Role of Quinones in Electron Transfer of PQQ–Glucose Dehydrogenase Anodes—Mediation or Orientation Effect

Polyazine nanoparticles as anchors of PQQ glucose dehydrogenase for its most efficient bioelectrocatalysis

We report on the drop-cast production of glucose biosensors based on the most efficient bioelectrocatalysis by pyrroloquinoline quinone dependent glucose dehydrogenase (PQQ GDH). To orient the enzyme upon immobilization we suggest using poly(Methylene Green) (p(MG)) nanoparticles acting as anchors. Synthesis of polymeric anchors has been carried out in course of Methylene Green electropolymerization, which allowed to tune polymer-to-monomer ratio in the drop-cast mixtures varying the monomer concentration and applying different number of potential sweep cycles. Except for elimination of electrochemical step for the electrode modification, opening the prospects for mass-production, the use of drop-cast enzyme anchors provides advantageous characteristics of the resulting biosensors. Catalytic current of PQQ GDH immobilized over p(MG) nanoparticles obtained in optimal conditions is increased only 2-2.5 times after addition of the freely diffusing mediator. Obviously, the lowest ratio of mediated-to-reagentless current points to the most efficient bioelectrocatalysis. The obtained ratio is 2.5 times lower than that for biosensors based on electropolymerized p(MG) films and practically an order of magnitude lower than that for the best reagentless sensors based on PQQ GDH immobilized over conductive nanomaterials. The achieved most efficient bioelectrocatalysis provides high sensitivity of the elaborated biosensors even at 0.0 V potential, which allows to operate them in power generation mode and control relative sweat glucose variation as a tool for non-invasive diabetes monitoring.

Computer-Aided Rational Construction of Mediated Bioelectrocatalysis with π-Conjugated (Hetero)cyclic Molecules: Toward Promoted Distant Electron Tunneling and Improved Biosensing

Highly π-conjugated (hetero)cyclic molecules having delocalized orbitals and tunable charge mobilities are attractive redox relays for mediated bioelectrocatalysis in manifold applications. As rigid molecules, their dynamics within the soft but confined intraprotein space becomes the crucial determinant of the enzyme-mediator electron-tunneling efficiency. However, it is rarely investigated in designing the mediated interface with a particular biocatalyst (e.g., oxidoreductase), which remains an empirical but try-and-error process. Herein, we present the computer-aided exploration of interactions between a flavin-containing reductive synthase and structurally diverse π-extended (hetero)cyclic mediators to realize reversed bioelectrocatalytic oxidation at low overpotentials. Compared to ring-fused systems with unbroken molecular planarity, heteroatom-bridged cyclic, and rotatable conjugated structures (e.g., indophenols) can experience unusually large dynamic torsion under biased forces of hydrogen bonding with enzyme residues. This behavior led to fast intraprotein reorientation (<50 ps) that shortened the electron-tunneling distance from 12 to 9 Å. Meanwhile, the lowest unoccupied molecular orbital level upon molecular torsion was decreased by 0.5 eV to further promote electron abstraction from the reduced flavin cofactor. An efficient distant electron tunneling also obviated mediator transport through the substrate channel, thus avoiding competitive inhibition on enzyme kinetics to broaden the operating concentration range. The resulting bioelectrocatalytic interface enables low-potential biosensing of glutamate with improved selectivity. Our finding provides new structural insights into constructing efficient long-range heterogeneous charge transport with biomacromolecular catalysts.

Anchoring PQQ-Glucose Dehydrogenase with Electropolymerized Azines for the Most Efficient Bioelectrocatalysis

Catalytic current of pyrroloquinoline quinone (PQQ)-glucose dehydrogenase (PQQ-GDH) immobilized over electropolymerized methylene green (MG) is increased only five times after the addition of the freely diffusing mediator. This value, being an efficiency criterion for bioelectrocatalysis, is several (three to six) times lower than that for the best reagentless glucose electrodes reported for this enzyme. Thermodynamics of the polyMG|PQQ-GDH electrode is determined by the enzyme-catalyzed reaction pointing to the direct bioelectrocatalysis. PQQ-GDH immobilized over polyMG displays the current plateau region from 0.0 to 0.2 V in the presence of glucose; at 0.00 V, being the optimal potential for biosensing applications, the catalytic current of the polyMG|PQQ-GDH electrode is 700-fold higher than that for the enzyme on a blank electrode. Successful glucose detection in human sweat by means of the corresponding enzyme electrode confirms that the reported bioelectrocatalytic system is attractive for advanced biosensors, as well as for biofuel cells.

