Ammonia Recycling

This pathway describes some of the routes by which ammonia is generated or re-used as part of nitrogen metabolism. Seven amino acids play a key role in this process: glutamate, glutamine, glycine, serine, histidine, aspartate and asparagines. Glutamate and glutamine are the two important amino acids in recycling ammonia in our body instead of excreting it as waste in form of urea. Glutamine is synthesized from glutamate by incorporation of an NH3 into the carboxyl group forming an amide. This step requires ATP and is catalyzed by glutamine synthetase. The coupling of glutamine synthesis with ATP hydrolysis renders the reaction irreversible. The back reaction – the regeneration of glutamate from glutamine – is catalyzed by glutaminase, which deaminates glutamine via a hydrolysis reaction. Transamination reactions also play an important role in nitrogen/ammonia metabolism. In particular, the interplay of two additional enzymes: glutamate transaminase and glutamate dehydrogenase is essential in the control of nitrogen balance in the body. Beyond the role of glutamate and glutamine in nitrogen metabolism, the action of asparaginase on asparagine (to generate aspartate and ammonia) is another route to generate and recycle ammonia. Ammonia is also generated or recycled through the action of the glycine cleavage system (ammoniamethyltransferase) on glycine. Furthermore, ammonia can be recycled through the action of histidine ammonia lyase which acts on histidine to generate urocanate and ammonia. Another amino acid, serine, can also play a part in ammonia recycling through the action of serine dehydratase. This enzyme cleaves serine into pyruvate and ammonia.

References

1. Lehninger, A.L. (2005) Lehninger principles of biochemistry (4 ^th ed.). New York: W.H Freeman.
2. Salway, J.G. (2004) Metabolism at a glance (3 ^rd ed.). Alden, Mass. : Blackwell Pub.
3. Kikuchi G (1973) The glycine cleavage system: composition, reaction mechanism, and physiological significance Mol. Cell. Biochem. 1 (2): 169–87.