Article
Refine
Year of publication
Document Type
- Article (58) (remove)
Language
- English (58) (remove)
Has Fulltext
- yes (58)
Is part of the Bibliography
- no (58)
Keywords
- Acetogenesis (4)
- Bioenergetics (4)
- Wood-Ljungdahl pathway (4)
- Acetogen (3)
- Biohydrogen (3)
- Hydrogenase (3)
- Thermoanaerobacter kivui (3)
- Thermophiles (3)
- Wood–Ljungdahl pathway (3)
- compatible solutes (3)
Institute
- Biowissenschaften (58) (remove)
Thermoanaerobacter kivui is an acetogenic model organism that reduces CO2 with electrons derived from H2 or CO, or from organic substrates in the Wood–Ljugdahl pathway (WLP). For the calculation of ATP yields, it is necessary to know the electron carriers involved in coupling of the oxidative and reductive parts of metabolism. Analyses of key catabolic oxidoreductases in cell-free extract (CFE) or with purified enzymes revealed the physiological electron carriers involved. The glyceraldehyde-3-phosphate dehydrogenase (GA3P-DH) assayed in CFE was NAD+-specific, NADP+ was used with less than 4% and ferredoxin (Fd) was not used. The methylene-THF dehydrogenase was NADP+-specific, NAD+ or Fd were not used. A Nfn-type transhydrogenase that catalyzes reduced Fd-dependent reduction of NADP+ with NADH as electron donor was also identified in CFE. The electron carriers used by the potential electron-bifurcating hydrogenase (HydABC) could not be unambiguously determined in CFE for technical reasons. Therefore, the enzyme was produced homologously in T. kivui and purified by affinity chromatography. HydABC contained 33.9 ± 4.5 mol Fe/mol of protein and FMN; it reduced NADP+ but not NAD+. The methylene-THF reductase (MetFV) was also produced homologously in T. kivui and purified by affinity chromatography. MetFV contained 7.2 ± 0.4 mol Fe/mol of protein and FMN; the complex did neither use NADPH nor NADH as reductant but only reduced Fd. In sum, these analysis allowed us to propose a scheme for entire electron flow and bioenergetics in T. kivui.
Thermoanaerobacter kivui is a thermophilic acetogen that can grow on carbon monoxide as sole carbon and energy source. To identify the gene(s) involved in CO oxidation, the genome sequence was analyzed. Two genes potentially encoding CO dehydrogenases were identified. One, cooS, potentially encodes a monofunctional CO dehydrogenase, whereas another, acsA, potentially encodes the CODH component of the CODH/ACS complex. Both genes were cloned, a His-tag encoding sequence was added, and the proteins were produced from a plasmid in T. kivui. His-AcsA copurified by affinity chromatography with AcsB, the acetyl-CoA synthase of the CO dehydrogenase/acetyl CoA synthase complex. His-CooS copurified with CooF1, a small iron-sulfur center containing protein likely involved in electron transport. Both protein complexes had CO:ferredoxin oxidoreductase as well as CO:methyl viologen oxidoreductase activity, but the activity of CooSF1 was 15-times and 231-times lower, respectively. To underline the importance of CooS, the gene was deleted in the CO-adapted strain. Interestingly, the ∆cooS deletion mutant did not grow on CO anymore. These experiments clearly demonstrated that CooS is essential for growth of T. kivui on CO. This is in line with the hypothesis that CooS is the CO-oxidizing enzyme in cells growing on CO.
The hydrogen-dependent carbon dioxide reductase is a soluble enzyme complex that directly utilizes hydrogen (H2) for the reduction of carbon dioxide (CO2) to formate in the first step of the acetyl-coenzyme A- or Wood-Ljungdahl pathway (WLP). HDCR consists of 2 catalytic subunits, a hydrogenase and a formate dehydrogenase (FDH) and two small subunits carrying iron-sulfur clusters. The enzyme complex has been purified and characterized from two acetogenic bacteria, from the mesophile Acetobacterium woodii and, recently, from the thermophile Thermoanaerobacter kivui. Physiological studies toward the importance of the HDCR for growth and formate metabolism in acetogens have not been carried out yet, due to the lack of genetic tools. Here, we deleted the genes encoding HDCR in T. kivui taking advantage of the recently developed genetic system. As expected, the deletion mutant (strain TKV_MB013) did not grow with formate as single substrate or under autotrophic conditions with H2 + CO2. Surprisingly, the strain did also not grow on any other substrate (sugars, mannitol or pyruvate), except for when formate was added. Concentrated cell suspensions quickly consumed formate in the presence of glucose only. In conclusion, HDCR provides formate which was essential for growth of the T. kivui mutant. Alternatively, extracellularly added formate served as terminal electron acceptor in addition to CO2, complementing the growth deficiency. The results show a tight coupling of multi-carbon substrate oxidation to the WLP. The metabolism in the mutant can be viewed as a coupled formate + CO2 respiration, which may be an ancient metabolic trait.
