Arg and Asp
The urea cycle consists of five reactions: two mitochondrial and three cytosolic. The cycle converts two amino groups, one from NH4+ and one from Asp, and a carbon atom from HCO3−, to the relatively nontoxic excretion product urea at
the cost of four "high-energy" phosphate bonds (3 ATP hydrolyzed to 2 ADP and one ATP to AMP). Orn is the carrier of these carbon and nitrogen atoms.