CueO-immobilized porous carbon electrode exhibiting improved performance of electrochemical reduction of dioxygen to water

Rational Surface Modification of Carbon Nanomaterials for Improved Direct Electron Transfer-Type Bioelectrocatalysis of Redox Enzymes

Interfacial electron transfer between redox enzymes and electrodes is a key step for enzymatic bioelectrocatalysis in various bioelectrochemical devices. Although the use of carbon nanomaterials enables an increasing number of redox enzymes to carry out bioelectrocatalysis involving direct electron transfer (DET), the role of carbon nanomaterials in interfacial electron transfer remains unclear. Based on the recent progress reported in the literature, in this mini review, the significance of carbon nanomaterials on DET-type bioelectrocatalysis is discussed. Strategies for the oriented immobilization of redox enzymes in rationally modified carbon nanomaterials are also summarized and discussed. Furthermore, techniques to probe redox enzymes in carbon nanomaterials are introduced.

Directed Evolution of a Bacterial Laccase (CueO) for Enzymatic Biofuel Cells

Escherichia coli's copper efflux oxidase (CueO) has rarely been employed in the cathodic compartment of enzymatic biofuel cells (EBFCs) due to its low redox potential (0.36 V vs. Ag/AgCl, pH 5.5) towards O₂ reduction. Herein, directed evolution of CueO towards a more positive onset potential was performed in an electrochemical screening system. An improved CueO variant (D439T/L502K) was obtained with a significantly increased onset potential (0.54 V), comparable to that of high-redox-potential fungal laccases. Upon coupling with an anodic compartment, the EBFC exhibited an open-circuit voltage (V_oc ) of 0.56 V. Directed enzyme evolution by tailoring enzymes to application conditions in EBFCs has been validated and might, in combination with molecular understanding, enable future breakthroughs in EBFC performance.

From fundamentals to applications of bioelectrocatalysis: bioelectrocatalytic reactions of FAD-dependent glucose dehydrogenase and bilirubin oxidase

In this review, I present the main highlights of my works in the development of bioelectrocatalysis, which can be used in widespread applications, particularly for the design of biosensor and biofuel cells. In particular, I focus on research progress made in two key bioelectrocatalytic reactions: glucose oxidation by flavin adenine dinucleotide-dependent glucose dehydrogenase and oxygen reduction by bilirubin oxidase. I demonstrate the fundamental principles of bioelectrocatalysis and the requirements for enhancing the catalytic performance, including the choice of a mediator of redox reactions, immobilization, and electrode materials. These methods can allow for achieving control of the bioelectrocatalytic reaction, thereby overcoming obstacles toward their industrial applications.

A L-proline/O2 biofuel cell using L-proline dehydrogenase (LPDH) from Aeropyrum pernix

To utilize amino acids from food waste as an energy source, L-proline/O₂ biofuel cell was constructed using a stable enzyme from hyperthermophilic archaeon for long-term operation. On the anode, the electrocatalytic oxidation of L-proline by L-proline dehydrogenase from Aeropyrum pernix was observed in the presence of ferrocenecarboxylic acid as mediator. On the cathode, electrocatalytic oxygen reduction was detected. Ketjenblack modification of carbon cloth substrate increased the current density due to increased laccase loading and enhanced electron transfer reaction. The biofuel cell using these electrodes achieved a current density of 6.00 µA/cm². We successfully constructed the first biofuel cell that generates power from L-proline.

Comprehensive Study of the Enzymatic Catalysis of the Electrochemical Oxygen Reduction Reaction (ORR) by Immobilized Copper Efflux Oxidase (CueO) From Escherichia coli

In recent years, enzymatic fuel cells have experienced a great development promoted by the availability of novel biological techniques that allow the access to a large number of enzymatic catalysts. One of the most important aspects in this area is the development of biocatalysts for the oxygen reduction reaction (ORR). Laccases from the group of enzymes called blue multi-cooper oxidases have received considerable attention because of their ability to catalyze the electrochemical oxygen reduction reaction to water when immobilized on metallic or carbonaceous electrode materials. In this paper we report a comprehensive study of the electrocatalytic activity of the enzyme Copper efflux oxidase (CueO) from Escherichia coli immobilized on different electrode materials. The influence of the electrode substrate employed for protein immobilization was evaluated using glassy carbon, gold or platinum electrodes. Gold and platinum electrodes were modified using different self-assembled monolayers (SAM) able to tune the electrostatic interaction between the protein and the substrate, depending on the nature of the terminal functional group in the SAM. The effects of protein immobilization time, electrode potential, solution pH and temperature, protein and O₂ concentration have been carefully investigated. Finally, direct electron transfer (DET) was investigated in the presence of the following inhibitors: fluoride (F⁻), chloride (Cl⁻) and azide (N3-).

Nano Copper Oxide-Modified Carbon Cloth as Cathode for a Two-Chamber Microbial Fuel Cell

In this work, Cu₂O nanoparticles were deposited on a carbon cloth cathode using a facile electrochemical method. The morphology of the modified cathode, which was characterized by scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) tests, showed that the porosity and specific surface area of the cathode improved with longer deposition times. X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV) results showed that cupric oxide and cuprous oxide coexisted on the carbon cloth, which improved the electrochemical activity of cathode. The cathode with a deposition time of 100 s showed the best performance, with a power density twice that of bare carbon cloth. Linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) results revealed that moderate deposition of nano copper oxide on carbon cloth could dramatically reduce the charge transfer resistance, which contributed to the enhanced electrochemical performance. The mediation mechanism of copper oxide nanocatalyst was illustrated by the fact that the recycled conversion between cupric oxide and cuprous oxide accelerated the electron transfer efficiency on the cathode.

