Detection and quantification of 10-formyltetrahydrofolate dehydrogenase (10-FTHFDH) in rat retina,optic nerve,and brain

Methanol poisoning is characterized by the accumulation of formic acid, a metabolite of methanol, which can lead to metabolic acidosis and ocular toxicity. Formate metabolism to CO₂ is governed by tissue H₄folate and 10-FTHFDH levels. Presumably, rats are not normally susceptible to formate toxicity because they possess high hepatic H₄folate and 10-FTHFDH levels. However, the ability of target tissues to metabolize formate is not known. Therefore, studies were performed to determine whether 10-FTHFDH was present in rat retina, optic nerve, and brain. 10-FTHFDH levels were determined using Western blot analysis of mitochondiral and postmitochondrial preparations from these tissues. Hepatic mitochandrial and postmitochondrial levels of 10-FTHFDH were 13 and 12 ng/μg protein, respectively. Postmitochondrial levels of 10-FTHFDH in rat retina, optic nerve and whole brain were 0.2, 1.3, and 2.1 ng/μg protein; mitochondrial values in retina and brain were 0.2 and 1.5 ng/μg protein, respectively. Postmitochondrial values obtained for rat brain regions were similar to those found for whole brain. These results suggest that, in rats, target tissues possess the capacity to metabolize formate to CO₂ and may be protected from formate toxicity through this folate-dependent system.     

Light Driven CO2 Fixation by Using Cyanobacterial Photosystem I and NADPH-Dependent Formate Dehydrogenase

The ultimate goal of this research is to construct a new direct CO₂ fixation system using photosystems in living algae. Here, we report light-driven formate production from CO₂ by using cyanobacterial photosystem I (PS I). Formate, a chemical hydrogen carrier and important industrial material, can be produced from CO₂ by using the reducing power and the catalytic function of formate dehydrogenase (FDH). We created a bacterial FDH mutant that experimentally switched the cofactor specificity from NADH to NADPH, and combined it with an in vitro-reconstituted cyanobacterial light-driven NADPH production system consisting of PS I, ferredoxin (Fd), and ferredoxin-NADP⁺-reductase (FNR). Consequently, light-dependent formate production under a CO₂ atmosphere was successfully achieved. In addition, we introduced the NADPH-dependent FDH mutant into heterocysts of the cyanobacterium Anabaena sp. PCC 7120 and demonstrated an increased formate concentration in the cells. These results provide a new possibility for photo-biological CO₂ fixation.     

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

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

Energetics and regulations of formate and hydrogen metabolism by Methanobacterium formicicum

Accumulation of formate to millimolar levels was observed during the growth of Methanobacterium formicicum species on H₂–CO₂. Hydrogen was also produced during formate metabolism by M. formicicum. The amount of formate accumulated in the medium or the amount H₂ released in gas phase was influenced by the bicarbonate concentration. The formate hydrogenlyase system was constitutive but regulated by formate. When methanogenesis was inhibited by addition of 2-bromoethane sulfonate, M. formicicum synthesized formate from H₂ plus HCO _inf3 ^sup- or produced H₂ from formate to a steady-state level at which point the Gibbs free energy (G) available for formate synthesis or H₂ production was approximately -2 to -3 kJ/reaction. Formate conversion to methane was inhibited in the presence of high H₂ pressure. The relative rates of conversion of formate and H₂ were apparently controlled by the G available for formate synthesis, hydrogen production, methane production from formate and methane production from H₂. Results from ¹⁴C-tracer tests indicated that a rapid isotopic exchange between HCOO^- and HCO _inf3 ^sup- occurred during the growth of M. formicicum on H₂–CO₂. Data from metabolism of ¹⁴C-labelled formate to methane suggested that formate was initially split to H₂ and HCO _inf3 ^sup- and then subsequently converted to methane. When molybdate was replaced with tungstate in the growth media, the growth of M. formicicum strain MF on H₂–CO₂ was inhibited although production of methane was not Formate synthesis from H₂ was also inhibited.     

