A metabolic control analysis of the glutamine synthetase/glutamate synthase cycle in isolated barley (Hordeum vulgare L.) chloroplasts
Ammonia assimilation in chloroplasts occurs via the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle. To determine the extent to which these enzymes contribute to the control of ammonia assimilation, a metabolic control analysis was performed on isolated barley (Hordeum vulgare L.) leaf chlo...
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Published in | Plant physiology (Bethesda) Vol. 105; no. 1; pp. 415 - 424 |
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Main Authors | , , , |
Format | Journal Article |
Language | English |
Published |
Rockville, MD
American Society of Plant Physiologists
01.05.1994
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Subjects | |
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Abstract | Ammonia assimilation in chloroplasts occurs via the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle. To determine the extent to which these enzymes contribute to the control of ammonia assimilation, a metabolic control analysis was performed on isolated barley (Hordeum vulgare L.) leaf chloroplasts. Pathway flux was measured polarographically as ammonium-plus-2-oxoglutarate-plus-glutamine-dependent evolution in illuminated chloroplasts. Enzyme activity was modulated by titration with specific, irreversible inhibitors of GS (phosphinothricin) and GOGAT (azaserine). Flux control coefficients were determined (a) by differentiation of best-fit hyperbolic curves of the data sets flux versus enzyme activity), and (b) from estimates of the deviation indices (deviation index). Both analyses gave similar values for the coefficients. The control coefficient for GS was relatively high and the value did not change significantly with changes in 2-oxoglutarate concentration (flux control coefficient = 0.58 at 5 millimole 2-oxoglutarate and 0.40 at 20 millimole 2-oxoglutarate). The control coefficient for GOGAT decreased with decreasing glutamine concentrations, from 0.76 at 20 millimole glutamine to 0.19 at 10 millimole glutamine. Thus, at high concentrations of glutamine, GOGAT exerts a major control over flux with a significant contribution also from GS. At lower concentrations of glutamine, however, GOGAT exerts far less control over pathway flux |
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AbstractList | Ammonia assimilation in chloroplasts occurs via the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle. To determine the extent to which these enzymes contribute to the control of ammonia assimilation, a metabolic control analysis was performed on isolated barley (Hordeum vulgare L.) leaf chloroplasts. Pathway flux was measured polarographically as ammonium-plus-2-oxoglutarate-plus-glutamine-dependent O2 evolution in illuminated chloroplasts. Enzyme activity was modulated by titration with specific, irreversible inhibitors of GS (phosphinothricin) and GOGAT (azaserine). Flux control coefficients (CJ0E0) were determined (a) by differentiation of best-fit hyperbolic curves of the data sets (flux versus enzyme activity), and (b) from estimates of the deviation indices (D/[prime]E0). Both analyses gave similar values for the coefficients. The control coefficient for GS was relatively high and the value did not change significantly with changes in 2-oxoglutarate concentration (C/0E0 = 0.58 at 5 mM 2-oxoglutarate and 0.40 at 20 mM 2-oxoglutarate). The control coefficient for GOGAT decreased with decreasing glutamine concentrations, from 0.76 at 20 mM glutamine to 0.19 at 10 mM glutamine. Thus, at high concentrations of glutamine, GOGAT exerts a major control over flux with a significant contribution also from GS. At lower concentrations of glutamine, however, GOGAT exerts far less control over pathway flux. Ammonia assimilation in chloroplasts occurs via the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle. To determine the extent to which these enzymes contribute to the control of ammonia assimilation, a metabolic control analysis was performed on