Employment of 1-Methoxy-5-Ethyl Phenazinium Ethyl Sulfate as a Stable Electron Mediator in Flavin Oxidoreductases-Based Sensors

His-tagged lactate oxidase production for industrial applications using fed-batch fermentation

L-lactate oxidase (LOX) is a biotechnologically important enzyme used in biosensors and colorimetric kits to detect lactate, a key biomarker in clinical diagnostics, sports medicine and the food industry. In this work, we produced a recombinant His-tagged Aerococcus viridans LOX (rLOX) in Escherichia coli and carried out its functional characterization for industrial applications. Our rLOX was evaluated in a colorimetric kit for human diagnostics and in an amperometric biosensor to measure the lactic acid in food products. The rLOX was fully functional for both applications, with a performance comparable to commercial untagged LOXs. As the industrial use of LOX enzyme requires a large-scale production, we scaled up the rLOX production in a fed-batch bioreactor culture and obtained a yield approximately ten times higher than that of the Erlenmeyer scale. The His-tag allowed an easy and highly efficient purification process, and a high-purity rLOX was recovered after this one-step affinity purification. In this study, we described a simple, rapid and cost-competitive approach for the production of a recombinant His-tagged LOX enzyme suitable for industrial use.

Computer-Aided Rational Construction of Mediated Bioelectrocatalysis with π-Conjugated (Hetero)cyclic Molecules: Toward Promoted Distant Electron Tunneling and Improved Biosensing

Highly π-conjugated (hetero)cyclic molecules having delocalized orbitals and tunable charge mobilities are attractive redox relays for mediated bioelectrocatalysis in manifold applications. As rigid molecules, their dynamics within the soft but confined intraprotein space becomes the crucial determinant of the enzyme-mediator electron-tunneling efficiency. However, it is rarely investigated in designing the mediated interface with a particular biocatalyst (e.g., oxidoreductase), which remains an empirical but try-and-error process. Herein, we present the computer-aided exploration of interactions between a flavin-containing reductive synthase and structurally diverse π-extended (hetero)cyclic mediators to realize reversed bioelectrocatalytic oxidation at low overpotentials. Compared to ring-fused systems with unbroken molecular planarity, heteroatom-bridged cyclic, and rotatable conjugated structures (e.g., indophenols) can experience unusually large dynamic torsion under biased forces of hydrogen bonding with enzyme residues. This behavior led to fast intraprotein reorientation (<50 ps) that shortened the electron-tunneling distance from 12 to 9 Å. Meanwhile, the lowest unoccupied molecular orbital level upon molecular torsion was decreased by 0.5 eV to further promote electron abstraction from the reduced flavin cofactor. An efficient distant electron tunneling also obviated mediator transport through the substrate channel, thus avoiding competitive inhibition on enzyme kinetics to broaden the operating concentration range. The resulting bioelectrocatalytic interface enables low-potential biosensing of glutamate with improved selectivity. Our finding provides new structural insights into constructing efficient long-range heterogeneous charge transport with biomacromolecular catalysts.

Wash-Free, Sandwich-Type Protein Detection Using Direct Electron Transfer and Catalytic Signal Amplification of Multiple Redox Labels

Direct electron transfer (DET) between a redox label and an electrode has been used for sensitive and selective sandwich-type detection without a wash step. However, applying DET is still highly challenging in protein detection, and a single redox label per probe is insufficient to obtain a high electrochemical signal. Here, we report a wash-free, sandwich-type detection of thrombin using DET and catalytic signal amplification of multiple redox labels. The detection scheme is based on (i) the redox label-catalyzed oxidation of a reductant, (ii) the conjugation of multiple redox labels per probe using a poly-linker, (iii) the low nonspecific adsorption of the conjugated poly-linker due to uncharged, reduced redox labels, and (iv) a facile DET using long, flexible poly-linker and spacer DNA. Amine-reactive phenazine ethosulfate and NADH were used as the redox label and reductant, respectively. N₃-terminated polylysine was used as the poly-linker for the conjugation between an aptamer probe and multiple redox labels. Approximately 11 redox labels per probe and rapid catalytic NADH oxidation enable high signal amplification. Thrombin in urine could be detected without a wash step with a detection limit of ∼50 pM, which is practically promising for point-of-care testing of proteins.