methylenetetrahydrofolate

Methylenetetrahydrofolate, typically referring to 5,10-methylene-tetrahydrofolate (5,10-CH2-THF), is a reduced form of folate that participates in mammalian one-carbon metabolism. As a one-carbon donor, it provides the methylene group for the synthesis of thymidine, via thymidylate synthase, from deoxyuridine monophosphate, a reaction essential for DNA replication and repair. It also serves as an intermediate in the folate cycle, linking nucleotide synthesis to methylation reactions.

5,10-CH2-THF is generated from dihydrofolate by dihydrofolate reductase and is produced by the serine hydroxymethyltransferase reaction, which transfers a one-carbon unit from serine to tetrahydrofolate. It can be reduced by methylenetetrahydrofolate reductase (MTHFR) to form 5-methyl-tetrahydrofolate (5-methyl-THF), the principal circulating folate species that donates a methyl group to homocysteine, converting it to methionine in a reaction catalyzed by methionine synthase with vitamin B12 as a cofactor.

Biological significance and clinical relevance: Because 5,10-CH2-THF feeds thymidylate synthesis, adequate availability is critical for DNA synthesis. Deficiency impairs DNA replication, contributing to megaloblastic anemia and, during development, neural tube defects. Genetic variations that reduce MTHFR activity, such as the C677T polymorphism, can lower 5-methyl-THF production and raise homocysteine levels, with potential cardiovascular and neurodevelopmental implications. Folate status and B12 status influence these pathways.

Dietary sources and medical use: Folate is found in leafy vegetables, legumes, and liver. In clinical settings, reduced-folate forms such as leucovorin (folinic acid) can be used to rescue thymidylate synthesis during antifolate chemotherapy, by replenishing folate cofactors, including 5,10-CH2-THF derivatives.