Propionate metabolism in a methanogenic enrichment culture. Direct reductive carboxylation and acetogenesis pathways

Abstract Serial dilutions of methanogenic sludges in propionate medium gave a methanogenic non-acetoclastic enrichment degrading 1 mol of propionate to 1.6 mol of acetate and 0.17 mol of methane, with a transient accumulation of butyrate. NMR recordings showed the conversion of [2-¹³C]- and [3-¹³C]-propionate to [3-¹³C]- and [4-¹³C]-butyrate, respectively, thus demonstrating a reductive carboxylation of propionate to butyrate. The labelling found in the accumulated acetate and fermentation balances also suggested that reductive carboxylation was the major pathway involved in propionate conversion to acetate.     

In vivo nuclear magnetic resonance studies of the metabolism of methanol and pyruvate by Methanosarcina barkeri

Abstract The metabolism of methanol and pyruvate by cells of Methanosarcina barkeri was probed in vivo by NMR taking advantage of the non-invasive characteristics of this technique. Upon administration of substrates, the kinetics of substrate consumption, the product formation and the energetic state of the cells was monitored using carbon-13, phosphorus-31 or proton NMR. The effects of several inhibitors and uncouplers were investigated. Cells supplied with pyruvate developed considerable levels of nucleotide triphosphate; methane production was monitored, as well as CO₂ and H₂ formation. Most of the pyruvate was utilized for the synthesis of valine or intermediates of the valine pathway. The origin of the carbon atom in methane was elucidated using ¹³C-labelled pyruvate.     

New insights into the butyric acid metabolism of Clostridium acetobutylicum

Biosynthesis of acetone and n-butanol is naturally restricted to the group of solventogenic clostridia with Clostridium acetobutylicum being the model organism for acetone-butanol-ethanol (ABE) fermentation. According to limited genetic tools, only a few rational metabolic engineering approaches were conducted in the past to improve the production of butanol, an advanced biofuel. In this study, a phosphotransbutyrylase-(Ptb) negative mutant, C. acetobutylicum ptb::int(87), was generated using the ClosTron methodology for targeted gene knock-out and resulted in a distinct butyrate-negative phenotype. The major end products of fermentation experiments without pH control were acetate (3.2?g/l), lactate (4.0?g/l), and butanol (3.4?g/l). The product pattern of the ptb mutant was altered to high ethanol (12.1?g/l) and butanol (8.0?g/l) titers in pH?≥?5.0-regulated fermentations. Glucose fed-batch cultivation elevated the ethanol concentration to 32.4?g/l, yielding a more than fourfold increased alcohol to acetone ratio as compared to the wildtype. Although butyrate was never detected in cultures of C. acetobutylicum ptb::int(87), the mutant was still capable to take up butyrate when externally added during the late exponential growth phase. These findings suggest that alternative pathways of butyrate re-assimilation exist in C. acetobutylicum, supposably mediated by acetoacetyl-CoA:acyl-CoA transferase and acetoacetate decarboxylase, as well as reverse reactions of butyrate kinase and Ptb with respect to previous studies.     

The effects of traT insertion mutations on detergent sensitivity and serum resistance of Escherichia coli and Salmonella typhimurium

The SS-A mutation of Salmonella typhimurium, which probably causes the production of a mutated TraT-like protein, sensitizes the bacteria to hydrophobic antibiotics. A similar phenotype is caused by insertion mutations in the cloned traT gene of R6-5. While the SS-A mutants are resistant to detergents and have unaltered serum resistance, the insertion mutations sensitize both S. typhimurium and Escherichia coli to detergents and abolish the increase in serum resistance caused by the wild-type traT gene product in E. coli.     

Impact of iron reduction on the metabolism of Clostridium acetobutylicum

Iron is essential for most living organisms. In addition, its biogeochemical cycling influences important processes in the geosphere (e.g., the mobilization or immobilization of trace elements and contaminants). The reduction of Fe(III) to Fe(II) can be catalysed microbially, particularly by metal-respiring bacteria utilizing Fe(III) as a terminal electron acceptor. Furthermore, Gram-positive fermentative iron reducers are known to reduce Fe(III) by using it as a sink for excess reducing equivalents, as a form of enhanced fermentation. Here, we use the Gram-positive fermentative bacterium Clostridium acetobutylicum as a model system due to its ability to reduce heavy metals. We investigated the reduction of soluble and solid iron during fermentation. We found that exogenous (resazurin, resorufin, anthraquinone-2,6-disulfonate) as well as endogenous (riboflavin) electron mediators enhance solid iron reduction. In addition, iron reduction buffers the pH, and elicits a shift in the carbon and electron flow to less reduced products relative to fermentation. This study underscores the role fermentative bacteria can play in iron cycling and provides insights into the metabolic profile of coupled fermentation and iron reduction with laboratory experiments and metabolic network modelling.     

