Catalytic mechanism and application of formate dehydrogenase

Structure of recombinant formate dehydrogenase from Methylobacterium extorquens (MeFDH1)

Formate dehydrogenase (FDH) is critical for the conversion between formate and carbon dioxide. Despite its importance, the structural complexity of FDH and difficulties in the production of the enzyme have made elucidating its unique physicochemical properties challenging. Here, we purified recombinant Methylobacterium extorquens AM1 FDH (MeFDH1) and used cryo-electron microscopy to determine its structure. We resolved a heterodimeric MeFDH1 structure at a resolution of 2.8 Å, showing a noncanonical active site and a well-embedded Fe-S redox chain relay. In particular, the tungsten bis-molybdopterin guanine dinucleotide active site showed an open configuration with a flexible C-terminal cap domain, suggesting structural and dynamic heterogeneity in the enzyme.

Engineering of formate dehydrogenase for improving conversion potential of carbon dioxide to formate

Formate dehydrogenase (FDH) is a D-2-hydroxy acid dehydrogenase, which can reversibly reduce CO2 to formate and thus act as non-photosynthetic CO2 reductase. In order to increase catalytic efficiency of formate dehydrogenase for CO2 reduction, two mutants V328I/F285W and V354G/F285W were obtained of which reduction activity was about two times more than the parent CbFDHM2, and the formate production from CO2 catalyzed by mutants were 2.9 and 2.7-fold higher than that of the parent CbFDHM2. The mutants had greater potential in CO2 reduction. The optimal temperature for V328I/F285W and V354G/F285W was 55 °C, and they showed increasement of relative activity under 45 °C to 55 °C compared with parent. The optimal pH for the mutants was 9.0, and they showed excellent stability in pH 4.0-11.5. The kcat/Km values of mutants were 1.75 times higher than that of the parent. Then the molecular basis for its improvement of biochemical characteristics were preliminarily elucidated by computer-aided methods. All of these results further established a solid foundation for molecular modification of formate dehydrogenase and CO2 reduction.

A Hitchhiker's Guide to Supplying Enzymatic Reducing Power into Synthetic Cells

The construction from scratch of synthetic cells by assembling molecular building blocks is unquestionably an ambitious goal from a scientific and technological point of view. To realize functional life-like systems, minimal enzymatic modules are required to sustain the processes underlying the out-of-equilibrium thermodynamic status hallmarking life, including the essential supply of energy in the form of electrons. The nicotinamide cofactors NAD(H) and NADP(H) are the main electron carriers fueling reductive redox reactions of the metabolic network of living cells. One way to ensure the continuous availability of reduced nicotinamide cofactors in a synthetic cell is to build a minimal enzymatic module that can oxidize an external electron donor and reduce NAD(P)+. In the diverse world of metabolism there is a plethora of potential electron donors and enzymes known from living organisms to provide reducing power to NAD(P)+ coenzymes. This perspective proposes guidelines to enable the reduction of nicotinamide cofactors enclosed in phospholipid vesicles, while avoiding high burdens of or cross-talk with other encapsulated metabolic modules. By determining key requirements, such as the feasibility of the reaction and transport of the electron donor into the cell-like compartment, we select a shortlist of potentially suitable electron donors. We review the most convenient proteins for the use of these reducing agents, highlighting their main biochemical and structural features. Noting that specificity toward either NAD(H) or NADP(H) imposes a limitation common to most of the analyzed enzymes, we discuss the need for specific enzymes─transhydrogenases─to overcome this potential bottleneck.

Life in Multi-Extreme Environments: Brines, Osmotic and Hydrostatic Pressure─A Physicochemical View

Elucidating the details of the formation, stability, interactions, and reactivity of biomolecular systems under extreme environmental conditions, including high salt concentrations in brines and high osmotic and high hydrostatic pressures, is of fundamental biological, astrobiological, and biotechnological importance. Bacteria and archaea are able to survive in the deep ocean or subsurface of Earth, where pressures of up to 1 kbar are reached. The deep subsurface of Mars may host high concentrations of ions in brines, such as perchlorates, but we know little about how these conditions and the resulting osmotic stress conditions would affect the habitability of such environments for cellular life. We discuss the combined effects of osmotic (salts, organic cosolvents) and hydrostatic pressures on the structure, stability, and reactivity of biomolecular systems, including membranes, proteins, and nucleic acids. To this end, a variety of biophysical techniques have been applied, including calorimetry, UV/vis, FTIR and fluorescence spectroscopy, and neutron and X-ray scattering, in conjunction with high pressure techniques. Knowledge of these effects is essential to our understanding of life exposed to such harsh conditions, and of the physical limits of life in general. Finally, we discuss strategies that not only help us understand the adaptive mechanisms of organisms that thrive in such harsh geological settings but could also have important ramifications in biotechnological and pharmaceutical applications.

