Role of glutamine synthetase in citric acid fermentation by Aspergillus niger.
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Date
1985-06
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Abstract
The activity of glutamine synthetase from Aspergillus niger was significantly
lowered under conditions of citric acid fermentation. The intracellular pH of the organism as
determined by bromophenol blue dye distribution and fluorescein diacetate uptake methods
was relatively constant between 6·0-6·5, when the pH of the external medium was varied
between 2·3-7·0. Aspergillus niger glutamine synthetase was rapidly inactivated under acidic
pH conditions and Mn2+ ions partially protected the enzyme against this inactivation. Mn2+-
dependent glutamine synthetase activity was higher at acidic pH (6·0) compared to Mg2+-
supported activity. While the concentration of Mg2+ required to optimally activate glutamine
synthetase at pH 6·0 was very high (≥ 50 mM), Mn2+ was effective at 4 mM. Higher
concentrations of Mn2+ were inhibitory. The inhibition of both Mn2+ and Mg2+-dependent
reactions by citrate, 2-oxoglutarate and ATP were probably due to their ability to chelate
divalent ions rather than as regulatory molecules. This suggestion was supported by the
observation that a metal ion chelator, EDTA also produced similar effects. Of the endproducts
of the pathway, only histidine, carbamyl phosphate, AMP and ADP inhibited
Aspergillus niger glutamine synthetase. The inhibitions were more pronounced when Mn2+
was the metal ion activator and greater inhibition was observed at lower pH values. These
results permit us to postulate that glutamine synthesis may be markedly inhibited when the
fungus is grown under conditions suitable for citric acid production and this block may result
in delinking carbon and nitrogen metabolism leading to acidogenesis.
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Keywords
Citric acid fermentation, glutamine synthetase, regulation by metal ions
Citation
Punekar N S, Vaidyanathan C S, Rao N Appaji. Role of glutamine synthetase in citric acid fermentation by Aspergillus niger. Journal of Biosciences. 1985 Jun; 7(3&4): 269-287.