Direct electron transfer-type bioelectrocatalysis by membrane-bound aldehyde dehydrogenase from Gluconobacter oxydans and cyanide effects on its bioelectrocatalytic properties

Direct Electron Transfer-Type Oxidoreductases for Biomedical Applications

Among the various types of enzyme-based biosensors, sensors utilizing enzymes capable of direct electron transfer (DET) are recognized as the most ideal. However, only a limited number of redox enzymes are capable of DET with electrodes, that is, dehydrogenases harboring a subunit or domain that functions specifically to accept electrons from the redox cofactor of the catalytic site and transfer the electrons to the external electron acceptor. Such subunits or domains act as built-in mediators for electron transfer between enzymes and electrodes; consequently, such enzymes enable direct electron transfer to electrodes and are designated as DET-type enzymes. DET-type enzymes fall into several categories, including redox cofactors of catalytic reactions, built-in mediators for DET with electrodes and by their protein hierarchic structures, DET-type oxidoreductases with oligomeric structures harboring electron transfer subunits, and monomeric DET-type oxidoreductases harboring electron transfer domains. In this review, we cover the science of DET-type oxidoreductases and their biomedical applications. First, we introduce the structural biology and current understanding of DET-type enzyme reactions. Next, we describe recent technological developments based on DET-type enzymes for biomedical applications, such as biosensors and biochemical energy harvesting for self-powered medical devices. Finally, after discussing how to further engineer and create DET-type enzymes, we address the future prospects for DET-type enzymes in biomedical engineering.

Low pH Stress Enhances Gluconic Acid Accumulation with Enzymatic Hydrolysate as Feedstock Using Gluconobacter oxydans

Gluconic acid has been increasingly in demand in recent years due to the wide applications in the food, healthcare and construction industries. Plant-derived biomass is rich in biopolymers that comprise glucose as the monomeric unit, which provide abundant feedstock for gluconic acid production. Gluconobacter oxydans can rapidly and incompletely oxidize glucose to gluconic acid and it is regarded as ideal industrial microorganism. Once glucose is depleted, the gluconic acid will be further bio-oxidized to 2-ketogluconic acid by Gluconobacter oxydans. The endpoint is difficult to be controlled, especially in an industrial fermentation process. In this study, it was found that the low pH environment (2.5~3.5) could limit the further metabolism of gluconic acid and that it resulted in a yield over 95%. Therefore, the low pH stress strategy for efficiently producing gluconic acid from biomass-derived glucose was put forward and investigated with enzymatic hydrolysate. As a result, 98.8 g/L gluconic acid with a yield of 96% could be obtained from concentrated corncob enzymatic hydrolysate that initially contained 100 g/L glucose with 1.4 g/L cells loading of Gluconobacter oxydans. In addition, the low pH stress strategy could effectively control end-point and decrease the risk of microbial contamination. Overall, this strategy provides a potential for industrial gluconic acid production from lignocellulosic materials.

pH regulatory divergent point for the selective bio-oxidation of primary diols during resting cell catalysis

Background

Hydroxyl acid is an important platform chemical that covers many industrial applications due to its dual functional modules. At present, the traditional technology for hydroxyl acid production mainly adopts the petroleum route with benzene, cyclohexane, butadiene and other non-renewable resources as raw materials which violates the development law of green chemistry. Conversely, it is well-known that biotechnology and bioengineering techniques possess several advantages over chemical methods, such as moderate reaction conditions, high chemical selectivity, and environmental-friendly. However, compared with chemical engineering, there are still some major obstacles in the industrial application of biotechnology. The critical issue of the competitiveness between bioengineering and chemical engineering is products titer and volume productivity. Therefore, based on the importance of hydroxyl acids in many fields, exploring a clean, practical and environmental-friendly preparation process of the hydroxyl acids is the core purpose of this study.

Results

To obtain high-purity hydroxyl acid, a microbiological regulation for its bioproduction by Gluconobacter oxydans was constructed. In the study, we found a critical point of chain length determine the end-products. Gluconobacter oxydans catalyzed diols with chain length ≤ 4, forming hydroxyl acids, and converting 1,5-pentylene glycol and 1,6-hexylene glycol to diacids. Based on this principle, we successfully synthesized 75.3 g/L glycolic acid, 83.2 g/L 3-hydroxypropionic acid, and 94.3 g/L 4-hydroxybutyric acid within 48 h. Furthermore, we directionally controlled the products of C5/C6 diols by adjusting pH, resulting in 102.3 g/L 5‑hydroxyvaleric acid and 48.8 g/L 6-hydroxycaproic acid instead of diacids. Combining pH regulation and cell-recycling technology in sealed-oxygen supply bioreactor, we prepared 271.4 g 5‑hydroxyvaleric acid and 129.4 g 6-hydroxycaproic acid in 6 rounds.

Conclusions

In this study, a green scheme of employing G. oxydans as biocatalyst for superior-quality hydroxyl acids (C2–C6) production is raised up. The proposed strategy commendably demonstrated a novel technology with simple pH regulation for high-value production of hydroxyl acids via green bioprocess developments.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13068-022-02171-5.