Production of 2-keto-L-gulonic acid by metabolically engineered Escherichia coli

The 2-keto-L-gulonic acid (2-KLG) is the direct precursor for industrial vitamin C production. The main biosynthetic method for 2-KLG production is the classical two-step fermentation route. However, disadvantages of this method are emerging, including high consumption of energy, difficulties in strain screening, complex operation, and poor stability. In this study, five recombinant Escherichia coli strains overexpressing different sorbose/sorbosone dehydrogenases were constructed and used for 2-KLG production. By optimizing catalytic conditions and further expressing pyrroloquinoline quinone in the recombinant strain, the titer of 2-KLG reached 72.4 g/L, with a conversion ratio from L-sorbose of 71.2% in a 5-L bioreactor. To achieve direct biosynthesis of 2-KLG from D-sorbitol, a co-culture system consisting of Gluconobacter oxydans and recombinant E. coli was designed. With this co-culture system, 16.8 g/L of 2-KLG was harvested, with a conversion ratio from D-sorbitol of 33.6%. The approaches developed here provide alternative routes for the efficient biosynthesis of 2-KLG.

Biosensing and electrochemical properties of flavin adenine dinucleotide (FAD)-Dependent glucose dehydrogenase (GDH) fused to a gold binding peptide

In the present work, direct electron transfer (DET) based biosensing system for the determination of glucose has been fabricated by utilizing gold binding peptide (GBP) fused flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) from Burkholderia cepacia. The GBP fused FAD-GDH was immobilized on the working electrode surface of screen-printed electrode (SPE) which consists of gold working electrode, a silver pseudo-reference electrode and a platinum counter electrode, to develop the biosensing system with compact design and favorable sensing ability. The bioelectrochemical and mechanical properties of GBP fused FAD-GDH (GDH-GBP) immobilized SPE (GDH-GBP/Au) were investigated. Here, the binding affinity of GDH-GBP on Au surface, was highly increased after fusion of gold binding peptide and its uniform monolayer was formed on Au surface. In the cyclic voltammetry (CV), GDH-GBP/Au displayed significantly high oxidative peak currents corresponding to glucose oxidation which is almost c.a. 10-fold enhanced value compared with that from native GDH immobilized SPE (GDH/Au). As well, GDH-GBP/Au has shown 92.37% of current retention after successive potential scans. In the chronoamperometry, its steady-state catalytic current was monitored in various conditions. The dynamic range of GDH-GBP/Au was shown to be 3-30 mM at 30 °C and exhibits high selectivity toward glucose in whole human blood. Additionally, temperature dependency of GDH-GBP/Au on DET capability was also investigated at 30-70 °C. Considering this efficient and stable glucose sensing with simple and easy sensor fabrication, GDH-GBP based sensing platform can provide new insight for future biosensor in research fields that rely on DET.

Advancing the fundamental understanding and practical applications of photo-bioelectrocatalysis

Photo-bioelectrocatalysis combines the natural and highly sophisticated process of photosynthesis in biological entities with an abiotic electrode surface, to perform semi-artificial photosynthesis. However, challenges must be overcome, from the establishment and understanding of the photoexcited electron harvesting process at the electrode to the electrochemical characterization of these biotic/abiotic systems, and their subsequent tuning for enhancing energy generation (chemical and/or electrical). This Feature Article discusses the various approaches utilized to tackle these challenges, particularly focusing on powerful multi-disciplinary approaches for understanding and improving photo-bioelectrocatalysis. Among them is the combination of experimental evidence and quantum mechanical calculations, the use of bioinformatics to understand photo-bioelectrocatalysis at a metabolic level, or bioengineering to improve and facilitate photo-bioelectrocatalysis. Key aspects for the future development of photo-bioelectrocatalysis are presented alongside future research needs and promising applications of semi-artificial photosynthesis.

