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Tungsten-containing formate dehydrogenase from Methylorubrum extroquens AM1 (FoDH1)—a promising biocatalyst for the interconversion of carbon dioxide/formate and nicotine adenine dinucleotide (NAD+)/NADH redox couples—was investigated using structural biology and bioelectrochemistry. FoDH1 is reported to be an enzyme that can realize "direct electron transfer (DET)-type bioelectrocatalysis." However, its 3-D structure, electrode-active sites, and electron transfer (ET) pathways remain unclear. The ET pathways were investigated using structural information, electrostatic interactions between the electrode and the enzyme, and the differences in the substrates. Two electrode-active sites and multiple ET pathways in FoDH1 were discovered. [ABSTRACT FROM AUTHOR]

Cell-to-cell communication via a local current caused by ion transport is elucidated using a model-cell system. To imitate tissues such as smooth muscles and cardiac muscles, liquid-membrane cells mimicking the function of K+ and Na+ channels were made. Connecting these channel-mimicking cells (K+ channel and voltage-gated Na+ channel) in parallel, model cells imitating living cell functions were constructed. Action-potential propagation within the cell aggregate model constructed by multiple model cells was investigated. When an action potential was generated at one cell, the cell behaved as an electric power source. Since a circulating current flowed around the cell, it flowed through neighboring model cells. Influx and efflux currents caused negative and positive shifts of the membrane potential, respectively, on the surface of neighboring model cells. The action potential was generated at the depolarized domain when the membrane potential exceeded the threshold of the voltage-gated Na+ channels. Thus, the action potential spread all over the cell system. When an external electric stimulus was applied to the layered cell-aggregate model system, propagation of the action potential was facilitated as if they were synchronized. [ABSTRACT FROM AUTHOR]

A flow-through bi-enzymatic reactor was developed to produce formate from carbon dioxide gas (CO2). The reaction operates under ambient conditions without hydrogen gas (H2) and involves a high performance formate dehydrogenase from Methylobacterium extorquens AM1 (MeFoDH1). The tandem reaction features in situ regeneration of the nicotinamide adenine dinucleotide (1,4-NADH) cofactor performed by a highly active mutant of the phosphite dehydrogenase from Pseudomonas stutzeri, without any side reaction. One of the highest turnover numbers reported to date in the literature was reached (6700) giving a high space–time yield of 0.6 mmolformate min−1 L−1reactor. [ABSTRACT FROM AUTHOR]

H2/O2 biofuel cells utilizing hydrogenases and multicopper oxidases as bioelectrocatalysts are clean, sustainable, and environmentally friendly power devices. In this study, we constructed a novel gas diffusion bioelectrode with a sheet of waterproof carbon cloth as the electrode base and optimized the hydrophilicity/hydrophobicity of the electrode for both high gas permeability and high direct electron transfer bioelectrocatalytic activity. The electrode exhibited a large current density of about 10 mA cm−2 in the steady-state for both H2 oxidation and O2 reduction. The biocathode and the bioanode were coupled to construct a gas diffusion H2/O2 biofuel cell. The dual gas diffusion system allowed the separate supply of gaseous substrates (H2 and O2) to the bioanode and biocathode, with consequent suppression of the oxidative inhibition of the hydrogenases. The cell exhibited a maximum power density of 8.4 mW cm−2 at a cell voltage of 0.7 V under quiescent conditions. [ABSTRACT FROM AUTHOR]

A new model system of nerve conduction, which has two sites (the potential-sending and the potential-receiving sites) was constructed by the use of some liquid-membrane cells which mimic the function of the K+ and Na+ channels. The model system setup was such that the membrane potential of the K+-channel cell (resting potential) was different from that of the Na+-channel cell (action potential). Initially, the K+-channel cell at the potential-sending site was connected to that at the potential-receiving site. After switching from the K+-channel cell to the Na+-channel cell at the potential-sending site, the membrane potential of the K+-channel cell at the potential-receiving site began to vary with the generation of the circulating current. By placing several K+-channel cells in parallel at the potential-receiving site, the propagation mechanism of the action potential was interpreted and the influence of the resistor and the capacitor on the propagation was evaluated. [ABSTRACT FROM AUTHOR]

The pseudo-steady-state current due to a mediated enzymatic reaction on a microelectrode is characterized on the basis of theoretical analysis and numerical simulation. The steady-state current is proportional to substrate concentration when the enzymatic reaction is considerably faster than substrate mass transport via nonlinear diffusion. Under such conditions, the reaction plane, where the mass flow of the substrate is converted to that of the mediator, exists near the electrode surface. The steady-state current increases as the diffusion coefficient of the substrate increases. In contrast, the diffusion coefficient and the concentration of the mediator have minor effects on the current. This difference can be explained on the basis of a change in the reaction plane location. When a sufficient amount of enzyme exists in a system, the system can be used as an amperometric biosensor, the response of which is independent of any change in enzyme activity. [ABSTRACT FROM AUTHOR]

