Purification and characterization of pyruvate ferredoxin oxidoreductase from the hyperthermophilic archaeon Pyrococcus furiosus

An energy-conserving reaction in amino acid metabolism catalyzed by arginine synthetase

All forms of life are presumed to synthesize arginine from citrulline via a two-step pathway consisting of argininosuccinate synthetase and argininosuccinate lyase using citrulline, adenosine 5'-triphosphate (ATP), and aspartate as substrates. Conversion of arginine to citrulline predominantly proceeds via hydrolysis. Here, from the hyperthermophilic archaeon Thermococcus kodakarensis, we identified an enzyme which we designate "arginine synthetase". In arginine synthesis, the enzyme converts citrulline, ATP, and free ammonia to arginine, adenosine 5'-diphosphate (ADP), and phosphate. In the reverse direction, arginine synthetase conserves the energy of arginine deimination and generates ATP from ADP and phosphate while releasing ammonia. The equilibrium constant of this reaction at pH 7.0 is [Cit][ATP][NH3]/[Arg][ADP][Pi] = 10.1 ± 0.7 at 80 °C, corresponding to a ΔG°' of -6.8 ± 0.2 kJ mol-1. Growth of the gene disruption strain was compared to the host strain in medium composed of amino acids. The results suggested that arginine synthetase is necessary in providing ornithine, the precursor for proline biosynthesis, as well as in generating ATP. Growth in medium supplemented with citrulline indicated that arginine synthetase can function in the direction of arginine synthesis. The enzyme is widespread in nature, including bacteria and eukaryotes, and catalyzes a long-overlooked energy-conserving reaction in microbial amino acid metabolism. Along with ornithine transcarbamoylase and carbamate kinase, the pathway identified here is designated the arginine synthetase pathway.

Phylogenetic Analysis of Pyruvate-Ferredoxin Oxidoreductase, a Redox Enzyme Involved in the Pharmacological Activation of Nitro-Based Prodrugs in Bacteria and Protozoa

The present frontrunners in the chemotherapy of infections caused by protozoa are nitro-based prodrugs that are selectively activated by PFOR-mediated redox reactions. This study seeks to analyze the distribution of PFOR in selected protozoa and bacteria by applying comparative genomics to test the hypothesis that PFOR in eukaryotes was acquired through horizontal gene transfer (HGT) from bacteria. Furthermore, to identify other putatively acquired genes, proteome-wide and gene enrichment analyses were used. A plausible explanation for the patchy occurrence of PFOR in protozoa is based on the hypothesis that bacteria are potential sources of genes that enhance the adaptation of protozoa in hostile environments. Comparative genomics of Entamoeba histolytica and the putative gene donor, Desulfovibrio vulgaris, identified eleven candidate genes for HGT involved in intermediary metabolism. If these results can be reproduced in other PFOR-possessing protozoa, it would provide more validated evidence to support the horizontal transfer of pfor from bacteria.

The secretome of Thermococcus barophilus in the presence of carbohydrates and the potential role of the TrmBL4 regulator

Global transcriptional regulators are crucial for supporting rapid adaptive responses in changing environments. In Thermococcales, the TrmB sugar-sensing regulator family is well represented but knowledge of the functional role/s of each of its members is limited. In this study, we examined the link between TrmBL4 and the degree of protein secretion in different sugar environments in the hyperthermophilic Archaeon Thermococcus barophilus. Although the absence of TrmBL4 did not induce any growth defects, proteomics analysis revealed different secretomes depending on the sugar and/or genetic contexts. Notably, 33 secreted proteins present in the supernatant were differentially detected. Some of these proteins are involved in sugar assimilation and transport, such as the protein encoded by TERMP_01455 (cyclomaltodextrin glucanotransferase), whereas others have intracellular functions, such as the protein encoded by TERMP_01556 (pyruvate: ferredoxin oxidoreductase Δsubunit). Then, using reverse transcription quantitative polymerase chain reaction experiments, we observed effective transcription regulation by TrmBL4 of the genes encoding at least two ABC-type transporters according to sugar availability.

Optimizing Strategies for Bio-Based Ethanol Production Using Genome-Scale Metabolic Modeling of the Hyperthermophilic Archaeon, Pyrococcus furiosus

A genome-scale metabolic model, encompassing a total of 623 genes, 727 reactions, and 865 metabolites, was developed for Pyrococcus furiosus, an archaeon that grows optimally at 100°C by carbohydrate and peptide fermentation. The model uses subsystem-based genome annotation, along with extensive manual curation of 237 gene-reaction associations including those involved in central carbon metabolism, amino acid metabolism, and energy metabolism. The redox and energy balance of P. furiosus was investigated through random sampling of flux distributions in the model during growth on disaccharides. The core energy balance of the model was shown to depend on high acetate production and the coupling of a sodium-dependent ATP synthase and membrane-bound hydrogenase, which generates a sodium gradient in a ferredoxin-dependent manner, aligning with existing understanding of P. furiosus metabolism. The model was utilized to inform genetic engineering designs that favor the production of ethanol over acetate by implementing an NADPH and CO-dependent energy economy. The P. furiosus model is a powerful tool for understanding the relationship between generation of end products and redox/energy balance at a systems-level that will aid in the design of optimal engineering strategies for production of bio-based chemicals and fuels. IMPORTANCE The bio-based production of organic chemicals provides a sustainable alternative to fossil-based production in the face of today's climate challenges. In this work, we present a genome-scale metabolic reconstruction of Pyrococcus furiosus, a well-established platform organism that has been engineered to produce a variety of chemicals and fuels. The metabolic model was used to design optimal engineering strategies to produce ethanol. The redox and energy balance of P. furiosus was examined in detail, which provided useful insights that will guide future engineering designs.

Manipulating Fermentation Pathways in the Hyperthermophilic Archaeon Pyrococcus furiosus for Ethanol Production up to 95°C Driven by Carbon Monoxide Oxidation

Genetic engineering of hyperthermophilic organisms for the production of fuels and other useful chemicals is an emerging biotechnological opportunity. In particular, for volatile organic compounds such as ethanol, fermentation at high temperatures could allow for straightforward separation by direct distillation. Currently, the upper growth temperature limit for native ethanol producers is 72°C in the bacterium Thermoanaerobacter ethanolicus JW200, and the highest temperature for heterologously-engineered bioethanol production was recently demonstrated at 85°C in the archaeon Pyrococcus furiosus. Here, we describe an engineered strain of P. furiosus that synthesizes ethanol at 95°C, utilizing a homologously-expressed native alcohol dehydrogenase, termed AdhF. Ethanol biosynthesis was compared at 75°C and 95°C with various engineered strains. At lower temperatures, the acetaldehyde substrate for AdhF is most likely produced from acetate by aldehyde ferredoxin oxidoreductase (AOR). At higher temperatures, the effect of AOR on ethanol production is negligible, suggesting that acetaldehyde is produced by pyruvate ferredoxin oxidoreductase (POR) via oxidative decarboxylation of pyruvate, a reaction known to occur only at higher temperatures. Heterologous expression of a carbon monoxide dehydrogenase complex in the AdhF overexpression strain enabled it to use CO as a source of energy, leading to increased ethanol production. A genome reconstruction model for P. furiosus was developed to guide metabolic engineering strategies and understand outcomes. This work opens the door to the potential for 'bioreactive distillation' since fermentation can be performed well above the normal boiling point of ethanol. IMPORTANCE Previously, the highest temperature for biological ethanol production was 85°C. Here, we have engineered ethanol production at 95°C by the hyperthermophilic archaeon Pyrococcus furiosus. Using mutant strains, we showed that ethanol production occurs by different pathways at 75°C and 95°C. In addition, by heterologous expression of a carbon monoxide dehydrogenase complex, ethanol production by this organism was driven by the oxidation of carbon monoxide. A genome reconstruction model for P. furiosus was developed to guide metabolic engineering strategies and understand outcomes.

Obligate autotrophy at the thermodynamic limit of life in a new acetogenic bacterium

One of the important current issues of bioenergetics is the establishment of the thermodynamic limits of life. There is still no final understanding of what is the minimum value of the energy yield of a reaction that is sufficient to be used by an organism (the so-called "biological quantum of energy"). A reasonable model for determination of the minimal energy yield would be microorganisms capable of living on low-energy substrates, such as acetogenic prokaryotes. The most prominent metabolic feature of acetogens is autotrophic growth with molecular hydrogen and carbon dioxide as the substrates, which is hardly competitive in environments. Most probably, that is why only facultative autotrophic acetogens have been known so far. Here, we describe the first obligately autotrophic acetogenic bacterium Aceticella autotrophica gen. nov., sp. nov., strain 3443-3Ac^T. Phylogenetically, the new genus falls into a monophyletic group of heterotrophic bacteria of the genera Thermoanaerobacterium, Thermoanaerobacter, and Caldanaerobacter (hereinafter referred to as TTC group), where the sole acetogenic representative has so far been the facultatively autotrophic Thermoanaerobacter kivui. A. autotrophica and T. kivui both are acetogens employing energy-converting hydrogenase (Ech-acetogens) that are likely to have inherited the acetogenesis capacity vertically from common ancestor. However, their acetogenic machineries have undergone different adjustments by gene replacements due to horizontal gene transfers from different donors. Obligate autotrophy of A. autotrophica is associated with the lack of many sugar transport systems and carbohydrate catabolism enzymes that are present in other TTC group representatives, including T. kivui.