Bioinformatics-Structural Approach to the Search for New D-Amino Acid Oxidases

D-amino acid oxidase (DAAO, EC 1.2.1.2) plays an important role in the functioning of prokaryotes as well as of lower (yeast and fungi) and higher eukaryotes (mammals). DAAO genes have not yet been found in archaean genomes. D-amino acid oxidase is increasingly used in various fields, which requires the development of new variants of the enzyme with specific properties. However, even within one related group (bacteria, yeasts and fungi, mammals), DAAOs show very low homology between amino acid sequences. In particular, this fact is clearly observed in the case of DAAO from bacteria. The high variability in the primary structures of DAAO severely limits the search for new enzymes in known genomes. As a result, many (if not most) DAAO genes remain either unannotated or incorrectly annotated. We propose an approach that uses bioinformatic methods in combination with general 3D structure and active center structure analysis to confirm that the gene found encodes D-amino acid oxidase and to predict the possible type of its substrate specificity. Using a homology search, we obtained a set of candidate sequences, modelled the tertiary structure of the selected enzymes, and compared them with experimental and model structures of known DAAOs. The effectiveness of the proposed approach for discrimination of DAAOs and glycine oxidases is shown. Using this approach, new DAAO genes were found in the genomes of six strains of extremophilic bacteria, and for the first time in the world, one gene was identified in the genome of halophilic archaea. Preliminary experiments confirmed the predicted specificity of DAAO from Natronosporangium hydrolyticum ACPA39 with D-Leu and D-Phe.

Synergy of metal nanoparticles and organometallic complex in NAD(P)H regeneration via relay hydrogenation

Most, if not all, of the hydrogenation reactions are catalyzed by organometallic complexes (M) or heterogeneous metal catalysts, but to improve both the activity and selectivity simultaneously in one reaction via a rational combination of the two types of catalysts remains largely unexplored. In this work, we report a hydrogenation mode though H species relay from supported metal nanoparticles (NPs) to M, where the former is responsible for H2 dissociation, and M is for further hydride transferring to reactants. The synergy between metal NPs and M yields an efficient NAD(P)H regeneration system with >99% selectivity and a magnitude higher activity than the corresponding metal NPs and M. The modularizing of hydrogenation reaction into hydrogen activation with metal NPs and substrate activation with metal complex paves a new way to rationally address the challenging hydrogenation reactions.

Delayed In Vivo Encapsulation of Enzymes Alters the Catalytic Activity of Virus-Like Particle Nanoreactors

Encapsulation of enzymes inside protein cage structures, mimicking protein-based organelle structures found in nature, has great potential for the development of new catalytic materials with enhanced properties. In vitro and in vivo methodologies have been developed for the encapsulation of enzymes within protein cage structures of several types, particularly virus-like particles (VLPs), with the ability to retain the activity of the encapsulated enzymes. Here, we examine the in vivo encapsulation of enzymes within the bacteriophage P22 derived VLP and show that some enzymes may require a delay in encapsulation to allow proper folding and maturation before they can be encapsulated inside P22 as fully active enzymes. Using a sequential expression strategy, where enzyme cargoes are first expressed, allowed to fold, and later encapsulated by the expression of the P22 coat protein, altered enzymatic activities are obtained in comparison to enzymes encapsulated in P22 VLPs using a simultaneous coexpression strategy. The strategy and results discussed here highlight important considerations for researchers investigating the encapsulation of enzymes inside confined reaction environments via in vivo routes and provide a potential solution for those that have been unable to produce active enzymes upon encapsulation.

Minimal Pathway for the Regeneration of Redox Cofactors

Effective metabolic pathways are essential for the construction of in vitro systems mimicking the biochemical complexity of living cells. Such pathways require the inclusion of a metabolic branch that ensures the availability of reducing equivalents. Here, we built a minimal enzymatic pathway confinable in the lumen of liposomes, in which the redox status of the nicotinamide cofactors NADH and NADPH is controlled by an externally provided formate. Formic acid permeates the membrane where a luminal formate dehydrogenase uses NAD⁺ to form NADH and carbon dioxide. Carbon dioxide diffuses out of the liposomes, leaving only the reducing equivalents in the lumen. A soluble transhydrogenase subsequently utilizes NADH for reduction of NADP⁺ thereby making NAD⁺ available again for the first reaction. The pathway is functional in liposomes ranging from a few hundred nanometers in diameter (large unilamellar vesicles) up to several tens of micrometers (giant unilamellar vesicles) and remains active over a period of 7 days. We demonstrate that the downstream biochemical process of reduction of glutathione disulfide can be driven by the transfer of reducing equivalents from formate via NAD(P)H, thereby providing a versatile set of electron donors for reductive metabolism.

Insights into the Biosynthesis of Nanoparticles by the Genus Shewanella

The exploitation of microorganisms for the fabrication of nanoparticles (NPs) has garnered considerable research interest globally. The microbiological transformation of metals and metal salts into respective NPs can be achieved under environmentally benign conditions, offering a more sustainable alternative to chemical synthesis methods. Species of the metal-reducing bacterial genus Shewanella are able to couple the oxidation of various electron donors, including lactate, pyruvate, and hydrogen, to the reduction of a wide range of metal species, resulting in biomineralization of a multitude of metal NPs. Single-metal-based NPs as well as composite materials with properties equivalent or even superior to physically and chemically produced NPs have been synthesized by a number of Shewanella species. A mechanistic understanding of electron transfer-mediated bioreduction of metals into respective NPs by Shewanella is crucial in maximizing NP yields and directing the synthesis to produce fine-tuned NPs with tailored properties. In addition, thorough investigations into the influence of process parameters controlling the biosynthesis is another focal point for optimizing the process of NP generation. Synthesis of metal-based NPs using Shewanella species offers a low-cost, eco-friendly alternative to current physiochemical methods. This article aims to shed light on the contribution of Shewanella as a model organism in the biosynthesis of a variety of NPs and critically reviews the current state of knowledge on factors controlling their synthesis, characterization, potential applications in different sectors, and future prospects.