Using planktonic microorganisms to supply the unpurified multi-copper oxidases laccase and copper efflux oxidases at a biofuel cell cathode

The feasibility to apply crude culture supernatants that contain the multicopper oxidases laccase or copper efflux oxidase (CueO) as oxygen reducing catalysts in a biofuel cell cathode is shown. As enzyme-secreting recombinant planktonic microorganisms, the yeast Yarrowia lipolytica and the bacterium Escherichia coli were investigated. The cultivation and operation conditions (choice of medium, pH) had distinct effects on the electro-catalytic performance. The highest current density of 119 ± 23 μA cm(-2) at 0.400 V vs. NHE was obtained with the crude culture supernatant of E. coli cells overexpressing CueO and tested at pH 5.0. In comparison, at pH 7.4 the electrode potential at 100 μA cm(-2) is 0.25 V lower. Laccase-containing supernatants of Y. lipolytica yielded a maximum current density of 6.7 ± 0.4 μAcm(-2) at 0.644 V vs. NHE. These results open future possibilities to circumvent elaborate enzyme purification procedures and realize cost effective and easy-to-operate enzymatic biofuel cells.

Catalysis of dioxygen reduction by Thermus thermophilus strain HB27 laccase on ketjen black electrodes

We present electrochemical analyses of the catalysis of dioxygen reduction by Thermus thermophilus strain HB27 laccase on ketjen black substrates. Our cathodes reliably produce 0.56 mA cm(-2) at 0.0 V vs Ag|AgCl reference at 30 °C in air-saturated buffer, under conditions of nonlimiting O(2) flux. We report the electrochemical activity of this laccase as a function of temperature, pH, time, and the efficiency of its conversion of dioxygen to water. We have measured the surface concentration of electrochemically active species, permitting the extraction of electron transfer rates at the enzyme-electrode interface: 1 s(-1) for this process at zero driving force at 30 °C and a limiting rate of 23 s(-1) at 240 mV overpotential at 50 °C.

Recent Developments of Nanostructured Electrodes for Bioelectrocatalysis of Dioxygen Reduction

The recent development of nanostructured electrodes for bioelectrocatalytic dioxygen reduction catalysed by two copper oxidoreductases, laccase and bilirubin oxidase, is reviewed. Carbon-based nanomaterials as carbon nanotubes or carbon nanoparticles are frequently used for electrode modification, whereas there are only few examples of biocathodes modified with metal or metal oxide nanoparticles. These nanomaterials are adsorbed on the electrode surface or embedded in multicomponent film. The nano-objects deposited act as electron shuttles between the enzyme and the electrode substrate providing favourable conditions for mediatorless bioelectrocatalysis.

How the local geometry of the Cu-binding site determines the thermal stability of blue copper proteins

Identifying the factors that govern the thermal resistance of cupredoxins is essential for understanding their folding and stability, and for improving our ability to design highly stable enzymes with potential biotechnological applications. Here, we show that the thermal unfolding of plastocyanins from two cyanobacteria--the mesophilic Synechocystis and the thermophilic Phormidium--is closely related to the short-range structure around the copper center. Cu K-edge X-ray absorption spectroscopy shows that the bond length between Cu and the S atom from the cysteine ligand is a key structural factor that correlates with the thermal stability of the cupredoxins in both oxidized and reduced states. These findings were confirmed by an additional study of a site-directed mutant of Phormidium plastocyanin showing a reverse effect of the redox state on the thermal stability of the protein.

Type-zero copper proteins

Copper proteins play key roles in biological processes such as electron transfer and dioxygen activation; the active site of each of these proteins is classified as either type 1, 2, or 3, depending on its optical and electron paramagnetic resonance properties. We have built a new type of site that we call “type zero copper” by incorporating leucine, isoleucine, or phenylalanine in place of methionine at position 121 in C112D Pseudomonas aeruginosa azurin. X-ray crystallographic analysis shows that these sites adopt distorted tetrahedral geometries, with an unusually short Cu-O(G45 carbonyl) bond (2.35–2.55 Å). Relatively weak absorption near 800 nm and narrow parallel hyperfine splittings in EPR spectra are the spectroscopic signatures of type zero copper. Copper K-edge x-ray absorption spectra suggest elevated Cu(II) 4p character in the d-electron ground state. Cyclic voltammetric experiments demonstrate that the electron transfer reactivities of type zero azurins are enhanced relative to that of the corresponding type 2 (C112D) protein.

Four-electron reduction of dioxygen by a multicopper oxidase, CueO, and roles of Asp112 and Glu506 located adjacent to the trinuclear copper center

The mechanism of the four-electron reduction of dioxygen by a multicopper oxidase, CueO, was studied based on reactions of single and double mutants with Cys(500), a type I copper ligand, and the noncoordinating Asp(112) and Glu(506), which form hydrogen bonds with the trinuclear copper center directly and indirectly via a water molecule. The reaction of C500S containing a vacant type I copper center produced intermediate I in an EPR-silent peroxide-bound form. The formation of intermediate I from C500S/D112N was restricted due to a reduction in the affinity of the trinuclear copper center for dioxygen. The state of intermediate I was realized to be the resting form of C500S/E506Q and C500S of the truncated mutant Deltaalpha5-7CueO, in which the 50 amino acids covering the substrate-binding site were removed. Reactions of the recombinant CueO and E506Q afforded intermediate II, a fully oxidized form different from the resting one, with a very broad EPR signal, g < 2, detectable only at cryogenic temperatures and unsaturated with high power microwaves. The lifetime of intermediate II was prolonged by the mutation at Glu(506) involved in the donation of protons. The structure of intermediates I and II and the mechanism of the four-electron reduction of dioxygen driven by Asp(112) and Glu(506) are discussed.