Anaerobic Degradation of Uric Acid by Gut Bacteria of Termites

A study was done of anaerobic degradation of uric acid (UA) by representative strains of uricolytic bacteria isolated from guts of Reticulitermes flavipes termites. Streptococcus strain UAD-1 degraded UA incompletely, secreting a fluorescent compound into the medium, unless formate (or a formicogenic compound) was present as a cosubstrate. Formate functioned as a reductant, and its oxidation to CO₂ by formate dehydrogenase provided 2H⁺ + 2e⁻ needed to drive uricolysis to completion. Uricolysis by Streptococcus UAD-1 thus corresponded to the following equation: 1UA + 1formate → 4CO₂ + 1acetate + 4NH₃. Urea did not appear to be an intermediate in CO₂ and NH₃ formation during uricolysis by strain UAD-1. Formate dehydrogenase and uricolytic activities of strain UAD-1 were inducible by growth of cells on UA. Bacteroides termitidis strain UAD-50 degraded UA as follows: 1UA → 3.5 CO₂ + 0.75acetate + 4NH₃. Exogenous formate was neither required for nor stimulatory to uricolysis by strain UAD-50. Studies of UA catabolism by Citrobacter strains were limited, because only small amounts of UA were metabolized by cells in liquid medium. Uricolytic activity of such bacteria in situ could be important to the carbon, nitrogen, and energy economy of R. flavipes.     

Photosynthesis in the Higher Plant Vicia faba: V. Role of Malate as a Precursor of the Tricarboxylic Acid Cycle

In the higher plant Vicia faba, anomalous labeling patterns in the organic acids and related amino acids of the tricarboxylic acid cycle which result from photosynthetic ¹⁴CO₂ fixation (in conjunction with an enzyme localization pattern unique to plant mitochondria) suggest that the tricarboxylic acid cycle functions primarily as a pathway leading to glutamic acid biosynthesis during autotrophic growth. The distribution of isotope in citrate indicates little recycling of oxaloacetate for the resynthesis of citrate. Rather, malate appears to provide both the C₂ and C₄ fragments for the synthesis of citrate, and [³H]formate and ¹⁴CO₂-labeling patterns implicate serine as the ultimate C₃ precursor of malate.     

Efficient CO2-Reducing Activity of NAD-Dependent Formate Dehydrogenase from Thiobacillus sp. KNK65MA for Formate Production from CO2 Gas

NAD-dependent formate dehydrogenase (FDH) from Candida boidinii (CbFDH) has been widely used in various CO₂-reduction systems but its practical applications are often impeded due to low CO₂-reducing activity. In this study, we demonstrated superior CO₂-reducing properties of FDH from Thiobacillus sp. KNK65MA (TsFDH) for production of formate from CO₂ gas. To discover more efficient CO₂-reducing FDHs than a reference enzyme, i.e. CbFDH, five FDHs were selected with biochemical properties and then, their CO₂-reducing activities were evaluated. All FDHs including CbFDH showed better CO₂-reducing activities at acidic pHs than at neutral pHs and four FDHs were more active than CbFDH in the CO₂ reduction reaction. In particular, the FDH from Thiobacillus sp. KNK65MA (TsFDH) exhibited the highest CO₂-reducing activity and had a dramatic preference for the reduction reaction, i.e., a 84.2-fold higher ratio of CO₂ reduction to formate oxidation in catalytic efficiency (k _cat/K _B) compared to CbFDH. Formate was produced from CO₂ gas using TsFDH and CbFDH, and TsFDH showed a 5.8-fold higher formate production rate than CbFDH. A sequence and structural comparison showed that FDHs with relatively high CO₂-reducing activities had elongated N- and C-terminal loops. The experimental results demonstrate that TsFDH can be an alternative to CbFDH as a biocatalyst in CO₂ reduction systems.     

Reevaluation of the role of bicarbonate and formate in the regulation of photosynthetic electron flow in broken chloroplasts

The stimulation of the Hill reaction in CO₂-depleted broken chloroplasts (Pisum sativum L. cv Rondo) by the total amount of dissolved CO₂ and HCO₃⁻ (bicarbonate^*) was measured at several formate concentrations. Formate appears to be a competitive inhibitor of the bicarbonate^* stimulation of electron flow. From these experiments we have obtained a reactivation constant (K_r) of 78 ± 31 micromolar NaHCO₃ and an inhibition constant (K_i) of 2.0 ± 0.7 millimolar HCOONa at pH 6.5. In the absence of formate, significant electron flow was measured at a bicarbonate^* concentration well below K_r, suggesting that electron flow from Q, the primary electron acceptor of photosystem II, to plastoquinone can proceed when no bicarbonate^* is bound to the regulatory site at the Q_B-protein. If so, bicarbonate^* stimulation of electron flow is mainly a diminution of the inhibition of electron flow by formate. In view of the results, it is proposed that regulation of linear electron flow by bicarbonate^* and formate is a mechanism that could link cell metabolism to photosynthetic electron flow.     