Directional enhancement of 2-keto-gluconic acid production from enzymatic hydrolysate by acetic acid-mediated bio-oxidation with Gluconobacter oxydans

An acetic acid-mediated bio-oxidation strategy with Gluconobacter oxydans was developed to produce valuable 2-ketogluconic acid from lignocellulosic biomass. Metabolically, glucose is firstly oxidized to gluconic acid and further oxidized to 2-keto-gluconic acid by Gluconobacter oxydans. As a specific inhibitor for microbial fermentation generated from pretreatment, acetic acid was validated to have a down-regulated effect on bio-oxidizing glucose to gluconic acid. Nevertheless, it significantly facilitated 2-keto-gluconic acid accumulation and improved gluconate dehydrogenase activity. In the presence of 5.0 g/L acetic acid, the yield of 2-keto-gluconic acid increased from 38.0% to 80.5% using pure glucose as feedstock with 1.5 g/L cell loading. Meanwhile, 44.6 g/L 2-keto-gluconic acid with a yield of 83.5% was also achieved from the enzymatic hydrolysate. 2-keto-gluconic acid production, found in this study, laid a theoretical foundation for the industrial production of 2-keto-gluconic acid by Gluconobacter oxydans using lignocellulosic materials.

Cyanide sensitivity in direct electron transfer-type bioelectrocatalysis by membrane-bound alcohol dehydrogenase from Gluconobacter oxydans

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.

Hybrid catalyst cascade for enhanced oxidation of glucose in glucose/air biofuel cell

Redox enzymes are capable of harvesting electrical energy from biofuels in high catalytic activity and under mild condition. However, it is difficult to achieve efficient electron transfer and deep oxidation of biofuels simultaneously in a single-enzyme catalytic system. Herein, we report a hybrid catalyst cascade consisting of an organic oxidation catalyst, 2,2,6,6-tetramethyl-1-piperidine N-oxyl (TEMPO), and an enzyme, glucose oxidase (GOx), for electrochemical oxidation of glucose. It is found that TEMPO is capable of mediating electron transfer between the redox center of GOx and the electrode surface. While glucose can be oxidized into glucuronic acid under neutral conditions. Thus, combining GOx and TEMPO, we are able to achieve 4e- electrooxidation of glucose using the hybrid enzymatic and organic cascade (HEOC) system. When coupled with an air-breathing Pt cathode, the resulting glucose/air biofuel cell using the proposed HEOC anode exhibits a maximum power density of 38.1 μW cm-2 with a short-circuit current of 651.4 μA cm-2, which can be attributed to the enhanced energetic efficiency, enabling TEMPO a promising catalyst for glucose oxidation in bioelectronics applications.

Bio-Electrochemical System Depollution Capabilities and Monitoring Applications: Models, Applicability, Advanced Bio-Based Concept for Predicting Pollutant Degradation and Microbial Growth Kinetics via Gene Regulation Modelling

Microbial fuel cells (MFC) are an emerging technology for waste, wastewater and polluted soil treatment. In this manuscript, pollutants that can be treated using MFC systems producing energy are presented. Furthermore, the applicability of MFC in environmental monitoring is described. Common microbial species used, release of genome sequences, and gene regulation mechanisms, are discussed. However, although scaling-up is the key to improving MFC systems, it is still a difficult challenge. Mathematical models for MFCs are used for their design, control and optimization. Such models representing the system are presented here. In such comprehensive models, microbial growth kinetic approaches are essential to designing and predicting a biosystem. The empirical and unstructured Monod and Monod-type models, which are traditionally used, are also described here. Understanding and modelling of the gene regulatory network could be a solution for enhancing knowledge and designing more efficient MFC processes, useful for scaling it up. An advanced bio-based modelling concept connecting gene regulation modelling of specific metabolic pathways to microbial growth kinetic models is presented here; it enables a more accurate prediction and estimation of substrate biodegradation, microbial growth kinetics, and necessary gene and enzyme expression. The gene and enzyme expression prediction can also be used in synthetic and systems biology for process optimization. Moreover, various MFC applications as a bioreactor and bioremediator, and in soil pollutant removal and monitoring, are explored.

Direct Bioelectrocatalytic Oxidation of Glucose by Gluconobacter oxydans Membrane Fractions in PEDOT:PSS/TEG-Modified Biosensors

Recent years have witnessed an ever-increasing interest in developing electrochemical biosensors based on direct electron transfer-type bioelectrocatalysis. This work investigates the bioelectrocatalytic oxidation of glucose by membrane fractions of Gluconobacter oxydans cells on screen-printed electrodes modified with thermally expanded graphite and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Electrooxidation of glucose was shown to occur without the presence of electron transport mediators. Chronoamperometric and cyclic voltametric characteristics showed an increase of anodic currents at electrode potentials of 0–500 mV relative to the reference electrode (Ag/AgCl). The direct electron transfer effect was observed for non-modified PEDOT:PSS as well as for PEDOT:PSS linked with crosslinkers and conductive fillers such as polyethylene glycol diglycidyl or dimethyl sulfoxide. Bioelectrodes with this composite can be successfully used in fast reagent-free glucose biosensors.