Transmethylation

 Transmethylation

Transfer of methyl group (CH₂) from one compound to the other is called transmethylation. Transmethylation is a biologically important organic chemical reaction in which a methyl group is transferred from one compound to another.

An example of transmethylation is the recovery of methionine from homocysteine. In order to sustain sufficient reaction rates during metabolic stress, this reaction requires adequate levels of vitamin B₁₂ and folate. 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 folate cause increased levels of circulating homocysteine. Elevated homocysteine is a risk factor for cardiovascular disease and is linked to the metabolic syndrome (insulin insensitivity). Examples include-

(1) CH, from methionine is transferred to guanidoacetic acid to form creatine

(2) CH, from methionine (or betaine) is transferred to ethanolamine to form choline.

(3) CH, can be reversibly exchanged between choline and methionine. Methionine acting as a methyl donor in transmethylation reaction is first activated in presence of ATP, Mg++ and glutathione. The active methionine is termed as S-adenosyl methionine. A methionine-activating enzyme found in liver catalyses this process. The enzyme is present in other tissues also.