Genomes of Thaumarchaeota from deep sea sediments reveal specific adaptations of three independently evolved lineages

Marine sediments represent a vast habitat for complex microbiomes. Among these, ammonia oxidizing archaea (AOA) of the phylum Thaumarchaeota are one of the most common, yet little explored, inhabitants, which seem extraordinarily well adapted to the harsh conditions of the subsurface biosphere. We present 11 metagenome-assembled genomes of the most abundant AOA clades from sediment cores obtained from the Atlantic Mid-Ocean ridge flanks and Pacific abyssal plains. Their phylogenomic placement reveals three independently evolved clades within the order Nitrosopumilales, of which no cultured representative is known yet. In addition to the gene sets for ammonia oxidation and carbon fixation known from other AOA, all genomes encode an extended capacity for the conversion of fermentation products that can be channeled into the central carbon metabolism, as well as uptake of amino acids probably for protein maintenance or as an ammonia source. Two lineages encode an additional (V-type) ATPase and a large repertoire of DNA repair systems that may allow to overcome the challenges of high hydrostatic pressure. We suggest that the adaptive radiation of AOA into marine sediments occurred more than once in evolution and resulted in three distinct lineages with particular adaptations to this extremely energy-limiting and high-pressure environment.

Immobilization of formate dehydrogenase in metal organic frameworks for enhanced conversion of carbon dioxide to formate

Hydrogenation of carbon dioxide (CO₂) to formic acid by the enzyme formate dehydrogenase (FDH) is a promising technology for reducing CO₂ concentrations in an environmentally friendly manner. However, the easy separation of FDH with enhanced stability and reusability is essential to the practical and economical implementation of the process. To achieve this, the enzyme must be used in an immobilized form. However, conventional immobilization by physical adsorption is prone to leaching, resulting in low stability. Although other immobilization methods (such as chemical adsorption) enhance stability, they generally result in low activity. In addition, mass transfer limitations are a major problem with most conventional immobilized enzymes. In this review paper, the effectiveness of metal organic frameworks (MOFs) is assessed as a promising alternative support for FDH immobilization. Kinetic mechanisms and stability of wild FDH from various sources were assessed and compared to those of cloned and genetically modified FDH. Various techniques for the synthesis of MOFs and different immobilization strategies are presented, with special emphasis on in situ and post synthetic immobilization of FDH in MOFs for CO₂ hydrogenation.

Ustilaginaceae Biocatalyst for Co-Metabolism of CO2-Derived Substrates toward Carbon-Neutral Itaconate Production

The family Ustilaginaceae (belonging to the smut fungi) are known for their plant pathogenicity. Despite the fact that these plant diseases cause agricultural yield reduction, smut fungi attracted special attention in the field of industrial biotechnology. Ustilaginaceae show a versatile product spectrum such as organic acids (e.g., itaconate, malate, succinate), polyols (e.g., erythritol, mannitol), and extracellular glycolipids, which are considered value-added chemicals with potential applications in the pharmaceutical, food, and chemical industries. This study focused on itaconate as a platform chemical for the production of resins, plastics, adhesives, and biofuels. During this work, 72 different Ustilaginaceae strains from 36 species were investigated for their ability to (co-) consume the CO₂-derived substrates acetate and formate, potentially contributing toward a carbon-neutral itaconate production. The fungal growth and product spectrum with special interest in itaconate was characterized. Ustilago maydis MB215 and Ustilago rabenhorstiana NBRC 8995 were identified as promising candidates for acetate metabolization whereas Ustilago cynodontis NBRC 7530 was identified as a potential production host using formate as a co-substrate enhancing the itaconate production. Selected strains with the best itaconate production were characterized in more detail in controlled-batch bioreactor experiments confirming the co-substrate utilization. Thus, a proof-of-principle study was performed resulting in the identification and characterization of three promising Ustilaginaceae biocatalyst candidates for carbon-neutral itaconate production contributing to the biotechnological relevance of Ustilaginaceae.

Proteomic comparison of biofilm vs. planktonic Staphylococcus epidermidis cells suggests key metabolic differences between these conditions

Previous studies have shown that biofilm-forming bacteria are deficient in tricarboxylic acid (TCA) cycle metabolites, suggesting a relationship between these cellular processes. In this work, we compared the proteomes of planktonic vs biofilm cells from a clinical strain of Staphylococcus epidermidis using LC-MS/MS. A total of 168 proteins were identified from both growth conditions. The biofilm cells showed enrichment of proteins participating in glycolysis for the formation of pyruvate; however, the absence of TCA cycle proteins and the presence of lactate dehydrogenase, formate acetyltransferase, and acetoin reductase suggested that pyruvate was catabolized to their respective products: lactate, formate and acetoin. On the other hand, planktonic cells showed proteins participating in glycolysis and the TCA cycle, the pentose phosphate pathway, gluconeogenesis, ATP generation and the oxidative stress response. Functional networks with higher interconnection were predicted for planktonic proteins. We propose that in S. epidermidis, the relative absence of TCA cycle proteins is associated with the formation of biofilms and that lactate, formate and acetoin are the end products of partial glucose metabolism.

