Question

Which of these statements are true regarding urea cycle?
1. Fumarate acts as a connecting link between urea cycle and Krebs cycle.2. Urea cycle solely occurs in mitochondria only.3. Citrulline and arginine reacts to form argininosuccinate.4. Ornithine formed in the last reaction is directly excreted along with urea.5. Arginase promotes cleavage of arginine to urea and ornithine.

• A. 1, 2, 5
• B. 1, 2
• C. 3, 4
• D. 1, 5

Answer 1

The correct option is D 1, 5

1. Fumarate is a side product formed during the cleavage of argininosuccinate along with arginine. It is used as a starting material in gluconeogenesis and also it is involved in Krebs cycle.
2. The enzymes for the first two steps of the urea cycle occurs in the mitochondria and rest in the cytosol. So it occurs in both mitochondria and cytosol
3. Formation of argininosuccinate occurs by the condensation reaction of citrulline and aspartate by the consumption of an ATP.
4. Ornithine is not excreted out directly with urea. It is involved in the further urea cycles.
5. In the last stage of the urea cycle, the formation of urea and ornithine from arginine is promoted by the enzyme arginase.
   

The correct statements in these are 1 and 5. So the correct option is 'D'.