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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)

Under anaerobic conditions, cells of Entamoeba histolytica grown with bacteria produce H2 and acetate while cells grown axenically produce neither. Aerobically, acetate is produced and O2 is consumed by amebae from either type of cells. Centrifuged extracts, 2.4 x 106 x g x min, from both types of cells contain pyruvate synthase (EC 1.2.7.1) and an acetate thiokinase which, together, form a system capable of converting pyruvate to acetate. Pyruvate synthase catalyzes the reaction: pyruvate + CoA leads to CO2 + acetyl-CoA + 2E. Electron acceptors which function with this enzyme are FAD, FMN, riboflavin, ferredoxin, and methyl viologen, but not NAD or NADP. The amebal acetate thiokinase catalyzes the reaction acetyl-CoA + ADP + Pi leads to acetate + ATP + CoA. For this apparently new enzyme we suggest the trivial name acetyl-CoA-synthetase (ADP-forming). Extracts from axenic amebae do not contain hydrogenase, but extracts from cells grown with bacteria do. It is postulated that in bacteria-grown amebae electrons generated at the pyruvate synthase step are utilized anaerobically to produce H2 via the hydrogenase and that the acetyl-CoA is converted to acetate in an energy-conserving step catalyzed by amebal acetyl-CoA synthetase. Aerobically, cells grown under either regimen may utilize the energy-conserving pyruvate-to-acetate pathway since O2 then serves as the ultimate electron acceptor.
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PMID:An energy-conserving pyruvate-to-acetate pathway in Entamoeba histolytica. Pyruvate synthase and a new acetate thiokinase. 1 76

CO dehydrogenase/acetyl-coenzyme A synthase (CODH) is the central enzyme in the pathway of acetyl-coenzyme A biosynthesis in Clostridium thermoaceticum. It catalyzes the interconversion of CO and CO2 and the synthesis of acetyl-coenzyme A from the methylated corrinoid/iron sulfur protein, CO, and coenzyme A. It is a nickel-iron-sulfur protein and contains two subunits in the form (alpha beta)3. Reported here is the cloning and sequencing of the genes for both subunits of CODH. The gene for the alpha subunit codes for a protein with 729 amino acids and a molecular weight of 81,730, and the beta gene for a protein with 674 amino acids and a molecular weight of 72,928. The alpha subunit follows the beta subunit by 23 bases and the genes for both subunits are preceded by a sequence which is similar to the Shine-Dalgarno sequence of Escherichia coli. No significant amino acid sequence homology has been found to any known sequence. Labeling CODH with 2,4-dinitrophenylsulfenyl chloride and isolating labeled peptide fragments demonstrated that a tryptophan, residue 418 of the alpha subunit, is protected by coenzyme A and thus may be considered a potential part of the coenzyme A site.
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PMID:The primary structure of the subunits of carbon monoxide dehydrogenase/acetyl-CoA synthase from Clostridium thermoaceticum. 174 56

Carnitine acetyltransferase was isolated from yeast Saccharomyces cerevisiae with an apparent molecular weight of 400,000. The enzyme contains identical subunits of 65,000 Da. The Km values of the isolated enzyme for acetyl-CoA and for carnitine were 17.7 microM and 180 microM, respectively. Carnitine acetyltransferase is an inducible enzyme, a 15-fold increase in the enzyme activity was found when the cells were grown on glycerol instead of glucose. Carnitine acetyltransferase, similarly to citrate synthase, has a double localization (approx. 80% of the enzyme is mitochondrial), while acetyl-CoA synthetase was found only in the cytosol. In the mitochondria carnitine acetyltransferase is located in the matrix space. The incorporation of 14C into CO2 and in lipids showed a similar ratio, 2.9 and 2.6, when the substrate was [1-14C]acetate and [1-14C]acetylcarnitine, respectively. Based on these results carnitine acetyltransferase can be considered as an enzyme necessary for acetate metabolism by transporting the activated acetyl group from the cytosol into the mitochondrial matrix.
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PMID:Isolation and characterization of carnitine acetyltransferase from S. cerevisiae. 189 91