The capability of osmoadaptation is a prerequisite of organisms that live in an environment with changing salinities. Halobacillus halophilus is a moderately halophilic bacterium that grows between 0.4 and 3 M NaCl by accumulating both chloride and compatible solutes as osmolytes. Chloride is absolutely essential for growth and, moreover, was shown to modulate gene expression and activity of enzymes involved in osmoadaptation. The synthesis of different compatible solutes is strictly salinity- and growth phase-dependent. This unique hybrid strategy of H. halophilus will be reviewed here taking into account the recently published genome sequence. Based on identified genes we will speculate about possible scenarios of the synthesis of compatible solutes and the uptake of potassium ion which would complete our knowledge of the fine-tuned osmoregulation and intracellular osmolyte balance in H. halophilus.
The stress protectant trehalose is synthesized in Acinetobacter baumannii from UPD‐glucose and glucose‐6‐phosphase via the OtsA/OtsB pathway. Previous studies proved that deletion of otsB led to a decreased virulence, the inability to grow at 45°C and a slight reduction of growth at high salinities indicating that trehalose is the cause of these phenotypes. We have questioned this conclusion by producing ∆otsA and ∆otsBA mutants and studying their phenotypes. Only deletion of otsB, but not deletion of otsA or otsBA, led to growth impairments at high salt and high temperature. The intracellular concentrations of trehalose and trehalose‐6‐phosphate were measured by NMR or enzymatic assay. Interestingly, none of the mutants accumulated trehalose any more but the ∆otsB mutant with its defect in trehalose‐6‐phosphate phosphatase activity accumulated trehalose‐6‐phosphate. Moreover, expression of otsA in a ∆otsB background under conditions where trehalose synthesis is not induced led to growth inhibition and the accumulation of trehalose‐6‐phosphate. Our results demonstrate that trehalose‐6‐phosphate affects multiple physiological activities in A. baumannii ATCC 19606.
Na+ homeostasis in Acinetobacter baumannii is facilitated via the activity of the Mrp antiporter
(2022)
The human opportunistic pathogen Acinetobacter baumannii is a global threat to healthcare institutions worldwide, since it developed very efficient strategies to evade host defence and to adapt to the different environmental conditions of the host. This work focused on the importance of Na+ homeostasis in A. baumannii with regards to pathobiological aspects. In silico studies revealed a homologue of a multicomponent Na+/H+ antiporter system. Inactivation of the Mrp antiporter through deletion of the first gene (mrpA′) resulted in a mutant that was sensitive to increasing pH values. Furthermore, the strain was highly sensitive to increasing Na+ and Li+ concentrations. Increasing Na+ sensitivity is thought to be responsible for growth impairment in human fluids. Furthermore, deletion of mrpA′ is associated with energetic defects, inhibition of motility and survival under anoxic and dry conditions.
The anaerobic acetogenic bacterium Acetobacterium woodii has a novel Na(+)-translocating electron transport chain that couples electron transfer from reduced ferredoxin to NAD(+) with the generation of a primary electrochemical Na(+) potential across its cytoplasmic membrane. In previous assays in which Ti(3+) was used to reduce ferredoxin, Na(+) transport was observed, but not a Na(+) dependence of the electron transfer reaction. Here, we describe a new biological reduction system for ferredoxin in which ferredoxin is reduced with CO, catalyzed by the purified acetyl-CoA synthase/CO dehydrogenase from A. woodii. Using CO-reduced ferredoxin, NAD(+) reduction was highly specific and strictly dependent on ferredoxin and occurred at a rate of 50 milliunits/mg of protein. Most important, this assay revealed for the first time a strict Na(+) dependence of this electron transfer reaction. The Km was 0.2 mm. Na(+) could be partly substituted by Li(+). Na(+) dependence was observed at neutral and acidic pH values, indicating the exclusive use of Na(+) as a coupling ion. Electron transport from reduced ferredoxin to NAD(+) was coupled to electrogenic Na(+) transport, indicating the generation of ΔμNa(+). Vice versa, endergonic ferredoxin reduction with NADH as reductant was possible, but only in the presence of ΔμNa(+), and was accompanied by Na(+) efflux out of the vesicles. This is consistent with the hypothesis that Rnf also catalyzes ferredoxin reduction at the expense of an electrochemical Na(+) gradient. The physiological significance of this finding is discussed.
Background: Ferredoxin:NAD+-oxidoreductases (Rnf) found in many bacteria are novel ion-translocating electron transport chains.
Results: A Na+ requirement for the reaction and its reversible coupling to the transmembrane Na+ gradient are demonstrated.