Transient Production of Formate During Chemolithotrophic Growth of Anaerobic Microorganisms on Hydrogen

The homoacetogenic bacteria Acetobacterium woodii, A. carbinolicum, Sporomusa ovata, and Eubacterium limosum, the methanogenic archaeon Methanobacterium formicicum, and the sulfate-reducing bacterium Desulfotomaculum orientis all produced formate as an intermediate when they were growing chemolithoautotrophically with H₂ and CO₂ as sources of energy, electrons, and carbon. The sulfate-reducing bacterium Desulfovibrio vulgaris grew chemolithoheterotrophically with H₂ and CO₂ using acetate as carbon source, but also produced formate when growth was limited by sulfate. All these bacteria were also able to grow on formate as energy source. Formate accumulated transiently while H₂ was consumed. The maximum formate concentrations measured in cultures of A. woodii and A. carbinolicum were proportional to the initial H₂ partial pressure, giving a ratio of about 0.5 mM formate per 10 kPa H₂. The methanogen Methanobacterium bryantii, on the other hand, was unable to grow on formate and did not produce formate during chemolithoautotrophic growth on H₂. The results indicate that the ability to utilize formate, that is, to possess a formate dehydrogenase, was the precondition for the production of formate during chemolithotrophic growth on H₂. Received: 24 November 1998 / Accepted: 30 December 1998     

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.     

The use of 3-amino-1,2,4-triazole to investigate the short-term effects of oxygen toxicity on carbon assimilation by Pisum sativum seedlings

To study the in vivo short-term effect of hydrogen peroxide on plant metabolism, 2 mol m^?3 3-amino-1,2,4-triazole, a catalase inhibitor, was applied through the transpiration stream to Pisum sativum seedlings, and gas exchange characteristics, ascorbate peroxidase, glutathione reductase and catalase activities, and levels of hydrogen peroxide and formate were determined. Carbon dioxide assimilation rates were inhibited after the addition of aminotriazole: photorespiratory conditions exacerbated this inhibition. Carbon dioxide response curves showed that aminotriazole reduced both the RuBP regeneration rate and the efficiency of the carboxylation reaction of Rubisco. Catalase activity was completely inhibited 200 min after the application of this inhibitor, but no concomitant increase in H₂O₂ concentration was found. Under enhanced photorespiratory conditions, H₂O₂ concentrations increased. This suggests that under normal environmental conditions hydrogen peroxide is metabolized via alternative mechanisms. The aminotriazole treatment had no effect on the ascotbate peroxidase and glutathione reductase activities, but caused a substantial increase in the formate pool size. These results suggest that hydrogen peroxide is metabolized by reacting with glyoxylate to produce formate and CO₂. The increased production of formate may reduce the flow of carbon through the normal photorespiratory pathway and may also be used anaplerotically as a precursor of products of 1-C metabolism other than serine. This would prevent the return of photorespiratory carbon to the RPP pathway, leading to a smaller RuBP pool size which would in turn result in a decrease in carboxylation conductance (carboxylation efficiency) and regeneration rate of RuBP.     

Control of Interspecies Electron Flow during Anaerobic Digestion: Significance of Formate Transfer versus Hydrogen Transfer during Syntrophic Methanogenesis in Flocs

Microbial formate production and consumption during syntrophic conversion of ethanol or lactate to methane was examined in purified flocs and digestor contents obtained from a whey-processing digestor. Formate production by digestor contents or purified digestor flocs was dependent on CO₂ and either ethanol or lactate but not H₂ gas as an electron donor. During syntrophic methanogenesis, flocs were the primary site for formate production via ethanol-dependent CO₂ reduction, with a formate production rate and methanogenic turnover constant of 660 μM/h and 0.044/min, respectively. Floc preparations accumulated fourfold-higher levels of formate (40 μM) than digestor contents, and the free flora was the primary site for formate cleavage to CO₂ and H₂ (90 μM formate per h). Inhibition of methanogenesis by CHCl₃ resulted in formate accumulation and suppression of syntrophic ethanol oxidation. H₂ gas was an insignificant intermediary metabolite of syntrophic ethanol conversion by flocs, and its exogenous addition neither stimulated methanogenesis nor inhibited the initial rate of ethanol oxidation. These results demonstrated that >90% of the syntrophic ethanol conversion to methane by mixed cultures containing primarily Desulfovibrio vulgaris and Methanobacterium formicicum was mediated via interspecies formate transfer and that <10% was mediated via interspecies H₂ transfer. The results are discussed in relation to biochemical thermodynamics. A model is presented which describes the dynamics of a bicarbonate-formate electron shuttle mechanism for control of carbon and electron flow during syntrophic methanogenesis and provides a novel mechanism for energy conservation by syntrophic acetogens.     

