Primary metabolic role of vitamin B12?

• A. Cofactor for methionine synthase and methylmalonyl-CoA mutase; essential for DNA synthesis
• B. Antioxidant
• C. Coenzyme in glycolysis
• D. Carrier for iron in heme synthesis

Answer: (A)

Vitamin B12 acts as a cofactor for two key enzymes, linking folate metabolism to DNA synthesis and to energy production from certain amino acids and fats. In the first reaction, a methyl group is transferred from 5-methyl-tetrahydrofolate to homocysteine to form methionine, and the folate is recycled to a form usable for nucleotide synthesis. This connection to folate is crucial because it keeps enough tetrahydrofolate available for thymidine (and thus DNA) synthesis; without B12, folate becomes trapped in a form that cannot support DNA replication, leading to impaired cell division and megaloblastic anemia. In the second reaction, B12 is required for methylmalonyl-CoA mutase to convert methylmalonyl-CoA into succinyl-CoA, channeling certain amino acid and odd-chain fatty acid catabolic products into the TCA cycle for energy. Because of these roles, vitamin B12 is not an antioxidant, not a coenzyme in glycolysis, and not a carrier for iron in heme synthesis.