An account is presented of the recent discovery of a pathway of growth by bacteria in which CO or CO2 and H2 are sources of carbon and energy. The Calvin cycle and subsequently other cycles were discovered in the 1950s, and in each the initial reaction of CO2 involved adding CO2 to an organic compound formed during the cyclic pathway (for example, CO2 and ribulose diphosphate). Studies were initiated in the 1950s with the thermophylic anaerobic organism Clostridium thermoaceticum, which Barker and Kamen had found fixed CO2 in both carbons of acetate during fermentation of glucose. The pathway of acetyl-CoA biosynthesis differs from all others in that two CO2 are combined with coenzyme A (CoASH) forming acetyl CoA, which then serves as the source of carbon for growth. This mechanism is designated the acetyl CoA pathway and some have called it the Wood pathway. A unique feature is the role of the enzyme carbon monoxide dehydrogenase (CODH), which catalyzes the conversion of CoASH, CO, and a methyl group to acetyl CoA, the final step of the pathway. The pathway involves the reduction of CO2 to formate, which then combines with tetrahydrofolate (THF) to form formyl THF. It in turn is reduced to CH3-THF. The methyl is then transferred to the cobalt on a corrinoid-containing enzyme. From there the methyl is transferred to CODH, and CO and CoASH bind with the enzyme at separate sites. Acetyl CoA is then synthesized. CODH would more properly be called carbon monoxide dehydrogenase-acetyl CoA synthase as it catalyzes oxidation of CO to CO2 and the synthesis of acetyl CoA. The solution of the mechanism of this pathway required more than 30 years, in part because the intermediate compounds are bound to enzymes, the enzymes are extremely sensitive to O2 and must be isolated under strictly anerobic conditions, and the role of a corrinoid and CODH was unprecedented. It is now apparent that this pathway occurs (perhaps with some modification) in many bacteria including the methane and sulfur bacteria. In some humans this pathway is catalyzed by the bacteria of the gut and acetate is produced rather than methane; it is calculated that 2.3 x 10(6) metric tons of acetate are formed daily from CO2. A similar synthesis occurs in the hind gut of termites. It is becoming apparent that the acetyl CoA pathway plays a significant role in the carbon cycle.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Life with CO or CO2 and H2 as a source of carbon and energy. 190 Jul 93

The physiology of Saccharomyces cerevisiae CBS 8066 was studied in glucose-limited chemostat cultures. Below a dilution rate of 0.30 h-1 glucose was completely respired, and biomass and CO2 were the only products formed. Above this dilution rate acetate and pyruvate appeared in the culture fluid, accompanied by disproportional increases in the rates of oxygen consumption and carbon dioxide production. This enhanced respiratory activity was accompanied by a drop in cell yield from 0.50 to 0.47 g (dry weight) g of glucose-1. At a dilution rate of 0.38 h-1 the culture reached its maximal oxidation capacity of 12 mmol of O2 g (dry weight)-1 h-1. A further increase in the dilution rate resulted in aerobic alcoholic fermentation in addition to respiration, accompanied by an additional decrease in cell yield from 0.47 to 0.16 g (dry weight) g of glucose-1. Since the high respiratory activity of the yeast at intermediary dilution rates would allow for full respiratory metabolism of glucose up to dilution rates close to mumax, we conclude that the occurrence of alcoholic fermentation is not primarily due to a limited respiratory capacity. Rather, organic acids produced by the organism may have an uncoupling effect on its respiration. As a result the respiratory activity is enhanced and reaches its maximum at a dilution rate of 0.38 h-1. An attempt was made to interpret the dilution rate-dependent formation of ethanol and acetate in glucose-limited chemostat cultures of S. cerevisiae CBS 8066 as an effect of overflow metabolism at the pyruvate level. Therefore, the activities of pyruvate decarboxylase, NAD+- and NADP+-dependent acetaldehyde dehydrogenases, acetyl coenzyme A (acetyl-CoA) synthetase, and alcohol dehydrogenase were determined in extracts of cells grown at various dilution rates. From the enzyme profiles, substrate affinities, and calculated intracellular pyruvate concentrations, the following conclusions were drawn with respect to product formation of cells growing under glucose limitation. (i) Pyruvate decarboxylase, the key enzyme of alcoholic fermentation, probably already is operative under conditions in which alcoholic fermentation is absent. The acetaldehyde produced by the enzyme is then oxidized via acetaldehyde dehydrogenases and acetyl-CoA synthetase. The acetyl-CoA thus formed is further oxidized in the mitochondria. (ii) Acetate formation results from insufficient activity of acetyl-CoA synthetase, required for the complete oxidation of acetate. Ethanol formation results from insufficient activity of acetaldehyde dehydrogenases.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Enzymic analysis of the crabtree effect in glucose-limited chemostat cultures of Saccharomyces cerevisiae. 256 99

