Homocysteine can be remethylated to methionine via two pathways. Which enzyme uses 5-methyltetrahydrofolate as the methyl donor and requires vitamin B12 as cofactor?

• A Methionine synthase (5-methyltetrahydrofolate-homocysteine methyltransferase) ✓
• B Betaine-homocysteine methyltransferase (BHMT)
• C Cystathionine beta-synthase (CBS)
• D Methylmalonyl-CoA mutase

Explanation

Methionine synthase (MS) catalyses: homocysteine + 5-methyl-THF → methionine + THF. This reaction requires methylcobalamin (vitamin B12 as cofactor) and links the folate cycle to the methionine cycle. Vitamin B12 deficiency traps folate as 5-methyl-THF (the 'methylfolate trap'), depleting THF for purine/thymidylate synthesis. BHMT uses betaine as the methyl donor and is a B12-independent alternative mainly in liver and kidney. CBS is a PLP-dependent enzyme catalysing the transsulfuration pathway (homocysteine → cystathionine).

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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