Electrochemically Mediated Reduction of Horseradish Peroxidase by 1,1‘-Ferrocenedimethanol in Organic Solvents

Multifaceted personality and roles of heme enzymes in industrial biotechnology

Heme enzymes are the most prominent category of iron-containing metalloenzymes with the capability of catalyzing an astonishingly wide range of reactions like epoxidation, hydroxylation, demethylation, desaturation, reduction, sulfoxidation, and decarboxylation. Various enzymes in this category are P450s, heme peroxidases, catalases, myoglobin, cytochrome C, and others. Besides this, the natural promiscuity and amenability of these enzymes to protein engineering and evolution have also added several non-native reactions such as C-H, N-H, S-H insertions, cyclopropanation, and other industrially important reactions to their capabilities. Surprisingly, all of these reactions and their wide substrate scopes are attributed to changes in the active site scaffold of different heme enzymes as the center of all enzymes is constituted by a porphyrin ring containing iron. Multiple prominent research groups across the world, including 2018, Nobel Laureate Frances Arnold's group, have shown keen interest in engineering and evolving these enzymes for utilizing their industrial potential. Besides engineering the active site, researchers have also explored the possibility of these enzymes catalyzing non-native reactions by replacing the center porphyrin ring with other cofactors or by changing the iron in the porphyrin ring with other metal ions along with engineering the active site and thereby creating novel artificial metalloenzymes. Thus, in this mini-review from our group, for the first time, we are trying to catalog various activities catalyzed by heme enzymes and their engineered variants and their active usage in various industries along with shedding light on their potential for use in various applications in the future.

Enzymatic self-wiring in nanopores and its application in direct electron transfer biofuel cells

A synthetic enzymatic activity in nanopores leading to the direct fabrication of modified electrodes applicable as biosensors and/or biofuel cell elements is reported. We demonstrate the heterogeneous enzymatic implanting of platinum nanoclusters, PtNCs, in glucose oxidase, GOx, immobilized on mesoporous carbon nanoparticles, MPCNP-modified surface. As the pores confine the growth of the clusters, the PtNC@GOx/MPCNP assembly becomes electrically wired to the matrix, demonstrating direct electron transfer, DET, bioelectrocatalytic properties that correlate with the applied duration of synthesis and cluster size. This inside-out nanocluster growth from the cofactor to the matrix is investigated and further compared to a reversed outside-in strategy which follows the electrochemical deposition of the Pt clusters inside the pores and their electrically induced expansion towards the FAD center of the enzyme. While the inside-out and outside-in methodologies provide, for the first time, synthetic bidirectional direct wiring routes of an enzyme to a surface, we highlight an asymmetry in the wiring efficiency associated with the different assemblies. The results indicate the existence of a shorter gap between the FAD cofactor and the PtNCs in the enzymatically implanted assembly, resulting in elevated bioelectrocatalytic currents, lower overpotential, and a higher turnover rate, 2580 e^- s^-1. The implanted assembly is then coupled to a bilirubin oxidase-adsorbed MPCNP cathode to yield an all-DET biofuel cell. Due to the superior electrical contact of the inside-out-synthesized anode, this cell demonstrates enhanced discharge potential and power outputs as compared to similar systems employing electrochemically synthesized outside-in-grown PtNC-GOx/MPCNPs or even GOx-modified MPCNPs diffusionally mediated by ferrocenemethanol.

Amplified electron transfer at poly-ethylene-glycol (PEG) grafted electrodes

Electron transfer at pegylated electrode surfaces is suppressed for Fe(CN)₆^3−/4− and then recovered in the presence of ferrocene-dimethanol.