Effects of various alcoholic supplements on the growth rate of Clostridium acetobutylicum ATCC 824

Summary The inhibitory effect of various alkanols, benzyl alcohol and phenethyl alcohol on the growth rate of Clostridium acetobutylicum ATCC 824 was investigated. Inhibition of cell growth was studied by treating cultures with varied concentrations of alcohols. There was a threshold concentration above which growth inhibition occurred. The degree of inhibition was a linear function of the alcohol concentration used. The natural logarithm of the inhibition constant was shown to be: (1) a linear function of the chain length of the alkanols, (2) a linear function of the natural logarithm of the octanol/water partition coefficient for both aliphatic and cyclic alcohols.     

Accumulation and disposal of tricarboxylic acid cycle intermediates during propionate oxidation in the isolated perfused rat heart

The role of the metabolite disposal mechanisms in the regulation of the tricarboxylic acid cycle pool size was studied in isolated perfused rat hearts oxidizing 2 mM propionate. Malate and succinate accumulated during the propionate metabolism. A further 118% increase in the malate concentration and 600% increase in the succinate concentration and a slight inhibition of the propionate uptake were observed during a subsequent KCl-induced arrest of the heart metabolizing propionate. When the mechanical activity of the heart was restored, the malate and succinate concentrations returned to the same levels as before the arrest of the heart, but the propionate uptake did not rise significantly. The mean disposal rates of the tricarboxylic acid cycle metabolites during the cardiac arrest and subsequent restoration of the activity were 1.4 and 2.4 μmol/min per g dry weight, respectively. During cardiac arrest the malate carbon disposed was almost totally recovered as C₃ compounds, whereas after the increase in the ATP-consumption most of it was oxidized. The results show that propionate is oxidized by heart muscle at an appreciable rate but the disposal rate of the tricarboxylic acid cycle intermediates is not tightly regulated by the cellular energy state. Although the metabolite pool size of the tricarboxylic acid cycle responds to change in the ATP consumption, the energy state appears to have a greater effect on the fate of the C₃ compounds formed than on the actual rate of C₄ compound disposition.     

H2 and CO2 production from methanol or formaldehyde by the methanogenic bacterium strain Gö1 treated with 2-bromoethanesulfonic acid

Abstract In cell suspensions of the methanogenic bacterium strain Gö1 or Methanosarcina barkeri H₂ formation from methanol in the presence of 2-bromoethanesulfonic acid (BES) was strictly dependent on sodium ions; apparent K _S for Na⁺, 1.3±0.3 mM.H₂ formation was inhibited by the uncoupler tetrachlorosalicylanilide (TCS), but this inhibition could be temporarily overcome, when a sodium pulse (100 mM) was given to the cell suspension. On the other hand, H₂ formation from formaldehyde in the presence of BES (rate: 300 nmol H₂/h·mg protein as compared to 25 nmol H₂/h·mg protein from methanol) was not sodium-dependent, not TCS-sensitive and not inhibited by addition of monensin. H₂ formation was accompanied by CO₂ formation in stoichiometric amounts, 3 H₂:1 CO₂ for methanol and 2 H₂:1 CO₂ for formaldehyde oxidation.     

Measurement of amino acid metabolism derived from [1-13C]glucose in the rat brain using13C magnetic resonance spectroscopy

To clarify the unique characteristics of amino acid metabolism derived from glucose in the central nervous system (CNS), we injected [1-¹³C]glucose intraperitoneally to the rat, and extracted the free amino acids from several kinds of tissues and measured the amount of incorporation of¹³C derived from [1-¹³C]glucose into each amino acid using¹³C-magnetic resonance spectroscopy (NMR). In the adult rat brain, the intensities of resonances from¹³C-amino acids were observed in the following order: glutamate, glutamine, aspartate, -aminobutyrate (GABA) and alanine. There seemed no regional difference on this labeling pattern in the brain. However, only in the striatum and thalamus, the intensities of resonances from [2-¹³C]GABA were larger than that from [2,3-¹³C]aspartate. In the other tissues, such as heart, kidney, liver, spleen, muscle, lung and small intestine, the resonances from GABA were not detected and every intensity of resonances from¹³C-amino acids, except¹³C-alanine, was much smaller than those in the brain and spinal cord. In the serum,¹³C-amino acid was not detected at all. When the rats were decapitated, in the brain, the resonances from [1-¹³C]glucose greatly reduced and the intensities of resonances from [3-¹³C]lactate, [3-¹³C]alanine, [2, 3, 4-¹³C]GABA and [2-¹³C]glutamine became larger as compared with those in the case that the rats were sacrificed with microwave. In other tissues, the resonances from [1-¹³C]glucose were clearly detected even after the decapitation. In the glioma induced by nitrosoethylurea in the spinal cord, the large resonances from glutamine and alanine were observed; however, the intensities of resonances from glutamate were considerably reduced and the resonances from GABA and aspartate were not detected. These results show that the pattern of¹³C label incorporation into amino acids is unique in the central nervous tissues and also suggest that the metabolic compartmentalization could exist in the CNS through the metabolic trafficking between neurons and astroglia.Abbreviations NMR nuclear magnetic resonance - GABA -aminobutyrate - GFAP glial fibrillary acidic protein Special issue dedicated to Dr. Bernard W. Agranoff.     