Biochemical and genetic examination of two aminotransferases from the hyperthermophilic archaeon Thermococcus kodakarensis

The hyperthermophilic archaeon Thermococcus kodakarensis utilizes amino acids as a carbon and energy source. Multiple aminotransferases, along with glutamate dehydrogenase, are presumed to be involved in the catabolic conversion of amino acids. T. kodakarensis harbors seven Class I aminotransferase homologs on its genome. Here we examined the biochemical properties and physiological roles of two Class I aminotransferases. The TK0548 protein was produced in Escherichia coli and the TK2268 protein in T. kodakarensis. Purified TK0548 protein preferred Phe, Trp, Tyr, and His, and to a lower extent, Leu, Met and Glu. The TK2268 protein preferred Glu and Asp, with lower activities toward Cys, Leu, Ala, Met and Tyr. Both proteins recognized 2-oxoglutarate as the amino acceptor. The TK0548 protein exhibited the highest k _cat/K _m value toward Phe, followed by Trp, Tyr, and His. The TK2268 protein exhibited highest k _cat/K _m values for Glu and Asp. The TK0548 and TK2268 genes were individually disrupted, and both disruption strains displayed a retardation in growth on a minimal amino acid medium, suggesting their involvement in amino acid metabolism. Activities in the cell-free extracts of the disruption strains and the host strain were examined. The results suggested that the TK0548 protein contributes to the conversion of Trp, Tyr and His, and the TK2268 protein to that of Asp and His. Although other aminotransferases seem to contribute to the transamination of Phe, Trp, Tyr, Asp, and Glu, our results suggest that the TK0548 protein is responsible for the majority of aminotransferase activity toward His in T. kodakarensis. The genetic examination carried out in this study provides insight into the contributions of the two aminotransferases toward specific amino acids in vivo, an aspect which had not been thoroughly considered thus far.

Obligately aerobic human gut microbe expresses an oxygen resistant tungsten-containing oxidoreductase for detoxifying gut aldehydes

Brevibacillus massiliensis strain phR is an obligately aerobic microbe that was isolated from human feces. Here, we show that it readily takes up tungsten (W), a metal previously associated only with anaerobes. The W is incorporated into an oxidoreductase enzyme (BmWOR) that was purified from native biomass. BmWOR consists of a single 65 kDa subunit and contains a single W-pyranopterin cofactor and a single [4Fe-4S] cluster. It exhibited high aldehyde-oxidizing activity with very high affinities (apparent K_m < 6 μM) for aldehydes common in the human gut and in cooked foods, including furfural, propionaldehyde, benzaldehyde and tolualdehyde, suggesting that BmWOR plays a key role in their detoxification. B. massiliensis converted added furfural to furoic acid when grown in the presence of W, but not in the presence of the analogous element molybdenum. B. massiliensis ferredoxin (BmFd) served as the electron acceptor (apparent K_m < 5 μM) for BmWOR suggesting it is the physiological electron carrier. Genome analysis revealed a Fd-dependent rather than NADH-dependent Complex I, suggesting that WOR not only serves a detoxification role but its aldehyde substrates could also serve as a source of energy. BmWOR is the first tungstoenzyme and the first member of the WOR family to be obtained from a strictly aerobic microorganism. Remarkably, BmWOR oxidized furfural in the presence of air (21% O₂, v/v) but only if BmFd was also present. BmWOR is the first characterized member of the Clade 83 WORs, which are predominantly found in extremely halophilic and aerobic archaea (Clade 83A), with many isolated from food sources, while the remaining bacterial members (Clade 83B) include both aerobes and anaerobes. The potential advantages for microbes found in foods and involved in human gut health that harbor O₂-resistant WORs, including in Bacillus and Brevibacillus based-probiotics, are discussed.

A purified energy-converting hydrogenase from Thermoanaerobacter kivui demonstrates coupled H+-translocation and reduction in vitro

Energy-converting hydrogenases (Ech) are ancient, membrane-bound enzymes that use reduced ferredoxin (Fd) as an electron donor to reduce protons to molecular H₂. Experiments with whole cells, membranes and vesicle-fractions suggest that proton reduction is coupled to proton translocation across the cytoplasmatic membrane, but this has never been demonstrated with a purified enzyme. To this end, we produced a His-tagged Ech complex in the thermophilic and anaerobic bacterium Thermoanaerobacter kivui. The enzyme could be purified by affinity chromatography from solubilized membranes with full retention of its eight subunits, as well as full retention of physiological activities, i.e., H₂-dependent Fd reduction and Fd^2--dependent H₂ production. We found the purified enzyme contained 34.2 ± 12.2 mol of iron/mol of protein, in accordance with seven predicted [4Fe-4S]-clusters and one [Ni-Fe]-center. The pH and temperature optima were at 7 to 8 and 66 °C, respectively. Notably, we found that the enzymatic activity was inhibited by N,N′-dicyclohexylcarbodiimide, an agent known to bind ion-translocating glutamates or aspartates buried in the cytoplasmic membrane and thereby inhibiting ion transport. To demonstrate the function of the Ech complex in ion transport, we further established a procedure to incorporate the enzyme complex into liposomes in an active state. We show the enzyme did not require Na⁺ for activity and did not translocate ²²Na⁺ into the proteoliposomal lumen. In contrast, Ech activity led to the generation of a pH gradient and membrane potential across the proteoliposomal membrane, demonstrating that the Ech complex of T. kivui is a H⁺-translocating, H⁺-reducing enzyme.

Evolution of Thermophilic Microbial Communities from a Deep-Sea Hydrothermal Chimney under Electrolithoautotrophic Conditions with Nitrate

Recent studies have shown the presence of an abiotic electrical current across the walls of deep-sea hydrothermal chimneys, allowing the growth of electroautotrophic microbial communities. To understand the role of the different phylogenetic groups and metabolisms involved, this study focused on electrotrophic enrichment with nitrate as electron acceptor. The biofilm density, community composition, production of organic compounds, and electrical consumption were monitored by FISH confocal microscopy, qPCR, metabarcoding, NMR, and potentiostat measurements. A statistical analysis by PCA showed the correlation between the different parameters (qPCR, organic compounds, and electron acceptors) in three distinct temporal phases. In our conditions, the Archaeoglobales have been shown to play a key role in the development of the community as the first colonizers on the cathode and the first producers of organic compounds, which are then used as an organic source by heterotrophs. Finally, through subcultures of the community, we showed the development of a greater biodiversity over time. This observed phenomenon could explain the biodiversity development in hydrothermal contexts, where energy sources are transient and unstable.

Energy conservation under extreme energy limitation: the role of cytochromes and quinones in acetogenic bacteria

Acetogenic bacteria are a polyphyletic group of organisms that fix carbon dioxide under anaerobic, non-phototrophic conditions by reduction of two mol of CO₂ to acetyl-CoA via the Wood–Ljungdahl pathway. This pathway also allows for lithotrophic growth with H₂ as electron donor and this pathway is considered to be one of the oldest, if not the oldest metabolic pathway on Earth for CO₂ reduction, since it is coupled to the synthesis of ATP. How ATP is synthesized has been an enigma for decades, but in the last decade two ferredoxin-dependent respiratory chains were discovered. Those respiratory chains comprise of a cytochrome-free, ferredoxin-dependent respiratory enzyme complex, which is either the Rnf or Ech complex. However, it was discovered already 50 years ago that some acetogens contain cytochromes and quinones, but their role had only a shadowy existence. Here, we review the literature on the characterization of cytochromes and quinones in acetogens and present a hypothesis that they may function in electron transport chains in addition to Rnf and Ech.