Apoenzyme reconstitution as a chemical tool for structural enzymology and biotechnology

Many enzymes contain a nondiffusible organic cofactor, often termed a prosthetic group, which is located in the active site and essential for the catalytic activity of the enzyme. These cofactors can often be extracted from the protein to yield the respective apoenzyme, which can subsequently be reconstituted with an artificial analogue of the native cofactor. Nowadays a large variety of synthetic cofactors can be used for the reconstitution of apoenzymes and, thus, generate novel semisynthetic enzymes. This approach has been refined over the past decades to become a versatile tool of structural enzymology to elucidate structure-function relationships of enzymes. Moreover, the reconstitution of apoenzymes can also be used to generate enzymes possessing enhanced or even entirely new functionality. This Review gives an overview on historical developments and the current state-of-the-art on apoenzyme reconstitution.

Electroenzymatic reactions with oxygen on laccase-modified electrodes in anhydrous (pure) organic solvent

The electroenzymatic reactions of Trametes hirsuta laccase in the pure organic solvent dimethyl sulfoxide (DMSO) have been investigated within the framework for potential use as a catalytic reaction scheme for oxygen reduction. The bioelectrochemical characteristics of laccase were investigated in two different ways: (i) by studying the electroreduction of oxygen in anhydrous DMSO via a direct electron transfer mechanism without proton donors and (ii) by doing the same experiments in the presence of laccase substrates, which display in pure organic solvents both the properties of electron donors as well as the properties of weak acids. The results obtained with laccase in anhydrous DMSO were compared with those obtained previously in aqueous buffer. It was shown that in the absence of proton donors under oxygenated conditions, formation of superoxide anion radicals is prevented at bare glassy carbon and graphite electrodes with adsorbed laccase. The influence of the time for drying the laccase solution at the electrode surface on the electroreduction of oxygen was studied. Investigating the electroenzymatic oxidation reaction of catechol and hydroquinone in DMSO reveals the formation of various intermediates of the substrates with different electrochemical activity under oxygenated conditions. The influence of the content of aqueous buffer in the organic solvent on the electrochemical behaviour of hydroquinone/1,4-benzoquinone couple was also studied.

Electrochemical biosensors in nonaqueous solutions and their applications

This review first describes the invention of functional interfaces to promote biochemical redox reactions between substrates in dipolar aprotic solvents and enzymes or related compounds immobilized at the interface. The interfaces contain hydrophilic polymer membranes, a gold nanoparticle self-assembled electrode constructed by using rigid rod dithiols, and binary self-assembled monolayers composed of amino and carboxyl terminal groups. Other topics covered are: the electrochemical characterization of the hydrophilic polymer membrane; the development of biosensors to obtain reaction parameters of enzymatic and of electrochemical kinetics; and applications to the study of materials involved in metabolism.

On-line monitoring of methanol in n-hexane by an organic-phase alcohol biosensor

An organic-phase alcohol biosensor has been developed by co-entrapping alcohol oxidase and horseradish peroxidase within an ionotropy polymer hydrogel matrix fabricated from silica gel particles, hydroxyethyl carboxymethylcellulose, an adduct of 3-methoxy-4-ethoxybenzaldehyde and 4-tert-butylpyridinium acetohydrazone, and octadecylsilica particles. The viability of the immobilised enzymes for the biocatalytic reaction of methanol in n-hexane was comparatively studied by using a bulk cell or a volume-changeable flow-through cell coupled with an oxygen optical transducer. It was found that the microenvironment around the enzyme, the deterioration property of the enzyme, the substrate throughput and the mass transfer process of the reactant in the bioreactor were the crucial parameters affecting the performance of the alcohol organic-phase biosensor. Our optimal biosensor was constructed from a flow-through cell packed with small particles of immobilised enzymes and it could maintain the biocatalytic reaction at high and stable rate for on-line detection of methanol in n-hexane under flow operation mode. The biosensor had an analytical working range of 2.3-90 mM methanol in n-hexane. The response times (t95) were 4.5 and 7.5 min for 60 and 10 mM methanol, respectively. The operational lifetime of the biosensor was more than 45 assays and the shelf lifetime was longer than 2 weeks. The biosensor has been successfully applied to determine the methanol content in a commercial gasoline-methanol blend sample with good recovery.

Horseradish and soybean peroxidases: comparable tools for alternative niches?

