Interaction of Fructose Dehydrogenase with a Sulfonated Polyaniline: Application for Enhanced Bioelectrocatalysis

Bioelectrocatalytic performance of d-fructose dehydrogenase

This review summarizes the bioelectrocatalytic properties of d-fructose dehydrogenase (FDH), while taking into consideration its enzymatic characteristics. FDH is a membrane-bound flavohemo-protein with a molecular mass of 138 kDa, and it catalyzes the oxidation of d-fructose to 5-keto-d-fructose. The characteristic feature of FDH is its strong direct-electron-transfer (DET)-type bioelectrocatalytic activity. The pathway of the DET-type reaction is discussed. An overview of the application of FDH-based bioelectrocatalysis to biosensors and biofuel cells is also presented, and the benefits and problems associated with it are extensively discussed.

The influence of pH and divalent/monovalent cations on the internal electron transfer (IET), enzymatic activity, and structure of fructose dehydrogenase

We report on the influence of pH and monovalent/divalent cations on the catalytic current response, internal electron transfer (IET), and structure of fructose dehydrogenase (FDH) by using amperometry, spectrophotometry, and circular dichroism (CD). Amperometric measurements were performed on graphite electrodes, onto which FDH was adsorbed and the effect on the response current to fructose was investigated when varying the pH and the concentrations of divalent/monovalent cations in the contacting buffer. In the presence of 10 mM CaCl₂, a current increase of up to ≈ 240% was observed, probably due to an intra-complexation reaction between Ca²⁺ and the aspartate/glutamate residues found at the interface between the dehydrogenase domain and the cytochrome domain of FDH. Contrary to CaCl₂, addition of MgCl₂ did not show any particular influence, whereas addition of monovalent cations (Na⁺ or K⁺) led to a slight linear increase in the maximum response current. To complement the amperometric investigations, spectrophotometric assays were carried out under homogeneous conditions in the presence of a 1-electron non-proton-acceptor, cytochrome c, or a 2-electron-proton acceptor, 2,6-dichloroindophenol (DCIP), respectively. In the case of cytochrome c, it was possible to observe a remarkable increase in the absorbance up to 200% when 10 mM CaCl₂ was added. However, by further increasing the concentration of CaCl₂ up to 50 mM and 100 mM, a decrease in the absorbance with a slight inhibition effect was observed for the highest CaCl₂ concentration. Addition of MgCl₂ or of the monovalent cations shows, surprisingly, no effect on the electron transfer to the electron acceptor. Contrary to the case of cytochrome c, with DCIP none of the cations tested seem to affect the rate of catalysis. In order to correlate the results obtained by amperometric and spectrophotometric measurements, CD experiments have been performed showing a great structural change of FDH when increasing the concentration CaCl₂ up to 50 mM, at which the enzyme molecules start to agglomerate, hindering the substrate access to the active site probably due to a chelation reaction occurring at the enzyme surface with the glutamate/aspartate residues.

Graphical Abstract

Integration of Enzymes in Polyaniline-Sensitized 3D Inverse Opal TiO2 Architectures for Light-Driven Biocatalysis and Light-to-Current Conversion

Inspired by natural photosynthesis, coupling of artificial light-sensitive entities with biocatalysts in a biohybrid format can result in advanced photobioelectronic systems. Herein, we report on the integration of sulfonated polyanilines (PMSA1) and PQQ-dependent glucose dehydrogenase (PQQ-GDH) into inverse opal TiO₂ (IO-TiO₂) electrodes. While PMSA1 introduces sensitivity for visible light into the biohybrid architecture and ensures the efficient wiring between the IO-TiO₂ electrode and the biocatalytic entity, PQQ-GDH provides the catalytic activity for the glucose oxidation and therefore feeds the light-driven reaction with electrons for an enhanced light-to-current conversion. Here, the IO-TiO₂ electrodes with pores of around 650 nm provide a suitable interface and morphology needed for the stable and functional assembly of polymer and enzyme. The IO-TiO₂ electrodes have been prepared by a template approach applying spin coating, allowing an easy scalability of the electrode height and surface area. The successful integration of the polymer and the enzyme is confirmed by the generation of an anodic photocurrent, showing an enhanced magnitude with increasing glucose concentrations. Compared to flat and nanostructured TiO₂ electrodes, the three-layered IO-TiO₂ electrodes give access to a 24-fold and 29-fold higher glucose-dependent photocurrent due to the higher polymer and enzyme loading in IO films. The three-dimensional IO-TiO₂|PMSA1|PQQ-GDH architecture reaches maximum photocurrent densities of 44.7 ± 6.5 μA cm^-2 at low potentials in the presence of glucose (for a three TiO₂ layer arrangement). The onset potential for the light-driven substrate oxidation is found to be at -0.315 V vs Ag/AgCl (1 M KCl) under illumination with 100 mW cm^-2, which is more negative than the redox potential of the enzyme. The results demonstrate the advantageous properties of IO-TiO₂|PMSA1|PQQ-GDH biohybrid architectures for the light-driven glucose conversion with improved performance.

Towards a novel bioelectrocatalytic platform based on "wiring" of pyrroloquinoline quinone-dependent glucose dehydrogenase with an electrospun conductive polymeric fiber architecture

Electrospinning is known as a fabrication technique for electrode architectures that serve as immobilization matrices for biomolecules. The current work demonstrates a novel approach to construct a conductive polymeric platform, capable not only of immobilization, but also of electrical connection of the biomolecule with the electrode. It is produced upon electrospinning from mixtures of three different highly conductive sulfonated polyanilines and polyacrylonitrile on ITO electrodes. The resulting fiber mats are with a well-retained conductivity. After coupling the enzyme pyrroloquinoline quinone-dependent glucose dehydrogenase (PQQ-GDH) to polymeric structures and addition of the substrate glucose an efficient bioelectrocatalysis is demonstrated. Depending on the choice of the sulfonated polyanilline mediatorless bioelectrocatalysis starts at low potentials; no large overpotential is needed to drive the reaction. Thus, the electrospun conductive immobilization matrix acts here as a transducing element, representing a promising strategy to use 3D polymeric scaffolds as wiring agents for active enzymes. In addition, the mild and well reproducible fabrication process and the active role of the polymer film in withdrawing electrons from the reduced PQQ-GDH lead to a system with high stability. This could provide access to a larger group of enzymes for bioelectrochemical applications including biosensors and biofuel cells.

Conducting polymer based electrochemical biosensors

Conducting polymer (CP)-based electrochemical biosensors have gained great attention as such biosensor platforms are easy and cost-effective to fabricate, and provide a direct electrical readout of the presence of biological analytes with high sensitivity and selectivity.