The Pyrococcus furiosus ironome is dominated by [Fe4S4]2+ clusters or thioferrate-like iron depending on the availability of elemental sulfur

Pyrococcus furiosus is a hyperthermophilic anaerobic archaeon whose metabolism depends on whether elemental sulfur is (+S⁰) or is not (-S⁰) included in growth medium. Under +S⁰ conditions, expression of respiratory hydrogenase declines while respiratory membrane-bound sulfane reductase and the putative iron-storage protein IssA increase. Our objective was to investigate the iron content of WT and ΔIssA cells under these growth conditions using Mössbauer spectroscopy. WT-S⁰ cells contained ∼1 mM Fe, with ∼85% present as two spectroscopically distinct forms of S = 0 [Fe₄S₄]²⁺ clusters; the remainder was mainly high-spin Fe^II. WT+S⁰ cells contained 5 to 9 mM Fe, with 75 to 90% present as magnetically ordered thioferrate-like (TFL) iron nanoparticles. TFL iron was similar to chemically defined thioferrates; both consisted of Fe^III ions coordinated by an S₄ environment, and both exhibited strong coupling between particles causing high applied fields to have little spectral effect. At high temperatures with magnetic hyperfine interactions abolished, TFL iron exhibited two doublets overlapping those of [Fe₄S₄]²⁺ clusters in -S⁰ cells. This coincidence arose because of similar coordination environments of TFL iron and cluster iron. The TFL structure was more heterogeneous in the presence of IssA. Presented data suggest that IssA may coordinate insoluble iron sulfides as TFL iron, formed as a byproduct of anaerobic sulfur respiration under high iron conditions, which thereby reduces its toxicity to the cell. This was the first Mössbauer characterization of the ironome of an archaeon, and it illustrates differences relative to the iron content of better-studied bacteria such as Escherichia coli.

The pyruvate:ferredoxin oxidoreductase of the thermophilic acetogen, Thermoanaerobacter kivui

Pyruvate:ferredoxin oxidoreductase (PFOR) is a key enzyme in bacterial anaerobic metabolism. Since a low‐potential ferredoxin (Fd²⁻) is used as electron carrier, PFOR allows for hydrogen evolution during heterotrophic growth as well as pyruvate synthesis during lithoautotrophic growth. The thermophilic acetogenic model bacterium Thermoanaerobacter kivui can use both modes of lifestyle, but the nature of the PFOR in this organism was previously unestablished. Here, we have isolated PFOR to apparent homogeneity from cells grown on glucose. Peptide mass fingerprinting revealed that it is encoded by pfor1. PFOR uses pyruvate as an electron donor and methylene blue (1.8 U·mg⁻¹) and ferredoxin (Fd; 27.2 U·mg⁻¹) as electron acceptors, and the reaction is dependent on thiamine pyrophosphate, pyruvate, coenzyme A, and Fd. The pH and temperature optima were 7.5 and 66 °C, respectively. We detected 13.6 mol of iron·mol of protein⁻¹, consistent with the presence of three predicted [4Fe–4S] clusters. The ability to provide reduced Fd makes PFOR an interesting auxiliary enzyme for enzyme assays. To simplify and speed up the purification procedure, we established a protocol for homologous protein production in T. kivui. Therefore, pfor1 was cloned and expressed in T. kivui and the encoded protein containing a genetically engineered His‐tag was purified in only two steps to apparent homogeneity. The homologously produced PFOR1 had the same properties as the enzyme from T. kivui. The enzyme can be used as auxiliary enzyme in enzymatic assays that require reduced Fd as electron donor, such as electron‐bifurcating enzymes, to keep a constant level of reduced Fd.

Glucose Metabolism and Acetate Switch in Archaea: the Enzymes in Haloferax volcanii

The halophilic archaeon Haloferax volcanii has been proposed to degrade glucose via the semiphosphorylative Entner-Doudoroff (spED) pathway. Following our previous studies on key enzymes of this pathway, we now focus on the characterization of enzymes involved in 3-phosphoglycerate conversion to pyruvate, in anaplerosis, and in acetyl coenzyme A (acetyl-CoA) formation from pyruvate. These enzymes include phosphoglycerate mutase, enolase, pyruvate kinase, phosphoenolpyruvate carboxylase, and pyruvate-ferredoxin oxidoreductase. The essential function of these enzymes were shown by transcript analyses and growth experiments with respective deletion mutants. Furthermore, we show that H. volcanii-during aerobic growth on glucose-excreted significant amounts of acetate, which was consumed in the stationary phase (acetate switch). The enzyme catalyzing the conversion of acetyl-CoA to acetate as part of the acetate overflow mechanism, an ADP-forming acetyl-CoA synthetase (ACD), was characterized. The functional involvement of ACD in acetate formation and of AMP-forming acetyl-CoA synthetases (ACSs) in activation of excreted acetate was proven by using respective deletion mutants. Together, the data provide a comprehensive analysis of enzymes of the spED pathway and of anaplerosis and report the first genetic evidence of the functional involvement of enzymes of the acetate switch in archaea.IMPORTANCE In this work, we provide a comprehensive analysis of glucose degradation via the semiphosphorylative Entner-Doudoroff pathway in the haloarchaeal model organism Haloferax volcanii The study includes transcriptional analyses, growth experiments with deletion mutants. and characterization of all enzymes involved in the conversion of 3-phosphoglycerate to acetyl coenzyme A (acetyl-CoA) and in anaplerosis. Phylogenetic analyses of several enzymes indicate various lateral gene transfer events from bacteria to haloarchaea. Furthermore, we analyzed the key players involved in the acetate switch, i.e., in the formation (overflow) and subsequent consumption of acetate during aerobic growth on glucose. Together, the data provide novel aspects of glucose degradation, anaplerosis, and acetate switch in H. volcanii and thus expand our understanding of the unusual sugar metabolism in archaea.

The Rnf complex is a Na+ coupled respiratory enzyme in a fermenting bacterium, Thermotoga maritima

rnf genes are widespread in bacteria and biochemical and genetic data are in line with the hypothesis that they encode a membrane-bound enzyme that oxidizes reduced ferredoxin and reduces NAD and vice versa, coupled to ion transport across the cytoplasmic membrane. The Rnf complex is of critical importance in many bacteria for energy conservation but also for reverse electron transport to drive ferredoxin reduction. However, the enzyme has never been purified and thus, ion transport could not be demonstrated yet. Here, we have purified the Rnf complex from the anaerobic, fermenting thermophilic bacterium Thermotoga maritima and show that is a primary Na⁺ pump. These studies provide the proof that the Rnf complex is indeed an ion (Na⁺) translocating, respiratory enzyme. Together with a Na⁺-F₁F_O ATP synthase it builds a simple, two-limb respiratory chain in T. maritima. The physiological role of electron transport phosphorylation in a fermenting bacterium is discussed.

From a fermenting bacterium, Thermotoga maritima, Kuhns, Trifunovi ć et al. purify a complex that includes a respiratory enzyme, Rnf. They find that the Rnf complex requires Na⁺ for activity and that it catalyzes Na⁺ transport in liposomes. This study shows that the Rnf complex is indeed an ion translocating, respiratory enzyme.

Effects of high-level expression of A1-ATPase on H2 production in Thermococcus kodakarensis

The hyperthermophilic archaeon Thermococcus kodakarensis can grow on pyruvate or maltooligosaccharides through H₂ fermentation. H₂ production levels of members of the Thermococcales are high, and studies to improve their production potential have been reported. Although H₂ production is primary metabolism, here we aimed to partially uncouple cell growth and H₂ production of T. kodakarensis. Additional A₁-type ATPase genes were introduced into T. kodakarensis KU216 under the control of two promoters; the strong constitutive cell surface glycoprotein promoter, P_csg, and the sugar-inducible fructose-1,6-bisphosphate aldolase promoter, P_fba. Whereas cells with the A₁-type ATPase genes under the control of P_csg displayed only trace levels of growth, cells with P_fba (strain KUA-PF) displayed growth sufficient for further analysis. Increased levels of A₁-type ATPase protein were detected in KUA-PF cells grown on pyruvate or maltodextrin, when compared to the levels in the host strain KU216. The growth and H₂ production levels of strain KUA-PF with pyruvate or maltodextrin as a carbon and electron source were analyzed and compared to those of the host strain KU216. Compared to a small decrease in total H₂ production, significantly larger decreases in cell growth were observed, resulting in an increase in cell-specific H₂ production. Quantification of the substrate also revealed that ATPase overexpression led to increased cell-specific pyruvate and maltodextrin consumptions. The results clearly indicate that ATPase production results in partial uncoupling of cell growth and H₂ production in T. kodakarensis.

