Dissimilation of Carbon Monoxide to Acetic Acid by Glucose-Limited Cultures of Clostridium thermoaceticum

Formate-Dependent Acetogenic Utilization of Glucose by the Fecal Acetogen Clostridium bovifaecis

Acetogenic bacteria are a diverse group of anaerobes that use the reductive acetyl coenzyme A (acetyl-CoA) (Wood-Ljungdahl) pathway for CO₂ fixation and energy conservation. The conversion of 2 mol CO₂ into acetyl-CoA by using the Wood-Ljungdahl pathway as the terminal electron accepting process is the most prominent metabolic feature for these microorganisms. However, here, we describe that the fecal acetogen Clostridium bovifaecis strain BXX displayed poor metabolic capabilities of autotrophic acetogenesis, and acetogenic utilization of glucose occurred only with the supplementation of formate. Genome analysis of Clostridium bovifaecis revealed that it contains almost the complete genes of the Wood-Ljungdahl pathway but lacks the gene encoding formate dehydrogenase, which catalyzes the reduction of CO₂ to formate as the first step of the methyl branch of the Wood-Ljungdahl pathway. The lack of a gene encoding formate dehydrogenase was verified by PCR, reverse transcription-PCR analysis, enzyme activity assay, and its formate-dependent acetogenic utilization of glucose on DNA, RNA, protein, and phenotype level, respectively. The lack of a formate dehydrogenase gene may be associated with the adaption to a formate-rich intestinal environment, considering the isolating source of strain BXX. The formate-dependent acetogenic growth of Clostridium bovifaecis provides insight into a unique metabolic feature of fecal acetogens.IMPORTANCE The acetyl-CoA pathway is an ancient pathway of CO₂ fixation, which converts 2 mol of CO₂ into acetyl-CoA. Autotrophic growth with H₂ and CO₂ via the acetyl-CoA pathway as the terminal electron accepting process is the most unique feature of acetogenic bacteria. However, the fecal acetogen Clostridium bovifaecis strain BXX displayed poor metabolic capabilities of autotrophic acetogenesis, and acetogenic utilization of glucose occurred only with the supplementation of formate. The formate-dependent acetogenic growth of Clostridium bovifaecis was associated with its lack of a gene encoding formate dehydrogenase, which may result from adaption to a formate-rich intestinal environment. This study gave insight into a unique metabolic feature of fecal acetogens. Because of the requirement of formate for the acetogenic growth of certain acetogens, the ecological impact of acetogens could be more complex and important in the formate-rich environment due to their trophic interactions with other microbes.

Competing formate- and carbon dioxide-utilizing prokaryotes in an anoxic methane-emitting fen soil

Methanogenesis in wetlands is dependent on intermediary substrates derived from the degradation of biopolymers. Formate is one such substrate and is stimulatory to methanogenesis and acetogenesis in anoxic microcosms of soil from the fen Schlöppnerbrunnen. Formate dissimilation also yields CO(2) as a potential secondary substrate. The objective of this study was to resolve potential differences between anaerobic formate- and CO(2)-utilizing prokaryotes of this fen by stable isotope probing. Anoxic soil microcosms were pulsed daily with low concentrations of [(13)C]formate or (13)CO(2) (i.e., [(13)C]bicarbonate). Taxa were evaluated by assessment of 16S rRNA genes, mcrA (encoding the alpha-subunit of methyl-coenzyme M reductase), and fhs (encoding formyltetrahydrofolate synthetase). Methanogens, acetogens, and formate-hydrogen lyase-containing taxa appeared to compete for formate. Genes affiliated with Methanocellaceae, Methanobacteriaceae, Acetobacteraceae, and Rhodospirillaceae were (13)C enriched (i.e., labeled) in [(13)C]formate treatments, whereas genes affiliated with Methanosarcinaceae, Conexibacteraceae, and Solirubrobacteraceae were labeled in (13)CO(2) treatments. [(13)C]acetate was enriched in [(13)C]formate treatments, but labeling of known acetogenic taxa was not detected. However, several phylotypes were affiliated with acetogen-containing taxa (e.g., Sporomusa). Methanosaetaceae-affiliated methanogens appeared to participate in the consumption of acetate. Twelve and 58 family-level archaeal and bacterial 16S rRNA phylotypes, respectively, were detected, approximately half of which had no isolated representatives. Crenarchaeota constituted half of the detected archaeal 16S rRNA phylotypes. The results highlight the unresolved microbial diversity of the fen Schlöppnerbrunnen, suggest that differing taxa competed for the same substrate, and indicate that Methanocellaceae, Methanobacteriaceae, Methanosarcinaceae, and Methanosaetaceae were linked to the production of methane, but they do not clearly resolve the taxa responsible for the apparent conversion of formate to acetate.

