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Query: EC:6.2.1.1 (ACS)
 78,556 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The acetate activating system of Acetobacter aceti has been studied. The enzyme responsible, acetyl-CoA synthetase, has been purified about 500-fold from crude cell extracts and was approximately 85% pure as judged by polyacrylamide gel electrophoresis in sodium dodecyl sulphate. The purified enzyme showed optimal activity at pH 7.6 in both Tris-HCL and potassium phosphate buffers. In its purest form, the enzyme was stable at 4 degrees-C but denatured upon freezing. The Km values for CoA, ATP and acetate were found to be 0.104 mM, 0.36 mM and 0.25 mM respectively; propionate and acrylate were also activated by the enzyme but not butyrate, isobutyrate or valerate. GTP, UTP, CTP and ADP could not replace ATP in the reaction, and cysteine or pantetheine failed to replace CoA. The cationic requirements were studied and of the divalent cations tested, only Mn2+ could significantly replace Mg2+ in the reaction; K+ and NH4+ stimulated enzyme activity but inhibited at high concentrations; Na+ was a poor activator, but did not inhibit at higher concentrations. The effect of a number of glucose and other metabolites on enzyme activity has been tested.
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PMID:Characterization of the acetyl-CoA synthetase of Acetobacter aceti. 1

Acetyl-CoA synthetase (ADP-forming) is an enzyme in Archaea that catalyzes the formation of acetate from acetyl-CoA and couples this reaction with the synthesis of ATP from ADP and Pi (acetyl-CoA + ADP + Pi --> acetate + ATP + CoA) [Schifer, T., Selig, M. & Schonheit, P. (1993) Arch. Microbiol. 159, 72-83]. The enzyme from the anaerobic hyperthermophile Pyrococcus furiosus was purified 96-fold with a yield of 20% to apparent electrophoretic homogeneity. The oxygen-stable enzyme had an apparent molecular mass of 145 kDa and was composed of two subunits with apparent molecular masses of 47 kDa and 25 kDa, indicating an alpha2beta2 structure. The N-terminal amino acid sequences of both subunits were determined; they do not show significant identity to other proteins in databases. The purified enzyme catalyzed the reversible conversion of acetyl-CoA, ADP and Pi to acetate, ATP and CoA. The apparent Vmax value in the direction of acetate formation was 18 U/mg (55 degrees C), the apparent Km values for acetyl-CoA, ADP and Pi were 17 microM, 60 microM and 200 microM, respectively. ADP and Pi could not be replaced by AMP and PPi, defining the enzyme as an ADP-forming rather than an AMP-forming acetyl-CoA synthetase. The apparent Vmax value in the direction of acetyl-CoA formation was about 40 U/mg (55 degrees C), and the apparent Km values for acetate, ATP and CoA were 660 microM, 80 microM and 30 microM, respectively. The purified enzyme was not specific for acetyl-CoA or acetate, in addition to acetyl-CoA (100%), the enzyme accepts propionyl-CoA (110%) and butyryl-CoA (92%), and in addition to acetate (100%), the enzyme accepts propionate (100%), butyrate (92%), isobutyrate (79%), valerate (36%) and isovalerate (34%), indicating that the enzyme functions as an acyl-CoA synthetase (ADP-forming) with a broad substrate spectrum. Succinate, phenylacetate and indoleacetate did not serve as substrates for the enzyme (<3%). In addition to ADP (100%), GDP (220%) and IDP (250%) were used, and in addition to ATP (100%), GTP (210%) and ITP (320%) were used. Pyrimidine nucleotides were not accepted. The enzyme was dependent on Mg2+, which could be partly substituted by Mn2+ and Co2+. The pH optimum was pH 7. The enzyme has a temperature optimum at 90 degrees C, which is in accordance with its physiological function under hyperthermophilic conditions. The enzyme was stabilized against heat inactivation by salts. In the presence of KCI (1 M), which was most effective, the enzyme did not loose activity after 2 h incubation at 100 degrees C.
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PMID:Purification and properties of acetyl-CoA synthetase (ADP-forming), an archaeal enzyme of acetate formation and ATP synthesis, from the hyperthermophile Pyrococcus furiosus. 911 24

