Role of glutamine synthetase in citric acid fermentation by Aspergillus niger

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 end-products 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.