A metabolic control analysis of the glutamine synthetase/glutamate synthase cycle in isolated barley (Hordeum vulgare L.) chloroplasts

• Published in: Plant physiology (Bethesda) Vol. 105; no. 1; pp. 415 - 424
• Main Authors: Baron, A.C, Tobin, T.H, Wallsgrove, R.M, Tobin, A.K
• Format: Journal Article
• Language: English
• Published: Rockville, MD American Society of Plant Physiologists 01.05.1994
• Subjects: 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; 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
• DOI: 10.1104/pp.105.1.415
• ISSN: 0032-0889

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