Measuring Metabolic Enzyme Performance

Understanding the performance of key metabolic enzymes is critical to metabolic engineering. It is important to know the kinetic parameters of both native enzymes and heterologously expressed enzymes that play key roles in pathway performance (Zeldes et al., Front Microbiol 6:1209, 2015; Keller et al., Metab Eng 27:101-106, 2015). This step cannot be overlooked as gene expression is not always a good indicator of the production of fully active enzymes, especially those requiring cofactor assembly and processing (Zeldes et al., Front Microbiol 6:1209, 2015; Chandrayan et al., J Biol Chem 287:3257-3264, 2012; Basen et al., MBio 3:e00053-e00012, 2012). Additionally, knowing kinetic parameters and having accurate and reproducible assays allows for the use of powerful computational and in vitro pathway optimization tools that can inform metabolic engineering efforts that in turn can lead to improvements in pathway performance (Keller et al., Metab Eng 27:101-106, 2015; Copeland et al., Metab Eng 14:270-280, 2012). To take full advantage of these tools, understanding the roles of both enzymes directly involved in a pathway of interest, together with those in related pathways that may syphon off key intermediates, is ideal (Keller et al., Metab Eng 27:101-106, 2015; Thorgersen et al., Metab Eng 22:83-88; Lin et al., Metab Engi 31:44-52, 2015).

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.

Determining the roles of the three alcohol dehydrogenases (AdhA, AdhB and AdhE) in Thermoanaerobacter ethanolicus during ethanol formation

Thermoanaerobacter ethanolicus is a promising candidate for biofuel production due to the broad range of substrates it can utilize and its high ethanol yield compared to other thermophilic bacteria, such as Clostridium thermocellum. Three alcohol dehydrogenases, AdhA, AdhB and AdhE, play key roles in ethanol formation. To study their physiological roles during ethanol formation, we deleted them separately and in combination. Previously, it has been thought that both AdhB and AdhE were bifunctional alcohol dehydrogenases. Here we show that AdhE has primarily acetyl-CoA reduction activity (ALDH) and almost no acetaldehyde reduction (ADH) activity, whereas AdhB has no ALDH activity and but high ADH activity. We found that AdhA and AdhB have similar patterns of activity. Interestingly, although deletion of both adhA and adhB reduced ethanol production, a single deletion of either one actually increased ethanol yields by 60-70%.

Crystal structures of archaeal 2-oxoacid:ferredoxin oxidoreductases from Sulfolobus tokodaii

As the first three-dimensional structure of the two-subunit type 2-oxoacid:ferredoxin oxidoreductases (OFOR) from archaea, we solved the crystal structures of STK_23000/STK_22980 (StOFOR1) and STK_24350/STK_24330 (StOFOR2) from Sulfolobus tokodaii. They showed similar overall structures, consisting of two a- and b-subunit heterodimers containing thiamin pyrophosphate (TPP) cofactor and [4Fe-4S] cluster, but lack an intramolecular ferredoxin domain. Unlike other OFORs, StOFORs can utilize both pyruvate and 2-oxoglutarate, playing a key role in the central metabolism. In the structure of StOFOR2 in unreacted pyruvate complex form, carboxylate group of pyruvate is recognized by Arg344 and Thr257 from the a-subunit, which are conserved in pyruvate:ferredoxin oxidoreductase from Desulfovbrio africanus (DaPFOR). In the structure of StOFOR1 co-crystallized with 2-oxobutyrate, electron density corresponding to a 1-hydroxypropyl group (post-decarboxylation state) was observed at the thiazole ring of TPP. The binding pockets of the StOFORs surrounding the methyl or propyl group of the ligands are wider than that of DaPFOR. Mutational analyses indicated that several residues were responsible for the broad 2-oxoacid specificity of StOFORs. We also constructed a possible complex structural model by placing a Zn(2+)-containing dicluster ferredoxin of S. tokodaii into the large pocket of StOFOR2, providing insight into the electron transfer between the two redox proteins.

Ancestral genome reconstruction identifies the evolutionary basis for trait acquisition in polyphosphate accumulating bacteria

The evolution of complex traits is hypothesized to occur incrementally. Identifying the transitions that lead to extant complex traits may provide a better understanding of the genetic nature of the observed phenotype. A keystone functional group in wastewater treatment processes are polyphosphate accumulating organisms (PAOs), however the evolution of the PAO phenotype has yet to be explicitly investigated and the specific metabolic traits that discriminate non-PAO from PAO are currently unknown. Here we perform the first comprehensive investigation on the evolution of the PAO phenotype using the model uncultured organism Candidatus Accumulibacter phosphatis (Accumulibacter) through ancestral genome reconstruction, identification of horizontal gene transfer, and a kinetic/stoichiometric characterization of Accumulibacter Clade IIA. The analysis of Accumulibacter's last common ancestor identified 135 laterally derived genes, including genes involved in glycogen, polyhydroxyalkanoate, pyruvate and NADH/NADPH metabolisms, as well as inorganic ion transport and regulatory mechanisms. In contrast, pathways such as the TCA cycle and polyphosphate metabolism displayed minimal horizontal gene transfer. We show that the transition from non-PAO to PAO coincided with horizontal gene transfer within Accumulibacter's core metabolism; likely alleviating key kinetic and stoichiometric bottlenecks, such as anaerobically linking glycogen degradation to polyhydroxyalkanoate synthesis. These results demonstrate the utility of investigating the derived genome of a lineage to identify key transitions leading to an extant complex phenotype.

Hydrogen Production and Enzyme Activities in the Hyperthermophile Thermococcus paralvinellae Grown on Maltose, Tryptone, and Agricultural Waste

Thermococcus may be an important alternative source of H2 in the hot subseafloor in otherwise low H2 environments such as some hydrothermal vents and oil reservoirs. It may also be useful in industry for rapid agricultural waste treatment and concomitant H2 production. Thermococcus paralvinellae grown at 82°C without sulfur produced up to 5 mmol of H2 L(-1) at rates of 5-36 fmol H2 cell(-1) h(-1) on 0.5% (wt vol(-1)) maltose, 0.5% (wt vol(-1)) tryptone, and 0.5% maltose + 0.05% tryptone media. Two potentially inhibiting conditions, the presence of 10 mM acetate and low pH (pH 5) in maltose-only medium, did not significantly affect growth or H2 production. Growth rates, H2 production rates, and cell yields based on H2 production were the same as those for Pyrococcus furiosus grown at 95°C on the same media for comparison. Acetate, butyrate, succinate, isovalerate, and formate were also detected as end products. After 100 h, T. paralvinellae produced up to 5 mmol of H2 L(-1) of medium when grown on up to 70% (vol vol(-1)) waste milk from cows undergoing treatment for mastitis with the bacterial antibiotic Ceftiofur and from untreated cows. The amount of H2 produced by T. paralvinellae increased with increasing waste concentrations, but decreased in P. furiosus cultures supplemented with waste milk above 1% concentration. All mesophilic bacteria from the waste milk that grew on Luria Bertani, Sheep's Blood (selective for Staphylococcus, the typical cause of mastitis), and MacConkey (selective for Gram-negative enteric bacteria) agar plates were killed by heat during incubation at 82°C. Ceftiofur, which is heat labile, was below the detection limit following incubation at 82°C. T. paralvinellae also produced up to 6 mmol of H2 L(-1) of medium when grown on 0.1-10% (wt vol(-1)) spent brewery grain while P. furiosus produced < 1 mmol of H2 L(-1). Twelve of 13 enzyme activities in T. paralvinellae showed significant (p < 0.05) differences across six different growth conditions; however, methyl viologen-dependent membrane hydrogenase activity remained constant across all media types. The results demonstrate the potential of at least some Thermococcus species to produce H2 if protein and α-glucosides are present as substrates.

Overproduction of the membrane-bound [NiFe]-hydrogenase in Thermococcus kodakarensis and its effect on hydrogen production

The hyperthermophilic archaeon Thermococcus kodakarensis can utilize sugars or pyruvate for growth. In the absence of elemental sulfur, the electrons via oxidation of these substrates are accepted by protons, generating molecular hydrogen (H₂). The hydrogenase responsible for this reaction is a membrane-bound [NiFe]-hydrogenase (Mbh). In this study, we have examined several possibilities to increase the protein levels of Mbh in T. kodakarensis by genetic engineering. Highest levels of intracellular Mbh levels were achieved when the promoter of the entire mbh operon (TK2080-TK2093) was exchanged to a strong constitutive promoter from the glutamate dehydrogenase gene (TK1431) (strain MHG1). When MHG1 was cultivated under continuous culture conditions using pyruvate-based medium, a nearly 25% higher specific hydrogen production rate (SHPR) of 35.3 mmol H₂ g-dcw⁻¹ h⁻¹ was observed at a dilution rate of 0.31 h⁻¹. We also combined mbh overexpression using an even stronger constitutive promoter from the cell surface glycoprotein gene (TK0895) with disruption of the genes encoding the cytosolic hydrogenase (Hyh) and an alanine aminotransferase (AlaAT), both of which are involved in hydrogen consumption (strain MAH1). At a dilution rate of 0.30 h⁻¹, the SHPR was 36.2 mmol H₂ g-dcw⁻¹ h⁻¹, corresponding to a 28% increase compared to that of the host T. kodakarensis strain. Increasing the dilution rate to 0.83 h⁻¹ or 1.07 h⁻¹ resulted in a SHPR of 120 mmol H₂ g-dcw⁻¹ h⁻¹, which is one of the highest production rates observed in microbial fermentation.

