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Table 1 Kinetic properties of the four M. truncatula GS isoenzymes

From: Novel aspects of glutamine synthetase (GS) regulation revealed by a detailed expression analysis of the entire GS gene family of Medicago truncatulaunder different physiological conditions

  GS1a GS1b GS2a GS2b
K m Glu (mM) 2.76 ± 0.22 1.56 ± 0.13 4.47 ± 0.30 6.79 ± 0.58
K m ATP mM 2.34 ± 0.18 1.67 ± 0.17 1.73 ± 0.14 4.76 ± 0.61
K 0.5 Hydroxylamine (mM) 0.90 ± 0.09 0.83 ± 0.08 0.97 ± 0.10 2.09 ± 0.27
h Hydroxylamine 1.97 ± 0.20 1.73 ± 0.16 1.73 ± 0.17 1.57 ± 0.14
GSs (μmol min-1 mg-1) 4.78 2.04 4.46 9.58
GSt (μmol min-1 mg-1) 413 296 251 12
Ratio GSt:GSs 87 145 56 1.2
pH optimum 7.0 7.0 7.5 7.5
  1. The affinity of each GS isoenzyme for the substrates was estimated using the synthetase reaction and 10 μg (GS1a; GS2a and GS2b) or 20 μg (GS1b) of purified protein per assay. Values were automatically calculated by Prism5® software (Graphpad software Inc.) by non-linear least squares regression method assuming either a Michaelis-Menten kinetics (hyperbolic) or an allosteric sigmoidal kinetics. K m – Michaelis-Menten constant; K 0.5 – Half-saturation constant; h – hill coefficient.