Bioelectrocatalysis is an interdisciplinary research field combining biocatalysis and electrocatalysis via the utilization of materials derived from biological systems as catalysts to catalyze the redox reactions occurring at an electrode. Bioelectrocatalysis synergistically couples the merits of both biocatalysis and electrocatalysis. The advantages of biocatalysis include high activity, high selectivity, wide substrate scope, and mild reaction conditions. The advantages of electrocatalysis include the possible utilization of renewable electricity as an electron source and high energy conversion efficiency. These properties are integrated to achieve selective biosensing, efficient energy conversion, and the production of diverse products. This review seeks to systematically and comprehensively detail the fundamentals, analyze the existing problems, summarize the development status and applications, and look toward the future development directions of bioelectrocatalysis. First, the structure, function, and modification of bioelectrocatalysts are discussed. Second, the essentials of bioelectrocatalytic systems, including electron transfer mechanisms, electrode materials, and reaction medium, are described. Third, the application of bioelectrocatalysis in the fields of biosensors, fuel cells, solar cells, catalytic mechanism studies, and bioelectrosyntheses of high-value chemicals are systematically summarized. Finally, future developments and a perspective on bioelectrocatalysis are suggested.

Petroleum hydrocarbons are our major energy source and an important feedstock for the chemical industry. With the exception of combustion, the deep conversion of chemically inert hydrocarbons to more valuable chemicals is of considerable interest. However, two challenges hinder this conversion. One is the regioselective activation of inert carbon–hydrogen (C–H) bonds. The other is designing a pathway to realize this complicated conversion. In response to the two challenges, a multistep bioelectrocatalytic system was developed to realize the one-pot deep conversion from heptane to N-heptylhepan-1-imine under mild conditions. First, in this enzymatic cascade, a bioelectrocatalytic C–H bond oxyfunctionalization step based on alkane hydroxylase (alkB) was applied to regioselectively convert heptane to 1-heptanol. By integrating subsequent alcohol oxidation and bioelectrocatalytic reductive amination steps based on an engineered choline oxidase (AcCO6) and a reductive aminase (NfRedAm), the generated 1-heptanol was successfully converted to N-heptylhepan-1-imine. The electrochemical architecture provided sufficient electrons to drive the bioelectrocatalytic C–H bond oxyfunctionalization and reductive amination steps with neutral red (NR) as electron mediator. The highest concentration of N-heptylhepan-1-imine achieved was 0.67 mM with a Faradaic efficiency of 45% for C–H bond oxyfunctionalization and 70% for reductive amination. Hexane, octane, and ethylbenzene were also successfully converted to the corresponding imines. Via regioselective C–H bond oxyfunctionalization, intermediate oxidation, and reductive amination, the bioelectrocatalytic hydrocarbon deep conversion system successfully realized the challenging conversion from inert hydrocarbons to imines that would have been impossible by using organic synthesis methods and provided a new methodology for the comprehensive conversion and utilization of inert hydrocarbons.

Increasing attention has been paid to bioelectrochemical nitrogen fixation (e-BNF) as a promising approach to achieve the NH 3 synthesis under mild conditions. However, currently developed microbial e-BNF systems all rely on diffusible mediators to deliver redox equivalents inside the bacteria. Challenges of using diffusible mediators include toxicity, inefficient transmembrane diffusion, mediator inactivation, mediator contamination, and low energy efficiency. To date, e-BNF through transmembrane electron uptake without using diffusible electron mediators has not yet been reported. Herein, we describe a genetic strategy to engineer cyanobacterium Synechococcus elongatus PCC 7942 with transmembrane electron transfer (TET) ability to realize e-BNF without the addition of soluble mediators. The engineered S. elongatus PCC 7942 strain Se-nif with N 2 fixation activity was further transformed with an outer membrane protein cytochrome S OmcS, which contributes for the extracellular electron transfer (EET) ability of Geobacter sp. The engineered Senifom strain exhibited enhanced TET ability resulting in an approximately 13-fold higher NH 3 production rate than the corresponding Se-nif strain. The Faradaic efficiency of the Senifom e-BNF system was calculated to be approximately 23.3%, which is higher than the previously reported e-BNF systems. The electron pathway of the obtained extracellular electron was briefly analyzed and an extracellular electron uptake mechanism in the Senifom strain was proposed. This work demonstrates that a genetically engineered conduit can facilitate transmembrane electronic communication from the electrode to living cells, thereby providing insights into bioelectrosynthesis technology, especially the e-BNF systems and ammonium production.

Direct electron transfer (DET) between enzymes and electrodes is a key issue for practical use of bioelectrocatalytic devices as a bioenergy process, such as enzymatic electrosynthesis, biosensors, and enzyme biofuel cells. To date, based on the DET of bioelectrocatalysis, less than 1% of the calculated theoretical current was transferred to final electron acceptor due to energy loss at enzyme-electrode interface. This study describes the design and construction of a synthetic glucose dehydrogenase (GDH; α and γ subunits) combined with a gold-binding peptide at its amino or carboxy terminus for direct contact between enzyme and electrode. The fused gold-binding peptide facilitated stable immobilization of GDH and constructed uniform monolayer of GDH onto a Au electrode. Depending on the fused site of binding peptide to the enzyme complex, nine combinations of recombinant GDH proteins on the electrode show significantly different direct electron-transfer efficiency across the enzyme-electrode interface. The fusion of site-specific binding peptide to the catalytic subunit (α subunit, carboxy terminus) of the enzyme complex enabled apparent direct electron transfer (DET) across the enzyme-electrode interface even in the absence of the electron-transfer subunit (i.e., β subunit having cytochrome domain). The catalytic glucose oxidation current at an onset potential of ca. (-)0.46 V vs Ag/AgCl was associated with the appearance of an flavin adenine dinucleotide (FAD)/FADH redox wave and a stabilized bioelectrocatalytic current of more than 100 μA, determined from chronoamperometric analysis. Electron recovery was 7.64%, and the catalytic current generation was 249 μA per GDH enzyme loading unit (U), several orders of magnitude higher than the values reported previously. These observations corroborated that the last electron donor facing to electrode was controlled to be in close proximity without electron-transfer intermediates and the native affinity for glucose was preserved. The design and construction of the site-specific "sticky-ended" proteins without loss of catalytic activity could be applied to other redox enzymes having a buried active site.

Substrate channeling, where an intermediate in a multistep reaction is directed toward a reaction center rather than freely diffusing, offers several advantages when employed in catalytic cascades. Here we present a fusion enzyme comprised of an alcohol and aldehyde dehydrogenase, that is computationally designed to facilitate electrostatic substrate channeling using a cationic linker bridging the two structures. Rosetta protein folding software was utilized to determine an optimal linker placement, added to the truncated termini of the proteins, which is as close as possible to the active sites of the enzymes without disrupting critical catalytic residues. With improvements in stability, product selectivity (90%), and catalyst turnover frequency, representing 500-fold increased activity compared to the unbound enzymes and nearly 140-fold for a neutral-linked fusion enzyme, this design strategy holds promise for making other multistep catalytic processes more sustainable and efficient.