The kinetic mechanism of the reactions catalyzed by the glutamate synthase from Azospirillum brasilense

M. A. Vanoni, L. Nuzzi, M. Rescigno, G. Zanetti, B. Curti

Research output: Contribution to journalArticle

26 Citations (Scopus)

Abstract

The reactions catalyzed by glutamate synthase from Azospirillum brasilense have been investigated by a combination of absorption spectroscopy, steady-state kinetic measurements and experiments with stereospecifically laballed substrate. The data show that both L-glutamine-dependent and ammonia-dependent reactions of the glutamate synthase from A. brasilense follow an identical two-site uni-uni bi-bi kinetic mechanism, in which the enzyme is alternately reduced by NADPH and oxidized by the iminoglutarate formed on addition of ammonia to the C2 of 2-oxoglutarate. The spectroscopic experiments support the involvement of the enzyme chromophores (flavins and iron-sulfur centers) in both reactions. Finally, using stereospecifically labelled NADPH, we showed that the enzyme from Azospirillum is specific for the transfer of the 4S hydrogen of NADPH. During the catalysis of both L-glutamine-dependent and ammonia-dependent reactions, this hydrogen atom equilibrates with the solvent. The data obtained with glutamate synthase from A. brasilense, a diazotroph, differ significantly from those regarding the ammonia-dependent reaction of other glutamate synthases. The ammonia-dependent activity of glutamate synthase from Azospirillum is not physiologically significant, representing only a segment of the overall physiological L-glutamine-dependent activity and requiring the enzyme flavins and iron-sulfur centers. Finally, the data are not consistent with the hypothesis [Geary, L.E. and Meister, A. (1977) J. Biol. Chem. 252, 3501-3508] that the small subunit of glutamate synthase is endowed with a glutamate-dehydrogenase-like activity.

Original languageEnglish
Pages (from-to)181-189
Number of pages9
JournalEuropean Journal of Biochemistry
Volume202
Issue number1
Publication statusPublished - 1991

Fingerprint

Glutamate Synthase
Azospirillum brasilense
Ammonia
Azospirillum
Glutamine
Kinetics
NADP
Flavins
Enzymes
Sulfur
Hydrogen
Iron
Glutamate Dehydrogenase
Chromophores
Catalysis
Absorption spectroscopy
Spectrum Analysis
Experiments
Atoms
Substrates

ASJC Scopus subject areas

  • Biochemistry

Cite this

The kinetic mechanism of the reactions catalyzed by the glutamate synthase from Azospirillum brasilense. / Vanoni, M. A.; Nuzzi, L.; Rescigno, M.; Zanetti, G.; Curti, B.

In: European Journal of Biochemistry, Vol. 202, No. 1, 1991, p. 181-189.

Research output: Contribution to journalArticle

@article{e64d9347f4ba42af89de74856f1c5d12,
title = "The kinetic mechanism of the reactions catalyzed by the glutamate synthase from Azospirillum brasilense",
abstract = "The reactions catalyzed by glutamate synthase from Azospirillum brasilense have been investigated by a combination of absorption spectroscopy, steady-state kinetic measurements and experiments with stereospecifically laballed substrate. The data show that both L-glutamine-dependent and ammonia-dependent reactions of the glutamate synthase from A. brasilense follow an identical two-site uni-uni bi-bi kinetic mechanism, in which the enzyme is alternately reduced by NADPH and oxidized by the iminoglutarate formed on addition of ammonia to the C2 of 2-oxoglutarate. The spectroscopic experiments support the involvement of the enzyme chromophores (flavins and iron-sulfur centers) in both reactions. Finally, using stereospecifically labelled NADPH, we showed that the enzyme from Azospirillum is specific for the transfer of the 4S hydrogen of NADPH. During the catalysis of both L-glutamine-dependent and ammonia-dependent reactions, this hydrogen atom equilibrates with the solvent. The data obtained with glutamate synthase from A. brasilense, a diazotroph, differ significantly from those regarding the ammonia-dependent reaction of other glutamate synthases. The ammonia-dependent activity of glutamate synthase from Azospirillum is not physiologically significant, representing only a segment of the overall physiological L-glutamine-dependent activity and requiring the enzyme flavins and iron-sulfur centers. Finally, the data are not consistent with the hypothesis [Geary, L.E. and Meister, A. (1977) J. Biol. Chem. 252, 3501-3508] that the small subunit of glutamate synthase is endowed with a glutamate-dehydrogenase-like activity.",
author = "Vanoni, {M. A.} and L. Nuzzi and M. Rescigno and G. Zanetti and B. Curti",
year = "1991",
language = "English",
volume = "202",
pages = "181--189",
journal = "European Journal of Biochemistry",
issn = "0014-2956",
publisher = "Wiley-Blackwell",
number = "1",

