Transmethylation is a biologically important organic chemical reaction in which a methyl group

Methyl group

Methyl group is a functional group derived from methane, containing one carbon atom bonded to three hydrogen atoms —CH3. The group is often abbreviated Me. Such hydrocarbon groups occur in many organic compounds. The methyl group can be found in three forms: anion, cation and radical. The anion...

 is transferred from one compound to another.

An example of transmethylation is the recovery of methionine from homocysteine

Homocysteine

Homocysteine is a non-protein amino acid with the formula HSCH2CH2CHCO2H. It is a homologue of the amino acid cysteine, differing by an additional methylene group. It is biosynthesized from methionine by the removal of its terminal Cε methyl group...

. In order to sustain sufficient reaction rates during metabolic stress, this reaction requires adequate levels of vitamin B₁₂ and folic acid. Methyl tetrahydrofolate delivers methyl groups to form the active methyl form of vitamin B₁₂ that is required for methylation of homocysteine. Deficiencies of vitamin B₁₂ or folic acid cause increased levels of circulatiing homocysteine. Elevated homocysteine is a risk factor for cardiovascular disease and is linked to the metabolic syndrome (insulin insensitivity). Transmethylation is decreased sometimes in children with autism.