design and preparation of an alternative naphthoquinone-based redox polymer hydrogel that is able to facilitate enzymatic glucose oxidation at low oxidation potentials while simultaneously producing high catalytic current densities. When coupled with an enzymatic biocathode, the resulting glucose/O2 EFC possessed an open-circuit potential of 0.864 ± 0.006 V, with an associated maximum current density of 5.4 ± 0.5 mA cm-2. Moreover, the EFC delivered its maximum power density (2.3 ± 0.2 mW cm-2) at a high operational potential of 0.55 V.