On the basis of enzyme activities detected in extracts of Selenomonas ruminantium HD4 grown in glucose-limited continuous culture, at a slow (0.11 h-1) and a fast (0.52 h-1) dilution rate, a pathway of glucose catabolism to lactate, acetate, succinate, and propionate was constructed. Glucose was catabolized to phosphoenol pyruvate (PEP) via the Emden-Meyerhoff-Parnas pathway. PEP was converted to either pyruvate (via pyruvate kinase) or oxalacetate (via PEP carboxykinase). Pyruvate was reduced to L-lactate via a NAD-dependent lactate dehydrogenase or oxidatively decarboxylated to acetyl coenzyme A (acetyl-CoA) and CO2 by pyruvate:ferredoxin oxidoreductase. Acetyl-CoA was apparently converted in a single enzymatic step to acetate and CoA, with concomitant formation of 1 molecule of ATP; since acetyl-phosphate was not an intermediate, the enzyme catalyzing this reaction was identified as acetate thiokinase. Oxalacetate was converted to succinate via the activities of malate dehydrogenase, fumarase and a membrane-bound fumarate reductase. Succinate was then excreted or decarboxylated to propionate via a membrane-bound methylmalonyl-CoA decarboxylase. Pyruvate kinase was inhibited by Pi and activated by fructose 1,6-bisphosphate. PEP carboxykinase activity was found to be 0.054 mumol min-1 mg of protein-1 at a dilution rate of 0.11 h-1 but could not be detected in extracts of cells grown at a dilution rate of 0.52 h-1. Several potential sites for energy conservation exist in S. ruminantium HD4, including pyruvate kinase, acetate thiokinase, PEP carboxykinase, fumarate reductase, and methylmalonyl-CoA decarboxylase. Possession of these five sites for energy conservation may explain the high yields reported here (56 to 78 mg of cells [dry weight] mol of glucose-1) for S. ruminantium HD4 grown in glucose-limited continuous culture.
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PMID:Pathway and sites for energy conservation in the metabolism of glucose by Selenomonas ruminantium. 314 85

The metabolism of acetaldehyde was studied in isolated dog, rat and guinea-pig kidney-cortex tubules. In contrast with previous observations of Cederbaum and Rubin in rat kidney mitochondria (Archs Biochem. Biophys. 179, 46-66 1977) acetaldehyde was found to be metabolized by the tubules at high rates and in a dose-dependent manner at concentrations up to 5-10 mM. At high acetaldehyde concentrations (1-10 mM) acetaldehyde removal was accompanied by a high rate of acetate accumulation which explained most of the acetaldehyde metabolized in dog and guinea-pig but not in rat kidney tubules. These species differences in acetaldehyde metabolism can be explained by the differences in activities of aldehyde dehydrogenase (EC 1.2.1.3) and acetyl-CoA synthetase (EC6.2.1.1), the enzymes involved in renal acetaldehyde metabolism which were measured in the renal cortex of the three species. The acetaldehyde carbon removed and not accounted for by acetate accumulation was completely oxidized to CO2 as demonstrated by the measurement of [U-14C]-acetaldehyde conversion into 14CO2. At "physiological" acetaldehyde concentrations (0.1 and 0.2 mM) acetaldehyde utilization was also concentration-dependent but no acetate accumulation was observed.
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PMID:Characteristics of acetaldehyde metabolism in isolated dog, rat and guinea-pig kidney tubules. 368 31

