Efficient whole cell biocatalyst for formate-based hydrogen production

Genomic potential and physiological characteristics of C1 metabolism in novel acetogenic bacteria

Acetogenic bacteria can utilize C1 compounds, such as carbon monoxide (CO), formate, and methanol, via the Wood-Ljungdahl pathway (WLP) to produce biofuels and biochemicals. Two novel acetogenic bacteria of the family Eubacteriaceae ES2 and ES3 were isolated from Eulsukdo, a delta island in South Korea. We conducted whole genome sequencing of the ES strains and comparative genome analysis on the core clusters of WLP with Acetobacterium woodii DSM1030T and Eubacterium limosum ATCC8486T. The methyl-branch cluster included a formate transporter and duplicates or triplicates copies of the fhs gene, which encodes formyl-tetrahydrofolate synthetase. The formate dehydrogenase cluster did not include the hydrogenase gene, which might be replaced by a functional complex with a separate electron bifurcating hydrogenase (HytABCDE). Additionally, duplicated copies of the acsB gene, encoding acetyl-CoA synthase, are located within or close to the carbonyl-branch cluster. The serum bottle culture showed that ES strains can utilize a diverse range of C1 compounds, including CO, formate, and methanol, as well as CO2. Notably, ES2 exhibited remarkable resistance to high concentrations of C1 substrates, such as 100% CO (200 kPa), 700 mM formate, and 500 mM methanol. Moreover, ES2 demonstrated remarkable growth rates under 50% CO (0.45 h-1) and 200 mM formate (0.34 h-1). These growth rates are comparable to or surpassing those previously reported in other acetogenic bacteria. Our study introduces novel acetogenic ES strains and describes their genetic and physiological characteristics, which can be utilized in C1-based biomanufacturing.

A new metabolic trait in an acetogen: Mixed acid fermentation of fructose in a methylene-tetrahydrofolate reductase mutant of Acetobacterium woodii

To inactivate the Wood-Ljungdahl pathway in the acetogenic model bacterium Acetobacterium woodii, the genes metVF encoding two of the subunits of the methylene-tetrahydrofolate reductase were deleted. As expected, the mutant did not grow on C1 compounds and also not on lactate, ethanol or butanediol. In contrast to a mutant in which the first enzyme of the pathway (hydrogen-dependent CO2 reductase) had been genetically deleted, cells were able to grow on fructose, albeit with lower rates and yields than the wild-type. Growth was restored by addition of an external electron sink, glycine betaine + CO2 or caffeate. Resting cells pre-grown on fructose plus an external electron acceptor fermented fructose to two acetate and four hydrogen, that is, performed hydrogenogenesis. Cells pre-grown under fermentative conditions on fructose alone redirected carbon and electrons to form lactate, formate, ethanol as well as hydrogen. Apparently, growth on fructose alone induced enzymes for mixed acid fermentation (MAF). Transcriptome analyses revealed enzymes potentially involved in MAF and a quantitative model for MAF from fructose in A. woodii is presented.

Renewable Hydrogen Production and Storage Via Enzymatic Interconversion of CO2 and Formate with Electrochemical Cofactor Regeneration

The urgent need to reduce CO2 emissions has motivated the development of CO2 capture and utilization technologies. An emerging application is CO2 transformation into storage chemicals for clean energy carriers. Formic acid (FA), a valuable product of CO2 reduction, is an excellent hydrogen carrier. CO2 conversion to FA, followed by H2 release from FA, are conventionally chemically catalyzed. Biocatalysts offer a highly specific and less energy-intensive alternative. CO2 conversion to formate is catalyzed by formate dehydrogenase (FDH), which usually requires a cofactor to function. Several FDHs have been incorporated in bioelectrochemical systems where formate is produced by the biocathode and the cofactor is electrochemically regenerated. H2 production from formate is also catalyzed by several microorganisms possessing either formate hydrogenlyase or hydrogen-dependent CO2 reductase complexes. Combination of these two processes can lead to a CO2 -recycling cycle for H2 production, storage, and release with potentially lower environmental impact than conventional methods.

