They are phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine. These core histones are rich in lysine and arginine residues. Both lysine and arginine residues are known to be methylated. The histone methyltransferases are specific to either lysine or arginine. Aspartate kinase becomes downregulated by the presence of threonine or lysine. All histones have a highly positively charged N-terminus with many lysine and arginine residues. Tranexamic acid is a synthetic analog of the amino acid lysine. These enzymes play a key role in the biosynthesis of lysine, threonine, and methionine. Cadaverine is the decarboxylation product of the amino acid lysine. Some grains are deficient in the essential amino acid, lysine. The major constituents of the cake are essential amino acids such as lysine and glutamine. Proteins are usually ubiquinated at lysine residues. G6PD is negatively regulated by acetylation on lysine 403 (Lys403), an evolutionarily conserved residue. ThrA and LysC are also feed-back inhibited by threonine and lysine. This diagram shows the progressive methylation of a lysine residue.