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