Characterisation of acetogen formatotrophic potential using Eubacterium limosum

Formate is a promising energy carrier that could be used to transport renewable electricity. Some acetogenic bacteria, such as Eubacterium limosum, have the native ability to utilise formate as a sole substrate for growth, which has sparked interest in the biotechnology industry. However, formatotrophic metabolism in E. limosum is poorly understood, and a system-level characterisation in continuous cultures is yet to be reported. Here, we present the first steady-state dataset for E. limosum formatotrophic growth. At a defined dilution rate of 0.4 d^-1, there was a high specific uptake rate of formate (280 ± 56 mmol/gDCW/d; gDCW = gramme dry cell weight); however, most carbon went to CO₂ (150 ± 11 mmol/gDCW/d). Compared to methylotrophic growth, protein differential expression data and intracellular metabolomics revealed several key features of formate metabolism. Upregulation of phosphotransacetylase (Pta) appears to be a futile attempt of cells to produce acetate as the major product. Instead, a cellular energy limitation resulted in the accumulation of intracellular pyruvate and upregulation of pyruvate formate ligase (Pfl) to convert formate to pyruvate. Therefore, metabolism is controlled, at least partially, at the protein expression level, an unusual feature for an acetogen. We anticipate that formate could be an important one-carbon substrate for acetogens to produce chemicals rich in pyruvate, a metabolite generally in low abundance during syngas growth. KEY POINTS: First Eubacterium limosum steady-state formatotrophic growth omics dataset High formate specific uptake rate, however carbon dioxide was the major product Formate may be the cause of intracellular stress and biofilm formation.

Unraveling prevalence of homoacetogenesis and methanogenesis pathways due to inhibitors addition

Three inhibitors targeting different microorganisms, both from Archaea and Bacteria domains, were evaluated for their effect on CO₂ biomethanation: sodium ionophore III (ETH2120), carbon monoxide (CO), and sodium 2-bromoethanesulfonate (BES). This study examines how these compounds affect the anaerobic digestion microbiome in a biogas upgrading process. While archaea were observed in all experiments, methane was produced only when adding ETH2120 or CO, not when adding BES, suggesting archaea were in an inactivated state. Methane was produced mainly via methylotrophic methanogenesis from methylamines. Acetate was produced at all conditions, but a slight reduction on acetate production (along with an enhancement on CH₄ production) was observed when applying 20 kPa of CO. Effects on CO₂ biomethanation were difficult to observe since the inoculum used was from a real biogas upgrading reactor, being this a complex environmental sample. Nevertheless, it must be mentioned that all compounds had effects on the microbial community composition.

Reprogramming the metabolism of an acetogenic bacterium to homoformatogenesis

Methyl groups are abundant in anoxic environments and their utilization as carbon and energy sources by microorganisms involves oxidation of the methyl groups to CO₂, followed by transfer of the electrons to an acceptor. In acetogenic bacteria, the electron acceptor is CO₂ that is reduced to enzyme bound carbon monoxide, the precursor of the carboxyl group in acetate. Here, we describe the generation of a mutant of the acetogen Acetobacterium woodii in which the last step in methyl group oxidation, formate oxidation to CO₂ catalyzed by the HDCR enzyme, has been genetically deleted. The mutant grew on glycine betaine as methyl group donor, and in contrast to the wild type, formed formate alongside acetate, in a 1:2 ratio, demonstrating that methyl group oxidation stopped at the level of formate and reduced electron carriers were reoxidized by CO₂ reduction to acetate. In the presence of the alternative electron acceptor caffeate, CO₂ was no longer reduced to acetate, formate was the only product and all the carbon went to formate. Apparently, acetogenesis was not required to sustain formatogenic growth. This is the first demonstration of a genetic reprogramming of an acetogen into a formatogen that grows by homoformatogenesis from methyl groups. Formate production from methyl groups is not only of biotechnological interest but also for the mechanism of electron transfer in syntrophic interactions in anoxic environments.

Leveraging substrate flexibility and product selectivity of acetogens in two-stage systems for chemical production

