Question

Which of the following steps are ATP dependent in urea cycle?
I. Combination of ammonia and $C{O}_2$
II. Conversion of ornithine to citrulline
III. Reaction of citrulline with aspartate
IV. Formation of urea

• A. I only
• B. I, II, III, and IV only
• C. I and III only
• D. I and IV only

Answer 1

The correct option is C I and III only

In urea cycle total three ATPs are utilised.
Ammonia and carbon dioxide combine to enter the cycle. 2 ATP molecules are used up in this process. Ornithine along with another intermediate forms citrulline. Citrulline binds to aspartate; this requires another ATP molecule. This finally leads to formation of arginine, and in the process fumarate is released. Arginine is converted to ornithine, in which reaction water is used up and urea is produced. The ornithine regenerated in this step again takes part in the cycle.