Since 1995, crystal structures have been determined for many transition-metal enzymes, in particular those containing the rarely used transition metals vanadium, molybdenum, tungsten, manganese, cobalt and nickel. Accordingly, our understanding of how an enzyme uses the unique properties of a specific transition metal has been substantially increased in the past few years. The different functions of nickel in catalysis are highlighted by describing the active sites of six nickel enzymes - methyl-coenyzme M reductase, urease, hydrogenase, superoxide dismutase, carbon monoxide dehydrogenase and acetyl-coenzyme A synthase.
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PMID:Active sites of transition-metal enzymes with a focus on nickel. 991 55

Reactive oxygen species are heavily involved in the pathogenesis of diabetes mellitus (DM) because the insulin-producing beta cells are particularly vulnerable to free-radical-mediated cytotoxicity. Catalytic anti-oxidants have been successfully applied for attenuation of DM and its consequences, but most recent research revealed that preventing the nitration of vital proteins/enzymes might be an even more powerful strategy. We now report an unprecedented efficiency of manganese(III) corroles regarding the protection of rat pancreatic beta cells against intracellular nitration by peroxynitrite and subsequent cell death. A comparison between analogous corroles and porphyrin metal complexes reveals significant superiority of the former in all examined aspects. This is particularly true for the positively-charged manganese(III) corrole, which decomposes peroxynitrite fast enough and through a unique catalytic mechanism that is devoid of potentially nitrating reaction intermediates.
ACS Chem Biol 2009 Nov 20
PMID:Manganese corroles prevent intracellular nitration and subsequent death of insulin-producing cells. 1971 43

While some organisms, including humans, eliminate oxidized purines to get rid of excess nitrogen, for many others the recovery of the purine ring nitrogen is vital. In the so-called ureide pathway, nitrogen is released as ammonia from allantoate through a series of reactions starting with allantoate amidohydrolase (AAH), a manganese-dependent enzyme found in plants and bacteria. We report NMR evidence that the true product of the AAH reaction is S-ureidoglycine, a nonstandard alpha-amino acid that spontaneously releases ammonia in vitro. Using gene proximity and logical genome analysis, we identified a candidate gene (ylbA) for S-ureidoglycine metabolism. The proteins encoded by Escherichia coli and Arabidopsis thaliana genes catalyze the manganese-dependent release of ammonia through hydrolysis of S-ureidoglycine. Hydrolysis then inverts the configuration and yields S-ureidoglycolate. S-Ureidoglycine aminohydrolase (UGHY) is cytosolic in bacteria, whereas in plants it is localized, like allantoate amidohydrolase, in the endoplasmic reticulum. These findings strengthen the basis for the known sensitivity of the ureide pathway to Mn availability and suggest a further rationale for the active transport of Mn in the endoplasmic reticulum of plant cells.
ACS Chem Biol 2010 Feb 19
PMID:Chemical basis of nitrogen recovery through the ureide pathway: formation and hydrolysis of S-ureidoglycine in plants and bacteria. 2003 85

The influence of synthesizing temperature of manganese dioxide (MnO(2)) powders on their electrochemical reactivity in 1 M KOH was investigated. These powders were prepared chemically by the hydrothermal method by oxidation of Mn(2+) by ammonium peroxodisulphate. The observations by scanning electronic microscopy, energy-dispersive X-ray analyses, and transmission electron microscopy techniques on MnO(2) obtained at different temperatures show the formation of many nanometre scale sticks lumped together to form a spherical particle of several micrometers. The results obtained by BET and BJH methods reveal mesoporous texture, and the MnO(2) synthesized at 90 degrees C presents the largest expanded surface area. The electrochemical reactivity of these powders in 1 M KOH was characterized with microcavity electrode by cyclic voltammetry and electrochemical impedance spectroscopy. The results illustrate that the nanostructured MnO(2) powder synthesized at 90 degrees C shows the highest electrochemical reactivity in agreement with BET data. The X-ray powder diffraction identified the gamma-MnO(2), known as the most reactive species.
ACS Appl Mater Interfaces 2009 Feb
PMID:Reactivity of nanostructured MnO(2) in alkaline medium studied with a micro-cavity electrode: effect of synthesizing temperature. 2035 33

