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.

Fundamentals, Applications, and Future Directions of Bioelectrocatalysis.

The anaerobic oxidation of methane coupled to sulfate reduction is a microbially mediated process requiring a syntrophic partnership between anaerobic methanotrophic (ANME) archaea and sulfate-reducing bacteria (SRB). Based on genome taxonomy, ANME lineages are polyphyletic within the phylum Halobacterota , none of which have been isolated in pure culture. Here, we reconstruct 28 ANME genomes from environmental metagenomes and flow sorted syntrophic consortia. Together with a reanalysis of previously published datasets, these genomes enable a comparative analysis of all marine ANME clades. We review the genomic features that separate ANME from their methanogenic relatives and identify what differentiates ANME clades. Large multiheme cytochromes and bioenergetic complexes predicted to be involved in novel electron bifurcation reactions are well distributed and conserved in the ANME archaea, while significant variations in the anabolic C1 pathways exists between clades. Our analysis raises the possibility that methylotrophic methanogenesis may have evolved from a methanotrophic ancestor. 

Comparative genomics reveals electron transfer and syntrophic mechanisms differentiating methanotrophic and methanogenic archaea

The biological production of methane is vital to the global carbon cycle and accounts for ca. 74% of total methane emissions. The organisms that facilitate this process, methanogenic archaea, belong to a large and phylogenetically diverse group that thrives in a wide range of anaerobic environments. Two main subgroups exist within methanogenic archaea: those with and those without cytochromes. Although a variety of metabolisms exist within this group, the reduction of growth substrates to methane using electrons from molecular hydrogen is, in a phylogenetic sense, the most widespread methanogenic pathway. Methanogens without cytochromes typically generate methane by the reduction of CO2 with electrons derived from H2, formate, or secondary alcohols, generating a transmembrane ion gradient for ATP production via an Na+-translocating methyltransferase (Mtr). These organisms also conserve energy with a novel flavin-based electron bifurcation mechanism, wherein the endergonic reduction of ferredoxin is facilitated by the exergonic reduction of a disulfide terminal electron acceptor coupled to either H2 or formate oxidation. Methanogens that utilize cytochromes have a broader substrate range, and can convert acetate and methylated compounds to methane, in addition to the ability to reduce CO2 Cytochrome-containing methanogens are able to supplement the ion motive force generated by Mtr with an H+-translocating electron transport system. In both groups, enzymes known as hydrogenases, which reversibly interconvert protons and electrons to molecular hydrogen, play a central role in the methanogenic process. This review discusses recent insight into methanogen metabolism and energy conservation mechanisms with a particular focus on the genus Methanosarcina.

Energy Conservation and Hydrogenase Function in Methanogenic Archaea, in Particular the Genus Methanosarcina.

The co-occurrence of Geobacter and Methanosarcinales is often used as a proxy for the manifestation of direct interspecies electron transfer (DIET) in man-made and natural aquatic environments. We previously reported that not all Geobacter are capable of DIET with Methanosarcina. Here we tested 15 new artificial co-culture combinations with methanogens and electrogenic bacteria, including an electrogen outside of the Geobacter clade – Rhodoferax ferrireducens. Consistently, highly effective electrogenic bacteria (G. metallireducens, G. hydrogenophilus and R. ferrireducens) formed successful associations with Methanosarcinales. Highly effective electrogens could not sustain the growth of H2-utilizing methanogens of the genera Methanococcus, Methanobacterium, Methanospirillum, Methanolacinia or Methanoculleus. Methanosarcinales, including strict non-hydrogenotrophic methanogens of the genus Methanothrix (Mtx. harundinacea and Mtx. shoeghenii) and Methanosarcina horonobensis, conserved their ability to interact with electrogens. Methanosarcinales were classified as the only methanogens containing c-type cytochromes, unlike strict hydrogenotrophic methanogens. It was then hypothesized that multiheme c-type cytochromes give Methanosarcinales their ability to retrieve extracellular electrons. However, multiheme c-type cytochromes are neither unique to this group of methanogens nor universal. Only two of the seven Methanosarcinales tested had multiheme c-type cytochromes (MCH). In one of these two species - M. mazei a deletion mutant for its MCH was readily available. Here we tested if the absence of this MHC impacts extracellular electron uptake. Deletion of the MHC in M. mazei did not impact the ability of this methanogens to retrieve extracellular electrons from G. metallireducens or a poised cathode. Since Methanosarcina did not require multiheme c-type cytochromes for direct electron uptake we proposed an alternative strategy for extracellular electron uptake.

https://biorxiv.org/content/biorxiv/early/2019/08/28/747485.full-text.pdf

Extracellular electron uptake in Methanosarcinales is independent of multiheme c-type cytochromes

