scholarly journals Carbon isotope effects on the decarboxylation of carboxylic acids. Comparison of the lactate oxidase reaction and the degradation of pyruvate by H2O2

1988 ◽  
Vol 252 (3) ◽  
pp. 913-915 ◽  
Author(s):  
E Melzer ◽  
H L Schmidt
Keyword(s):  
Carbon Isotope ◽  
Isotope Effect ◽  
Isotope Fractionation ◽  
Hydrogen Transfer ◽  
Isotope Effects ◽  
Model Reaction ◽  
Lactate Oxidase ◽  
Rate Determining Step ◽  
Enzymic Reaction ◽  
Oxidase Reaction

The isotope effect at C-1 on the H2O2-catalysed decarboxylation of pyruvate (used as a model reaction for the enzymic reaction) increases between pH 3 and 10 from 1.0007 +/- 0.0004 to 1.0283 +/- 0.0014 (25 degrees C). This result indicates a change in the rate-determining step from formation of the tetrahedral intermediate to decarboxylation of this intermediate. Practically no isotope fractionation at C-1 (1.0011 +/- 0.0002, pH 6.0, 25 degrees C) is found in the lactate oxidase-catalysed decarboxylation of lactate, which is indicative for the existence of an irreversible O2-dependent step prior to the enzyme-catalysed decarboxylation. In addition, the result provides further evidence that dissociation of pyruvate and H2O2 from the enzyme can be excluded. The isotope effect at C-2 of lactate in the enzymic reaction (1.0048 +/- 0.0004) is attributed to the hydrogen transfer step from lactate to the coenzyme.

scholarly journals Oxygen isotope fractionation during N<sub>2</sub>O production by soil denitrification

2016 ◽  
Vol 13 (4) ◽  
pp. 1129-1144 ◽  
Author(s):  
Dominika Lewicka-Szczebak ◽  
Jens Dyckmans ◽  
Jan Kaiser ◽  
Alina Marca ◽  
Jürgen Augustin ◽  
...  
Keyword(s):  
Oxygen Isotope ◽  
Soil Water ◽  
Isotope Effect ◽  
Isotope Exchange ◽  
Isotope Fractionation ◽  
Isotope Effects ◽  
N2o Production ◽  
Oxygen Isotope Fractionation ◽  
Oxygen Isotope Exchange ◽  
Incubation Experiments

Abstract. The isotopic composition of soil-derived N2O can help differentiate between N2O production pathways and estimate the fraction of N2O reduced to N2. Until now, δ18O of N2O has been rarely used in the interpretation of N2O isotopic signatures because of the rather complex oxygen isotope fractionations during N2O production by denitrification. The latter process involves nitrate reduction mediated through the following three enzymes: nitrate reductase (NAR), nitrite reductase (NIR) and nitric oxide reductase (NOR). Each step removes one oxygen atom as water (H2O), which gives rise to a branching isotope effect. Moreover, denitrification intermediates may partially or fully exchange oxygen isotopes with ambient water, which is associated with an exchange isotope effect. The main objective of this study was to decipher the mechanism of oxygen isotope fractionation during N2O production by soil denitrification and, in particular, to investigate the relationship between the extent of oxygen isotope exchange with soil water and the δ18O values of the produced N2O. In our soil incubation experiments Δ17O isotope tracing was applied for the first time to simultaneously determine the extent of oxygen isotope exchange and any associated oxygen isotope effect. We found that N2O formation in static anoxic incubation experiments was typically associated with oxygen isotope exchange close to 100 % and a stable difference between the 18O ∕ 16O ratio of soil water and the N2O product of δ18O(N2O ∕ H2O)  =  (17.5 ± 1.2) ‰. However, flow-through experiments gave lower oxygen isotope exchange down to 56 % and a higher δ18O(N2O ∕ H2O) of up to 37 ‰. The extent of isotope exchange and δ18O(N2O ∕ H2O) showed a significant correlation (R2 = 0.70, p <  0.00001). We hypothesize that this observation was due to the contribution of N2O from another production process, most probably fungal denitrification. An oxygen isotope fractionation model was used to test various scenarios with different magnitudes of branching isotope effects at different steps in the reduction process. The results suggest that during denitrification, isotope exchange occurs prior to isotope branching and that this exchange is mostly associated with the enzymatic nitrite reduction mediated by NIR. For bacterial denitrification, the branching isotope effect can be surprisingly low, about (0.0 ± 0.9) ‰, in contrast to fungal denitrification where higher values of up to 30 ‰ have been reported previously. This suggests that δ18O might be used as a tracer for differentiation between bacterial and fungal denitrification, due to their different magnitudes of branching isotope effects.