Corpus cavernosal smooth muscle relaxation effect of a novel AMPK activator, beta-lapachone

INTRODUCTION

Adenosine monophosphate-activated protein kinase (AMPK) activation is suggested to relax smooth muscle by endothelial nitric oxide synthase (eNOS) phosphorylation.

AIM

To assess the mechanism and effect of a novel AMPK activator, beta-lapachone, upon cavernosal smooth muscle relaxation and the therapeutic potential for erectile dysfunction.

METHODS

Human umbilical vein endothelial cells (HUVECs) were treated with beta-lapachone. The lysates were blotted with specific antibodies for phosphorylated AMPK (p-AMPK) or phosphorylated eNOS (p-eNOS). The membranes were re-blotted for total AMP total eNOS, or beta-actin. The eNOS activity was measured by the conversion of L-14C-arginine to L-14C-citrulline in HUVECs lysates. In a separated experiment, cavernosal strips from New Zealand white rabbits were harvested for organ bath study and the relaxation effect of beta-lapachone on phenylephrine-induced contracted strips was evaluated and compared with sodium nitroprusside, zaprinast, metformin, and aminoimidazole carboxamide ribonucleotide (AICAR). Methylene blue and L-NAME were used to assess the inhibition of cyclic guanosine monophosphate/nitric oxide pathway. Zinc-protoporphyrin-IX (ZnPP) was also used to investigate the contribution of mevalonate pathway.

MAIN OUTCOME MEASURES

The expression of p-AMPK, p-eNOS, AMPK and eNOS induced by beta-lapachone in HUVECs study and the percent relaxation of cavernosal tissue in organ bath study.

RESULTS

Beta-lapachone clearly induced AMPK phosphorylation and, as a consequence, eNOS phosphorylation in HUVECs. Beta-lapachone-induced upregulation of eNOS activity was also observed in HUVECs and steadily increased up to 1 hour. In organ bath study, beta-lapachone significantly relaxed the phenylephrine pretreated strips in a dose-dependent manner. This relaxation effect was not totally blocked by methylene blue or L-NAME. After removing endothelium, the relaxation was totally blocked by ZnPP.

CONCLUSIONS

A novel AMPK activator, beta-lapachone has a strong relaxation effect on precontracted cavernosal smooth muscle strips in the rabbit. And phosphorylation of AMPK and eNOS strongly related to the action of beta-lapachone. Mevalonate pathway also might be considered as a suggestive mechanism.

Diversity and ecophysiological features of thermophilic carboxydotrophic anaerobes

Both natural and anthropogenic hot environments contain appreciable levels of carbon monoxide (CO). Anaerobic microbial communities play an important role in CO conversion in such environments. CO is involved in a number of redox reactions. It is biotransformed by thermophilic methanogens, acetogens, hydrogenogens, sulfate reducers, and ferric iron reducers. Most thermophilic CO-oxidizing anaerobes have diverse metabolic capacities, but two hydrogenogenic species are obligate carboxydotrophs. Among known thermophilic carboxydotrophic anaerobes, hydrogenogens are most numerous, and based on available data they are most important in CO biotransformation in hot environments.

Old acetogens, new light

Acetogens utilize the acetyl-CoA Wood-Ljungdahl pathway as a terminal electron-accepting, energy-conserving, CO(2)-fixing process. The decades of research to resolve the enzymology of this pathway (1) preceded studies demonstrating that acetogens not only harbor a novel CO(2)-fixing pathway, but are also ecologically important, and (2) overshadowed the novel microbiological discoveries of acetogens and acetogenesis. The first acetogen to be isolated, Clostridium aceticum, was reported by Klaas Tammo Wieringa in 1936, but was subsequently lost. The second acetogen to be isolated, Clostridium thermoaceticum, was isolated by Francis Ephraim Fontaine and co-workers in 1942. C. thermoaceticum became the most extensively studied acetogen and was used to resolve the enzymology of the acetyl-CoA pathway in the laboratories of Harland Goff Wood and Lars Gerhard Ljungdahl. Although acetogenesis initially intrigued few scientists, this novel process fostered several scientific milestones, including the first (14)C-tracer studies in biology and the discovery that tungsten is a biologically active metal. The acetyl-CoA pathway is now recognized as a fundamental component of the global carbon cycle and essential to the metabolic potentials of many different prokaryotes. The acetyl-CoA pathway and variants thereof appear to be important to primary production in certain habitats and may have been the first autotrophic process on earth and important to the evolution of life. The purpose of this article is to (1) pay tribute to those who discovered acetogens and acetogenesis, and to those who resolved the acetyl-CoA pathway, and (2) highlight the ecology and physiology of acetogens within the framework of their scientific roots.