Replacing the Calvin cycle with the reductive glycine pathway in Cupriavidus necator

Formate can be directly produced from CO₂ and renewable electricity, making it a promising microbial feedstock for sustainable bioproduction. Cupriavidus necator is one of the few biotechnologically-relevant hosts that can grow on formate, but it uses the Calvin cycle, the high ATP cost of which limits biomass and product yields. Here, we redesign C. necator metabolism for formate assimilation via the synthetic, highly ATP-efficient reductive glycine pathway. First, we demonstrate that the upper pathway segment supports glycine biosynthesis from formate. Next, we explore the endogenous route for glycine assimilation and discover a wasteful oxidation-dependent pathway. By integrating glycine biosynthesis and assimilation we are able to replace C. necator's Calvin cycle with the synthetic pathway and achieve formatotrophic growth. We then engineer more efficient glycine metabolism and use short-term evolution to optimize pathway activity. The final growth yield we achieve (2.6 gCDW/mole-formate) nearly matches that of the WT strain using the Calvin Cycle (2.9 gCDW/mole-formate). We expect that further rational and evolutionary optimization will result in a superior formatotrophic C. necator strain, paving the way towards realizing the formate bio-economy.     

Eubacterium acidaminophilum sp. nov., a versatile amino acid-degrading anaerobe producing or utilizing H2 or formate

An obligately anaerobic, rod-shaped bacterium was isolated on alanine in co-culture with H₂-scavenging Desulfovibrio and obtained in pure culture with glycine as sole fermentation substrate. The isolated strain, al-2, was motile by a polar to subpolar flagellum and stained Gram-positive. The guanine plus cytosine content of the DNA was 44.0 mol%. Strain al-2 grew in defined, reduced glycine media supplemented with biotin. The pure culture fermented 4 mol glycine to 3 mol acetate, 4 mol ammonia and 2 mol CO₂. Under optimum conditions (34°C, pH 7.3), the doubling time on glycine was 60 min and the molar growth yield 7.6 g cell dry mass. Serine was fermented to acetate, ethanol, CO₂, H₂ and ammonia. In addition, betaine, sarcosine or creatine served as substrates for growth and acetate production if H₂, formate or e.g. valine were added as H-donors. In pure culture on alanine under N₂, strain al-2 grew very poorly and produced H₂ up to a partial pressure of 3.6 kPa (0.035 atm). Desulfovibrio species, Methanospirillum hungatei and Acetobacterium woodii served as H₂-scavengers that allowed good syntrophic growth on alanine. The co-cultures also grew on aspartate, leucine, valine or malate. Alanine and aspartate were stoichiometrically degraded to acetate and ammonia, whereas the reducing equivalents were recovered as H₂S, CH₄ or newly synthetized acetate, respectively. Growth of strain al-2 in co-culture with the hydrogenase-negative, formate-utilizing Desulfovibrio baarsii indicated that a syntrophy was also possible by interspecies formate transfer. Growth on glycine, or on betaine, sarcosine or creatine (plus H-donors) depended strictly on the addition of selenite (0.1 M); selenite was not required for fermentation of serine, or for degradation of alanine, aspartate or valine by the co-cultures. Cell-free extracts of glycine-grown cells contained active glycine reductase, glycine decarboxylase and reversible methyl viologen-dependent formate dehydrogenase in addition to the other enzymes necessary for an oxidation to CO₂. In all reactions NADP was the preferred H-carrier. Both formate and glycine could be synthesized from bicarbonate. Serine-grown cells did not contain serine hydroxymethyl transferase but serine dehydratase and other enzymes commonly involved in pyruvate metabolism to acetate, CO₂ and H₂. The enzymes involved in glycine metabolism were repressed during growth on serine. By its morphology and physiology, strain al-2 did not resemble described amino acid-degrading species. Therefore, the new isolate is proposed as type strain of a new species, Eubacterium acidaminophilum.     

Relationships between glycollate and folate metabolism in Euglena gracilis

When division synchronized cultures of Euglena gracilis Klebs (strain Z) were aerated with 5% CO₂ in air the specific activity of glycollate dehydrogenase was only 13% of that in cultures receiving unsupplemented air. The concentrations of 10-formyltetrahydrofolate synthetase (EC 6.3.4.3) and formylfolate derivatives were also lowered by this treatment. In contrast, the specific activity of serine hydroxymethyltransferase (EC 2.1.2.1) and the concentration of methylfolates were raised by supplying CO₂-supplemented air. These effects on enzyme levels were reversed when air was supplied following a period of CO₂ treatment. The levels of glycollate dehydrogenase, 10-formyl-tetrahydrofolate synthetase and formylfolate derivatives were decreased when cells were aerated in media containing 5 mM α-hydroxy-2-pyridinemethane sulphonate. Cell free extracts had the ability to decarboxylate glyoxylate, producing ca equal amounts of CO₂ and formate from C-1 and C-2 respectively. Cells receiving 5% CO₂ in air had a decreased ability to incorporate formate-[¹⁴C] into serine and methionine. It is concluded that during growth at low CO₂ concentrations glycollate metabolism will provide substrate for the formyltetrahydrofolate synthetase reaction.     