scholarly journals Evidence of Substantial Carbon Isotope Fractionation among Substrate, Inorganic Carbon, and Biomass during Aerobic Mineralization of 1,2-Dichloroethane byXanthobacter autotrophicus

2000 ◽  
Vol 66 (11) ◽  
pp. 4870-4876 ◽  
Author(s):  
D. Hunkeler ◽  
R. Aravena
Keyword(s):  
Carbon Isotope ◽  
Isotope Effect ◽  
Isotope Fractionation ◽  
Kinetic Isotope Effect ◽  
Inorganic Carbon ◽  
Mechanistic Model ◽  
Fractionation Factor ◽  
Carbon Isotope Fractionation ◽  
Kinetic Isotope ◽  
Factor Α

ABSTRACT Carbon isotope fractionation during aerobic mineralization of 1,2-dichloroethane (1,2-DCA) by Xanthobacter autotrophicusGJ10 was investigated. A strong enrichment of 13C in residual 1,2-DCA was observed, with a mean fractionation factor α ± standard deviation of 0.968 ± 0.0013 to 0.973 ± 0.0015. In addition, a large carbon isotope fractionation between biomass and inorganic carbon occurred. A mechanistic model that links the fractionation factor α to the rate constants of the first catabolic enzyme was developed. Based on the model, it was concluded that the strong enrichment of 13C in 1,2-DCA arises because the first irreversible step of the initial enzymatic transformation of 1,2-DCA consists of an SN2 nucleophilic substitution. SN2 reactions are accompanied by a large kinetic isotope effect. The substantial carbon isotope fractionation between biomass and inorganic carbon could be explained by the kinetic isotope effect associated with the initial 1,2-DCA transformation and by the metabolic pathway of 1,2-DCA degradation. Carbon isotope fractionation during 1,2-DCA mineralization leads to 1,2-DCA, inorganic carbon, and biomass with characteristic carbon isotope compositions, which may be used to trace the process in contaminated environments.

Viewpoint: Carbon isotope effect predictions for enzymes involved in the primary carbon metabolism of plant leaves

2005 ◽  
Vol 32 (4) ◽  
pp. 277 ◽  
Author(s):  
Guillaume Tcherkez ◽  
Graham D. Farquhar
Keyword(s):  
Carbon Isotope ◽  
Isotope Effect ◽  
Carbon Metabolism ◽  
Citrate Synthase ◽  
Isotope Effects ◽  
Krebs Cycle ◽  
Equilibrium Reactions ◽  
Theoretical Predictions ◽  
Carbon Isotope Effect

Carbon isotope effects of enzymes involved in primary carbon metabolism are key parameters in our understanding of plant metabolism. Nevertheless, some of them are poorly known because of the lack of in vitro experimental data on purified enzymes. Some studies have focused on theoretical predictions of isotope effects. Here we show how quantum chemical calculations can be adapted for calculation of isotope effects for the Rubisco-catalysed carboxylation and oxygenation reactions and the citrate synthase reaction. The intrinsic isotope effect of the carboxylation by Rubisco appears to be much smaller than previously thought, being close to the overall isotope effect of the reaction that is, between 25 and 30 per mil. The same applies to the enzyme citrate synthase, that catalyses the first step of the Krebs cycle, with an isotope effect of around 23 per mil. Combined with the isotope effects of equilibrium reactions calculated with β-factors, the Krebs cycle then has an overall isotope effect that depletes organic acids in 13C.

Isotope effects in the diphenylpicrylhydrazyl-inhibited thermal polymerization of 2,6-dideuteriostyrene

1981 ◽  
Vol 59 (21) ◽  
pp. 3090-3094 ◽  
Author(s):  
Karl R. Kopecky ◽  
Michael C. Hall
Keyword(s):  
Isotope Effect ◽  
Rate Constant ◽  
Hydrogen Transfer ◽  
Isotope Effects ◽  
Thermal Polymerization ◽  
Diels Alder ◽  
Inverse Isotope Effect

There is an inverse isotope effect in the reaction between 2,2-diphenyl-1-picrylhydrazyl DPPH and 2,6-dideuteriostyrene of 0.75 ± 0.07 at 75 °C in degassed neat styrene. This result is consistent with the proposal that the reaction involves hydrogen transfer to DPPH from a Diels–Alder dimer of styrene. The rate constant for dimerization of styrene to this dimer is calculated to be 1.8 × 10−10 L mol−1 s−1 at 75 °C.