Acetate Metabolism in Anaerobes from the Domain Archaea

Acetate and acetyl-CoA play fundamental roles in all of biology, including anaerobic prokaryotes from the domains Bacteria and Archaea, which compose an estimated quarter of all living protoplasm in Earth's biosphere. Anaerobes from the domain Archaea contribute to the global carbon cycle by metabolizing acetate as a growth substrate or product. They are components of anaerobic microbial food chains converting complex organic matter to methane, and many fix CO2 into cell material via synthesis of acetyl-CoA. They are found in a diversity of ecological habitats ranging from the digestive tracts of insects to deep-sea hydrothermal vents, and synthesize a plethora of novel enzymes with biotechnological potential. Ecological investigations suggest that still more acetate-metabolizing species with novel properties await discovery.

Molecular and biochemical characterization of bifunctional pyruvate decarboxylases and pyruvate ferredoxin oxidoreductases from Thermotoga maritima and Thermotoga hypogea

Hyperthermophilic bacteria Thermotoga maritima and Thermotoga hypogea produce ethanol as a metabolic end product, which is resulted from acetaldehyde reduction catalysed by an alcohol dehydrogenase (ADH). However, the enzyme that is involved in the production of acetaldehyde from pyruvate is not well characterized. An oxygen sensitive and coenzyme A-dependent pyruvate decarboxylase (PDC) activity was found to be present in cell free extracts of T. maritima and T. hypogea. Both enzymes were purified and found to have pyruvate ferredoxin oxidoreductase (POR) activity, indicating their bifunctionality. Both PDC and POR activities from each of the purified enzymes were characterized in regards to their optimal assay conditions including pH dependency, oxygen sensitivity, thermal stability, temperature dependency and kinetic parameters. The close relatedness of the PORs that was shown by sequence analysis could be an indication of the presence of such bifunctionality in other hyperthermophilic bacteria. This is the first report of a bifunctional PDC/POR enzyme in hyperthermophilic bacteria. The PDC and the previously reported ADHs are most likely the key enzymes catalysing the production of ethanol from pyruvate in bacterial hyperthermophiles.

Anaerobic degradation of aromatic amino acids by the hyperthermophilic archaeon Ferroglobus placidus

Ferroglobus placidus was discovered to oxidize completely the aromatic amino acids tyrosine, phenylalanine and tryptophan when Fe(III) oxide was provided as an electron acceptor. This property had not been reported previously for a hyperthermophilic archaeon. It appeared that F. placidus follows a pathway for phenylalanine and tryptophan degradation similar to that of mesophilic nitrate-reducing bacteria, Thauera aromatica and Aromatoleum aromaticum EbN1. Phenylacetate, 4-hydroxyphenylacetate and indole-3-acetate were formed during anaerobic degradation of phenylalanine, tyrosine and tryptophan, respectively. Candidate genes for enzymes involved in the anaerobic oxidation of phenylalanine to phenylacetate (phenylalanine transaminase, phenylpyruvate decarboxylase and phenylacetaldehyde : ferredoxin oxidoreductase) were identified in the F. placidus genome. In addition, transcription of candidate genes for the anaerobic phenylacetate degradation, benzoyl-CoA degradation and glutaryl-CoA degradation pathways was significantly upregulated in microarray and quantitative real-time-PCR studies comparing phenylacetate-grown cells with acetate-grown cells. These results suggested that the general strategies for anaerobic degradation of aromatic amino acids are highly conserved amongst bacteria and archaea living in both mesophilic and hyperthermophilic environments. They also provided insights into the diverse metabolism of Archaeoglobaceae species living in hyperthermophilic environments.

Genetic examination and mass balance analysis of pyruvate/amino acid oxidation pathways in the hyperthermophilic archaeon Thermococcus kodakarensis

The present study investigated the simultaneous oxidation of pyruvate and amino acids during H2-evolving growth of the hyperthermophilic archaeon Thermococcus kodakarensis. The comparison of mass balance between a cytosolic hydrogenase (HYH)-deficient strain (the ΔhyhBGSL strain) and the parent strain indicated that NADPH generated via H2 uptake by HYH was consumed by reductive amination of 2-oxoglutarate catalyzed by glutamate dehydrogenase. Further examinations were done to elucidate functions of three enzymes potentially involved in pyruvate oxidation: pyruvate formate-lyase (PFL), pyruvate:ferredoxin oxidoreductase (POR), and 2-oxoisovalerate:ferredoxin oxidoreductase (VOR) under the HYH-deficient background in T. kodakarensis. No significant change was observed by deletion of pflDA, suggesting that PFL had no critical role in pyruvate oxidation. The growth properties and mass balances of ΔporDAB and ΔvorDAB strains indicated that POR and VOR specifically functioned in oxidation of pyruvate and branched-chain amino acids, respectively, and the lack of POR or VOR was compensated for by promoting the oxidation of another substrate driven by the remaining oxidoreductase. The H2 yields from the consumed pyruvate and amino acids were increased from 31% by the parent strain to 67% and 82% by the deletion of hyhBGSL and double deletion of hyhBGSL and vorDAB, respectively. Significant discrepancies in the mass balances were observed in excess formation of acetate and NH3, suggesting the presence of unknown metabolisms in T. kodakarensis grown in the rich medium containing pyruvate.

Carbohydrate metabolism in Archaea: current insights into unusual enzymes and pathways and their regulation

The metabolism of Archaea, the third domain of life, resembles in its complexity those of Bacteria and lower Eukarya. However, this metabolic complexity in Archaea is accompanied by the absence of many "classical" pathways, particularly in central carbohydrate metabolism. Instead, Archaea are characterized by the presence of unique, modified variants of classical pathways such as the Embden-Meyerhof-Parnas (EMP) pathway and the Entner-Doudoroff (ED) pathway. The pentose phosphate pathway is only partly present (if at all), and pentose degradation also significantly differs from that known for bacterial model organisms. These modifications are accompanied by the invention of "new," unusual enzymes which cause fundamental consequences for the underlying regulatory principles, and classical allosteric regulation sites well established in Bacteria and Eukarya are lost. The aim of this review is to present the current understanding of central carbohydrate metabolic pathways and their regulation in Archaea. In order to give an overview of their complexity, pathway modifications are discussed with respect to unusual archaeal biocatalysts, their structural and mechanistic characteristics, and their regulatory properties in comparison to their classic counterparts from Bacteria and Eukarya. Furthermore, an overview focusing on hexose metabolic, i.e., glycolytic as well as gluconeogenic, pathways identified in archaeal model organisms is given. Their energy gain is discussed, and new insights into different levels of regulation that have been observed so far, including the transcript and protein levels (e.g., gene regulation, known transcription regulators, and posttranslational modification via reversible protein phosphorylation), are presented.

Cysteine desulphurase plays an important role in environmental adaptation of the hyperthermophilic archaeon Thermococcus kodakarensis

The sulphur atoms of sulphur-containing cofactors that are essential for numerous cellular functions in living organisms originate from L-cysteine via cysteine desulphurase (CSD) activity. However, many (hyper)thermophilic archaea, which thrive in solfataric fields and are positioned near the root of the evolutionary tree of life, lack CSD orthologues. The existence of CSD orthologues in a subset of (hyper)thermophilic archaea is of interest with respect to the evolution of sulphur-trafficking systems for the cofactors. This study demonstrates that the disruption of the csd gene of Thermococcus kodakarensis, a facultative elemental sulphur (S(0))-reducing hyperthermophilic archaeon, encoding Tk-CSD, conferred a growth defect evident only in the absence of S(0), and that growth can be restored by the addition of S(0), but not sulphide. We show that the csd gene is not required for biosynthesis of thiamine pyrophosphate or molybdopterin, irrespective of the presence or absence of S(0), but is necessary for iron-sulphur cluster biosynthesis in the absence of S(0). Recombinant form of Tk-CSD expressed in Escherichia coli was obtained and it was found to catalyse the desulphuration of L-cysteine. The obtained data suggest that hyperthermophiles might benefit from a capacity for CSD-dependent iron-sulphur cluster biogenesis, which allows them to thrive outside solfataric environments.

