Histone modifications regulate gene expression in a cell by switching between the activation and repression of gene transcription. The methylation of histones is a unique type of chemical modification where up to three methyl groups can be added to a single lysine residue, resulting in mono-, di-, and tri-methylated states (me1, me2, me3). These modifications play a role in either activating or repressing gene expression, depending on where the lysine residues are located on the histone proteins. Histone lysine methyltransferases (KMTs) are enzymes responsible for transferring methyl groups from a compound called AdoMet to histone lysine residues, generating adenosylhomocysteine and three methylated products.
Histone acetylation levels are controlled by two groups of enzymes with opposing functions: histone acetyltransferases (HATs) and histone deacetylases (HDACs). HATs add an acetyl group from acetyl-CoA to lysine residues in histone tails, neutralizing their positive charge. This weakens the interaction between histones and DNA, making chromatin less tightly packed and more accessible for gene transcription. On the other hand, HDACs remove acetyl groups from histone lysine residues, acting as repressors of gene expression.