scholarly journals Isotope fractionation (δ 13C, δ15N) and microbial community response in degradation of petroleum hydrocarbons by biostimulation in contaminated soil

2021 ◽  
Author(s):  
Heng Liu ◽  
Manli Wu ◽  
Xiqian Guo ◽  
Huan Gao ◽  
Yinrui Xu
Keyword(s):  
Isotope Effect ◽  
Contaminated Soil ◽  
Isotope Fractionation ◽  
Isotope Effects ◽  
Alpha Diversity ◽  
Nitrogen Isotope ◽  
Community Response ◽  
Carbon And Nitrogen ◽  
Stable Isotope Fractionation ◽  
Compost Amendment

Abstract This study investigated the isotope effects of δ13C and δ15N and microbial response during biodegradation of hydrocarbons by biostimulation with nitrate or compost in the petroleum-contaminated soil. Compost and KNO3 amendments promoted the total petroleum hydrocarbon (TPH) removal accompanied by a significant increase of Actinobacteria and Firmicutes phyla. Soil alpha diversity decreased after 90 days of biostimulation. An inverse significant carbon isotope effect (εc = 16.6 ± 0.8‰) and strong significant nitrogen isotope effect (εN = -24.20 ± 9.54‰) were shown by the KNO3 supplementation. For compost amendment, significant carbon and nitrogen isotope effect were εc = 38.8 ± 1.1‰ and εN = -79.49 ± 16.41‰, respectively. A clear difference of the carbon and nitrogen stable isotope fractionation was evident by KNO3 or compost amendment, which indicated the mechanisms of petroleum degradation by adding compost or KNO3 are different.

scholarly journals The mechanism of oxygen isotope fractionation during N<sub>2</sub>O production by denitrification

2015 ◽  
Vol 12 (20) ◽  
pp. 17009-17049
Author(s):  
D. Lewicka-Szczebak ◽  
J. Dyckmans ◽  
J. Kaiser ◽  
A. Marca ◽  
J. Augustin ◽  
...  
Keyword(s):  
Oxygen Isotope ◽  
Soil Water ◽  
Isotope Effect ◽  
Isotope Exchange ◽  
Isotope Fractionation ◽  
Isotope Effects ◽  
Nitrite Reduction ◽  
N2o Production ◽  
Oxygen Isotope Fractionation ◽  
Oxygen Isotope Exchange

Abstract. The isotopic composition of soil-derived N2O can help differentiate between N2O production pathways and estimate the fraction of N2O reduced to N2. Until now, δ18O of N2O has been rarely used in the interpretation of N2O isotopic signatures because of the rather complex oxygen isotope fractionations during N2O production by denitrification. The latter process involves nitrate reduction mediated through the following three enzymes: nitrate reductase (NAR), nitrite reductase (NIR) and nitric oxide reductase (NOR). Each step removes one oxygen atom as water (H2O), which gives rise to a branching isotope effect. Moreover, denitrification intermediates may partially or fully exchange oxygen isotopes with ambient water, which is associated with an exchange isotope effect. The main objective of this study was to decipher the mechanism of oxygen isotope fractionation during N2O production by denitrification and, in particular, to investigate the relationship between the extent of oxygen isotope exchange with soil water and the δ18O values of the produced N2O. We performed several soil incubation experiments. For the first time, Δ17O isotope tracing was applied to simultaneously determine the extent of oxygen isotope exchange and any associated oxygen isotope effect. We found bacterial denitrification to be typically associated with almost complete oxygen isotope exchange and a stable difference in δ18O between soil water and the produced N2O of δ18O(N2O / H2O) = (17.5 ± 1.2) ‰. However, some experimental setups yielded oxygen isotope exchange as low as 56 % and a higher δ18O(N2O / H2O) of up to 37 ‰. The extent of isotope exchange and δ18O(N2O / H2O) showed a very significant correlation (R2 = 0.70, p < 0.00001). We hypothesise that this observation was due to the contribution of N2O from another production process, most probably fungal denitrification. An oxygen isotope fractionation model was used to test various scenarios with different magnitudes of branching isotope effects at different steps in the reduction process. The results suggest that during denitrification the isotope exchange occurs prior to the isotope branching and that the mechanism of this exchange is mostly associated with the enzymatic nitrite reduction mediated by NIR. For bacterial denitrification, the branching isotope effect can be surprisingly low, about (0.0 ± 0.9) ‰; in contrast to fungal denitrification where higher values of up to 30 ‰ have been reported previously. This suggests that δ18O might be used as a tracer for differentiation between bacterial and fungal denitrification, due to their different magnitudes of branching isotope effects.

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