Insights into palladium nanoparticles produced by Shewanella oneidensis MR-1: Roles of NADH dehydrogenases and hydrogenases

Biologically synthesized palladium nanoparticles (bio-Pd) have attracted considerable interest as promising green catalysts for environmental remediation. However, the mechanisms by which microorganisms produce bio-Pd remain unclear. In the present study, we investigated the roles of Shewanella oneidensis MR-1 and its NADH dehydrogenases and hydrogenases (HydA and HyaB) in bio-Pd production using formate as the electron donor. The roles of NADH dehydrogenases and hydrogenases were studied by inhibiting NADH dehydrogenases and using hydrogenase mutants (ΔhydA, ΔhyaB, and ΔhydAΔhyaB), respectively. The results showed ~97% reduction of palladium by S. oneidensis MR-1 after 24 h using 250 μM palladium and 500 μM formate. Electron microscopy images showed the presence of bio-Pd on both the outer and cytoplasmic membranes of S. oneidensis MR-1. However, the inhibition of NADH dehydrogenases in S. oneidensis MR-1 resulted in only ~61% reduction of palladium after 24 h, and bio-Pd were not found on the outer membrane. The mutants lacking one or two hydrogenases removed 91-96% of palladium ions after 24 h and showed more cytoplasmic bio-Pd but less periplasmic bio-Pd. To the best of our knowledge, this is the first study to demonstrate the role of NADH dehydrogenases of S. oneidensis MR-1 in the formation of bio-Pd on the outer membrane. It also demonstrates that the hydrogenases (especially HyaB) of S. oneidensis MR-1 contribute to the formation of bio-Pd in the periplasmic space. This study provides mechanistic insights into the production of biogenic metal nanoparticles towards their possible use in industrial and environmental applications.

Structure-Guided Design of Formate Dehydrogenase for Regeneration of a Non-Natural Redox Cofactor

Formate dehydrogenase (FDH) has been widely used for the regeneration of the reduced nicotinamide adenine dinucleotide (NADH). To utilize nicotinamide cytosine dinucleotide (NCD) as a non-natural redox cofactor, it remains challenging as NCDH, the reduced form of NCD, has to be efficiently regenerated. Here we demonstrate successful engineering of FDH for NCDH regeneration. Guided by the structural information of FDH from Pseudomonas sp. 101 (pseFDH) and the NAD-pseFDH complex, semi-rational strategies were applied to design mutant libraries and screen for NCD-linked activity. The most active mutant reached a cofactor preference switch from NAD to NCD by 3700-fold. Homology modeling analysis showed that these mutants had reduced cofactor binding pockets and dedicated hydrophobic interactions for NCD. Efficient regeneration of NCDH was implemented by powering an NCD-dependent D-lactate dehydrogenase for stoichiometric and stereospecific reduction of pyruvate to D-lactate at the expense of formate.

Role of Protein Motions in Catalysis by Formate Dehydrogenase

We have analyzed the reaction catalyzed by formate dehydrogenase using transition path sampling. This system has recently received experimental attention using infrared spectroscopy and heavy-enzyme studies. Some of the experimental results point to the possible importance of protein motions that are coupled to the chemical step. We found that the residue Val123 that lies behind the nicotinamide ring occasionally comes into van der Waals contact with the acceptor and that in all reactive trajectories, the barrier-crossing event is preceded by this contact, meaning that the motion of Val123 is part of the reaction coordinate. Experimental results have been interpreted with a two-dimensional formula for the chemical rate, which cannot capture effects such as the one we describe.