Conclusion: Na+ is the coupling ion. Rnf not only generates a Na+ potential but also uses it to drive the reverse reaction.
Significance: Evidence for a function of Rnf in ferredoxin reduction is provided.
BACKGROUND: Acetogenic bacteria are able to use CO2 as terminal electron acceptor of an anaerobic respiration, thereby producing acetate with electrons coming from H2. Due to this feature, acetogens came into focus as platforms to produce biocommodities from waste gases such as H2+CO2 and/or CO. A prerequisite for metabolic engineering is a detailed understanding of the mechanisms of ATP synthesis and electron-transfer reactions to ensure redox homeostasis. Acetogenesis involves the reduction of CO2 to acetate via soluble enzymes and is coupled to energy conservation by a chemiosmotic mechanism. The membrane-bound module, acting as an ion pump, was of special interest for decades and recently, an Rnf complex was shown to couple electron flow from reduced ferredoxin to NAD+ with the export of Na+ in Acetobacterium woodii. However, not all acetogens have rnf genes in their genome. In order to gain further insights into energy conservation of non-Rnf-containing, thermophilic acetogens, we sequenced the genome of Thermoanaerobacter kivui.
RESULTS: The genome of Thermoanaerobacter kivui comprises 2.9 Mbp with a G+C content of 35% and 2,378 protein encoding orfs. Neither autotrophic growth nor acetate formation from H2+CO2 was dependent on Na+ and acetate formation was inhibited by a protonophore, indicating that H+ is used as coupling ion for primary bioenergetics. This is consistent with the finding that the c subunit of the F1FO ATP synthase does not have the conserved Na+ binding motif. A search for potential H+-translocating, membrane-bound protein complexes revealed genes potentially encoding two different proton-reducing, energy-conserving hydrogenases (Ech).
CONCLUSIONS: The thermophilic acetogen T. kivui does not use Na+ but H+ for chemiosmotic ATP synthesis. It does not contain cytochromes and the electrochemical proton gradient is most likely established by an energy-conserving hydrogenase (Ech). Its thermophilic nature and the efficient conversion of H2+CO2 make T. kivui an interesting acetogen to be used for the production of biocommodities in industrial micobiology. Furthermore, our experimental data as well as the increasing number of sequenced genomes of acetogenic bacteria supported the new classification of acetogens into two groups: Rnf- and Ech-containing acetogens.
Filamentous enzymes have been found in all domains of life, but the advantage of filamentation is often elusive1. Some anaerobic, autotrophic bacteria have an unusual filamentous enzyme for CO2 fixation—hydrogen-dependent CO2 reductase (HDCR)2,3—which directly converts H2 and CO2 into formic acid. HDCR reduces CO2 with a higher activity than any other known biological or chemical catalyst4,5, and it has therefore gained considerable interest in two areas of global relevance: hydrogen storage and combating climate change by capturing atmospheric CO2. However, the mechanistic basis of the high catalytic turnover rate of HDCR has remained unknown. Here we use cryo-electron microscopy to reveal the structure of a short HDCR filament from the acetogenic bacterium Thermoanaerobacter kivui. The minimum repeating unit is a hexamer that consists of a formate dehydrogenase (FdhF) and two hydrogenases (HydA2) bound around a central core of hydrogenase Fe-S subunits, one HycB3 and two HycB4. These small bacterial polyferredoxin-like proteins oligomerize through their C-terminal helices to form the backbone of the filament. By combining structure-directed mutagenesis with enzymatic analysis, we show that filamentation and rapid electron transfer through the filament enhance the activity of HDCR. To investigate the structure of HDCR in situ, we imaged T. kivui cells with cryo-electron tomography and found that HDCR filaments bundle into large ring-shaped superstructures attached to the plasma membrane. This supramolecular organization may further enhance the stability and connectivity of HDCR to form a specialized metabolic subcompartment within the cell.
Methanol is the simplest of all alcohols, is universally distributed in anoxic sediments as a result of plant material decomposition and is constantly attracting attention as an interesting substrate for anaerobes like acetogens that can convert bio-renewable methanol into value-added chemicals. A major drawback in the development of environmentally friendly but economically attractive biotechnological processes is the present lack of information on biochemistry and bioenergetics during methanol conversion in these bacteria. The mesophilic acetogen Eubacterium callanderi KIST612 is naturally able to consume methanol and produce acetate as well as butyrate. To grasp the full potential of methanol-based production of chemicals, we analysed the genes and enzymes involved in methanol conversion to acetate and identified the redox carriers involved. We will display a complete model for methanol-derived acetogenesis and butyrogenesis in Eubacterium callanderi KIST612, tracing the electron transfer routes and shed light on the bioenergetics during the process.