Influence of pH and undissociated butyric acid on the production of acetone and butanol in batch cultures of Clostridium acetobutylicum

Summary The kinetics of growth and acid and solvent production are examined in batch fermentation of Clostridium acetobutylicum at pH between 4.5 and 6.0. At the lower pH, growth occurs in two consecutive phases and solvents are the main excreted metabolites. At the higher pH, there is a single growth phase with only acid formation. The influence of the pH can be correlated with a critical role of the concentration of undissociated butyric acid in the medium: cellular growth is inhibited above 0.5 g/l and solvent production starts at an undissociated acid level of 1.5 g/l. Reducing the intracellular acid dissociation by lowering the intracellular pH also favours the production of acetone and butanol.     

Investigation of the fumarate metabolism of the syntrophic propionate-oxidizing bacterium strain MPOB

The growth of the syntrophic propionate-oxidizing bacterium strain MPOB in pure culture by fumarate disproportionation into carbon dioxide and succinate and by fumarate reduction with propionate, formate or hydrogen as electron donor was studied. The highest growth yield, 12.2 g dry cells/mol fumarate, was observed for growth by fumarate disproportionation. In the presence of hydrogen, formate or propionate, the growth yield was more than twice as low: 4.8, 4.6, and 5.2 g dry cells/mol fumarate, respectively. The location of enzymes that are involved in the electron transport chain during fumarate reduction in strain MPOB was analyzed. Fumarate reductase, succinate dehydrogenase, and ATPase were membrane-bound, while formate dehydrogenase and hydrogenase were loosely attached to the periplasmic side of the membrane. The cells contained cytochrome c, cytochrome b, menaquinone-6 and menaquinone-7 as possible electron carriers. Fumarate reduction with hydrogen in membranes of strain MPOB was inhibited by 2-(heptyl)-4-hydroxyquinoline-N-oxide (HOQNO). This inhibition, together with the activity of fumarate reductase with reduced 2,3-dimethyl-1,4-naphtoquinone (DMNH₂) and the observation that cytochrome b of strain MPOB was oxidized by fumarate, suggested that menequinone and cytochrome b are involved in the electron transport during fumarate reduction in strain MPOB. The growth yields of fumarate reduction with hydrogen or formate as electron donor were similar to the growth yield of Wolinella succinogenes. Therefore, it can be assumed that strain MPOB gains the same amount of ATP from fumarate reduction as W. succinogenes, i.e. 0.7 mol ATP/mol fumarate. This value supports the hypothesis that syntrophic propionate-oxidizing bacteria have to invest two-thirds of an ATP via reversed electron transport in the succinate oxidation step during the oxidation of propionate. The same electron transport chain that is involved in fumarate reduction may operate in the reversed direction to drive the energetically unfavourable oxidation of succinate during syntrophic propionate oxidation since (1) cytochrome b was reduced by succinate and (2) succinate oxidation was similarly inhibited by HOQNO as fumarate reduction. Received: 18 March 1997 / Accepted: 10 November 1997     

Effect of different acetate:propionate ratios on the methanogenic community during thermophilic anaerobic digestion in batch experiments

The present study investigated the metabolism of different acetate:propionate ratios (0.25, 0.33, 0.5, 1.0, 2.0, 3.0, 4.0) in equimolar carbon concentration during an anaerobic decomposition process under defined laboratory conditions and evaluated the engaged methanogenic community. Significant differences on a metabolic level (gas production, gas composition, volatile fatty acid (VFA) concentration) were observed between acetate:propionate ratios ≤1 and ≥2. Generally ratios ≥2 resulted in a faster methane production and VFA decomposition compared to ratios ≤1. Regarding the composition of the methanogenic community as well as the abundance of Methanosarcinales these differences were not reflected in an appropriate manner when DNA based methods (dHPLC and qPCR) were applied. However, by a culture based approach these differences could be documented showing a significant difference in the number of cultivable methanogens between initial acetate:propionate ratios ≤1 and ≥2.     