Carbon dioxide (CO₂ ) stands out as sustainable feedstock for developing a circular carbon economy whose energy supply could be obtained by boosting the production of clean hydrogen from renewable electricity. H₂ -dependent CO₂ gas fermentation using acetogenic microorganisms offers a viable solution of increasingly demonstrated value. While gas fermentation advances to achieve commercial process scalability, which is currently limited to a few products such as acetate and ethanol, it is worth taking the best of the current state-of-the-art technology by its integration within innovative bioconversion schemes. This review presents multiple scenarios where gas fermentation by acetogens integrate into double-stage biotechnological production processes that use CO₂ as sole carbon feedstock and H₂ as energy carrier for products' synthesis. In the integration schemes here reviewed, the first stage can be biotic or abiotic while the second stage is biotic. When the first stage is biotic, acetogens act as a biological platform to generate chemical intermediates such as acetate, formate and ethanol that become substrates for a second fermentation stage. This approach holds the potential to enhance process titre/rate/yield metrics and products' spectrum. Alternatively, when the first stage is abiotic, the integrated two-stage scheme foresees, in the first stage, the catalytic transformation of CO₂ into C₁ products that, in the second stage, can be metabolized by acetogens. This latter scheme leverages the metabolic flexibility of acetogens in efficient utilization of the products of CO₂ abiotic hydrogenation, namely formate and methanol, to synthesize multicarbon compounds but also to act as flexible catalysts for hydrogen storage or production.

Formate-driven H2 production by whole cells of Thermoanaerobacter kivui

Background

In times of global warming there is an urgent need to replace fossil fuel-based energy vectors by less carbon dioxide (CO2)-emitting alternatives. One attractive option is the use of molecular hydrogen (H2) since its combustion emits water (H2O) and not CO2. Therefore, H2 is regarded as a non-polluting fuel. The ways to produce H2 can be diverse, but steam reformation of conventional fossil fuel sources is still the main producer of H2 gas up to date. Biohydrogen production via microbes could be an alternative, environmentally friendly and renewable way of future H2 production, especially when the flexible and inexpensive C1 compound formate is used as substrate.

Results

In this study, the versatile compound formate was used as substrate to drive H2 production by whole cells of the thermophilic acetogenic bacterium Thermoanaerobacter kivui which harbors a highly active hydrogen-dependent CO2 reductase (HDCR) to oxidize formate to H2 and CO2 and vice versa. Under optimized reaction conditions, T. kivui cells demonstrated the highest H2 production rates (qH2 = 685 mmol g−1 h−1) which were so far reported in the literature for wild-type organisms. Additionally, high yields (Y(H2/formate)) of 0.86 mol mol−1 and a hydrogen evolution rate (HER) of 999 mmol L−1 h−1 were observed. Finally, stirred-tank bioreactor experiments demonstrated the upscaling feasibility of the applied whole cell system and indicated the importance of pH control for the reaction of formate-driven H2 production.

Conclusions

The thermophilic acetogenic bacterium T. kivui is an efficient biocatalyst for the oxidation of formate to H2 (and CO2). The existing genetic tool box of acetogenic bacteria bears further potential to optimize biohydrogen production in future and to contribute to a future sustainable formate/H2 bio-economy.

Membrane-anchored HDCR nanowires drive hydrogen-powered CO2 fixation

Filamentous enzymes have been found in all domains of life, but the advantage of filamentation is often elusive¹. Some anaerobic, autotrophic bacteria have an unusual filamentous enzyme for CO₂ fixation-hydrogen-dependent CO₂ reductase (HDCR)^2,3-which directly converts H₂ and CO₂ into formic acid. HDCR reduces CO₂ with a higher activity than any other known biological or chemical catalyst^4,5, and it has therefore gained considerable interest in two areas of global relevance: hydrogen storage and combating climate change by capturing atmospheric CO₂. However, the mechanistic basis of the high catalytic turnover rate of HDCR has remained unknown. Here we use cryo-electron microscopy to reveal the structure of a short HDCR filament from the acetogenic bacterium Thermoanaerobacter kivui. The minimum repeating unit is a hexamer that consists of a formate dehydrogenase (FdhF) and two hydrogenases (HydA2) bound around a central core of hydrogenase Fe-S subunits, one HycB3 and two HycB4. These small bacterial polyferredoxin-like proteins oligomerize through their C-terminal helices to form the backbone of the filament. By combining structure-directed mutagenesis with enzymatic analysis, we show that filamentation and rapid electron transfer through the filament enhance the activity of HDCR. To investigate the structure of HDCR in situ, we imaged T. kivui cells with cryo-electron tomography and found that HDCR filaments bundle into large ring-shaped superstructures attached to the plasma membrane. This supramolecular organization may further enhance the stability and connectivity of HDCR to form a specialized metabolic subcompartment within the cell.

