One-carbon metabolism

One-carbon metabolism is a network of reactions that transfers single-carbon units among folate and methionine pathways to support biosynthesis and methylation.

Folate cycle: A set of reactions that carries one-carbon units on folate cofactors in different oxidation states. It supplies one-carbon units for nucleotide synthesis and remethylation of homocysteine.
Tetrahydrofolate: The reduced, active form of folate that carries one-carbon units in several oxidation states. Its nitrogen atoms bind the carbon units central to the folate cycle.
Purine biosynthesis: The biochemical pathway that produces adenine and guanine nucleotides from smaller precursors. Two steps use folate-bound one-carbon units to build purine rings.
Transsulfuration pathway: A pathway that converts homocysteine into cystathionine and cysteine through vitamin B6-dependent reactions. It removes homocysteine rather than remethylating it within the methionine cycle.
Methionine cycle: A metabolic cycle that regenerates methionine and S-adenosylmethionine while producing homocysteine. It converts folate-derived methyl groups into the universal methyl donor S-adenosylmethionine.
Folate: A family of water-soluble B vitamins and related compounds needed for one-carbon transfer reactions. Dietary folate supplies the precursor from which cells make tetrahydrofolate.
Thymidylate synthesis: The conversion of deoxyuridine monophosphate into deoxythymidine monophosphate for DNA synthesis. It consumes a folate-bound carbon unit and regenerates dihydrofolate.
Betaine-homocysteine S-methyltransferase: An enzyme that transfers a methyl group from betaine to homocysteine, producing methionine and dimethylglycine. It remethylates homocysteine without using folate or vitamin B12.
Serine hydroxymethyltransferase: An enzyme that transfers a carbon from serine to tetrahydrofolate, producing glycine and 5,10-methylene-tetrahydrofolate. It is a major entry point for carbon units into the folate cycle.
Vitamin B12: A cobalt-containing vitamin required by methionine synthase and methylmalonyl-CoA mutase. Methionine synthase uses it to transfer folate-bound methyl groups to homocysteine.

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