Influence of cold exposure on liver amino acid metabolism enzymes of the rat

The activities of alanine, aspartate and branched-chain amino acid transaminases, glutamate dehydrogenase, glutamine synthetase and adenylate deaminase have been studied in liver of male rats exposed [12 hours at 4 degrees C] or acclimated [15 days at 4 degrees C] to cold temperature. Cold temperature induced an increase of the activities of glutamate dehydrogenase and alanine and aspartate transaminases both in cold-exposed and cold-acclimated animals; adenylate deaminase activity diminished after 15-day cold acclimation. There were not significant changes induced by cold temperature in the activities of the other two enzymes studied. These results agree with a possible direct implication of amino acid utilization by the liver in the context of the overall thermogenic response to cold temperature.     

Effect of insulin on glucose oxidation and amino isobutyric acid transport and binding of insulin in chicken thymocytes

In chicken thymocytes isolated from 15–40 day-old chickens, after a 2 h incubation at 37°C, insulin stimulated amino isobutyric acid uptake (maximal response: 40–50% of increase at 1 μg insulin/ml and half maximal response at 60 ng/ml) by specifically stimulating the influx without altering the efflux. Insulin also stimulated glucose oxidation (maximal response: 11% of increase at 1 μg insulin/ml). Binding of ¹²⁵I-labelled chicken insulin to thymocytes was rapid and higher at 15°C than at 37°C. At steady state, (90 min at 15°C), chicken, porcine and goose insulins were equipotent in inhibiting the binding of ¹²⁵I-labelled chicken insulin. Maximal binding capacity was estimated at 1250 pg insulin/10⁸ cells, i.e., 1250 binding sites/cell with an apparent dissociation constant of 200 ng insulin/ml at 15°C. Degradation of ¹²⁵I-labelled chicken insulin in the incubation medium was negligible at 15°C but very noticeable at 37°C. Therefore, the low level of insulin binding at 15°C reflects a true scarcity of insulin receptors in chicken thymocytes as compared to rat thymocytes.     

Influence of substrate carbon on the metabolism of Clostridium thermohydrosulfuricum

The concentration of carbon sources has a significant influence on the growth, carbohydrate uptake and metabolite distribution in Clostridium thermohydrosulfuricum. The growing concentrations of glucose or starch increase the production of ethanol and lactate, the intracellular fructose-1,6-diphosphate (FDP) and the specific activity of lactate dehydrogenase (LDH), but decrease the ethanol/lactate ratio.     

The metabolism of [3-(13)C]lactate in the rat brain is specific of a pyruvate carboxylase-deprived compartment

Lactate metabolism in the adult rat brain was investigated in relation with the concept of lactate trafficking between astrocytes and neurons. Wistar rats were infused intravenously with a solution containing either [3-(13)C]lactate (534 mM) or both glucose (750 mM) and [3-(13)C]lactate (534 mM). The time courses of both the concentration and (13)C enrichment of blood glucose and lactate were determined. The data indicated the occurrence of [3-(13)C]lactate recycling through liver gluconeogenesis. The yield of glucose labeling was, however, reduced when using the glucose-containing infusate. After a 20-min or 1-h infusion, perchloric acid extracts of the brain tissue were prepared and subsequently analyzed by (13)C- and (1)H-observed/(13)C-edited NMR spectroscopy. The (13)C labeling of amino acids indicated that [3-(13)C]lactate was metabolized in the brain. Based on the alanine C3 enrichment, lactate contribution to brain metabolism amounted to 35% under the most favorable conditions used. By contrast with what happens with [1-(13)C]glucose metabolism, no difference in glutamine C2 and C3 labeling was evidenced, indicating that lactate was metabolized in a compartment deprived of pyruvate carboxylase activity. This result confirms, for the first time from an in vivo study, that lactate is more specifically a neuronal substrate.     

The internal pH of Clostridium acetobutylicum and its effect on the shift from acid to solvent formation

Clostridium acetobutylicum was unable to keep a constant pH inside the cells when grown on a phosphatelimited synthetic medium which allowed production of organic acids in a first phase and of solvents in a second phase. At external pH values between 5.9 and 4.3, the cells kept a constant pH of 0.9 to 1.3. A similar pH was measured in continuous culture under solventproducing conditions. The pH was abolished by protonovorous uncouplers, such as tetrachlorosalicylanilide (TCS) or carbonyl-p-trifluormethoxyphenylhydrazone (FCCP). n-Butanol at concentration of 150 mM and above led also to a complete abolition of the pH gradient.The internal pH stayed above 5.5 in cultures that shifted from acid to solvent formation. It is concluded that this is a prerequisite for the shift. The possible function of high internal concentrations of butyrate, butyryl phosphate and butyryl coenzyme A in the triggering mechanisms of the shift is discussed.Abbreviations TCS Tetrachlorosalicylanilide - FCCP carbonyl-p-trifluormethyoxyphenylhydrazone