Effect of Metal Ions on the Activity of Ten NAD-Dependent Formate Dehydrogenases

NAD-dependent formate dehydrogenase (FDH) enzymes are frequently used in industrial and scientific applications. FDH is a reversible enzyme that reduces the NAD molecule to NADH and produces CO₂ by oxidation of the formate ion, whereas it causes CO₂ reduction in the reverse reaction. Some transition metal elements - Fe³⁺, Mo⁶⁺ and W^{6 +} - can be found in the FDH structure of anaerobic and archaeal microorganisms, and these enzymes require cations and other redox-active cofactors for their FDH activity. While NAD-dependent FDHs do not necessarily require any metal cations, the presence of various metal cations can still affect FDH activities. To study the effect of 11 different metal ions, NAD-dependent FDH enzymes from ten different microorganisms were tested: Ancylobacter aquaticus (AaFDH), Candida boidinii (CboFDH), Candida methylica (CmFDH), Ceriporiopsis subvermispora (CsFDH), Chaetomium thermophilum (CtFDH), Moraxella sp. (MsFDH), Myceliophthora thermophila (MtFDH), Paracoccus sp. (PsFDH), Saccharomyces cerevisiae (ScFDH) and Thiobacillus sp. (TsFDH). It was found that metal ions (mainly Cu²⁺ and Zn²⁺) could have quite strong inhibition effects on several enzymes in the forward reaction, whereas several cations (Li⁺, Mg²⁺, Mn²⁺, Fe³⁺ and W⁶⁺) could increase the forward reaction of two FDHs. The highest activity increase (1.97 fold) was caused by Fe³⁺ in AaFDH. The effect on the reverse reaction was minimal. The modelled structures of ten FDHs showed that the active site is formed by 15 highly conserved amino acid residues spatially settling around the formate binding site in a conserved way. However, the residue differences at some of the sites close to the substrate do not explain the activity differences. The active site space is very tight, excluding water molecules, as observed in earlier studies. Structural examination indicated that smaller metal ions might be spaced close to the active site to affect the reaction. Metal ion size showed partial correlation to the effect on inhibition or activation. Affinity of the substrate may also affect the sensitivity to the metal's effect. In addition, amino acid differences on the protein surface may also be important for the metal ion effect.

Conserved Amino Acid Residues that Affect Structural Stability of Candida boidinii Formate Dehydrogenase

The NAD⁺-dependent formate dehydrogenase (FDH; EC 1.2.1.2) from Candida boidinii (CboFDH) has been extensively used in NAD(H)-dependent industrial biocatalysis as well as in the production of renewable fuels and chemicals from carbon dioxide. In the present work, the effect of amino acid residues Phe285, Gln287, and His311 on structural stability was investigated by site-directed mutagenesis. The wild-type and mutant enzymes (Gln287Glu, His311Gln, and Phe285Thr/His311Gln) were cloned and expressed in Escherichia coli. Circular dichroism (CD) spectroscopy was used to determine the effect of each mutation on thermostability. The results showed the decisive roles of Phe285, Gln287, and His311 on enhancing the enzyme's thermostability. The melting temperatures for the wild-type and the mutant enzymes Gln287Glu, His311Gln, and Phe285Thr/His311Gln were 64, 70, 77, and 73 °C, respectively. The effects of pH and temperature on catalytic activity of the wild-type and mutant enzymes were also investigated. Interestingly, the mutant enzyme His311Gln exhibits a large shift of pH optimum at the basic pH range (1 pH unit) and substantial increase of the optimum temperature (25 °C). The present work supports the multifunctional role of the conserved residues Phe285, Gln287, and His311 and further underlines their pivotal roles as targets in protein engineering studies.

Highly-Active Recombinant Formate Dehydrogenase from Pathogenic Bacterium Staphylococcus aureus: Preparation and Crystallization

# These authors contributed equally to the work. NAD+-dependent formate dehydrogenase from Staphylococcus aureus (SauFDH) is one of the key enzymes responsible for the survival of this pathogen in the form of biofilms. 3D structure of the enzyme might be helpful in the search for highly specific SauFDH inhibitors that can be used as antibacterial agents exactly against S. aureus biofilms. Here, we prepared a recombinant SauFDH in Escherichia coli cells with a yield of 1 g target protein per liter medium. The developed procedure for the enzyme purification allowed to obtain 400 mg of homogenous enzyme with 61% yield. The specific activity of the purified recombinant SauFDH was 20 U per mg protein, which was 2 times higher than the previously reported activities of formate dehydrogenases. We also found crystallization conditions in the course of two rounds of optimization and obtained 200- and 40-µm crystals for the SauFDH apo- and holoenzymes, respectively. X-ray analysis using synchrotron X-ray sources produced diffraction data sufficient for solving the three-dimensional structures of the apo- and holoenzymes with the resolution of 2.2 and 2.7 Å, respectively. Crystals of the apo- and holoforms of SauFDH had different crystal space groups, which suggest coenzyme binding in the SauFDH holoenzyme.

Growth of E. coli on formate and methanol via the reductive glycine pathway

Engineering a biotechnological microorganism for growth on one-carbon intermediates, produced from the abiotic activation of CO₂, is a key synthetic biology step towards the valorization of this greenhouse gas to commodity chemicals. Here we redesign the central carbon metabolism of the model bacterium Escherichia coli for growth on one-carbon compounds using the reductive glycine pathway. Sequential genomic introduction of the four metabolic modules of the synthetic pathway resulted in a strain capable of growth on formate and CO₂ with a doubling time of ~70 h and growth yield of ~1.5 g cell dry weight (gCDW) per mol-formate. Short-term evolution decreased doubling time to less than 8 h and improved biomass yield to 2.3 gCDW per mol-formate. Growth on methanol and CO₂ was achieved by further expression of a methanol dehydrogenase. Establishing synthetic formatotrophy and methylotrophy, as demonstrated here, paves the way for sustainable bioproduction rooted in CO₂ and renewable energy.

