Which of these are reactants in urea cycle? (i) CO2 (ii) Aspartic acid (iii) Arginine (iv) Ornithine
i and ii
ii and iii
iii and iv
i and iv
The correct option is A i and ii In the first step of the urea cycle, ammonia and carbon dioxide combine to enter the cycle. 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 water is used up and urea is produced. The ornithine regenerated in this step again takes part in the cycle.
Therefore, CO2 and aspartic acid are reactants in this cycle while arginine and ornithine are intermediates.