In chemistry, biochemistry, and biology methylation refers to the addition of methyl groups (-CH3; the addition of 12 mass units or delta mass = 15 dalton) to organic compounds. This reaction type is a specific case of alkylation.
The term alkylation refers to the transfer of an alkyl group, such as the isopropyl group, –CH(CH3)2, or the methyl group, -CH3, from one molecule to another. Alkyl groups may be transferred as alkyl groups with a positively charged carbon atom, a free radical, a carbanion or a carbine. In chemistry, some typical methylation reagents are dimethylsulfate, methanol, methyl halides and diazomethane.
In cellular proteins many protein or peptide motifs that contain lysine residues can be methylated or acetylated. These modified motifs can lead to the recognition by other protein domains such as chromo-domains or bromo-domains. Such domains are found in proteins regulating chromatin structure and gene expression.
The flexible N-terminal and C-terminal ends of histones are known to contain lysine modifications important for the coupling of histones to changes in chromatin organization and the epigenetic control of gene expression. Typically, a single chromo- or bromo-domain recognizes a suitable modified lysine residue within a short peptide motif sequence.