The bifunctional pyruvate decarboxylase/pyruvate ferredoxin oxidoreductase from Thermococcus guaymasensis

The hyperthermophilic archaeon Thermococcus guaymasensis produces ethanol as a metabolic end product, and an alcohol dehydrogenase (ADH) catalyzing the reduction of acetaldehyde to ethanol has been purified and characterized. However, the enzyme catalyzing the formation of acetaldehyde has not been identified. In this study an enzyme catalyzing the production of acetaldehyde from pyruvate was purified and characterized from T. guaymasensis under strictly anaerobic conditions. The enzyme had both pyruvate decarboxylase (PDC) and pyruvate ferredoxin oxidoreductase (POR) activities. It was oxygen sensitive, and the optimal temperatures were 85°C and >95°C for the PDC and POR activities, respectively. The purified enzyme had activities of 3.8 ± 0.22 U mg(-1) and 20.2 ± 1.8 U mg(-1), with optimal pH-values of 9.5 and 8.4 for each activity, respectively. Coenzyme A was essential for both activities, although it did not serve as a substrate for the former. Enzyme kinetic parameters were determined separately for each activity. The purified enzyme was a heterotetramer. The sequences of the genes encoding the subunits of the bifunctional PDC/POR were determined. It is predicted that all hyperthermophilic β -keto acids ferredoxin oxidoreductases are bifunctional, catalyzing the activities of nonoxidative and oxidative decarboxylation of the corresponding β -keto acids.

Characterization of ten heterotetrameric NDP-dependent acyl-CoA synthetases of the hyperthermophilic archaeon Pyrococcus furiosus

The hyperthermophilic archaeon Pyrococcus furiosus grows by fermenting peptides and carbohydrates to organic acids. In the terminal step, acyl-CoA synthetase (ACS) isoenzymes convert acyl-CoA derivatives to the corresponding acid and conserve energy in the form of ATP. ACS1 and ACS2 were previously purified from P. furiosus and have α₂β₂ structures but the genome contains genes encoding three additional α-subunits. The ten possible combinations of α and β genes were expressed in E. coli and each resulted in stable and active α₂β₂ isoenzymes. The α-subunit of each isoenzyme determined CoA-based substrate specificity and between them they accounted for the CoA derivatives of fourteen amino acids. The β-subunit determined preference for adenine or guanine nucleotides. The GTP-generating isoenzymes are proposed to play a role in gluconeogenesis by producing GTP for GTP-dependent phosphoenolpyruvate carboxykinase and for other GTP-dependent processes. Transcriptional and proteomic data showed that all ten isoenzymes are constitutively expressed indicating that both ATP and GTP are generated from the metabolism of most of the amino acids. A phylogenetic analysis showed that the ACSs of P. furiosus and other members of the Thermococcales are evolutionarily distinct from those found throughout the rest of biology, including those of other hyperthermophilic archaea.

Four Cys residues in heterodimeric 2-oxoacid:ferredoxin oxidoreductase are required for CoA-dependent oxidative decarboxylation but not for a non-oxidative decarboxylation

Heterodimeric 2-oxoacid:ferredoxin oxidoreductase (OFOR) from Sulfolobus tokodaii (StOFOR) has only one [4Fe-4S]²⁺ cluster, ligated by 4 Cys residues, C12, C15, C46, and C197. The enzyme has no other Cys. To elucidate the role of these Cys residues in holding of the iron-sulfur cluster in the course of oxidative decarboxylation of a 2-oxoacid, one or two of these Cys residues was/were substituted with Ala to yield C12A, C15A, C46A, C197A and C12/15A mutants. All the mutants showed the loss of iron-sulfur cluster, except the C197A one which retained some unidentified type of iron-sulfur cluster. On addition of pyruvate to OFOR, the wild type enzyme exhibited a chromophore at 320nm and a stable large EPR signal corresponding to a hydroxyethyl-ThDP radical, while the mutant enzymes did not show formation of any radical intermediate or production of acetyl-CoA, suggesting that the intact [4Fe-4S] cluster is necessary for these processes. The stable radical intermediate in wild type OFOR was rapidly decomposed upon addition of CoA in the absence of an electron acceptor. Non-oxidative decarboxylation of pyruvate, yielding acetaldehyde, has been reported to require CoA for other OFORs, but StOFOR catalyzed acetaldehyde production from pyruvate independent of CoA, regardless of whether the iron-sulfur cluster is intact [4Fe-4S] type or not. A comprehensive reaction scheme for StOFOR with a single cluster was proposed.

Heat-stable enzymes from extremely thermophilic and hyperthermophilic microorganisms

Only in the last decade have microorganisms been discovered which grow near or above 100°C. The enzymes that are formed by these extremely thermophilic (growth temperature 65 to 85°C) and hyperthermophilic (growth temperature 85 to 110°C) microorganisms are of great interest. This review covers the extracellular and intracellular enzymes of these exotic microorganisms that have recently been described. Polymer-hydrolysing enzymes, such as amylolytic, cellulolytic, hemicellulolytic and proteolytic enzymes, will be discussed. In addition, the properties of the intracellular enzymes involved in carbohydrate and amino-acid metabolism and DNA-binding and chaperones and chaperone-like proteins from hyperthermophiles are described. Due to the unusual properties of these heat-stable enzymes, they are expected to fill the gap between biological and chemical processes.

Characterization of two members among the five ADP-forming acyl coenzyme A (Acyl-CoA) synthetases reveals the presence of a 2-(Imidazol-4-yl)acetyl-CoA synthetase in Thermococcus kodakarensis

The genome of Thermococcus kodakarensis, along with those of most Thermococcus and Pyrococcus species, harbors five paralogous genes encoding putative α subunits of nucleoside diphosphate (NDP)-forming acyl coenzyme A (acyl-CoA) synthetases. The substrate specificities of the protein products for three of these paralogs have been clarified through studies on the individual enzymes from Pyrococcus furiosus and T. kodakarensis. Here we have examined the biochemical properties of the remaining two acyl-CoA synthetase proteins from T. kodakarensis. The TK0944 and TK2127 genes encoding the two α subunits were each coexpressed with the β subunit-encoding TK0943 gene. In both cases, soluble proteins with an α2β2 structure were obtained and their activities toward various acids in the ADP-forming reaction were examined. The purified TK0944/TK0943 protein (ACS IIITk) accommodated a broad range of acids that corresponded to those generated in the oxidative metabolism of Ala, Val, Leu, Ile, Met, Phe, and Cys. In contrast, the TK2127/TK0943 protein exhibited relevant levels of activity only toward 2-(imidazol-4-yl)acetate, a metabolite of His degradation, and was thus designated 2-(imidazol-4-yl)acetyl-CoA synthetase (ICSTk), a novel enzyme. Kinetic analyses were performed on both proteins with their respective substrates. In T. kodakarensis, we found that the addition of histidine to the medium led to increases in intracellular ADP-forming 2-(imidazol-4-yl)acetyl-CoA synthetase activity, and 2-(imidazol-4-yl)acetate was detected in the culture medium, suggesting that ICSTk participates in histidine catabolism. The results presented here, together with those of previous studies, have clarified the substrate specificities of all five known NDP-forming acyl-CoA synthetase proteins in the Thermococcales.

Construction of an in vitro bypassed pyruvate decarboxylation pathway using thermostable enzyme modules and its application to N-acetylglutamate production

Background

Metabolic engineering has emerged as a practical alternative to conventional chemical conversion particularly in biocommodity production processes. However, this approach is often hampered by as yet unidentified inherent mechanisms of natural metabolism. One of the possible solutions for the elimination of the negative effects of natural regulatory mechanisms on artificially engineered metabolic pathway is to construct an in vitro pathway using a limited number of enzymes. Employment of thermostable enzymes as biocatalytic modules for pathway construction enables the one-step preparation of catalytic units with excellent selectivity and operational stability. Acetyl-CoA is a central precursor involved in the biosynthesis of various metabolites. In this study, an in vitro pathway to convert pyruvate to acetyl-CoA was constructed and applied to N-acetylglutamate production.

Results

A bypassed pyruvate decarboxylation pathway, through which pyruvate can be converted to acetyl-CoA, was constructed by using a coupled enzyme system consisting of pyruvate decarboxylase from Acetobacter pasteurianus and the CoA-acylating aldehyde dehydrogenase from Thermus thermophilus. To demonstrate the applicability of the bypassed pathway for chemical production, a cofactor-balanced and CoA-recycling synthetic pathway for N-acetylglutamate production was designed by coupling the bypassed pathway with the glutamate dehydrogenase from T. thermophilus and N-acetylglutamate synthase from Thermotoga maritima. N-Acetylglutamate could be produced from an equimolar mixture of pyruvate and α-ketoglutarate with a molar yield of 55% through the synthetic pathway consisting of a mixture of four recombinant E. coli strains having either one of the thermostable enzymes. The overall recycling number of CoA was calculated to be 27.