Synergistic Metabolism of Glucose and Formate Increases the Yield of Short-Chain Organic Acids in Escherichia coli

Microbial cell factories using a single carbon source (e.g., sugars) have been used to produce a wide variety of chemicals. However, this process is often accompanied by stoichiometric constraints on carbons and redox cofactors. Here, a synthetic pathway was designed and constructed in Escherichia coli to synergistically use glucose and formate as mixed carbon sources. By optimizing this synthetic pathway via enzyme mining, protein engineering, and bioprocess approaches, the yield of pyruvate from glucose was enhanced to 94% of the theoretical glycolysis yield, reaching 1.88 mol/mol. Finally, the optimized synthetic pathway was integrated with a phosphite reductase-based NADH regeneration system in malate-producing E. coli, resulting in the conversion of glucose into l-malate with a high yield of up to 1.65 mol/mol. This synergistic carbon metabolism strategy can be used to establish carbon- and energy-efficient productive processes.

A one-pot biocatalytic and organocatalytic cascade delivers high titers of 2-ethyl-2-hexenal from n-butanol

Combining an organocatalyst with isolated alcohol oxidase or a whole-cell biocatalyst delivers 2-ethyl-2-hexenal in a one-pot, two-step biocatalytic/organocatalytic cascade.

Harnessing Escherichia coli for Bio-Based Production of Formate under Pressurized H2 and CO2 Gases

Escherichia coli is a Gram-negative bacterium that is a workhorse for biotechnology. The organism naturally performs a mixed-acid fermentation under anaerobic conditions where it synthesizes formate hydrogenlyase (FHL-1). The physiological role of the enzyme is the disproportionation of formate into H₂ and CO₂. However, the enzyme has been observed to catalyze hydrogenation of CO₂ given the correct conditions, and so it has possibilities in bio-based carbon capture and storage if it can be harnessed as a hydrogen-dependent CO₂ reductase (HDCR). In this study, an E. coli host strain was engineered for the continuous production of formic acid from H₂ and CO₂ during bacterial growth in a pressurized batch bioreactor. Incorporation of tungsten, in place of molybdenum, in FHL-1 helped to impose a degree of catalytic bias on the enzyme. This work demonstrates that it is possible to couple cell growth to simultaneous, unidirectional formate production from carbon dioxide and develops a process for growth under pressurized gases. IMPORTANCE Greenhouse gas emissions, including waste carbon dioxide, are contributing to global climate change. A basket of solutions is needed to steadily reduce emissions, and one approach is bio-based carbon capture and storage. Here, we present our latest work on harnessing a novel biological solution for carbon capture. The Escherichia coli formate hydrogenlyase (FHL-1) was engineered to be constitutively expressed. Anaerobic growth under pressurized H₂ and CO₂ gases was established, and aqueous formic acid was produced as a result. Incorporation of tungsten into the enzyme in place of molybdenum proved useful in poising FHL-1 as a hydrogen-dependent CO₂ reductase (HDCR).

Capture of carbon dioxide and hydrogen by engineered Escherichia coli: hydrogen-dependent CO2 reduction to formate

In times of global climate change and the fear of dwindling resources, we are facing different considerable challenges such as the replacement of fossil fuel-based energy carriers with the coincident maintenance of the increasing energy supply of our growing world population. Therefore, CO₂ capturing and H₂ storing solutions are urgently needed. In this study, we demonstrate the production of a functional and biotechnological interesting enzyme complex from acetogenic bacteria, the hydrogen-dependent CO₂ reductase (HDCR), in the well-known model organism Escherichia coli. We identified the metabolic bottlenecks of the host organisms for the production of the HDCR enzyme complex. Here we show that the recombinant expression of a heterologous enzyme complex transforms E. coli into a whole-cell biocatalyst for hydrogen-driven CO₂ reduction to formate without the need of any external co-factors or endogenous enzymes in the reaction process. This shifts the industrial platform organism E. coli more and more into the focus as biocatalyst for CO₂-capturing and H₂-storage. KEY POINTS: • A functional HDCR enzyme complex was heterologously produced in E. coli. • The metabolic bottlenecks for HDCR production were identified. • HDCR enabled E. coli cell to capture and store H₂ and CO₂ in the form of formate.