The fructose/dioxygen biofuel cell, one of the direct electron transfer (DET)-type bioelectrochemical devices, utilizes fructose dehydrogenase (FDH) on the anode and multi-copper oxidase such as bilirubin oxidase (BOD) on the cathode as catalysts. The power density in the literature is limited by the biocathode performance. We show that the DET-type biocathode performance is greatly improved, when bilirubin or some related substances are adsorbed on electrodes before the BOD adsorption. Several data show that the substrate modification induces the appropriate orientation of BOD on the electrode surface for the DET. The substrate-modification method has successfully been applied to air-breathing gas-diffusion-type biocathodes. We have also optimized the conditions of the FDH adsorption on carbon cryogel electrodes. Finally, a one-compartment DET-type biofuel cell without separators has been constructed, and the maximum power density of 2.6 mW cm−2 was achieved at 0.46 V of cell voltage under quiescent (passive) and air atmospheric conditions. [ABSTRACT FROM AUTHOR]

We demonstrate the efficient direct electron transfer (DET) from an electrode to an engineered laccase isolated from a metagenome. The enzyme has a unique homotrimeric architecture with a two-domain-type laccase subunit. The recombinant laccase-modified mesoporous carbon electrode exhibits an effective catalytic current for oxygen reduction, which depends on the affinity tags attached near the electroactive Cu site of the enzyme. We also investigated the effect of the affinity tags on the orientation of the enzyme on functional thiol-modified Au electrodes. The results suggest that a poly-histidine tag (His-tag) functions as an anchor to control the orientation of the enzyme to enhance the current density of the DET-type bioelectrocatalysis. [ABSTRACT FROM AUTHOR]

We report the first example of a liposome-based energy conversion system that is useful for entrapping enzymes and NAD coenzyme to accelerate multi-step enzymatic reactions. The liposome generates a much higher catalytic current compared with the non-liposome system, which is in good consistency with numerical simulations. [ABSTRACT FROM AUTHOR]

We developed an efficient bioelectrocatalytic system for glucose oxidation by introducing hydrophilic glucose-permeable antibiotic channels into liposomes.

Uncoupler-mediated cation transport has been investigated by cyclic voltammetry for the ion transfer from one aqueous phase (W1) to another (W2) across a bilayer lipid membrane (BLM) in the presence of typical uncouplers, 3,5-di(tert-butyl)-4-hydroxybenzylidenemalononitril (SF6847) and 2,4-dinitrophenol (DNP). The voltammograms for the ion transfer were in a steady state and exhibited a rotated sigmoidal shape that was almost symmetrical about the origin (0 V, 0 A). The plot of the ion transfer current against pH was a bell-type curve centered on pH approximately = pK(a) + 1, K(a) being the dissociation constant of the uncouplers in the aqueous phase. Taking into account the ion transfer reactions at the W1|BLM and the BLM|W2 interfaces, these properties were well explained by our proposed model which considers that the ion transfer current is attributable to the facilitated transfers of H(+) and Na(+). The buffer action in the aqueous phase was found to play an important role in the facilitated H(+)-transfer across the BLM. The nature of the pH-dependence of the ion transfer current was reasonably explained from an electrochemical viewpoint based on the distribution coefficient of the anionic and neutral forms of SF6847, as estimated from its absorption spectra in liposomal membrane. The proposed model is also valuable for understanding the pH-dependence of uncoupling activity in mitochondria in the literature.

One-compartment biofuel cells without separators have been constructed, in which d-fructose dehydrogenase (FDH) from Gluconobacter sp. and laccase from Trametes sp. (TsLAC) work as catalysts of direct electron transfer (DET)-type bioelectrocatalysis in the two-electron oxidation of d-fructose and four-electron reduction of dioxygen as fuels, respectively. FDH adsorbs strongly and stably on Ketjen black (KB) particles that have been modified on carbon papers (CP) and produces the catalytic current with the maximum density of about 4 mA cm(-2) without mediators at pH 5. The catalytic wave of the d-fructose oxidation is controlled by the enzyme kinetics. The location and the shape of the catalytic waves suggest strongly that the electron is directly transferred to the KB particles from the heme c site in FDH, of which the formal potential has been determined to be 39 mV vs. Ag|AgCl|sat. KCl. Electrochemistry of three kinds of multi-copper oxidases has also been investigated and TsLAC has been selected as the best one of the DET-type bioelectrocatalyst for the four-electron reduction of dioxygen in view of the thermodynamics and kinetics at pH 5. In the DET-type bioelectrocatalysis, the electron from electrodes seems to be transferred to the type I copper site of multi-copper oxidases. TsLAC adsorbed on carbon aerogel (CG) particles with an average pore size of 22 nm, that have been modified on CP electrodes, produces the catalytic reduction current of dioxygen with a density of about 4 mA cm(-2), which is governed by the mass transfer of the dissolved dioxygen. The FDH-adsorbed KB-modified CP electrodes and the TsLAC-adsorbed CG-modified CP electrodes have been combined to construct one-compartment biofuel cells without separators. The open-circuit voltage was 790 mV. The maximum current density of 2.8 mA cm(-2) and the maximum power density of 850 microW cm(-2) have been achieved at 410 mV of the cell voltage under stirring.