Physiology of the thermophilic acetogen Moorella thermoacetica

Moorella thermoacetica (originally isolated as Clostridium thermoaceticum) has served as the primary acetogenic bacterium for the resolution of the acetyl coenzyme A (acetyl-CoA) or Wood-Lijungdahl pathway, a metabolic pathway that (i) autotrophically assimilates CO2 and (ii) is centrally important to the turnover of carbon in many habitats. The purpose of this article is to highlight the diverse physiological features of this model acetogen and to examine some of the consequences of its metabolic capabilities.

Physiology of the thermophilic acetogen Moorella thermoacetica

Moorella thermoacetica (originally isolated as Clostridium thermoaceticum) has served as the primary acetogenic bacterium for the resolution of the acetyl coenzyme A (acetyl-CoA) or Wood-Ljungdahl pathway, a metabolic pathway that (i) autotrophically assimilates CO2 and (ii) is centrally important to the turnover of carbon in many habitats. The purpose of this article is to highlight the diverse physiological features of this model acetogen and to examine some of the consequences of its metabolic capabilities.

Anaerobic biodegradation of methyl esters by Acetobacterium woodii and Eubacterium limosum

The ability of Acetobacterium woodii and Eubacterium limosum to degrade methyl esters of acetate, propionate, butyrate, and isobutyrate was examined under growing and resting-cell conditions. Both bacteria hydrolyzed the esters to the corresponding carboxylates and methanol under either condition. Methanol was further oxidized to formate under growing but not resting conditions. Unlike the metabolism of phenylmethylethers, no H2 requirement was evident for ester biotransformation. The hydrolysis of methyl carboxylates is thermodynamically favorable under standard conditions and the mixotrophic metabolism of ester/CO2 allowed for bacterial growth. These results suggest that the degradation of methyl carboxylates may be a heretofore unrecognized nutritional option for acetogenic bacteria.

Expression of an aromatic-dependent decarboxylase which provides growth-essential CO2 equivalents for the acetogenic (Wood) pathway of Clostridium thermoaceticum

The acetogen Clostridium thermoaceticum generates growth-essential CO2 equivalents from carboxylated aromatic compounds (e.g., 4-hydroxybenzoate), and these CO2 equivalents are likely integrated into the acetogenic pathway (T. Hsu, S. L. Daniel, M. F. Lux, and H. L. Drake, J. Bacteriol. 172:212-217, 1990). By using 4-hydroxybenzoate as a model substrate, an assay was developed to study the expression and activity of the decarboxylase involved in the activation of aromatic carboxyl groups. The aromatic-dependent decarboxylase was induced by carboxylated aromatic compounds in the early stages of growth and was not repressed by glucose or other acetogenic substrates; nonutilizable carboxylated aromatic compounds did not induce the decarboxylase. The decarboxylase activity displayed saturation kinetics at both whole-cell and cell extract levels, was sensitive to oxidation, and was not affected by exogenous energy sources. However, at the whole-cell level, metabolic inhibitors decreased the decarboxylase activity. Supplemental biotin or avidin did not significantly affect decarboxylation. The aromatic-dependent decarboxylase was specific for benzoates with a hydroxyl group in the para position of the aromatic ring; the meta position could be occupied by various substituent groups (-H, -OH, -OCH3, -Cl, or -F). The carboxyl carbon from [carboxyl-14C] vanillate went primarily to 14CO2 in short-term decarboxylase assays. During growth, the aromatic carboxyl group went primarily to CO2 under CO2-enriched conditions. However, under CO2-limited conditions, the aromatic carboxyl carbon went nearly totally to acetate, with equal distribution between the carboxyl and methyl carbons, thus demonstrating that acetate could be totally synthesized from aromatic carboxyl groups. In contrast, when cocultivated (i.e., supplemented) with CO under CO2-limited conditions, the aromatic carboxyl group went primarily to the methyl carbon of acetate.

