Question

Deoxyuridine monophosphate (dUMP) is converted to Thymidine monophosphate (TMP) by which of the following biochemical reaction?

• A. Reductive Methylation
• B. Carboxylation
• C. Deamination
• D. Oxidative decarboxylation

Hint:

Thymidine is just "Methylated Uracil."
This reaction is the "Choke Point" of DNA synthesis.
Drugs like Methotrexate stop this process by blocking the recycling of the Folate needed for this methylation.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question focuses on the biosynthesis of DNA nucleotides, specifically the formation of Thymidine (which is unique to DNA) from a Uracil precursor. This reaction is a key target for chemotherapy drugs.

Step 2: Detailed Explanation:

• The Reaction: The enzyme Thymidylate Synthase catalyzes the conversion of dUMP to dTMP (TMP).

• The Methyl Donor: This reaction requires the cofactor $N^5,N^{10$-methylene tetrahydrofolate} ($CH_2$-THF).

• Mechanism: Unlike most methylation reactions where THF simply acts as a carrier, here THF serves two roles:
- It provides the single carbon (methyl group).
- It provides the reducing power (hydrogen atoms).

• Reductive Nature: During the transfer, the folate is oxidized to Dihydrofolate (DHF). Because the carbon group is added and then reduced to a methyl group (using electrons from the folate ring), the process is classified as a reductive methylation.

• Significance: This is the only reaction in which THF is converted back to DHF. The cell must use the enzyme Dihydrofolate Reductase (DHFR) to regenerate THF to keep DNA synthesis going.

• Analyzing other options: Carboxylation adds $CO_2$. Deamination removes nitrogen. Oxidative decarboxylation removes carbon and $CO_2$ while producing energy (like in the Krebs cycle). None of these fit the dUMP $\rightarrow$ TMP conversion.

Step 3: Final Answer:
The conversion of dUMP to TMP involves the addition of a methyl group coupled with a reduction step, technically known as reductive methylation.