Horseradish and soybean peroxidases (HRP and SBP, respectively) are useful biotechnological tools. HRP is often termed the classical plant heme peroxidase and although it has been studied for decades, our understanding has deepened since its cloning and subsequent expression, enabling numerous mutational and protein engineering studies. SBP, however, has been neglected until recently, despite offering a real alternative to HRP: SBP actually outperforms HRP in terms of stability and is now used in numerous biotechnological applications, including biosensors. Review of both is timely. This article summarizes and discusses the main insights into the structure and mechanism of HRP, with special emphasis on HRP mutagenesis, and outlines its use in a variety of applications. It also reviews the current knowledge and applications to date of SBP, particularly biosensors. The final paragraphs speculate on the future of plant heme-based peroxidases, with probable trends outlined and explored.

Characterization of an organic phase peroxide biosensor based on horseradish peroxidase immobilized in Eastman AQ

Due to their frequent occurrence in food, cosmetics and pharmaceutical products, and their poor solubility in water, the detection of peroxides in organic solvents has aroused significant interest. For diagnostics or on-site testing, a fast and specific experimental approach is required. Although aqueous peroxide biosensors are well known, they are usually not suitable for nonaqueous applications due to their instability. Here we describe an organic phase biosensor for hydrogen peroxide based on horseradish peroxidase immobilized in an Eastman AQ 55 polymer matrix. Rotating disc amperometry was used to examine the effect of the solvent properties, the amount and pH of added buffer, the concentration of peroxide and ferrocene dimethanol, and the amount of Eastman AQ 55 and of enzyme on the response of the biosensor to hydrogen peroxide. The response of the biosensor was limited by diffusion. Linear responses (with detection limits to hydrogen peroxide given in parentheses) were obtained in methanol (1.2 microM), ethanol (0.6 microM), 1-propanol (2.8 microM), acetone (1.4 microM), acetonitrile (2.6 microM), and ethylene glycol (13.6 microM). The rate of diffusion of ferrocene dimethanol was more constrained than the rate of diffusion of hydrogen peroxide, resulting in a comparatively narrow linear range. The main advantages of the sensor are its ease of use and a high degree of reproducibility, together with good operational and storage stability.

Enzyme Electrochemistry — Biocatalysis on an Electrode

Oxidoreductase enzymes catalyze single- or multi-electron reduction/oxidation reactions of small molecule inorganic or organic substrates, and they are integral to a wide variety of biological processes including respiration, energy production, biosynthesis, metabolism, and detoxification. All redox enzymes require a natural redox partner such as an electron-transfer protein (e.g. cytochrome, ferredoxin, flavoprotein) or a small molecule cosubstrate (e.g. NAD(P)H, dioxygen) to sustain catalysis, in effect to balance the substrate/product redox half-reaction. In principle, the natural electron-transfer partner may be replaced by an electrochemical working electrode. One of the great strengths of this approach is that the rate of catalysis (equivalent to the observed electrochemical current) may be probed as a function of applied potential through linear sweep and cyclic voltammetry, and insight to the overall catalytic mechanism may be gained by a systematic electrochemical study coupled with theoretical analysis. In this review, the various approaches to enzyme electrochemistry will be discussed, including direct and indirect (mediated) experiments, and a brief coverage of the theory relevant to these techniques will be presented. The importance of immobilizing enzymes on the electrode surface will be presented and the variety of ways that this may be done will be reviewed. The importance of chemical modification of the electrode surface in ensuring an environment conducive to a stable and active enzyme capable of functioning natively will be illustrated. Fundamental research into electrochemically driven enzyme catalysis has led to some remarkable practical applications. The glucose oxidase enzyme electrode is a spectacularly successful application of enzyme electrochemistry. Biosensors based on this technology are used worldwide by sufferers of diabetes to provide rapid and accurate analysis of blood glucose concentrations. Other applications of enzyme electrochemistry are in the sensing of macromolecular complexation events such as antigen–antibody binding and DNA hybridization. The review will include a selection of enzymes that have been successfully investigated by electrochemistry and, where appropriate, discuss their development towards practical biotechnological applications.