Cofactor F420-Dependent Enzymes: An Under-Explored Resource for Asymmetric Redox Biocatalysis

The asymmetric reduction of enoates, imines and ketones are among the most important reactions in biocatalysis. These reactions are routinely conducted using enzymes that use nicotinamide cofactors as reductants. The deazaflavin cofactor F420 also has electrochemical properties that make it suitable as an alternative to nicotinamide cofactors for use in asymmetric reduction reactions. However, cofactor F420-dependent enzymes remain under-explored as a resource for biocatalysis. This review considers the cofactor F420-dependent enzyme families with the greatest potential for the discovery of new biocatalysts: the flavin/deazaflavin-dependent oxidoreductases (FDORs) and the luciferase-like hydride transferases (LLHTs). The characterized F420-dependent reductions that have the potential for adaptation for biocatalysis are discussed, and the enzymes best suited for use in the reduction of oxidized cofactor F420 to allow cofactor recycling in situ are considered. Further discussed are the recent advances in the production of cofactor F420 and its functional analog FO-5′-phosphate, which remains an impediment to the adoption of this family of enzymes for industrial biocatalytic processes. Finally, the prospects for the use of this cofactor and dependent enzymes as a resource for industrial biocatalysis are discussed.

Multienzymatic in situ hydrogen peroxide generation cascade for peroxygenase-catalysed oxyfunctionalisation reactions

There is an increasing interest in the application of peroxygenases in biocatalysis, because of their ability to catalyse the oxyfunctionalisation reaction in a stereoselective fashion and with high catalytic efficiencies, while using hydrogen peroxide or organic peroxides as oxidant. However, enzymes belonging to this class exhibit a very low stability in the presence of peroxides. With the aim of bypassing this fast and irreversible inactivation, we study the use of a gradual supply of hydrogen peroxide to maintain its concentration at stoichiometric levels. In this contribution, we report a multienzymatic cascade for in situ generation of hydrogen peroxide. In the first step, in the presence of NAD+ cofactor, formate dehydrogenase from Candida boidinii (FDH) catalysed the oxidation of formate yielding CO2. Reduced NADH was reoxidised by the reduction of the flavin mononucleotide cofactor bound to an old yellow enzyme homologue from Bacillus subtilis (YqjM), which subsequently reacts with molecular oxygen yielding hydrogen peroxide. Finally, this system was coupled to the hydroxylation of ethylbenzene reaction catalysed by an evolved peroxygenase from Agrocybe aegerita (rAaeUPO). Additionally, we studied the influence of different reaction parameters on the performance of the cascade with the aim of improving the turnover of the hydroxylation reaction.

Addition of formate dehydrogenase increases the production of renewable alkane from an engineered metabolic pathway

An engineered metabolic pathway consisting of reactions that convert fatty acids to aldehydes and eventually alkanes would provide a means to produce biofuels from renewable energy sources. The enzyme aldehyde-deformylating oxygenase (ADO) catalyzes the conversion of aldehydes and oxygen to alkanes and formic acid and uses oxygen and a cellular reductant such as ferredoxin (Fd) as co-substrates. In this report, we aimed to increase ADO-mediated alkane production by converting an unused by-product, formate, to a reductant that can be used by ADO. We achieved this by including the gene (fdh), encoding formate dehydrogenase from Xanthobacter sp. 91 (XaFDH), into a metabolic pathway expressed in Escherichia coli Using this approach, we could increase bacterial alkane production, resulting in a conversion yield of ∼50%, the highest yield reported to date. Measuring intracellular nicotinamide concentrations, we found that E. coli cells harboring XaFDH have a significantly higher concentration of NADH and a higher NADH/NAD⁺ ratio than E. coli cells lacking XaFDH. In vitro analysis disclosed that ferredoxin (flavodoxin):NADP⁺ oxidoreductase could use NADH to reduce Fd and thus facilitate ADO-mediated alkane production. As formic acid can decrease the cellular pH, the addition of formate dehydrogenase could also maintain the cellular pH in the neutral range, which is more suitable for alkane production. We conclude that this simple, dual-pronged approach of increasing NAD(P)H and removing extra formic acid is efficient for increasing the production of renewable alkanes via synthetic biology-based approaches.

Enzyme-Based Electrobiotechnological Synthesis

Oxidoreductases are enzymes with a high potential for organic synthesis, as their selectivity often exceeds comparable chemical syntheses. The biochemical cofactors of these enzymes need regeneration during synthesis. Several regeneration methods are available but the electrochemical approach offers an efficient and quasi mass-free method for providing the required redox equivalents. Electron transfer systems involving direct regeneration of natural and artificial cofactors, indirect electrochemical regeneration via a mediator, and indirect electroenzymatic cofactor regeneration via enzyme and mediator have been investigated. This chapter gives an overview of electroenzymatic syntheses with oxidoreductases, structured by the enzyme subclass and their usage of cofactors for electron relay. Particular attention is given to the productivity of electroenzymatic biotransformation processes. Because most electroenzymatic syntheses suffer from low productivity, we discuss reaction engineering concepts to overcome the main limiting factors, with a focus on media conductivity optimization, approaches to prevent enzyme inactivation, and the application of advanced cell designs. Graphical Abstract.

