One-Carbon Metabolism and Methylation

Core Idea

One-carbon metabolism transfers one-carbon units at the formyl, hydroxymethyl, and methyl oxidation levels, using folate as the carrier. These units are essential for nucleotide synthesis (purines and pyrimidines) and methylation reactions via S-adenosylmethionine. The pathway integrates amino acid degradation, nucleotide biosynthesis, and gene regulation.

Explainer

From your study of sulfur amino acid metabolism, you know that methionine is activated to S-adenosylmethionine (SAM), the universal methyl donor, and that its demethylation produces homocysteine. One-carbon metabolism is the broader network that regenerates methionine from homocysteine, supplies one-carbon units for building nucleotides, and connects these processes through a shared carrier: the B-vitamin folate (tetrahydrofolate, or THF).

Think of THF as a molecular taxi that picks up single-carbon fragments from amino acid breakdown and delivers them wherever the cell needs a one-carbon unit. The carbon can ride at different oxidation states — as a formyl group (–CHO, most oxidized), a methylene group (–CH₂–), or a methyl group (–CH₃, most reduced) — and the cell can interconvert between these forms. The primary source of one-carbon units is serine, which donates its hydroxymethyl group to THF via serine hydroxymethyltransferase, producing glycine and N⁵,N¹⁰-methylene-THF. This methylene-THF sits at a metabolic branch point: it can be oxidized to formyl-THF for purine synthesis (contributing carbons C2 and C8 of the purine ring), used directly by thymidylate synthase to methylate dUMP to dTMP (essential for DNA synthesis), or reduced to methyl-THF for methionine regeneration.

The methionine cycle closes the loop. N⁵-methyl-THF donates its methyl group to homocysteine via methionine synthase, a reaction that requires vitamin B₁₂ as a cofactor. This regenerates both methionine (which can be reactivated to SAM) and free THF (which can pick up another one-carbon unit). SAM then methylates dozens of substrates — DNA (gene silencing via CpG methylation), histones (chromatin regulation), neurotransmitters, phospholipids, and more. Every methylation reaction produces S-adenosylhomocysteine (SAH), which is hydrolyzed back to homocysteine, restarting the cycle.

The clinical importance of this pathway is enormous. Folate deficiency starves the cell of one-carbon units, impairing both DNA synthesis (causing megaloblastic anemia from failed cell division in bone marrow) and neural tube closure in embryos. B₁₂ deficiency traps folate as methyl-THF — the so-called "methyl trap" — because without B₁₂, methyl-THF cannot donate its methyl group and regenerate free THF. The result mimics folate deficiency even when folate intake is adequate. Elevated homocysteine, a marker of impaired one-carbon metabolism, is associated with cardiovascular risk. Drugs like methotrexate exploit this pathway by inhibiting dihydrofolate reductase, blocking THF regeneration and halting DNA synthesis in rapidly dividing cancer cells. One-carbon metabolism is thus the metabolic crossroads where amino acid catabolism, nucleotide biosynthesis, epigenetic regulation, and clinical pharmacology all converge.