Coenzyme Coupling Boosts Charge Transport through Single Bioactive Enzyme Junctions

Oxidation of formate to CO₂ is catalyzed via the donation of electrons from formate dehydrogenase (FDH) to nicotinamide adenine dinucleotide (NAD⁺), and thus the charge transport characteristics of FDH become essential but remain unexplored. Here, we investigated the charge transport through single-enzyme junctions of FDH using the scanning tunneling microscope break junction technique (STM-BJ). We found that the coupling of NAD⁺ with FDH boosts the charge transport by ∼2,100%, and the single-enzyme conductance highly correlates with the enzyme activity. The combined flicker noise analysis demonstrated the switching of the coenzyme-mediated charge transport pathway and supported by the significantly reduced HOMO-LUMO gap from calculations. Site-specific mutagenesis analysis demonstrated that FDH-NAD⁺ stably combined own higher bioactivity and boosts charge transport, and the coupling has been optimized via the natural selection. Our work provides evidence of hydrogen bond coupling in bioactivity but also bridges the charge transport through single-enzyme junctions and enzyme activities.

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Binding of NAD⁺ with FDH boosts the charge transport by more than 2,100%

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Single-enzyme conductance highly correlates with the enzyme activity

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Hydrogen bond bridges the charge transport and enzyme activities

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Experiments combined with calculations probe switching of charge transport pathway

PQQ-GDH - Structure, function and application in bioelectrochemistry

This review summarizes the basic features of the PQQ-GDH enzyme as one of the sugar converting biocatalysts. Focus is on the membrane -bound and the soluble form. Furthermore, the main principles of enzymatic catalysis as well as studies on the physiological importance are reviewed. A short overview is given on developments in protein engineering. The major part, however, deals with the different fields of application in bioelectrochemistry. This includes approaches for enzyme-electrode communication such as direct electron transfer, mediator-based systems, redox polymers or conducting polymers and holoenzyme reconstitution, and covers applied areas such as biosensing, biofuel cells, recycling schemes, enzyme competition, light-directed sensing, switchable detection schemes, logical operations by enzyme electrodes and immune sensing.

Purple bacteria photo-bioelectrochemistry: enthralling challenges and opportunities

Perspective of research directions exploring purple bacteria photo-bioelectrochemistry: from harvesting photoexcited electrons to bioelectrochemical systems development.

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.

Construction of Uniform Monolayer- and Orientation-Tunable Enzyme Electrode by a Synthetic Glucose Dehydrogenase without Electron-Transfer Subunit via Optimized Site-Specific Gold-Binding Peptide Capable of Direct Electron Transfer

Direct electron transfer (DET) between enzymes and electrodes is a key issue for practical use of bioelectrocatalytic devices as a bioenergy process, such as enzymatic electrosynthesis, biosensors, and enzyme biofuel cells. To date, based on the DET of bioelectrocatalysis, less than 1% of the calculated theoretical current was transferred to final electron acceptor due to energy loss at enzyme-electrode interface. This study describes the design and construction of a synthetic glucose dehydrogenase (GDH; α and γ subunits) combined with a gold-binding peptide at its amino or carboxy terminus for direct contact between enzyme and electrode. The fused gold-binding peptide facilitated stable immobilization of GDH and constructed uniform monolayer of GDH onto a Au electrode. Depending on the fused site of binding peptide to the enzyme complex, nine combinations of recombinant GDH proteins on the electrode show significantly different direct electron-transfer efficiency across the enzyme-electrode interface. The fusion of site-specific binding peptide to the catalytic subunit (α subunit, carboxy terminus) of the enzyme complex enabled apparent direct electron transfer (DET) across the enzyme-electrode interface even in the absence of the electron-transfer subunit (i.e., β subunit having cytochrome domain). The catalytic glucose oxidation current at an onset potential of ca. (-)0.46 V vs Ag/AgCl was associated with the appearance of an flavin adenine dinucleotide (FAD)/FADH₂ redox wave and a stabilized bioelectrocatalytic current of more than 100 μA, determined from chronoamperometric analysis. Electron recovery was 7.64%, and the catalytic current generation was 249 μA per GDH enzyme loading unit (U), several orders of magnitude higher than the values reported previously. These observations corroborated that the last electron donor facing to electrode was controlled to be in close proximity without electron-transfer intermediates and the native affinity for glucose was preserved. The design and construction of the site-specific "sticky-ended" proteins without loss of catalytic activity could be applied to other redox enzymes having a buried active site.