Formate Oxidase (FOx) from Aspergillus oryzae: One Catalyst Enables Diverse H2 O2 -Dependent Biocatalytic Oxidation Reactions

An increasing number of biocatalytic oxidation reactions rely on H2 O2 as a clean oxidant. The poor robustness of most enzymes towards H2 O2 , however, necessitates more efficient systems for in situ H2 O2 generation. In analogy to the well-known formate dehydrogenase to promote NADH-dependent reactions, we here propose employing formate oxidase (FOx) to promote H2 O2 -dependent enzymatic oxidation reactions. Even under non-optimised conditions, high turnover numbers for coupled FOx/peroxygenase catalysis were achieved.

Classification, substrate specificity and structural features of D-2-hydroxyacid dehydrogenases: 2HADH knowledgebase

BACKGROUND

The family of D-isomer specific 2-hydroxyacid dehydrogenases (2HADHs) contains a wide range of oxidoreductases with various metabolic roles as well as biotechnological applications. Despite a vast amount of biochemical and structural data for various representatives of the family, the long and complex evolution and broad sequence diversity hinder functional annotations for uncharacterized members.

RESULTS

We report an in-depth phylogenetic analysis, followed by mapping of available biochemical and structural data on the reconstructed phylogenetic tree. The analysis suggests that some subfamilies comprising enzymes with similar yet broad substrate specificity profiles diverged early in the evolution of 2HADHs. Based on the phylogenetic tree, we present a revised classification of the family that comprises 22 subfamilies, including 13 new subfamilies not studied biochemically. We summarize characteristics of the nine biochemically studied subfamilies by aggregating all available sequence, biochemical, and structural data, providing comprehensive descriptions of the active site, cofactor-binding residues, and potential roles of specific structural regions in substrate recognition. In addition, we concisely present our analysis as an online 2HADH enzymes knowledgebase.

CONCLUSIONS

The knowledgebase enables navigation over the 2HADHs classification, search through collected data, and functional predictions of uncharacterized 2HADHs. Future characterization of the new subfamilies may result in discoveries of enzymes with novel metabolic roles and with properties beneficial for biotechnological applications.

DMSO tolerant NAD(P)H recycler enzyme from a pathogenic bacterium, Burkholderia dolosa PC543: effect of N-/C-terminal His Tag extension on protein solubility and activity

NAD(P)⁺ dependent formate dehydrogenase (FDH) is an oxidoreductase used as a biocatalyst to regenerate NAD(P)H in reductase-mediated chiral synthesis reactions. Solvent stability and the need to reduce NADP⁺ to NADPH, due to the high cost of NADPH, are required features in the industrial usage of FDHs. Therefore, we aimed to identify a novel, robust NADP⁺ dependent FDH and evaluate the effect of N- and C- terminus His tag extensions on protein solubility and activity. Herein, we report a novel, DMSO tolerant formate dehydrogenase (BdFDH), which has dual coenzyme specificity and tolerance to acidic pH, from Burkholderia dolosa PC543. N- and C-terminus His-tagged BdFDHs were expressed separately in Escherichia coli BL21 (DE3). The C-terminal His-tagged BdFDH was soluble and active whereas the N-terminal version was not. The enzyme displays dual coenzyme specificity and resistance to some organic solvents, particularly DMSO, and is able to tolerate acidic pH conditions. The apparent K_M values for NADP⁺, NAD⁺ and sodium formate (with NADP⁺), are 1.17, 14.7 and 5.66 mM, respectively. As a result, due to its DMSO tolerance and coenzyme preference, this enzyme can be utilized as an NAD(P)H recycler in several biotransformations particularly when carried out under acidic conditions. Moreover, it can be said that the position of the His tag extension may affect the enzyme solubility and functionality.

Bacillus megaterium adapts to acid stress condition through a network of genes: Insight from a genome-wide transcriptome analysis

RNA-seq analysis of B. megaterium exposed to pH 7.0 and pH 4.5 showed differential expression of 207 genes related to several processes. Among the 207 genes, 11 genes displayed increased transcription exclusively in pH 4.5. Exposure to pH 4.5 induced the expression of genes related to maintenance of cell integrity, pH homeostasis, alternative energy generation and modification of metabolic processes. Metabolic processes like pentose phosphate pathway, fatty acid biosynthesis, cysteine and methionine metabolism and synthesis of arginine and proline were remodeled during acid stress. Genes associated with oxidative stress and osmotic stress were up-regulated at pH 4.5 indicating a link between acid stress and other stresses. Acid stress also induced expression of genes that encoded general stress-responsive proteins as well as several hypothetical proteins. Our study indicates that a network of genes aid B. megaterium G18 to adapt and survive in acid stress condition.

Providing reducing power by microalgal photosynthesis: a novel perspective towards sustainable biocatalytic production of bulk chemicals exemplified for aliphatic amines

A biotechnological process is reported, which enables an enzymatic reduction without the need for addition of an organic co-substrate for in situ-cofactor recycling. The process is based on merging the fields of enzymatic reductive amination with formate dehydrogenase-based in situ-cofactor recycling and algae biotechnology by means of the photoautotrophic microorganism Chlamydomonas reinhardtii, providing the needed formate in situ by formation from carbon dioxide, water and light. This biotransformation has been exemplified for the synthesis of various aliphatic amines known as bulk chemicals.