Conclusions

Assembly of thermostable enzymes enables the flexible design and construction of an in vitro metabolic pathway specialized for chemical manufacture. We herein report the in vitro construction of a bypassed pathway capable of an almost stoichiometric conversion of pyruvate to acetyl-CoA. This pathway is potentially applicable not only to N-acetylglutamate production but also to the production of a wide range of acetyl-CoA-derived metabolites.

Growth profile of Carboxydothermus hydrogenoformans on pyruvate

Carboxydothermus hydrogenoformans is a thermophilic anaerobic strain most widely known for its ability to produce hydrogen (H₂) when grown on carbon monoxide (CO). Although relatively well studied, growth characterization on pyruvate has never been assessed. The present work fully characterizes growth of the bacterium on pyruvate as a sole carbon source. C. hydrogenoformans demonstrated a growth rate of 0.03 h^-1, with pyruvate consumption ranging between 0.21 and 0.48 mol · g^-1 volatile suspended solid · d^-1. A lag phase was also observed when switching from pyruvate to CO. When grown simultaneously on pyruvate and CO, pyruvate consumption was initiated upon CO depletion. This was attributed to pyruvate oxidation inhibition by CO, and not to a diauxic phenomenom. The strain also showed homoacetogenic activity.

Decarboxylation of pyruvate to acetaldehyde for ethanol production by hyperthermophiles

Pyruvate decarboxylase (PDC encoded by pdc) is a thiamine pyrophosphate (TPP)-containing enzyme responsible for the conversion of pyruvate to acetaldehyde in many mesophilic organisms. However, no pdc/PDC homolog has yet been found in fully sequenced genomes and proteomes of hyper/thermophiles. The only PDC activity reported in hyperthermophiles was a bifunctional, TPP- and CoA-dependent pyruvate ferredoxin oxidoreductase (POR)/PDC enzyme from the hyperthermophilic archaeon Pyrococcus furiosus. Another enzyme known to be involved in catalysis of acetaldehyde production from pyruvate is CoA-acetylating acetaldehyde dehydrogenase (AcDH encoded by mhpF and adhE). Pyruvate is oxidized into acetyl-CoA by either POR or pyruvate formate lyase (PFL), and AcDH catalyzes the reduction of acetyl-CoA to acetaldehyde in mesophilic organisms. AcDH is present in some mesophilic (such as clostridia) and thermophilic bacteria (e.g., Geobacillus and Thermoanaerobacter). However, no AcDH gene or protein homologs could be found in the released genomes and proteomes of hyperthermophiles. Moreover, no such activity was detectable from the cell-free extracts of different hyperthermophiles under different assay conditions. In conclusion, no commonly-known PDCs was found in hyperthermophiles. Instead of the commonly-known PDC, it appears that at least one multifunctional enzyme is responsible for catalyzing the non-oxidative decarboxylation of pyruvate to acetaldehyde in hyperthermophiles.

Genetic examination of initial amino acid oxidation and glutamate catabolism in the hyperthermophilic archaeon Thermococcus kodakarensis

Amino acid catabolism in Thermococcales is presumed to proceed via three steps: oxidative deamination of amino acids by glutamate dehydrogenase (GDH) or aminotransferases, oxidative decarboxylation by 2-oxoacid:ferredoxin oxidoreductases (KOR), and hydrolysis of acyl-coenzyme A (CoA) by ADP-forming acyl-CoA synthetases (ACS). Here, we performed a genetic examination of enzymes involved in Glu catabolism in Thermococcus kodakarensis. Examination of amino acid dehydrogenase activities in cell extracts of T. kodakarensis KUW1 (ΔpyrF ΔtrpE) revealed high NADP-dependent GDH activity, along with lower levels of NAD-dependent activity. NADP-dependent activities toward Gln/Ala/Val/Cys and an NAD-dependent threonine dehydrogenase activity were also detected. In KGDH1, a gene disruption strain of T. kodakarensis GDH (Tk-GDH), only threonine dehydrogenase activity was detected, indicating that all other activities were dependent on Tk-GDH. KGDH1 could not grow in a medium in which growth was dependent on amino acid catabolism, implying that Tk-GDH is the only enzyme that can discharge the electrons (to NADP(+)/NAD(+)) released from amino acids in their oxidation to 2-oxoacids. In a medium containing excess pyruvate, KGDH1 displayed normal growth, but higher degrees of amino acid catabolism were observed compared to those for KUW1, suggesting that Tk-GDH functions to suppress amino acid oxidation and plays an anabolic role under this condition. We further constructed disruption strains of 2-oxoglutarate:ferredoxin oxidoreductase and succinyl-CoA synthetase. The two strains displayed growth defects in both media compared to KUW1. Succinate generation was not observed in these strains, indicating that the two enzymes are solely responsible for Glu catabolism among the multiple KOR and ACS enzymes in T. kodakarensis.

Chlamydomonas reinhardtii chloroplasts contain a homodimeric pyruvate:ferredoxin oxidoreductase that functions with FDX1

Eukaryotic algae have long been known to live in anoxic environments, but interest in their anaerobic energy metabolism has only recently gained momentum, largely due to their utility in biofuel production. Chlamydomonas reinhardtii figures remarkably in this respect, because it efficiently produces hydrogen and its genome harbors many genes for anaerobic metabolic routes. Central to anaerobic energy metabolism in many unicellular eukaryotes (protists) is pyruvate:ferredoxin oxidoreductase (PFO), which decarboxylates pyruvate and forms acetyl-coenzyme A with concomitant reduction of low-potential ferredoxins or flavodoxins. Here, we report the biochemical properties of the homodimeric PFO of C. reinhardtii expressed in Escherichia coli. Electron paramagnetic resonance spectroscopy of the recombinant enzyme (Cr-rPFO) showed three distinct [4Fe-4S] iron-sulfur clusters and a thiamine pyrophosphate radical upon reduction by pyruvate. Purified Cr-rPFO exhibits a specific decarboxylase activity of 12 µmol pyruvate min⁻¹ mg⁻¹ protein using benzyl viologen as electron acceptor. Despite the fact that the enzyme is very oxygen sensitive, it localizes to the chloroplast. Among the six known chloroplast ferredoxins (FDX1-FDX6) in C. reinhardtii, FDX1 and FDX2 were the most efficient electron acceptors from Cr-rPFO, with comparable apparent K(m) values of approximately 4 µm. As revealed by immunoblotting, anaerobic conditions that lead to the induction of CrPFO did not increase levels of either FDX1 or FDX2. FDX1, being by far the most abundant ferredoxin, is thus likely the partner of PFO in C. reinhardtii. This finding postulates a direct link between CrPFO and hydrogenase and provides new opportunities to better study and engineer hydrogen production in this protist.

Pyruvate:ferredoxin oxidoreductase is coupled to light-independent hydrogen production in Chlamydomonas reinhardtii

In anaerobiosis, the green alga Chlamydomonas reinhardtii evolves molecular hydrogen (H(2)) as one of several fermentation products. H(2) is generated mostly by the [Fe-Fe]-hydrogenase HYDA1, which uses plant type ferredoxin PETF/FDX1 (PETF) as an electron donor. Dark fermentation of the alga is mainly of the mixed acid type, because formate, ethanol, and acetate are generated by a pyruvate:formate lyase pathway similar to Escherichia coli. However, C. reinhardtii also possesses the pyruvate:ferredoxin oxidoreductase PFR1, which, like pyruvate:formate lyase and HYDA1, is localized in the chloroplast. PFR1 has long been suggested to be responsible for the low but significant H(2) accumulation in the dark because the catalytic mechanism of pyruvate:ferredoxin oxidoreductase involves the reduction of ferredoxin. With the aim of proving the biochemical feasibility of the postulated reaction, we have heterologously expressed the PFR1 gene in E. coli. Purified recombinant PFR1 is able to transfer electrons from pyruvate to HYDA1, using the ferredoxins PETF and FDX2 as electron carriers. The high reactivity of PFR1 toward oxaloacetate indicates that in vivo, fermentation might also be coupled to an anaerobically active glyoxylate cycle. Our results suggest that C. reinhardtii employs a clostridial type H(2) production pathway in the dark, especially because C. reinhardtii PFR1 was also able to allow H(2) evolution in reaction mixtures containing Clostridium acetobutylicum 2[4Fe-4S]-ferredoxin and [Fe-Fe]-hydrogenase HYDA.

