Question

Write short notes on the ornithine cycle

Answer 1

The Ornithine or urea cycle is a cycle of reactions that produces urea from ammonia. This cycle occurs in ureotelic organisms. The urea cycle converts highly toxic ammonia to urea for excretion. This cycle was the first metabolic cycle to be discovered.

The ornithine cycle consists of three main reactions: the conversion of ornithine into citrulline, the conversion of citrulline into arginine, and the splitting of arginine into urea and ornithine. Reactions (I) and (II) require expenditures of energy, which is provided in the form of adenosine triphosphate (ATP).

• Reaction (I) proceeds in two steps: (1) the formation of carbamoyl phosphate, which has an energy-rich phosphate bond, from ammonia, carbon dioxide, and two molecules of ATP (the reaction is activated by N-acetylmuramic acid; ammonia is apparently supplied to the liver in the form of glutamine, which is split by liver glutaminase into NH${}_3$ and glutamic acid); and (2) the formation of citrulline in the reaction of carbamyl phosphate with ornithine (the reaction proceeds using the energy of the carbamyl phosphate bond).
  

Reaction (II) also has two stages: (1) the reaction of citrulline with aspartic acid to form argininosuccinic acid (the reaction proceeds with the participation of ATP, with the liberation of adenylic acid [adenosine monophosphate, or AMP] and pyrophosphate, H4P2O7); and (2) the splitting of argininosuccinic acid to yield arginine and fumaric acid.

• In reaction (III), arginine is hydrolyzed to urea and ornithine, which reenters the cycle.