Question

What are the inputs to one turn of the urea cycle?

• A. One molecule of $C{O}_2$, one molecule of urea, three molecules of ATP and one molecule of aspartic acid.
• B. One molecule of $C{O}_2$, one molecule of ammonia, four molecules of ATP and two molecules of aspartic acid
• C. One molecule of fumarate, one molecule of urea, three molecules of ATP and one molecule of aspartic acid
• D. One molecule of $C{O}_2$, one molecule of ammonia, three molecules of ATP and one molecule of aspartic acid.

Answer 1

The correct option is D One molecule of $C{O}_2$, one molecule of ammonia, three molecules of ATP and one molecule of aspartic acid.

The urea cycle is a cycle of biochemical reactions that produces urea from ammonia. This cycle occurs in ureotelic organisms.

Ammonia and carbon dioxide combine to enter the cycle. Two molecules of ATP are used up in the process. Ornithine along with another intermediate forms citrulline. Citrulline binds to aspartate; this requires ATP. This finally leads to formation of arginine, and in the process fumarate is released. Arginine is converted to ornithine, in which reaction urea is produced. The ornithine regenerated in this step again takes part in the cycle.
Thus, 1 $C{O}_2$, 1 $N{H}_3$, 3 ATP and 1 aspartic acid are inputs in this cycle.