Search

Quantification of leuco-indigo is most important for Aizome, Japanese indigo-dyeing; however, there has been no convenient quantitative method. This study demonstrated that normal pulse voltammetry under quiescent conditions can be used to detect leuco-indigo. As a result of quantification of leuco-indigo in the depth direction in fermenting suspensions, the steady-state concentrations of leuco-indigo showed sigmoidal profiles in the depth direction. The steady state is caused by competitive reactions of microbial reduction of indigo and autoxidation of leuco-indigo by O

In enzyme-based biosensors, Ag

A pair of symmetrical cathodic and anodic peaks is observed in cyclic voltammograms for the ion transport across a bilayer lipid membrane (BLM) between two aqueous phases in the presence of tetraphenylborate (TPhB

Tungsten-containing formate dehydrogenase from Methylorubrum extroquens AM1 (FoDH1) was investigated from structural biology and bioelectrochemistry. The electron transfer (ET) pathways were investigated by 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.

The efficient utilization of biomass fuels is a critical component of a sustainable energy economy. Via respiration, acetic acid bacteria can oxidize biomass ethanol into acetic acid using membrane-bound alcohol and aldehyde dehydrogenases (ADH and AlDH, respectively). Focusing on the ability of these enzymes to interact directly and electrically with electrode materials, we constructed a mediatorless bioanode for ethanol oxidation based on a direct electron transfer (DET)-type bienzymatic cascade by ADH and AlDH. The three-dimensional structural data of ADH and AlDH elucidated by cryo-electron microscopy were valuable for effectively designing electrode platforms with multi-walled carbon nanotubes (MWCNTs) and pyrene derivatives. DET-type bioelectrocatalysis by ADH and AlDH was improved by using 1-pyrene carboxylic acid-functionalized MWCNT. The catalytic current densities for bienzymatic ethanol oxidation were recorded at the bioanodes modified by various ADH/AlDH ratios. The reaction model was constructed by focusing on the competitive ad-sorption of two enzymes on the electrode surface and the collection efficiency of the intermediately produced acetaldehyde. The power output of an ethanol/air biofuel cell using the bienzymatic bioanode reached 0.48 ± 0.01 mW cm–2, which is the highest value reported for ethanol biofuel cells. In addition, the Faraday efficiency of acetate production by the cell reached 100 ± 4%. This study will lead to efficient conversion of biomass fuels based on a multi-catalytic cascade system.

Salinity gradient energy generated by the contact between seawater and river water is one of the promising renewable energies. In the reverse electrodialysis (RED), salinity gradient energy is directly translated into the electricity. The representative problem is a large electrical resistance of river water or dilute solutions. The dilute solutions are poor electrically conductive. This results in a huge energy loss when an electrical current passes through it. In this study, sodium chloride (NaCl) or poly(sodium 4-styrenesulfonate) (NaPSS) was added to the dilute solutions to increase the conductivities and enhance the power outputs of the RED cells. When NaCl was added, the power output reached 11.4 ± 0.6 µW. On the other hand, when NaPSS was added, the power output increased up to 19.6 ± 0.6 µW.

Flavin adenine dinucleotide (FAD)-dependent fructose dehydrogenase (FDH) from Gluconobacter japonicus NBRC3260, a membrane-bound flavohemoprotein capable of direct electron transfer (DET)-type bioelectrocatalysis, was investigated from the viewpoints of structural biology and bioelectrochemistry. As FDH provides a strong DET-type catalytic signal, extensive research has been conducted. Structural analysis using cryo-electron microscopy (cryo-EM) and single-particle analysis revealed the entire FDH structure. The electron transfer (ET) pathway during the catalytic oxidation of D-fructose was investigated using thermodynamic and kinetic approaches in bioelectrochemistry, as well as structural information. Key amino acid residues that play important roles in substrate specificity and ET acceleration have also been proposed.