isolated barley (Hordeum vulgare L.) leaf chloroplasts. Pathway flux was measured polarographically as ammonium-plus-2-oxoglutarate-plus-glutamine-dependent O2 evolution in illuminated chloroplasts. Enzyme activity was modulated by titration with specific, irreversible inhibitors of GS (phosphinothricin) and GOGAT (azaserine). Flux control coefficients ($C_{E_{0}}^{J^{0}}$) were determined (a) by differentiation of best-fit hyperbolic curves of the data sets (flux versus enzyme activity), and (b) from estimates of the deviation indices ($D_{E_{\text{i}}^{\text{r}}}^{J^{r}}$). Both analyses gave similar values for the coefficients. The control coefficient for GS was relatively high and the value did not change significantly with changes in 2-oxoglutarate concentration ($C_{E_{0}}^{J^{0}}$ = 0.58 at 5 mM 2-oxoglutarate and 0.40 at 20 mM 2-oxoglutarate). The control coefficient for GOGAT decreased with decreasing glutamine concentrations, from 0.76 at 20 mM glutamine to 0.19 at 10 mM glutamine. Thus, at high concentrations of glutamine, GOGAT exerts a major control over flux with a significant contribution also from GS. At lower concentrations of glutamine, however, GOGAT exerts far less control over pathway flux. Ammonia assimilation in chloroplasts occurs via the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle. To determine the extent to which these enzymes contribute to the control of ammonia assimilation, a metabolic control analysis was performed on isolated barley (Hordeum vulgare L.) leaf chloroplasts. Pathway flux was measured polarographically as ammonium-plus-2-oxoglutarate-plus-glutamine-dependent evolution in illuminated chloroplasts. Enzyme activity was modulated by titration with specific, irreversible inhibitors of GS (phosphinothricin) and GOGAT (azaserine). Flux control coefficients were determined (a) by differentiation of best-fit hyperbolic curves of the data sets flux versus enzyme activity), and (b) from estimates of the deviation indices (deviation index). Both analyses gave similar values for the coefficients. The control coefficient for GS was relatively high and the value did not change significantly with changes in 2-oxoglutarate concentration (flux control coefficient = 0.58 at 5 millimole 2-oxoglutarate and 0.40 at 20 millimole 2-oxoglutarate). The control coefficient for GOGAT decreased with decreasing glutamine concentrations, from 0.76 at 20 millimole glutamine to 0.19 at 10 millimole glutamine. Thus, at high concentrations of glutamine, GOGAT exerts a major control over flux with a significant contribution also from GS. At lower concentrations of glutamine, however, GOGAT exerts far less control over pathway flux |
Author | Tobin, T.H Tobin, A.K Baron, A.C Wallsgrove, R.M |
AuthorAffiliation | School of Biological Sciences, S3.614 Stopford Building, University of Manchester, Manchester M13 9PT, United Kingdom (A.C.B., T.H.T., A.K.T.) |
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Keywords | Assimilation Monocotyledones Carbon-nitrogen ligases Hordeum vulgare Enzyme Metabolic cycle Cereal crop Ammonia Enzymatic activity Gramineae Ligases Angiospermae Spermatophyta Inhibitor Oxidoreductases Glutamate-ammonia ligase Chloroplast |
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SubjectTerms | ACTIVIDAD ENZIMATICA ACTIVITE ENZYMATIQUE Agronomy. Soil science and plant productions Ammonia Barley Biological and medical sciences Centrifugation CHLOROPLASTE Chloroplasts CLOROPLASTO Coefficients Enzymes FEUILLE Fundamental and applied biological sciences. Psychology HOJAS HORDEUM VULGARE LIGASAS LIGASE MATEMATICAS MATHEMATIQUE Metabolism Metabolism and Enzymology METABOLISME METABOLISMO Nitrogen metabolism OXIDORREDUCTASAS OXYDOREDUCTASE Plant physiology and development Plants Protoplasts REACCIONES QUIMICAS REACTION CHIMIQUE Silicones Titration VIA BIOQUIMICA DEL METABOLISMO VOIE BIOCHIMIQUE DU METABOLISME |
Title | A metabolic control analysis of the glutamine synthetase/glutamate synthase cycle in isolated barley (Hordeum vulgare L.) chloroplasts |
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