Characterization of the H2- and CO-dependent chemolithotrophic potentials of the acetogens Clostridium thermoaceticum and Acetogenium kivui

Strains of Clostridium thermoaceticum were tested for H2- and CO-dependent growth in a defined medium containing metals, minerals, vitamins, cysteine-sulfide, CO2-bicarbonate, and H2 or CO. Ten of the thirteen strains tested grew at the expense of H2 and CO, and C. thermoaceticum ATCC 39073 was chosen for further study. The doubling times for H2- and CO-dependent growth under chemolithotrophic conditions (the defined medium with nicotinic acid as sole essential vitamin and sulfide as sole reducer) were 25 and 10 h, respectively. Product stiochiometries for chemolithotrophic cultures approximated: 4.1H2 + 2.4CO2----CH3COOH + 0.1 cell C + 0.3 unrecovered C and 6.8CO----CH3COOH + 3.5CO2 + 0.4 cell C + 0.9 unrecovered C. H2-dependent growth produced significantly higher acetate concentrations per unit of biomass synthesized than did CO- or glucose-dependent growth. In contrast, the doubling time for H2-dependent growth under chemolithotrophic conditions (the defined medium without vitamins and sulfide as sole reducer) by Acetogenium kivui ATCC 33488 was 2.7 h; as a sole energy source, CO was not growth supportive for A. kivui. The YH2 values for A. kivui and C. thermoaceticum were 0.91 and 0.46 g of cell dry weight per mol of H2 consumed, respectively; the YCO value for C. thermoaceticum was 1.28 g of cell dry weight per mol of CO consumed. The specific activities of hydrogenase and CO dehydrogenase in both acetogens were influenced by the energy source utilized for growth and were significantly lower in C. thermoaceticum than in A. kivui. With extracts of H2-cultivated cells and benzyl viologen as electron acceptor, the Vmax values for hydrogenase from C. thermoaceticum and A. kivui were 155.7 and 1,670 micromoles of H2 oxidized per min mg of protein, respectively; the Vmax values for CO dehydrogenase from C. thermoaceticum and A. kivui were 90.6 and 2,973 micromoles of CO oxidized per min per mg of protein, respectively.

Biotransformations of carboxylated aromatic compounds by the acetogen Clostridium thermoaceticum: generation of growth-supportive CO2 equivalents under CO2-limited conditions

Clostridium thermoaceticum ATCC 39073 converted vanillate to catechol. Although carboxylated aromatic compounds which did not contain methoxyl groups were not by themselves growth supportive, protocatechuate and p-hydroxybenzoate (nonmethoxylated aromatic compounds) were converted to catechol and phenol, respectively, during carbon monoxide-dependent growth. Syringate is not subject to decarboxylation by C. thermoaceticum (Z. Wu, S. L. Daniel, and H. L. Drake, J. Bacteriol. 170:5705-5708, 1988), and sustained growth at the expense of syringate-derived methoxyl groups was dependent on supplemental CO2. In contrast, vanillate was growth supportive in the absence of supplemental CO2, and 14CO2 was the major 14C-labeled product during [carboxyl-14C]vanillate-dependent growth. Furthermore, the decarboxylation of protocatechuate and p-hydroxybenzoate supported methanol- and 1,2,3-trimethoxybenzene-dependent growth (CO2 is required for growth at the expense of these substrates) when supplemental CO2 was depleted from the growth medium, and the decarboxylation of protocatechuate was concomitant with improved cell yields of methanol cultures. These findings demonstrate that (i) C. thermoaceticum is competent in the decarboxylation of certain aromatic compounds and (ii) under certain conditions, decarboxylation may be integrated to the flow of carbon and energy during acetogenesis.

Carbon monoxide-dependent chemolithotrophic growth of Clostridium thermoautotrophicum

The acetogen Clostridium thermoautotrophicum was cultivated under CO-dependent chemolithotrophic conditions. CO-dependent growth profiles and energetics indicated that supplemental CO2 was fundamental to efficient growth at the expense of CO. Overall product stoichiometry approximated 6.5CO----CH3CO2H + 3.5CO2 + 0.6 cell C + 0.5 unrecovered C. Initial CO/CO2 ratios of 2 to 4 yielded optimal doubling times and cell yields. Maximal YCO values approximated 2.5 g of cell dry weight per mol of CO consumed; YH2 was considerably lower than YCO. Cross-transfer growth experiments and protein profiles indicated differential expression of genes between CO and methanol cultures.

Source of carbon and hydrogen in methane produced from formate by Methanococcus thermolithotrophicus

Methanococcus thermolithotrophicus is able to produce methane either from H2-CO2 or from formate. The route of formate entry into the methanogenic pathway was investigated by using 2H2O or [13C]formate and analysis by mass spectrometry. When cells (H2-CO2 or formate grown) were transferred to formate medium in 95% 2H water, the proportion of 2H in methane was 95%. When cells (H2-CO2 or formate grown) were transferred to media containing [13C]formate in the presence of H2-CO2 or He-CO2, the ratio of 13CH4 to 12CH4 increased over time parallel to the ratio of 13CO2 to 12CO2. The cells catalyzed a significant exchange of label between [13C]formate and 13CO2.