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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Archives of microbiology 112 (1977), S. 239-246 
    ISSN: 1432-072X
    Keywords: Hydrogen bacteria ; Alcaligenes eutrophus H 16 ; Leucine biosynthesis ; α-Isopropylmalate synthase ; Temperature anomaly ; Cold lability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract α-Isopropylmalate (IPM) synthase, the first enzyme in the biosynthesis of l-leucine, was purified to a specific activity of 12 μmole/min x mg protein from the valine-isoleucine double auxotrophic mutant A-81 of the hydrogen bacterium Alcaligenes eutrophus H 16. The activity in crude extracts of derepressed cells was 0.106 μmoles of isopropylmalate formed per min and per mg protein. Gel electrophoresis and regel electrophoresis of the isolated main band resulted in several distinct bands, which were not altered by the additions of substrate α-ketoisovalerate, feedback inhibitor leucine or other effectors. The isoelectric points of the enzyme protein was between 3.9 and 4.0. The molecular weight was 114500 daltons and 100000 respectively in the absence and presence of the feedback inhibitor leucine. The enzyme activity depended strongly on the pH, the optimum is at pH 8.2. The enzyme was could labile and exhibits temperature anomalies.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Archives of microbiology 112 (1977), S. 247-254 
    ISSN: 1432-072X
    Keywords: Hydrogen bacteria ; Alcaligenes eutrophus H 16 ; Leucine biosynthesis ; α-Isopropylmalate synthase ; Cooperativity changes ; Product inhibition ; Substrate specificity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The purified isopropylmalate synthase of Alcaligenes eutrophus H 16 reacted with the following α-keto acids and acyl-coenzyme A derivatives (in the sequence of decreasing affinities): α-ketoisovalerate, α-keto-n-valerate, α-ketobutyrate and pyruvate; acetyl-CoA, propionyl-CoA, butyryl-CoA. malonyl-CoA, valeryl-CoA, and crotonyl-CoA. α-Ketoisocaproate, however, is a strong inhibitor of the enzyme. All reactions catalyzed by isopropylmalate synthase were inhibited to the same extent by the endproduct l-leucine. the substrate saturation curves of α-ketoisovalerate or other α-keto acids and of acetyl-coenzyme A or other acyl-CoA derivatives had intermediary plateau regions; the Hill coefficient alternated between n H -values higher and lower than 1.0, indicating changes from positive to negative and from negative to positive cooperativity for the substrates. The products, isopropylmalate and free coenzyme A, showed competitive inhibition patterns against both substrates (α-ketoisovalerate and acetyl-CoA). Free coenzyme A (1 μM) inactivated the enzyme irreversibly. The 3′-phosphate of coenzyme A and the free carboxyl group of α-ketoisovalerate were involved in optimal binding of these substrates, but 3′-dephospho-acetyl-coenzyme A and the methylester of α-ketoisovalerate were also converted by this enzyme. A CH3−CH2-grouping of the α-keto acids seemed to be necessary for binding this substrate.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Archives of microbiology 114 (1977), S. 203-210 
    ISSN: 1432-072X
    Keywords: Hydrogen bacteria ; Alcaligenes eutrophus H 16 ; Leucine biosynthesis ; α-isopropylmalate synthase ; Regulation ; Feedback inhibition ; Relief of inhibition by valine and isoleucine ; Inhibition by α-ketoisocaproate ; Temperature anomaly
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The α-isopropylmalate synthase (EC 4.1.3.12) from Alcaligenes eutrophus H 16 was inhibited by l-leucine and α-ketoisocaproate. The extent of inhibition was influenced by substrate- and inhibitor concentrations as well as by the pH. Intermediary plateaus, which always appeared in the inhibition curves, suggested cooperative effects. The maximal Hill coefficient was found to be two. At low concentrations of leucine the inhibition mechanism was of the competitive type with respect to substrate acetyl coenzyme A and of the noncompetitive type with respect to substrate α-ketoisovalerate. The inhibition was specifically relieved by the addition of valine or isoleucine. The anomalous effect of temperature on enzyme activity was diminished by leucine. The Arrhenius energy of the reaction increased from about 11 kcal/mole in the absence of leucine to about 18 kcal/mole in the presence of leucine. The further addition of valine reversed this effect. The physiological relevance of the α-ketoisocaproate-mediated inhibition is discussed.
    Type of Medium: Electronic Resource
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