Electrochemical Polymerization of Hydroquinone on Graphite Felt as a Pseudocapacitive Material for Application in a Microbial Fuel Cell

Here we reported the use of electropolymerization to achieve the transformation of aqueous hydroquinone to solid-phase polyhydroquinone (PHQ) with pseudocapacitive characteristics, and the application of this redox-active product to shuttle electron transfer in the anode system of a microbial fuel cell (MFC). The microscopic and spectroscopic results showed that the treatment of the graphite felt (GF) substrate with acids was effective in improving the amounts of surface-bound oxygen-containing groups, enabling better adhesion of PHQ onto the GF surfaces. The electrochemical measurements indicated that the resulting PHQ–AGF (acid treated GF) possessed high pseudocapacitance due to the fast and reversible redox cycling between hydroquinone and benzoquinone. The MFC equipped with the PHQ–AGF anode achieved a maximum power density of 633.6 mW m⁻², which was much higher than 368.2, 228.8, and 119.7 mW m⁻² corresponding to the MFC with the reference PHQ–GF, AGF, and GF anodes, respectively. The increase in the power performance was attributed to the incorporation of the redox-active PHQ abundant in C–OH and C=O groups that were beneficial to the increased extracellular electron transfer and enhanced bacterial adhesion on the anode.

Outer membrane cytochromes/flavin interactions in Shewanella spp.-A molecular perspective

Extracellular electron transfer (EET) is intrinsically associated with the core phenomena of energy harvesting/energy conversion in natural ecosystems and biotechnology applications. However, the mechanisms associated with EET are complex and involve molecular interactions that take place at the "bionano interface" where biotic/abiotic interactions are usually explored. This work provides molecular perspective on the electron transfer mechanism(s) employed by Shewanella oneidensis MR-1. Molecular docking simulations were used to explain the interfacial relationships between two outer-membrane cytochromes (OMC) OmcA and MtrC and riboflavin (RF) and flavin mononucleotide (FMN), respectively. OMC-flavin interactions were analyzed by studying the electrostatic potential, the hydrophilic/hydrophobic surface properties, and the van der Waals surface of the OMC proteins. As a result, it was proposed that the interactions between flavins and OMCs are based on geometrical recognition event. The possible docking positions of RF and FMN to OmcA and MtrC were also shown.

The nature of the rate-limiting step of blue multicopper oxidases: Homogeneous studies versus heterogeneous

Multicopper oxidases (MCOs) catalyzed two half reactions (linked by an intramolecular electron transfer) through a Ping-Pong mechanism: the substrate oxidation followed by the O₂ reduction. MCOs have been characterized in details in solution or immobilized on electrode surfaces. The nature of the rate-limiting steps, which is controversial in the literature, is discussed in this mini review for both cases. Deciphering such rate-limiting steps is of particular importance to efficiently use MCOs in any applications requiring the reduction of O₂ to water.

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Limiting rate constant of multicopper oxidase in solution is discussed.

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Limiting rate constant of immobilized multicopper oxidase is discussed.

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Homogeneous and heterogeneous enzyme kinetics are compared.

Self-powered electrochemical systems as neurochemical sensors: toward self-triggered in vivo analysis of brain chemistry

This review highlights recent development of self-powered electrochemical systems for in vivo neurochemical sensing.

Hybrid molecular/enzymatic catalytic cascade for complete electro-oxidation of glycerol using a promiscuous NAD-dependent formate dehydrogenase from Candida boidinii

The formate dehydrogenase from Candida boidinii was combined with NH₂-TEMPO to form a novel hybrid anode to oxidize glycerol to carbon dioxide at near-neutral pH.

Protein-Support Interactions for Rationally Designed Bilirubin Oxidase Based Cathode: A Computational Study

An example of biocathode based on bilirubin oxidase (BOx) was used to demonstrate how density functional theory can be combined with docking simulations in order to study the interface interactions between the enzyme and specifically designed electrode surface. The electrode surface was modified through the adsorption of bilirubin, the natural substrate for BOx, and the prepared electrode was electrochemically characterized using potentiostatic measurements. The experimentally determined current densities showed that the presence of bilirubin led to significant improvement of the cathode operation. On the basis of the computationally calculated binding energies of bilirubin to the graphene support and BOx and the analysis of the positioning of bilirubin relative to the support and T1 Cu atom of the enzyme, we hypothesize that the bilirubin serves as a geometric and electronic extension of the support. The computational results further confirm that the modification of the electrode surface with bilirubin provides an optimal orientation of BOx toward the support but also show that bilirubin facilitates the interfacial electron transfer by decreasing the distance between the electrode surface and the T1 Cu atom.