Efficient electrocatalytic energy conversion requires the devices to function reversibly, i.e. deliver a significant current at minimal overpotential. Redox-active films can effectively embed and stabilise molecular electrocatalysts, but mediated electron transfer through the film typically makes the catalytic response irreversible. Here, we describe a redox-active film for bidirectional (oxidation or reduction) and reversible hydrogen conversion, consisting of [FeFe] hydrogenase embedded in a low-potential, 2,2'-viologen modified hydrogel. When this catalytic film served as the anode material in a H2/O2 biofuel cell, an open circuit voltage of 1.16 V was obtained - a benchmark value near the thermodynamic limit. The same film also acted as a highly energy efficient cathode material for H2 evolution. We explained the catalytic properties using a kinetic model, which shows that reversibility can be achieved despite intermolecular electron transfer being slower than catalysis. This understanding of reversibility simplifies the design principles of highly efficient and stable bioelectrocatalytic films, advancing their implementation in energy conversion.

https://hal.archives-ouvertes.fr/hal-03215114/document

Reversible H2 Oxidation and Evolution by Hydrogenase Embedded in a Redox Polymer Film.

Molecular hydrogen is a major high-energy carrier for future energy technologies, if produced from renewable electrical energy. Hydrogenase enzymes offer a pathway for bioelectrochemically producing hydrogen that is advantageous over traditional platforms for hydrogen production because of low overpotentials and ambient operating temperature and pressure. However, electron delivery from the electrode surface to the enzyme's active site is often rate-limiting. Here, it is shown that three different hydrogenases from Clostridium pasteurianum and Methanococcus maripaludis, when immobilized at a cathode in a cobaltocene-functionalized polyallylamine (Cc-PAA) redox polymer, mediate rapid and efficient hydrogen evolution. Furthermore, it is shown that Cc-PAA-mediated hydrogenases can operate at high faradaic efficiency (80-100 %) and low apparent overpotential (-0.578 to -0.593 V vs. SHE). Specific activities of these hydrogenases in the electrosynthetic Cc-PAA assay were comparable to their respective activities in traditional methyl viologen assays, indicating that Cc-PAA mediates electron transfer at high rates, to most of the embedded enzymes.

Enhanced Electrosynthetic Hydrogen Evolution by Hydrogenases Embedded in a Redox-Active Hydrogel.

While enzymes catalyze reactions with high selectivity and specificity at ambient temperature and pressure, electroactive enzymes retain such remarkable catalytic properties for catalyzing redox re...

Enzyme Electrochemistry for Industrial Energy Applications—A Perspective on Future Areas of Focus

Methanogenic archaea can be integrated into a sustainable, carbon-neutral cycle for producing organic chemicals from C1 compounds if the rate, yield, and titer of product synthesis can be improved using metabolic engineering. However, metabolic engineering techniques are limited in methanogens by insufficient methods for controlling cellular protein levels. We conducted a systematic approach to tune protein levels in Methanosarcina acetivorans C2A, a model methanogen, by regulating transcription and translation initiation. Rationally designed core promoter and ribosome binding site mutations in M. acetivorans C2A resulted in a predicable change in protein levels over a 60 fold range. The overall range of protein levels was increased an additional 3 fold by introducing the 5′ untranslated region of the mcrB transcript. This work demonstrates a wide range of precisely controlled protein levels in M. acetivorans C2A, which will help facilitate systematic metabolic engineering efforts in methanogens.

Fine-Tuned Protein Production in Methanosarcina acetivorans C2A

Synthetic fuels: what are they and where do they come from?

Hydrogenotrophic methanogens oxidize molecular hydrogen to reduce carbon dioxide to methane. In methanogens without cytochromes, the initial endergonic reduction of CO2 to formylmethanofuran with H2-derived electrons is coupled to the exergonic reduction of a heterodisulfide of coenzymes B and M by flavin-based electron bifurcation (FBEB). In Methanococcus maripaludis, FBEB is performed by a heterodisulfide reductase (Hdr) enzyme complex that involves hydrogenase (Vhu), although formate dehydrogenase (Fdh) has been proposed as an alternative to Vhu. We have identified and purified three Hdr complexes of M. maripaludis, where homodimeric Hdr complexes containing (Vhu)2 or (Fdh)2 were found, in addition to a heterocomplex that contains both Vhu and Fdh. Formate was found in in vitro assays using the purified Hdr complex to act directly as the electron donor for FBEB via the associated Fdh. Furthermore, while ferredoxin was slowly reduced to 30% [-360 mV vs the standard hydrogen electrode (SHE)] by H2 and formate (0.8 atm and 30 mM, according to thermodynamics), the addition of CoB-S-S-CoM as the high-potential electron acceptor ( E°' = -140 mV vs SHE; to induce FBEB) resulted in the rapid and more complete reduction of Fd to 94% (-455 mV vs SHE).

Methanococcus maripaludis Employs Three Functional Heterodisulfide Reductase Complexes for Flavin-Based Electron Bifurcation Using Hydrogen and Formate.

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