Part:BBa_K5378000:Hard Information

Usage and Biology

The glutamine synthetase (GS) is an ammonia scavenger that is located in perivenous hepatocytes (constituting around 6–7% of the parenchymal population) and catalyses the ATP-dependent amidation of glutamate to glutamine. There are documents proving that hepatic deletion of GS in mice results in hyperammonaemia, oxidative stress in brain tissue, behaviour abnormalities and reduced lifespan.

Functional Verification

In humans and other mammals, ammonia is majorly digested by the Urea Cycle in the liver. However, as it is shown before, the liver functions are destroyed in HE patients, so the urea cycle is hard to use. So we move our sight to plants. Plants utilize ammonium (NH₄⁺) as a nitrogen source through several processes, which are critical for their growth and development. Upon uptake by plant roots, ammonium is either directly assimilated into amino acids via the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway or converted to nitrate for further use in metabolic processes[4]. Ammonium is an energetically favorable form of nitrogen compared to nitrate(like the annamox bateria)[5], as it does not require reduction before assimilation. However, excessive ammonium can be toxic to plants, so its uptake and assimilation are tightly regulated to maintain optimal nitrogen levels within the plant tissues. As for the nitrate path, it may produce harmful side products such as nitrite, so we finally decided to mimic the GS-GOGAT cycle to metabolize ammonia in HE patients. So we expressed the glutamate synthase in EcN, and it can degrade ammonia into glutamine without harm to the body.

Reference

[1]Qvartskhava, N. et al. Hyperammonemia in gene-targeted mice lacking functional hepatic glutamine synthetase. Proc. Natl Acad. Sci. USA 112, 5521–5526 (2015).

[2]Frieg, B., Gorg, B., Gohlke, H. & Haussinger, D. Glutamine synthetase as a central element in hepatic glutamine and ammonia metabolism: novel aspects. Biol. Chem. 402, 1063–1072 (2021).

[3]Paluschinski, M. et al. Characterization of the scavenger cell proteome in mouse and rat liver. Biol. Chem. 402, 1073–1085 (2021).

[4] Haussinger, D. Nitrogen metabolism in liver: structural and functional organization and physiological relevance. Biochem. J. 267, 281–290 (1990).

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