Fig. 1.

The basic process of DNA methylation. S-adenosylmethionine (SAM) is used as the methyl (CH3) donor by the enzyme DNA methyltransferase (DNMT) to transfer a methyl group to cytosine rings of the DNA strands. TET proteins are dioxygenase enzymes that hydroxylate 5-methylcytosine residues to form 5-hydroxymethylcytosine (5hmC). They use a metabolite intermediate, α-ketoglutarate (α-KG), and molecular oxygen as enzyme cofactors for this reaction