Urea Cycle

Question 1

Ornithine transcarbamoylase

Answer 1

Converts Ornithine to Citrulline by transferring a carbamoyl group to Ornithine.

Question 2

Argininosuccinate synthetase

Answer 2

Converts Citrulline to Argininosuccinate by adding an aspartate to Citrulline in an ATP-dependent condensation reaction. The products of the reaction include AMP and PPi; the PPi is hydrolyzed to 2 PPi.
This step consumes 2 ATP.

Question 3

Argininosuccinase

Answer 3

Converts Argininosuccinate to Arginine by releasing fumarate (which represents the carbon skeleton of aspartate).

Question 4

Arginase

Answer 4

Converts Arginine to Ornithine by hydrolyzing arginine to generate Urea and regenerate Ornithine for another round of Urea.
Removing NH2 from Urea starts the process again.