methyldonor

A methyldonor, or methyl donor, is a molecule that transfers a methyl group (−CH3) to another molecule during a chemical reaction. In biology, methyl donors are central to one-carbon metabolism and to the regulation of gene expression through methylation. The most important biological methyl donor is S-adenosyl-L-methionine (SAM).

SAM is formed from methionine and ATP by the enzyme methionine adenosyltransferase. In most methylation reactions, SAM donates a methyl group to substrates such as DNA, RNA, proteins, and small molecules, mediated by methyltransferase enzymes. After donation, SAM becomes S-adenosyl-L-homocysteine (SAH), which is hydrolyzed to homocysteine and adenosine. The homocysteine can be remethylated back to methionine, using one-carbon units supplied by the folate cycle and vitamins B12 and B6, thereby regenerating SAM and sustaining methylation capacity.

Other physiological methyl donors feed into this network. Methionine itself is the amino acid precursor to SAM. Choline and its oxidation product betaine can donate methyl groups to homocysteine in remethylation pathways. Folate and other B vitamins influence the availability of one-carbon units required for these transfers.

Biological significance arises from the role of methyl donors in DNA and RNA methylation, protein methylation, and lipid methylation, processes that affect gene expression, development, and genome stability. Disruptions in methyl donor availability or metabolism can alter epigenetic patterns and are studied in nutrition, development, and disease contexts.

In chemistry outside biology, methyl donors also refer to reagents that transfer methyl groups in organic synthesis, but in biological systems SAM remains the canonical methyl donor.