Discovery of an acidic, thermostable and highly NADP+ dependent formate dehydrogenase from Lactobacillus buchneri NRRL B-30929

OBJECTIVES

To identify a robust NADP⁺ dependent formate dehydrogenase from Lactobacillus buchneri NRRL B-30929 (LbFDH) with unique biochemical properties.

RESULTS

A new NADP⁺ dependent formate dehydrogenase gene (fdh) was cloned from genomic DNA of L. buchneri NRRL B-30929. The recombinant construct was expressed in Escherichia coli BL21(DE3) with 6 × histidine at the C-terminus and the purified protein obtained as a single band of approx. 44 kDa on SDS-PAGE and 90 kDa on native-PAGE. The LbFDH was highly active at acidic conditions (pH 4.8-6.2). Its optimum temperature was 60 °C and 50 °C with NADP⁺ and NAD⁺, respectively and its T_m value was 78 °C. Its activity did not decrease after incubation in a solution containing 20% of DMSO and acetonitrile for 6 h. The K_M constants were 49.8, 0.12 and 1.68 mM for formate (with NADP⁺), NADP⁺ and NAD⁺, respectively.

CONCLUSIONS

An NADP⁺ dependent FDH from L. buchneri NRRL B-30929 was cloned, expressed and identified with its unusual characteristics. The LbFDH can be a promising candidate for NADPH regeneration through biocatalysis requiring acidic conditions and high temperatures.

Discovery of a new metal and NAD+-dependent formate dehydrogenase from Clostridium ljungdahlii

Over the next decades, with the growing concern of rising atmospheric carbon dioxide (CO₂) levels, the importance of investigating new approaches for its reduction becomes crucial. Reclamation of CO₂ for conversion into biofuels represents an alternative and attractive production method that has been studied in recent years, now with enzymatic methods gaining more attention. Formate dehydrogenases (FDHs) are NAD(P)H-dependent oxidoreductases that catalyze the conversion of formate into CO₂ and have been extensively used for cofactor recycling in chemoenzymatic processes. A new FDH from Clostridium ljungdahlii (ClFDH) has been recently shown to possess activity in the reverse reaction: the mineralization of CO₂ into formate. In this study, we show the successful homologous expression of ClFDH in Escherichia coli. Biochemical and kinetic characterization of the enzyme revealed that this homologue also demonstrates activity toward CO₂ reduction. Structural analysis of the enzyme through homology modeling is also presented.

Transcriptional Response to Lactic Acid Stress in the Hybrid Yeast Zygosaccharomyces parabailii

Lactic acid has a wide range of applications starting from its undissociated form, and its production using cell factories requires stress-tolerant microbial hosts. The interspecies hybrid yeast Zygosaccharomyces parabailii has great potential to be exploited as a novel host for lactic acid production, due to high organic acid tolerance at low pH and a fermentative metabolism with a high growth rate. Here we used mRNA sequencing (RNA-seq) to analyze Z. parabailii's transcriptional response to lactic acid added exogenously, and we explore the biological mechanisms involved in tolerance. Z. parabailii contains two homeologous copies of most genes. Under lactic acid stress, the two genes in each homeolog pair tend to diverge in expression to a significantly greater extent than under control conditions, indicating that stress tolerance is facilitated by interactions between the two gene sets in the hybrid. Lactic acid induces downregulation of genes related to cell wall and plasma membrane functions, possibly altering the rate of diffusion of lactic acid into cells. Genes related to iron transport and redox processes were upregulated, suggesting an important role for respiratory functions and oxidative stress defense. We found differences in the expression profiles of genes putatively regulated by Haa1 and Aft1/Aft2, previously described as lactic acid responsive in Saccharomyces cerevisiae. Furthermore, formate dehydrogenase (FDH) genes form a lactic acid-responsive gene family that has been specifically amplified in Z. parabailii in comparison to other closely related species. Our study provides a useful starting point for the engineering of Z. parabailii as a host for lactic acid production.

IMPORTANCE Hybrid yeasts are important in biotechnology because of their tolerance to harsh industrial conditions. The molecular mechanisms of tolerance can be studied by analyzing differential gene expression under conditions of interest and relating gene expression patterns to protein functions. However, hybrid organisms present a challenge to the standard use of mRNA sequencing (RNA-seq) to study transcriptional responses to stress, because their genomes contain two similar copies of almost every gene. Here we used stringent mapping methods and a high-quality genome sequence to study the transcriptional response to lactic acid stress in Zygosaccharomyces parabailii ATCC 60483, a natural interspecies hybrid yeast that contains two complete subgenomes that are approximately 7% divergent in sequence. Beyond the insights we gained into lactic acid tolerance in this study, the methods we developed will be broadly applicable to other yeast hybrid strains.

Secretion of functional formate dehydrogenase in Pichia pastoris

Biofuels are an important tool for the reduction of carbon dioxide and other greenhouse emissions. NAD+-dependent formate dehydrogenase has been previously shown to be capable of the electrochemical reduction of carbon dioxide into formate, which can be ultimately converted to methanol. We established that a functional enzyme, tagged for immobilization, could be continuously secreted by Pichia pastoris. The protein can be easily separated from the growth media and its activity remains constant over an extended period of time. This is an important first step in creating a self-sustaining system capable of producing biofuels with minimal resources and space required.