This work presents two points with respect to manganese oxide (MnO(x)) nanomaterials: their controllable synthesis with desired phases and shapes together with their applications as catalysts for oxygen reduction and Al/air batteries. Solid MnO(x) with crystalline phases of MnOOH, Mn(2)O(3), and MnO(2) as well as shapes of the sphere, wire, rod, and particle were prepared through a simple one-pot hydrothermal route. Selective preparation was achieved by adjusting the surfactant concentration that controls simultaneously the growth thermodynamic and dynamic parameters of MnO(x) nanocrystals. Electrochemical investigations show that the obtained Mn(2)O(3) nanowires, which possess a large aspect ratio and preferentially exposed (222) crystal surfaces, exhibit remarkable catalytic activity (comparable to Pt/C counterparts) toward the electroreduction of oxygen in alkaline media. The tailored MnO(x) nanostructures may find prospective applications as low-cost catalysts for alkaline fuel cells and metal/air batteries.
ACS Appl Mater Interfaces 2009 Feb
PMID:Selective synthesis of manganese oxide nanostructures for electrocatalytic oxygen reduction. 2035 37

The charge-storage mechanism in manganese dioxide (MnO2)-based electrochemical supercapacitors was investigated and discussed toward prepared MnO2 microstructures. The preparation of a series of MnO2 allotropic phases was performed by following dedicated synthetic routes. The resulting compounds are classified into three groups depending on their crystal structures based on 1D channels, 2D layers, or 3D interconnected tunnels. The 1D group includes pyrolusite, ramsdellite, cryptomelane, Ni-doped todorokite (Ni-todorokite), and OMS-5. The 2D and 3D groups are composed of birnessite and spinel, respectively. The prepared MnO2 powders were characterized using X-ray diffraction, scanning electron microscopy, the Brunauer-Emmett-Teller technique, cyclic voltammetry (CV), and electrochemical impedance spectroscopy. The influence of the MnO2 microstructure on the electrochemical performance of MnO2-based electrodes is commented on through the specific surface area and the electronic and ionic conductivities. It was demonstrated that the charge-storage mechanism in MnO2-based electrodes is mainly faradic rather than capacitive. The specific capacitance values are found to increase in the following order: pyrolusite (28 Fx g(-1)) < Ni-todorokite < ramsdellite < cryptomelane < OMS-5 < birnessite < spinel (241 Fx g(-1)). Thus, increasing the cavity size and connectivity results in the improvement of the electrochemical performance. In contrast with the usual assumption, the electrochemical performance of MnO2-based electrodes was not dependent on the specific surface area. The electronic conductivity was shown to have a limited impact as well. However, specific capacitances of MnO2 forms were strongly correlated with the corresponding ionic conductivities, which obviously rely on the microstructure. The CV experiments confirmed the good stability of all MnO2 phases during 500 charge/discharge cycles.
ACS Appl Mater Interfaces 2009 May
PMID:Microstructural effects on charge-storage properties in MnO2-based electrochemical supercapacitors. 2035 1

A composite of graphene oxide supported by needle-like MnO(2) nanocrystals (GO-MnO(2) nanocomposites) has been fabricated through a simple soft chemical route in a water-isopropyl alcohol system. The formation mechanism of these intriguing nanocomposites investigated by transmission electron microscopy and Raman and ultraviolet-visible absorption spectroscopy is proposed as intercalation and adsorption of manganese ions onto the GO sheets, followed by the nucleation and growth of the crystal species in a double solvent system via dissolution-crystallization and oriented attachment mechanisms, which in turn results in the exfoliation of GO sheets. Interestingly, it was found that the electrochemical performance of as-prepared nanocomposites could be enhanced by the chemical interaction between GO and MnO(2). This method provides a facile and straightforward approach to deposit MnO(2) nanoparticles onto the graphene oxide sheets (single layer of graphite oxide) and may be readily extended to the preparation of other classes of hybrids based on GO sheets for technological applications.
ACS Nano 2010 May 25
PMID:Graphene oxide--MnO2 nanocomposites for supercapacitors. 2038 18

Solid-state and flexible zinc carbon (or Leclanche) batteries are fabricated using a combination of functional nanostructured materials for optimum performance. Flexible carbon nanofiber mats obtained by electrospinning are used as a current collector and cathode support for the batteries. The cathode layer consists of manganese oxide particles combined with single-walled carbon nanotubes for improved conductivity. A polyethylene oxide layer containing titanium oxide nanoparticles forms the electrolyte layer, and a thin zinc foil is used as the anode. The battery is shown to retain its performance under mechanically stressed conditions. The results show that the above configuration can achieve solid-state mechanical flexibility and increased shelf life with little sacrifice in performance.
ACS Nano 2010 May 25
PMID:Nanomaterial-enhanced all-solid flexible zinc--carbon batteries. 2041 26


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