Definition: In biochemistry, methylation refers to the replacement of a hydrogen atom (H) with a methyl group (CH3), regardless of the substrate. In general chemistry, the term deals with delivery of a methyl group in a variety of ways, not necessarily replacing hydrogen.
In biological systems, methylation is catalyzed by enzymes; such methylation can be involved in modification of heavy metals, regulation of gene expression, regulation of protein function, and RNA metabolism. Methylation of heavy metals can also occur outside of biological systems. Chemical methylation of tissue samples is also one method for reducing certain histological staining artifacts.
Methylation contributing to epigenetic inheritance can occur either through DNA methylation or protein methylation.
DNA methylation in vertebrates typically occurs at CpG sites (that is, where a cytosine is directly followed by a guanine in the DNA sequence); this methylation results in the conversion of the cytosine to 5-methylcytosine. The formation of Me-CpG is catalyzed by the enzyme DNA methyltransferase. CpG sites are uncommon in vertebrate genomes but are often found at higher density near vertebrate gene promoters where they are collectively referred to as CpG islands. The methylation state of these CpG sites can have a major impact on gene activity/expression.
protein methylation typically takes place on arginine or lysine amino acid residues in the protein sequence. Protein methylation has been most well studied in the histones. The transfer of methyl groups from S-adenosyl methionine to histones is catalyzed by enzymes known as histone methyltransferases. Histones which are methylated on certain residues can act epigenetically to repress or activate "gene" expression (histone methylation). protein methylation is one type of post-translational modification.
See also
DNA methylation
protein methylation