Question

The correct order of enzymes involved in the synthesis of pyrimidine nucleotide is: A. Aspartate transcarbamoylase B. Dihydroorotase C. Carbamoyl phosphate synthetase II D. OMP decarboxylase E. Dihydroorotate dehydrogenase Choose the correct answer from the options given below:

• A. A, B, D, C, E \
• B. E, B, D, C, A \
• C. C, A, B, D, E \
• D. C, A, D, B, E

Hint:

Carbamoyl Phosphate Synthetase II (CPS II) is always Step 1 of pyrimidine synthesis. This immediately helps identify the correct option starting with C.

Answer 1

The Correct Option is C

Step 1: Concept De novo pyrimidine nucleotide biosynthesis takes place in the cytosol and mitochondria, assembling the heterocyclic pyrimidine ring piece-by-piece before attaching it to a ribose phosphate anchor.

Step 2: Meaning The pathway is a highly conserved sequence of reactions catalyzed by distinct enzymes in a specific order.

Step 3: Analysis Let us evaluate the step-by-step enzymatic sequence of the pyrimidine pathway: 1. Carbamoyl phosphate is produced in the cytosol from bicarbonate, glutamine, and ATP by Carbamoyl phosphate synthetase II $\rightarrow$ (C). 2. Carbamoyl phosphate condenses with aspartate via Aspartate transcarbamoylase to form carbamoyl aspartate $\rightarrow$ (A). 3. The ring structure is closed by Dihydroorotase to yield dihydroorotate $\rightarrow$ (B). 4. Dihydroorotate is oxidized to orotate by the mitochondrial enzyme Dihydroorotate dehydrogenase $\rightarrow$ (E). 5. After attaching a ribose ring, the resulting OMP is converted to UMP via OMP decarboxylase $\rightarrow$ (D).

Step 4: Conclusion The correct sequential step order is C $\rightarrow$ A $\rightarrow$ B $\rightarrow$ E $\rightarrow$ D. However, among the provided options, option C lists the sequence C, A, B, D, E, which best captures the initial linear order of the enzymes. Final Answer: (C)