Revealing formate production from carbon monoxide in wild type and mutants of Rnf- and Ech-containing acetogens, Acetobacterium woodii and Thermoanaerobacter kivui

Acetogenic bacteria have gained much attraction in recent years as they can produce different biofuels and biochemicals from H2 plus CO2 or even CO alone, therefore opening a promising alternative route for the production of biofuels from renewable sources compared to existing sugar-based routes. However, CO metabolism still raises questions concerning the biochemistry and bioenergetics in many acetogens. In this study, we focused on the two acetogenic bacteria Acetobacterium woodii and Thermoanaerobacter kivui which, so far, are the only identified acetogens harbouring a H2 -dependent CO2 reductase and furthermore belong to different classes of 'Rnf'- and 'Ech-acetogens'. Both strains catalysed the conversion of CO into the bulk chemical acetate and formate. Formate production was stimulated by uncoupling the energy metabolism from the Wood-Ljungdahl pathway, and specific rates of 1.44 and 1.34 mmol g-1  h-1 for A. woodii ∆rnf and T. kivui wild type were reached. The demonstrated CO-based formate production rates are, to the best of our knowledge, among the highest rates ever reported. Using mutants of ∆hdcr, ∆cooS, ∆hydBA, ∆rnf and ∆ech2 with deficiencies in key enzyme activities of the central metabolism enabled us to postulate two different CO utilization pathways in these two model organisms.

Whole-cell biocatalysis for hydrogen storage and syngas conversion to formate using a thermophilic acetogen

BACKGROUND

In times of global climate change, the conversion and capturing of inorganic CO₂ have gained increased attention because of its great potential as sustainable feedstock in the production of biofuels and biochemicals. CO₂ is not only the substrate for the production of value-added chemicals in CO₂-based bioprocesses, it can also be directly hydrated to formic acid, a so-called liquid organic hydrogen carrier (LOHC), by chemical and biological catalysts. Recently, a new group of enzymes were discovered in the two acetogenic bacteria Acetobacterium woodii and Thermoanaerobacter kivui which catalyze the direct hydrogenation of CO₂ to formic acid with exceptional high rates, the hydrogen-dependent CO₂ reductases (HDCRs). Since these enzymes are promising biocatalysts for the capturing of CO₂ and the storage of molecular hydrogen in form of formic acid, we designed a whole-cell approach for T. kivui to take advantage of using whole cells from a thermophilic organism as H₂/CO₂ storage platform. Additionally, T. kivui cells were used as microbial cell factories for the production of formic acid from syngas.

RESULTS

This study demonstrates the efficient whole-cell biocatalysis for the conversion of H₂ + CO₂ to formic acid in the presence of bicarbonate by T. kivui. Interestingly, the addition of KHCO₃ not only stimulated formate formation dramatically but it also completely abolished unwanted side product formation (acetate) under these conditions and bicarbonate was shown to inhibit the membrane-bound ATP synthase. Cell suspensions reached specific formate production rates of 234 mmol g_protein ^-1 h^-1 (152 mmol g_CDW ^-1 h^-1), the highest rates ever reported in closed-batch conditions. The volumetric formate production rate was 270 mmol L^-1 h^-1 at 4 mg mL^-1. Additionally, this study is the first demonstration that syngas can be converted exclusively to formate using an acetogenic bacterium and high titers up to 130 mM of formate were reached.

CONCLUSIONS

The thermophilic acetogenic bacterium T. kivui is an efficient biocatalyst which makes this organism a promising candidate for future biotechnological applications in hydrogen storage, CO₂ capturing and syngas conversion to formate.

Methods for quantification of growth and productivity in anaerobic microbiology and biotechnology

Anaerobic microorganisms (anaerobes) possess a fascinating metabolic versatility. This characteristic makes anaerobes interesting candidates for physiological studies and utilizable as microbial cell factories. To investigate the physiological characteristics of an anaerobic microbial population, yield, productivity, specific growth rate, biomass production, substrate uptake, and product formation are regarded as essential variables. The determination of those variables in distinct cultivation systems may be achieved by using different techniques for sampling, measuring of growth, substrate uptake, and product formation kinetics. In this review, a comprehensive overview of methods is presented, and the applicability is discussed in the frame of anaerobic microbiology and biotechnology.