Cells of the aerotolerant anaerobe Giardia lamblia respire in the presence of oxygen. Endogenous respiration is stimulated by glucose but not by other carbohydrates and Krebs cycle intermediates. Endogenous and glucose-stimulated respiration are insensitive to cyanide, malonate, and 2,4-dinitrophenol, but are inhibited by atabrin and iodoacetamide. G. lamblia produces ethanol, acetate and CO2 both aerobically and anaerobically either from endogenous reserves or exogenous glucose. Molecular hydrogen is not produced. The following enzyme activities were detected in homogenates: hexokinase, fructose-biphosphate aldolase, pyruvate kinase, phosphoenolpyruvate carboxykinase, malate dehydrogenase, malate dehydrogenase (decarboxylating), pyruvate synthase, acetyl-CoA synthetase, alcohol dehydrogenase (NADP+), NADH dehydrogenase, NADPH dehydrogenase, NADPH oxidoreductase and superoxide dismutase. The enzymes of energy and carbohydrate metabolism are nonsedimentable (109 000 x g for 30 min). Activities of lactate dehydrogenase, hydrogenase, phosphate acetyltransferase, acetate kinase, citrate synthase, succinate dehydrogenase, fumarate hydratase and catalase were below the limits of detection. The results suggest the occurrence of glycolysis, energy production by substrate level phosphorylation and a flavin, iron-sulfur protein mediated electron transport system as well as the absence of cytochrome mediated oxidative phosphorylation and functional Krebs cycle.
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PMID:Energy metabolism of the anaerobic protozoon Giardia lamblia. 610 7

Methanobacterium thermoautotrophicum growing on H2 plus CO2 as sole carbon and energy source was found to contain acetate thiokinase (Acetyl CoA synthetase; EC 6.2.1.1); Acetate + ATP + CoA leads to Acetyl CoA + AMP + PPi. The apparent Km value for acetate was 40 microM. Acetate kinase (EC 2.7.2.1) and phosphotransacetylase (EC 2.3.1.8) could not be detected. The specific activity of acetate thiokinase was high in cells grown with limited H2 and CO2 supply (approximately 100 nmol/min . mg protein), it was low in exponentially grown cells (2 nmol/min . mg protein). This corresponded with the finding that cells growing linearly in the presence of acetate assimilated the monocarboxylic acid in high amounts (greater than 10% of the cell carbon was derived from acetate), whereas exponentially growing cells did not (less than 1% of cell carbon was derived from acetate). These latter observations indicated that acetate thiokinase and free acetate are not involved in autotrophic CO2 fixation in M. thermoautotrophicum. The presence and some kinetic properties of succinate thiokinase (EC 6.2.1.5), adenylate kinase (EC 2.7.4.3), and inorganic pyrophosphatase (EC 3.6.1.1) are also described.
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PMID:Acetate thiokinase and the assimilation of acetate in methanobacterium thermoautotrophicum. 611

The physiology of Hanseniaspora uvarum K5 was studied in glucose-limited chemostat cultures and upon glucose pulse. Up to a dilution rate of 0.28 h-1, glucose was completely metabolized in biomass and CO2. Above this value, increase in the dilution rate was accompanied by sequential production of metabolites (glycerol, acetate and ethanol) and decrease in cell yield. Similar results were observed upon glucose pulse. From the enzyme activities (pyruvate dehydrogenase, pyruvate decarboxylase, NAD and NADP-dependent acetaldehyde dehydrogenases, acetyl coenzyme A synthetase and alcohol dehydrogenase) and substrate affinities, the following conclusions were drawn with respect to product formation of cells: (1) pyruvate was preferentially metabolized via pyruvate dehydrogenase, when biomass and CO2 were the only products formed; (2) acetaldehyde formed by pyruvate decarboxylase was preferentially oxidized in acetate by NADP-dependent aldehyde dehydrogenase; acetate accumulation results from insufficient activity of acetyl-CoA synthetase required for the complete oxidation of acetate; (3) acetaldehyde was oxidized in ethanol by alcohol dehydrogenase, in addition to acetate production.
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PMID:Glucose metabolism, enzymic analysis and product formation in chemostat culture of Hanseniaspora uvarum. 778 33


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