Urea Cycle Reactions

The urea cycle, as its name implies, is a cyclic pathway, but there is a point of entry of ammonium ion and an exit point of urea, which will be used below as the first and last reactions of the cycle. Figure 20.13 depicts the figure-8 scheme of the pathway, which explains the reactions below that follow the reactions leading to urea.

Reaction | Enzyme
NH4+ + CO2 + 2 ATP <=> Carbamoyl Phosphate + 2 ADP + Pi | Carbamoyl Phosphate Synthetase
Carbamoyl Phosphate + Ornithine <=> Citrulline + Pi | Ornithine Transcarbamoylase
Citrulline + Aspartate + ATP <=> Argininosuccinate + AMP + PPi | Argininosuccinate Synthetase
Argininosuccinate <=> Arginine + Fumarate | Argininosuccinase
Arginine + H2O <=> Urea + Ornithine | Arginase
Fumarate + H2O <=> Malate | Fumarate Hydratase
Malate + NAD+<=> Oxaloacetate + NADH + H+ | Malate Dehydrogenase
Oxaloacetate + Glutamate<=> Aspartate + -Ketoglutarate | Transaminase
-Ketoglutarate + NH4+ + NADH + H+ <=> Glutamate + H2O + NAD+ | Glutamate dehydrogenase

See also: Urea Cycle Description

INTERNET LINK: Urea Cycle and Metabolism of Amino Groups