}

TY - JOUR

T1 - The kinetic mechanism of the reactions catalyzed by the glutamate synthase from Azospirillum brasilense

AU - Vanoni, M. A.

AU - Nuzzi, L.

AU - Rescigno, M.

AU - Zanetti, G.

AU - Curti, B.

PY - 1991

Y1 - 1991

N2 - The reactions catalyzed by glutamate synthase from Azospirillum brasilense have been investigated by a combination of absorption spectroscopy, steady-state kinetic measurements and experiments with stereospecifically laballed substrate. The data show that both L-glutamine-dependent and ammonia-dependent reactions of the glutamate synthase from A. brasilense follow an identical two-site uni-uni bi-bi kinetic mechanism, in which the enzyme is alternately reduced by NADPH and oxidized by the iminoglutarate formed on addition of ammonia to the C2 of 2-oxoglutarate. The spectroscopic experiments support the involvement of the enzyme chromophores (flavins and iron-sulfur centers) in both reactions. Finally, using stereospecifically labelled NADPH, we showed that the enzyme from Azospirillum is specific for the transfer of the 4S hydrogen of NADPH. During the catalysis of both L-glutamine-dependent and ammonia-dependent reactions, this hydrogen atom equilibrates with the solvent. The data obtained with glutamate synthase from A. brasilense, a diazotroph, differ significantly from those regarding the ammonia-dependent reaction of other glutamate synthases. The ammonia-dependent activity of glutamate synthase from Azospirillum is not physiologically significant, representing only a segment of the overall physiological L-glutamine-dependent activity and requiring the enzyme flavins and iron-sulfur centers. Finally, the data are not consistent with the hypothesis [Geary, L.E. and Meister, A. (1977) J. Biol. Chem. 252, 3501-3508] that the small subunit of glutamate synthase is endowed with a glutamate-dehydrogenase-like activity.

AB - The reactions catalyzed by glutamate synthase from Azospirillum brasilense have been investigated by a combination of absorption spectroscopy, steady-state kinetic measurements and experiments with stereospecifically laballed substrate. The data show that both L-glutamine-dependent and ammonia-dependent reactions of the glutamate synthase from A. brasilense follow an identical two-site uni-uni bi-bi kinetic mechanism, in which the enzyme is alternately reduced by NADPH and oxidized by the iminoglutarate formed on addition of ammonia to the C2 of 2-oxoglutarate. The spectroscopic experiments support the involvement of the enzyme chromophores (flavins and iron-sulfur centers) in both reactions. Finally, using stereospecifically labelled NADPH, we showed that the enzyme from Azospirillum is specific for the transfer of the 4S hydrogen of NADPH. During the catalysis of both L-glutamine-dependent and ammonia-dependent reactions, this hydrogen atom equilibrates with the solvent. The data obtained with glutamate synthase from A. brasilense, a diazotroph, differ significantly from those regarding the ammonia-dependent reaction of other glutamate synthases. The ammonia-dependent activity of glutamate synthase from Azospirillum is not physiologically significant, representing only a segment of the overall physiological L-glutamine-dependent activity and requiring the enzyme flavins and iron-sulfur centers. Finally, the data are not consistent with the hypothesis [Geary, L.E. and Meister, A. (1977) J. Biol. Chem. 252, 3501-3508] that the small subunit of glutamate synthase is endowed with a glutamate-dehydrogenase-like activity.

UR - http://www.scopus.com/inward/record.url?scp=0026056062&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0026056062&partnerID=8YFLogxK

M3 - Article

VL - 202

SP - 181

EP - 189

JO - European Journal of Biochemistry

JF - European Journal of Biochemistry

SN - 0014-2956

IS - 1

ER -