Mechanism of oxygen detoxification by the surprisingly oxygen-tolerant hyperthermophilic archaeon, Pyrococcus furiosus

The anaerobic archaeon Pyrococcus furiosus grows by fermenting carbohydrates producing H(2), CO(2), and acetate. We show here that it is surprisingly tolerant to oxygen, growing well in the presence of 8% (vol/vol) O(2). Although cell growth and acetate production were not significantly affected by O(2), H(2) production was reduced by 50% (using 8% O(2)). The amount of H(2) produced decreased in a linear manner with increasing concentrations of O(2) over the range 2-12% (vol/vol), and for each mole of O(2) consumed, the amount of H(2) produced decreased by approximately 2 mol. The recycling of H(2) by the two cytoplasmic hydrogenases appeared not to play a role in O(2) resistance because a mutant strain lacking both enzymes was not more sensitive to O(2) than the parent strain. Decreased H(2) production was also not due to inactivation of the H(2)-producing, ferredoxin-dependent membrane-bound hydrogenase because its activity was unaffected by O(2) exposure. Electrons from carbohydrate oxidation must therefore be diverted to relieve O(2) stress at the level of reduced ferredoxin before H(2) production. Deletion strains lacking superoxide reductase (SOR) and putative flavodiiron protein A showed increased sensitivity to O(2), indicating that these enzymes play primary roles in resisting O(2). However, a mutant strain lacking the proposed electron donor to SOR, rubredoxin, was unaffected in response to O(2). Hence, electrons from sugar oxidation normally used to produce H(2) are diverted to O(2) detoxification by SOR and putative flavodiiron protein A, but the electron flow pathway from ferredoxin does not necessarily involve rubredoxin.|

The modular respiratory complexes involved in hydrogen and sulfur metabolism by heterotrophic hyperthermophilic archaea and their evolutionary implications

Hydrogen production is a vital metabolic process for many anaerobic organisms, and the enzyme responsible, hydrogenase, has been studied since the 1930s. A novel subfamily with unique properties was recently recognized, represented by the 14-subunit membrane-bound [NiFe] hydrogenase from the archaeon Pyrococcus furiosus. This so-called energy-converting hydrogenase links the thermodynamically favorable oxidation of ferredoxin with the formation of hydrogen and conserves energy in the form of an ion gradient. It is therefore a simple respiratory system within a single complex. This hydrogenase shows a modular composition represented by a Na(+)/H(+) antiporter domain (Mrp) and a [NiFe] hydrogenase domain (Mbh). An analysis of the large number of microbial genome sequences available shows that homologs of Mbh and Mrp tend to be clustered within the genomes of a limited number of archaeal and bacterial species. In several instances, additional genes are associated with the Mbh and Mrp gene clusters that encode proteins that catalyze the oxidation of formate, CO or NAD(P)H. The Mbh complex also shows extensive homology to a number of subunits within the NADH quinone oxidoreductase or complex I family. The respiratory-type membrane-bound hydrogenase complex appears to be closely related to the common ancestor of complex I and [NiFe] hydrogenases in general.

Indolepyruvate ferredoxin oxidoreductase: An oxygen-sensitive iron-sulfur enzyme from the hyperthermophilic archaeon Thermococcus profundus

Thermococcus profundus is a strictly anaerobic sulfur-dependent archaeon that grows optimally at 80°C by peptide fermentation. Indolepyruvate ferredoxin oxidoreductase (IOR), an enzyme involved in the peptide fermentation pathway, was purified to homogeneity from the archaeon under strictly anaerobic conditions. The maximal activity was obtained above the boiling temperature of water (105°C), with a half-life of 62min at 100°C and 20min at 105°C. IOR was oxygen-sensitive with a half-life of 7h at 25°C under aerobic conditions. The specific activity of T. profundus IOR was found to be dependent on the number of [4Fe-4S] clusters in the enzyme.

Radical reactions of thiamin pyrophosphate in 2-oxoacid oxidoreductases

Thiamin pyrophosphate (TPP) is essential in carbohydrate metabolism in all forms of life. TPP-dependent decarboxylation reactions of 2-oxo-acid substrates result in enamine adducts between the thiazolium moiety of the coenzyme and decarboxylated substrate. These central enamine intermediates experience different fates from protonation in pyruvate decarboxylase to oxidation by the 2-oxoacid dehydrogenase complexes, the pyruvate oxidases, and 2-oxoacid oxidoreductases. Virtually all of the TPP-dependent enzymes, including pyruvate decarboxylase, can be assayed by 1-electron redox reactions linked to ferricyanide. Oxidation of the enamines is thought to occur via a 2-electron process in the 2-oxoacid dehydrogenase complexes, wherein acyl group transfer is associated with reduction of the disulfide of the lipoamide moiety. However, discrete 1-electron steps occur in the oxidoreductases, where one or more [4Fe-4S] clusters mediate the electron transfer reactions to external electron acceptors. These radical intermediates can be detected in the absence of the acyl-group acceptor, coenzyme A (CoASH). The π-electron system of the thiazolium ring stabilizes the radical. The extensively delocalized character of the radical is evidenced by quantitative analysis of nuclear hyperfine splitting tensors as detected by electron paramagnetic resonance (EPR) spectroscopy and by electronic structure calculations. The second electron transfer step is markedly accelerated by the presence of CoASH. While details of the second electron transfer step and its facilitation by CoASH remain elusive, expected redox properties of potential intermediates limit possible scenarios. This article is part of a Special Issue entitled: Radical SAM enzymes and Radical Enzymology.

Biochemical and genetic characterization of the three metabolic routes in Thermococcus kodakarensis linking glyceraldehyde 3-phosphate and 3-phosphoglycerate

In the classical Embden-Meyerhof (EM) pathway for glycolysis, the conversion between glyceraldehyde 3-phosphate (GAP) and 3-phosphoglycerate (3-PGA) is reversibly catalysed by phosphorylating GAP dehydrogenase (GAPDH) and phosphoglycerate kinase (PGK). In the Euryarchaeota Thermococcus kodakarensis and Pyrococcus furiosus, an additional gene encoding GAP:ferredoxin oxidoreductase (GAPOR) and a gene similar to non-phosphorylating GAP dehydrogenase (GAPN) are present. In order to determine the physiological roles of the three routes that link GAP and 3-PGA, we individually disrupted the GAPOR, GAPN, GAPDH and PGK genes (gor, gapN, gapDH and pgk respectively) of T. kodakarensis. The Δgor strain displayed no growth under glycolytic conditions, confirming its proposed function to generate reduced ferredoxin for energy generation in glycolysis. Surprisingly, ΔgapN cells also did not grow under glycolytic conditions, suggesting that GAPN plays a key role in providing NADPH under these conditions. Disruption of gor and gapN had no effect on gluconeogenic growth. Growth experiments with the ΔgapDH and Δpgk strains indicated that, unlike their counterparts in the classical EM pathway, GAPDH/PGK play a major role only in gluconeogenesis. Biochemical analyses indicated that T. kodakarensis GAPN did not recognize aldehyde substrates other than d-GAP, preferred NADP(+) as cofactor and was dramatically activated with glucose 1-phosphate.

Enzymatic and electron paramagnetic resonance studies of anabolic pyruvate synthesis by pyruvate: ferredoxin oxidoreductase from Hydrogenobacter thermophilus

Pyruvate: ferredoxin oxidoreductase (POR; EC 1.2.7.1) catalyzes the thiamine pyrophosphate-dependent oxidative decarboxylation of pyruvate to form acetyl-CoA and CO(2). The thermophilic, obligate chemolithoautotrophic hydrogen-oxidizing bacterium, Hydrogenobacter thermophilus TK-6, assimilates CO(2) via the reductive tricarboxylic acid cycle. In this cycle, POR acts as pyruvate synthase catalyzing the reverse reaction (i.e. reductive carboxylation of acetyl-CoA) to form pyruvate. The pyruvate synthesis reaction catalyzed by POR is an energetically unfavorable reaction and requires a strong reductant. Moreover, the reducing equivalents must be supplied via its physiological electron mediator, a small iron-sulfur protein ferredoxin. Therefore, the reaction is difficult to demonstrate in vitro and the reaction mechanism has been poorly understood. In the present study, we coupled the decarboxylation of 2-oxoglutarate catalyzed by 2-oxoglutarate: ferredoxin oxidoreductase (EC 1.2.7.3), which generates sufficiently low-potential electrons to reduce ferredoxin, to drive the energy-demanding pyruvate synthesis by POR. We demonstrate that H. thermophilus POR catalyzes pyruvate synthesis from acetyl-CoA and CO(2), confirming the operation of the reductive tricarboxylic acid cycle in this bacterium. We also measured the electron paramagnetic resonance spectra of the POR intermediates in both the forward and reverse reactions, and demonstrate the intermediacy of a 2-(1-hydroxyethyl)- or 2-(1-hydroxyethylidene)-thiamine pyrophosphate radical in both reactions. The reaction mechanism of the reductive carboxylation of acetyl-CoA is also discussed.