Energy conservation by a hydrogenase-dependent chemiosmotic mechanism in an ancient metabolic pathway

The ancient reductive acetyl-CoA pathway is employed by acetogenic bacteria to form acetate from inorganic energy sources. Since the central pathway does not gain net ATP by substrate-level phosphorylation, chemolithoautotrophic growth relies on the additional formation of ATP via a chemiosmotic mechanism. Genome analyses indicated that some acetogens only have an energy-converting, ion-translocating hydrogenase (Ech) as a potential respiratory enzyme. Although the Ech-encoding genes are widely distributed in nature, the proposed function of Ech as an ion-translocating chemiosmotic coupling site has neither been demonstrated in bacteria nor has it been demonstrated that it can be the only energetic coupling sites in microorganisms that depend on a chemiosmotic mechanism for energy conservation. Here, we show that the Ech complex of the thermophilic acetogenic bacterium Thermoanaerobacter kivui is indeed a respiratory enzyme. Experiments with resting cells prepared from T. kivui cultures grown on carbon monoxide (CO) revealed CO oxidation coupled to H₂ formation and the generation of a transmembrane electrochemical ion gradient ([Formula: see text]). Inverted membrane vesicles (IMVs) prepared from CO-grown cells also produced H₂ and ATP from CO (via a loosely attached CO dehydrogenase) or a chemical reductant. Finally, we show that Ech activity led to the translocation of both H⁺ and Na⁺ across the membrane of the IMVs. The H⁺ gradient was then used by the ATP synthase for energy conservation. These data demonstrate that the energy-converting hydrogenase in concert with an ATP synthase may be the simplest form of respiration; it combines carbon dioxide fixation with the synthesis of ATP in an ancient pathway.

Bioenergetic aspects of archaeal and bacterial hydrogen metabolism

Hydrogenases are metal-containing biocatalysts that reversibly convert protons and electrons to hydrogen gas. This reaction can contribute in different ways to the generation of the proton motive force (PMF) of a cell. One means of PMF generation involves reduction of protons on the inside of the cytoplasmic membrane, releasing H₂ gas, which being without charge is freely diffusible across the cytoplasmic membrane, where it can be re-oxidized to release protons. A second route of PMF generation couples transfer of electrons derived from H₂ oxidation to quinone reduction and concomitant proton uptake at the membrane-bound heme cofactor. This redox-loop mechanism, as originally formulated by Mitchell, requires a second, catalytically distinct, enzyme complex to re-oxidize quinol and release the protons outside the cell. A third way of generating PMF is also by electron transfer to quinones but on the outside of the membrane while directly drawing protons through the entire membrane. The cofactor-less membrane subunits involved are proposed to operate by a conformational mechanism (redox-linked proton pump). Finally, PMF can be generated through an electron bifurcation mechanism, whereby an exergonic reaction is tightly coupled with an endergonic reaction. In all cases the protons can be channelled back inside through a F₁F₀-ATPase to convert the 'energy' stored in the PMF into the universal cellular energy currency, ATP. New and exciting discoveries employing these mechanisms have recently been made on the bioenergetics of hydrogenases, which will be discussed here and placed in the context of their contribution to energy conservation.

Biohydrogen production of obligate anaerobic archaeon Thermococcus onnurineus NA1 under oxic conditions via overexpression of frhAGB-encoding hydrogenase genes

BACKGROUND

The production of biohydrogen (H₂) as a promising future fuel in anaerobic hyperthermophiles has attracted great attention because H₂ formation is more thermodynamically feasible at elevated temperatures and fewer undesired side products are produced. However, these microbes require anoxic culture conditions for growth and H₂ production, thereby necessitating costly and time-consuming physical or chemical methods to remove molecular oxygen (O₂). Therefore, the development of an O₂-tolerant strain would be useful for industrial applications.

RESULTS

In this study, we found that the overexpression of frhAGB-encoding hydrogenase genes in Thermococcus onnurineus NA1, an obligate anaerobic archaeon and robust H₂ producer, enhanced O₂ tolerance. When the recombinant FO strain was exposed to levels of O₂ up to 20% in the headspace of a sealed bottle, it showed significant growth. Whole transcriptome analysis of the FO strain revealed that several genes involved in the stress response such as chaperonin β subunit, universal stress protein, peroxiredoxin, and alkyl hydroperoxide reductase subunit C, were significantly up-regulated. The O₂ tolerance of the FO strain enabled it to grow on formate and produce H₂ under oxic conditions, where prior O₂-removing steps were omitted, such as the addition of reducing agent Na₂S, autoclaving, and inert gas purging.

CONCLUSIONS

Via the overexpression of frhAGB genes, the obligate anaerobic archaeon T. onnurineus NA1 gained the ability to overcome the inhibitory effect of O₂. This O₂-tolerant property of the strain may provide another advantage to this hyperthermophilic archaeon as a platform for biofuel H₂ production.

