Question

How is glutamine used in nitrogen metabolism and Why is the activity of glutamine synthetase tightly regulated?

Answer 1

Glutamine plays the a key role in the nitrogen (N) flow of organism, it serve as both a N- donor and nitrogen acceptor. The enzymes glutamine synthetase (GS) is ubiquitous and found in all organisms and control conversion of ammonia into a amino acid glutamine. In animal system, GS is predominantly found in the liver, brain and kidneys. Along with glutamate, glutamine maintains nitrogen homeostasis in Brain. Within central nervous system the interaction between neurons, cerebral blood flow and protective astrocytes regulates the metabolism nitrogenous compounds: glutamine, glutamate and ammonia, also know as glutamate-glutamine cycle which is critical metabolic process integral to overall brain glutamate metabolism. Glutamine is also involved in reduction ammonia induced neurotoxicity. Reduced ammonia and higher level of glutamate (glutamate converts to glutamine) is required for proper functioning of brain. Because glutamate act as both: a neurotransmitter and a precursor for GABA, an another neurotransmitter. On the other hand, apart from changing brain blood pH, it also depletes 2-oxoglutarate levels, ultimately downregulation of Krebs cycle.

In Plant, GS is distributed in subcellular organelle such as chloroplast and cytoplasm of different tissues and organs. Depending on the growth rate and the developmental stage the distribution and the expression of GS differs, for example GS1 appears to play central role during leaf senescence. GS is involved in the ammonia assimilation and re-assimilation, and maintains carbon (C)/nitrogen ratio. Together, Glutamine and GS role is photorespiration, amino acid biosynthesis and secondary metabolite are also important. Several homologs of GS found in plants which were expressed at different level in different organs/tissue as the N-need and maintains N-homeostasis. Important factors that affects the activity of GS are C-status, nitrogen nutrition and light. Phosphorylation of GS is important, in dark it get phosphorylated; additionally, it binds to 14?3?3proteins which provides stability against degradation. In contrast, light induces de-phosphorylation of GS leading to its degradation. Similarly, GS is reciprocally regulated by C-status.