Bilirubin oxidase from Myrothecium verrucaria (mBOD) is a promising enzyme for catalyzing the four-electron reduction of dioxygen into water and realizes direct electron transfer (DET)-type bioelectrocatalysis. It has two N-linked glycans (N-glycans), and N472 and N482 are known as binding sites. Both binding sites located on opposite side of the type I (T1) Cu, which is the electrode-active site of BOD. We investigated the effect of N-glycans on DET-type bioelectrocatalysis by performing electrochemical measurements using electrodes with controlled surface charges. Two types of BODs with different N-glycans, mBOD and recombinant BOD overexpressed in Pichia pastoris (pBOD), and their deglycosylated forms (dg-mBOD and dg-pBOD) were used in this study. Kinetic analysis of the steady-state catalytic waves revealed that both size and composition of N-glycans affected the orientation of adsorbed BODs on the electrodes. Interestingly, the most favorable orientation was achieved with pBOD, which has the largest N-glycans. Furthermore, the effect of the orientation control by the N-glycans is cooperative with electrostatic interaction.

International audience; Copper efflux oxidases (CueOs) are key enzymes in copper homeostasis systems. The mechanisms involved are however largely unknown. CueO-type enzymes share a typical structural feature composed of Methionine-rich (Met-rich) domains that are proposed to be involved in copper homeostasis. Bioelectrocatalysis using CueO-type enzymes in the presence of Cu2+ recently highlighted a new Cu2+-dependent catalytic pathway related to a cuprous oxidase activity. In this work, we further investigated the effects of Cu2+ on direct electron transfer (DET)-type bioelectrocatalytic reduction of O2 by CueO at NH2-functionalized multi-walled carbon nanotubes. The DET-type bioelectrocatalytic activity of CueO decreased at low potential in the presence of Cu2+, showing unique peak-shaped voltammograms that we attribute to inactivation and reactivation processes. Chronoamperometry was used to kinetically analyze these processes, and the results suggested linear free energy relationships between the inactivation/reactivation rate constant and the electrode potential. Pseudo-steady-state analysis also indicated that Cu2+ uncompetitively inhibited the enzymatic activity. A detailed model for the Cu2+-dependent reductive inactivation of CueO was proposed to explain the electrochemical data, and the related thermodynamic and kinetic parameters. A CueO variant with truncated copper-binding α helices and bilirubin oxidase free of Met-rich domains also showed such reductive inactivation process, which suggests that multicopper oxidases contain copper-binding sites that lead to inactivation.

An overexpression system of membrane-bound alcohol dehydrogenase (ADH) from Gluconobacter oxydans was constructed to examine its bioelectrocatalytic characteristics. The effects of cyanide (CN-) addition on the kinetics of direct electron transfer (DET)-type bioelectrocatalysis by ADH were analyzed. CN- enhanced the bioelectrocatalytic activity, while the catalytic activity in the solution remained unchanged, even in the presence of CN-. Electrochemical methods and electron spin resonance spectroscopy showed the detailed electron transfer pathway in the DET-type bioelectrocatalysis by ADH. Briefly, ADH is suggested to communicate with an electrode via a CN--insensitive and H+-sensitive heme c in DET. These characteristics of ADH with respect to CN- suggest the involvement of ADH in CN--insensitive respiration in G. oxydans.

Aizome (Japanese indigo dyeing) is a unique dyeing method using microbial activity under anaerobic alkaline conditions. In indigo-dye fermenting suspensions, microorganisms reduce indigo into leuco-indigo with acetaldehyde as a reductant. In this study, we constructed a semi-microbial biofuel cell using an indigo-dye fermenting suspension. Carbon fiber and Pt mesh were used as the anode and cathode materials, respectively. The open-circuit voltage (OCV) was 0.6 V, and the maximum output power was 32 µW cm−2 (320 mW m−2). In addition, the continuous stability was evaluated under given conditions starting with the highest power density, the power density rapidly decreased in 0.5 h due to the degradation of the anode. Conversely, at the OCV, the anode potential exhibited high stability for two days. However, the OCV decreased by approximately 80 mV after 2 d compared with the initial value, which was attributed to the performance degradation of the gas-diffusion-cathode system caused by the evaporation of the dispersion solution. This is the first study to construct a semi-microbial biofuel cell using an indigo-dye fermenting suspension.