Question

In the skeletal form of ornithine cycle given above, some intermediate products are indicated by alphabets. Choose the answer in which these alphabets are properly matched with the names of corresponding products.

• A. A- Citrulline, B- Fumaric acid, C- Arginine
• B. A- Citrulline, B- Arginine, C- Fumaric acid
• C. A- Arginine, B- Succinic acid, C- Fumaric acid
• D. A- Citrulline, B- Arginine, C- Succinic acid

Answer 1

The correct option is B A- Citrulline, B- Arginine, C- Fumaric acid

Steps in urea cycle are as follows-
1. Synthesis of carbamoyl phosphate : Carbamoyl phosphate synthase I of mitochondria catalyses the condensation of $N{H}_4^{+}$ ions with $C{O}_2$ to form carbamoyl phosphate. This steps consumes two ATP and is irreversible, and rate-limiting step.
2. Formation of citrulline : Citrulline is synthesized from carbamoyl phosphate and ornithine by ornithine transcarbamoylase.
3. Synthesis of arginosuccinate : Arginosuccinate synthase ( in cytosol) condenses citrulline with aspartate to produce arginosuccinate. This step requires ATP, which is cleaved to AMP and pyrophosphate(PPi), which brokes down to inorganic phosphate (Pi).
4. Cleavage of arginosuccinate : Arginosuccinase cleaves arginosuccinate to give Arginine and fumarate.
5. Formation of urea : Arginase cleaves arginine to yield urea and ornithine. Ornithine, so regenerated, enters mitochondria for its reuse in the urea cycle.

So, 'A' is citrulline, 'B' is arginine and 'C' is fumaric acid.