Formation and metabolism of glycolate in the cyanobacterium Coccochloris peniocystis

The formation and metabolism of glycolate in the cyanobacterium Coccochloris peniocystis was investigated and the activities of enzymes of glycolate metabolism assayed. Photosynthetic ¹⁴CO₂ incorporation was O₂ insensitive and no labelled glycolate could be detected in cells incubated at 2 and 21% O₂. Under conditions of 100% O₂ glycolate comprised less than 1% of the acid-stable products indicating ribulose 1,5 bisphosphate (RuBP) oxidation only occurs under conditions of extreme O₂ stress. Metabolism of [1-¹⁴C] glycolate indicated that as much as 62% of ¹⁴C metabolized was released as ¹⁴CO₂ in the dark. Metabolism of labelled glycolate, particularly incorporation of ¹⁴C into glycine, was inhibited by the amino-transferase inhibitor amino-oxyacetate. Metabolism of [2-¹⁴C] glycine was not inhibited by the serine hydroxymethyltransferase inhibitor isonicotinic acid hydrazide and little or no labelled serine was detected as a result of ¹⁴C-glycolate metabolism. These findings indicate that a significant amount of metabolized glycolate is totally oxidized to CO₂ via formate. The remainder is converted to glycine or metabolized via a glyoxylate cycle. The conversion of glycine to serine contributes little to glycolate metabolism and the absence of hydroxypyruvate reductase confirms that the glycolate pathway is incomplete in this cyanobacterium.Abbreviations AAN aminoacetonitrile - AOA aminooxyacetate - DIC dissolved inorganic carbon - INH isonicotinic acid hydrazide - PEP phosphoenolpyruvate - PEPcase phosphoenolpyruvate carboxylase - PG phosphoglycolate - PGA phosphoglyceric acid - PGPase phosphoglycolate phosphatase - PR photorespiration - Rubisco ribulose-1,5-bisphosphate carboxylase oxygenase - TCA trichloroacetic acid - RuBP ribulose-1,5-bisphosphate     

Anaerobic Degradation of Propionate by a Mesophilic Acetogenic Bacterium in Coculture and Triculture with Different Methanogens

A mesophilic acetogenic bacterium (MPOB) oxidized propionate to acetate and CO₂ in cocultures with the formate- and hydrogen-utilizing methanogens Methanospirillum hungatei and Methanobacterium formicicum. Propionate oxidation did not occur in cocultures with two Methanobrevibacter strains, which grew only with hydrogen. Tricultures consisting of MPOB, one of the Methanobrevibacter strains, and organisms which are able to convert formate into H₂ plus CO₂ (Desulfovibrio strain G11 or the homoacetogenic bacterium EE121) also degraded propionate. The MPOB, in the absence of methanogens, was able to couple propionate conversion to fumarate reduction. This propionate conversion was inhibited by hydrogen and by formate. Formate and hydrogen blocked the energetically unfavorable succinate oxidation to fumarate involved in propionate catabolism. Low formate and hydrogen concentrations are required for the syntrophic degradation of propionate by MPOB. In triculture with Methanospirillum hungatei and the aceticlastic Methanothrix soehngenii, propionate was degraded faster than in biculture with Methanospirillum hungatei, indicating that low acetate concentrations are favorable for propionate oxidation as well.     

Mitochondrial one-carbon metabolism is adapted to the specific needs of yeast, plants and mammals

In eukaryotes, folate metabolism is compartmentalized between the cytoplasm and organelles. The folate pathways of mitochondria are adapted to serve the metabolism of the organism. In yeast, mitochondria support cytoplasmic purine synthesis through the generation of formate. This pathway is important but not essential for survival, consistent with the flexibility of yeast metabolism. In plants, the mitochondrial pathways support photorespiration by generating serine from glycine. This pathway is essential under photosynthetic conditions and the enzyme expression varies with photosynthetic activity. In mammals, the expression of the mitochondrial enzymes varies in tissues and during development. In embryos, mitochondria supply formate and glycine for purine synthesis, a process essential for survival; in adult tissues, flux through mitochondria can favor serine production. The differences in the folate pathways of mitochondria depending on species, tissues and developmental stages, profoundly alter the nature of their metabolic contribution.