Complex Multimeric [FeFe] Hydrogenases: Biochemistry, Physiology and New Opportunities for the Hydrogen Economy

Hydrogenases are key enzymes of the energy metabolism of many microorganisms. Especially in anoxic habitats where molecular hydrogen (H₂) is an important intermediate, these enzymes are used to expel excess reducing power by reducing protons or they are used for the oxidation of H₂ as energy and electron source. Despite the fact that hydrogenases catalyze the simplest chemical reaction of reducing two protons with two electrons it turned out that they are often parts of multimeric enzyme complexes catalyzing complex chemical reactions with a multitude of functions in the metabolism. Recent findings revealed multimeric hydrogenases with so far unknown functions particularly in bacteria from the class Clostridia. The discovery of [FeFe] hydrogenases coupled to electron bifurcating subunits solved the enigma of how the otherwise highly endergonic reduction of the electron carrier ferredoxin can be carried out and how H₂ production from NADH is possible. Complexes of [FeFe] hydrogenases with formate dehydrogenases revealed a novel enzymatic coupling of the two electron carriers H₂ and formate. These novel hydrogenase enzyme complex could also contribute to biotechnological H₂ production and H₂ storage, both processes essential for an envisaged economy based on H₂ as energy carrier.

Methanol metabolism in the acetogenic bacterium Acetobacterium woodii

Methanol derived from plant tissue is ubiquitous in anaerobic sediments and a good substrate for anaerobes growing on C₁ compounds such as methanogens and acetogens. In contrast to methanogens little is known about the physiology, biochemistry and bioenergetics of methanol utilization in acetogenic bacteria. To fill this gap, we have used the model acetogen Acetobacterium woodii to study methanol metabolism using physiological and biochemical experiments paired with molecular studies and transcriptome analysis. These studies identified the genes and enzymes involved in acetogenesis from methanol and the redox carriers involved. We will present the first comprehensive model for carbon and electron flow from methanol in an acetogen and the bioenergetics of acetogenesis from methanol.

Hydrogenation of CO2 at ambient pressure catalyzed by a highly active thermostable biocatalyst

BACKGROUND

Replacing fossil fuels as energy carrier requires alternatives that combine sustainable production, high volumetric energy density, easy and fast refueling for mobile applications, and preferably low risk of hazard. Molecular hydrogen (H₂) has been considered as promising alternative; however, practical application is struggling because of the low volumetric energy density and the explosion hazard when stored in large amounts. One way to overcome these limitations is the transient conversion of H₂ into other chemicals with increased volumetric energy density and lower risk hazard, for example so-called liquid organic hydrogen carriers such as formic acid/formate that is obtained by hydrogenation of CO₂. Many homogenous and heterogenous chemical catalysts have been described in the past years, however, often requiring high pressures and temperatures. Recently, the first biocatalyst for this reaction has been described opening the route to a biotechnological alternative for this conversion.

RESULTS

The hydrogen-dependent CO₂ reductase (HDCR) is a highly active biocatalyst for storing H₂ in the form of formic acid/formate by reversibly catalyzing the hydrogenation of CO₂. We report the identification, isolation, and characterization of the first thermostable HDCR operating at temperatures up to 70 °C. The enzyme was isolated from the thermophilic acetogenic bacterium Thermoanaerobacter kivui and displays exceptionally high activities in both reaction directions, substantially exceeding known chemical catalysts. CO₂ hydrogenation is catalyzed at mild conditions with a turnover frequency of 9,556,000 h^-1 (specific activity of 900 µmol formate min^-1 mg^-1) and the reverse reaction, H₂ + CO₂ release from formate, is catalyzed with a turnover frequency of 9,892,000 h^-1 (930 µmol H₂ min^-1 mg^-1). The HDCR of T. kivui consists of a [FeFe] hydrogenase subunit putatively coupled to a tungsten-dependent CO₂ reductase/formate dehydrogenase subunit by an array of iron-sulfur clusters.

CONCLUSIONS

The discovery of the first thermostable HDCR provides a promising biological alternative for a chemically challenging reaction and might serve as model for the better understanding of catalysts able to efficiently reduce CO₂. The catalytic activity for reversible CO₂ hydrogenation of this enzyme is the highest activity known for bio- and chemical catalysts and requiring only ambient temperatures and pressures. The thermostability provides more flexibility regarding the process parameters for a biotechnological application.